Automotive Why engine efficiency drops for light loads?

[physea]

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well.

Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well.

Thanks!

 

[anorlunda]

Well, at zero load the efficiency is zero by definition.

If the curve of efficiency versus load is continuous and smooth, it must turn toward the zero-zero point. I don't need to know anything about engines to say that.

 

[russ_watters]

The "why" is at least partly a matter of friction being constant at a particular rpm. So if friction loss is 20% of engine output at 100% power, it is 50% of engine output at 40% power (and as said previously, 100% at 20%).

My car burns about 30% as much gas at idle as it does at 60mph.

 

[RogueOne]

The "why" is at least partly a matter of friction being constant at a particular rpm. So if friction loss is 20% of engine output at 100% power, it is 50% of engine output at 40% power (and as said previously, 100% at 20%).

My car burns about 30% as much gas at idle as it does at 60mph.

Friction is not a constant. The friction increases with torque output due to side-loading on pistons and loads on bearings.

However, there are losses that correlate inversely with torque output. Pumping losses, for example, are not as present with higher throttle positions and the engine's corresponding torque output. When the throttle is closed, the engine has to do work to decompress the intake charge inside of the intake tract downstream of the throttle. Inhale through a straw vs your mouth wide open. It takes more work to draw a smaller amount of air.

Also, the dynamic and effective compression ratios change with RPM and Throttle position. When you draw a larger amounts of fuel and air in, the piston compresses it to higher pressures just prior to ignition (more mass in a fixed volume combustion chamber). More heat is released in a shorter amount of time, and a smaller portion of that heat is lost to transferring into the combustion chamber/cylinder/block before it is converted into torque.

Most engines make their peak BSFC (Brake-Specific-Fuel-Consumption) while 80% loaded at the RPM where the engine would make peak torque if it were at 100% throttle/load.

 

[Baluncore]

My car burns about 30% as much gas at idle as it does at 60mph.

Per hour or per mile?

 

[Never to Old]

Congratulations to the first three answers, you missed the obvious, easy answer.. Its simple, it requires a calculable amount of horse power/torque to propel a vehicle of a certain weight and air resistance down the road at 50 mph or 60 mph or speed of the factories choosing.. The engine for said vehicle is designed to travel at the speed desired is designed with that requirement in mind.. You go faster it takes more power, more fuel, you go slower you drop below the efficient torque curve of the engine.. More fuel is required going slow because you are operating below the efficient torque curve so you have to give more fuel to make the required power in the inefficient portion of the power/torque curve of the engine...

 

[Baluncore]

Congratulations to the first three answers, you missed the obvious, easy answer.. Its simple, ...

If only the question was better worded and the answer could be so simple.

 

[RogueOne]

Congratulations to the first three answers, you missed the obvious, easy answer.. Its simple, it requires a calculable amount of horse power/torque to propel a vehicle of a certain weight and air resistance down the road at 50 mph or 60 mph or speed of the factories choosing.. The engine for said vehicle is designed to travel at the speed desired is designed with that requirement in mind.. You go faster it takes more power, more fuel, you go slower you drop below the efficient torque curve of the engine.. More fuel is required going slow because you are operating below the efficient torque curve so you have to give more fuel to make the required power in the inefficient portion of the power/torque curve of the engine...

Actually, you just gave an incorrect answer to a question that was not asked. Automobile engines do not achieve peak thermal efficiency at 50 or 60 mph. Do you know what brake specific fuel consumption is? The amount of work done per unit of fuel energy administered to the engine peaks at high loads. Generally 80-90% load. The reason cars don't get better gas mileage at those high loads is because the parasitic losses on the chassis increase exponentially with speed. Your engine might be doing more work per liter of fuel take in, but it has to do a disproportionately larger amount of work to overcome the wind resistance at those higher speeds.

Engines have high brake-specific fuel consumption at low loads for the reasons I shared in post #4.

Automobiles, as a system, get good fuel mileage between 50-70 mph because that is the fastest speed they go before the aerodynamics cause too much load on the engine. The engine runs more efficiently in terms of power output per unit of fuel, but the system achieves lower fuel economy because it is doing work that is not considered productive (accelerating air around and in front of it).

This question, however, was about engines. The efficiency of one component of the system that is the vehicle. Not the overall system that is the vehicle. If engines were designed solely to give the vehicle peak fuel economy at a certain speed, they would be very small engines making around 30hp and cruising at nearly WOT. An understanding of BSFC will help you understand why that is the case.

 

[tech99]

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well.

Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well.

Thanks!

One factor is that there is time at slow speeds for heat to be lost from the charge (or air for a Diesel) as it is compressed, and from the hot gas as it expands.
For a Diesel we do not usually have a throttle, so do not have pumping losses, which is an advantage at low speeds. There are also losses caused by valve overlap, which will be working in conjunction with an inlet and exhaust tract designed for a fairly high speed. Another is that if using a turbo charger, it is ineffective at light loads, so the engine drops back to being naturally aspirated.

 

[Never to Old]

RogueOne... Com'on guy, you're picking fly poop out of pepper... Of course the design parameters for an engine to go into a particular type of automobile are far more extensive than what I described... I was mearly attempting to give a non-tech answer to the question... Didn't intend to ruffle your feathers and make you dig out all the big, impressive words from your textbook...

 

[Randy Beikmann]

RogueOne... Com'on guy, you're picking fly poop out of pepper... Of course the design parameters for an engine to go into a particular type of automobile are far more extensive than what I described... I was mearly attempting to give a non-tech answer to the question... Didn't intend to ruffle your feathers and make you dig out all the big, impressive words from your textbook...

Actually, RogueOne was right to point out the correct answer in precise physics terms, and to avoid combining the "part load" factor as posed by the OP, with the "low vehicle speed" factor. His explanation was thorough, and avoided any ambiguity. I would take the opportunity to look up any terms he used that you don't understand.

 

[Never to Old]

Actually, RogueOne was right to point out the correct answer in precise physics terms, and to avoid combining the "part load" factor as posed by the OP, with the "low vehicle speed" factor. His explanation was thorough, and avoided any ambiguity. I would take the opportunity to look up any terms he used that you don't understand.

Not going to argue yours or his points for a second.. I was only trying to give a non technical answer to a simply stated question.. I can go farther into detail of design and operations of the IC engine, as I'm sure both of you gentlemen are able, I just didn't see where the question warranted the lengthy, detailed answer..

 

[RogueOne]

RogueOne... Com'on guy, you're picking fly poop out of pepper... Of course the design parameters for an engine to go into a particular type of automobile are far more extensive than what I described... I was mearly attempting to give a non-tech answer to the question... Didn't intend to ruffle your feathers and make you dig out all the big, impressive words from your textbook...

I'll entertain this off-topic post because I am sorry if I come across as rude. I want to say that my focus is to make my posts information dense. The goal itself is not to post jargon or one-up other members. Those words are in common use within this industry. Also, technically detailed answers are always welcome here. This is an engineering forum on a website called "Physics Forums" . I did not mean to belittle your post or your information. Accurate high-level answers are good, but I think they should be used as introductory paragraphs followed up by precise articulation of the factors and phenomena in the equation.

 

[Never to Old]

I'll entertain this off-topic post because I am sorry if I come across as rude. I want to say that my focus is to make my posts information dense. The goal itself is not to post jargon or one-up other members. Those words are in common use within this industry. Also, technically detailed answers are always welcome here. This is an engineering forum on a website called "Physics Forums" . I did not mean to belittle your post or your information. Accurate high-level answers are good, but I think they should be used as introductory paragraphs followed up by precise articulation of the factors and phenomena in the equation.

RogueOne... Not to worry, my skin is pretty thick, I don't offend easily.. Nor was it my intention to slight you in any way.. We both obviously have a different approach when we answer questions from people.. Somewhere in my collection of files I hold a couple pieces of paper from an institutional of higher education that says I am one of those engineer guys, couldn't stand working in an office, I have to get my hands dirty.. I have found after 50 years, or so, of answering questions from co-workers and supervision that I'm best off not making my answer any more technical than the question.. If the person asking wants to dive in deeper, by all means I will.. It's just my way of doing things, made the mistake of making my supervisor, and his supervisor, feel like they didn't know what they were doing on some major electrical distribution for a job I was working.. Took those two guys almost 18 months to get past that incident.. Mad my life miserable for a while.. All due respect to you, Ill reserve myself to reading and keeping my comment to myself... .

 

[Andy SV]

One thing I don't think anyone has mentioned yet.
Engines run more efficiently with more compression, but low throttle has very little.
I run a line from my crankcase to pull blow by and another to my exhaust.
The exhaust line raises the compression with a neutral gas
The RPM goes up just from adding pure exhaust then I drop it back down with the idle screw.
Saves a bit of gas

 

[Work Hard Play Hard]

Powertrain matching:

http://r.search.yahoo.com/_ylt=A0LEVyt8rt1YPnAAxipXNyoA;_ylu=X3oDMTEyZXNqZ25rBGNvbG8DYmYxBHBvcwMyBHZ0aWQDQjM2MjRfMQRzZWMDc3I-/RV=2/RE=1490951932/RO=10/RU=http%3a%2f%2fwww.sae.org%2fstudents%2fpresentations%2fpowertrain_matching_by_john_bucknell.pdf/RK=0/RS=gkamsANAdEAnLjIepj_fx_keHoo-

 

[russ_watters]

One thing I don't think anyone has mentioned yet.
Engines run more efficiently with more compression, but low throttle has very little.

Sorry, no - the compression ratio of a piston engine is fixed due to the geometry of the piston/cylinder.

 

[jim hardy]

Thought experiment by a Non-mechanical engineer:
If i wanted to make a machine that only wastes energy -

i could build a cascade of two pumps
that draws air into a chamber at maybe 1/4 atmosphere through a partially open valve (that's throttling)
then pumps it back up to one atmosphere and exhausts it.

That's what a lightly loaded gas engine does.
As you approach WOT there's no throttling loss so that contribution to inefficiency disappears .

I've always thought that was part of a Diesel's inherent advantage, it has no throttle plate so no intake throttling loss at part load.
Am i correct that throttling process is irreversible?

 

[Andy SV]

Sorry, no - the compression ratio of a piston engine is fixed due to the geometry of the piston/cylinder.

Really?! Wow...no
are you not thinking?
what is in the cylinder?
How much compression do you get if there is no air to compress
Only the maximum capacity of the cylinder is fixed

 

[russ_watters]

How much compression do you get if there is no air to compress
Only the maximum capacity of the cylinder is fixed

A couple of things:
1. "Compression ratio" is not a pressure ratio, it is a volume ratio. Specifically, the ratio of the starting to the ending volume of the combustion chamber. Or to put it another way: "Compression" is the act of changing volume, not the act of changing pressure.
2. Even if it was a pressure ratio, it would still be all about what is happeing in the combustion chamber, not the ambient pressure outside of the engine.

See:

The static compression ratio of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.

https://en.wikipedia.org/wiki/Compression_ratio

If what you said were true, you'd be able to find a range or graph of compression ratios for engines instead of the fixed specification that it is.

Now that said, there is an energy loss associated with air trying to force its way - unsuccessfully - through the intake when it is throttled down. This would manifest as a drag on the cylinder's movement. On the other hand, the exhaust has an easier time getting out of the engine.

 

[jim hardy]

How much compression do you get if there is no air to compress

I know exactly what you mean, and it's what you read on a "Compression Tester" gage.

Actually a "Compression Tester" indicates cylinder pressure. "Compression" is a misnomer despite how common it is.

That term, compression, in engineering circles means compression ratio .
Cylinder Pressure is usually called by that name, either MEP for Mean Effective Pressure , see https://en.wikipedia.org/wiki/Mean_effective_pressure
or Peak Pressure for the instantaneous maximum reading which is what your compression tester gage shows .

Different strokes for different folks. This being a physics forum we ought to be precise in our wording and distinguish between "Compression Ratio" and "Cylinder Pressure". As last few posts in this thread show, impreciseness of vernacular terms results in miscommunication.

old jim

 

[Andy SV]

I agree with you on that
But I used the words I did on purpose
after I looked up the different definitions of compression I chose to use
the physics definition of "inward force" as that seems to be the hart of the word
I was a bit prickly about it though

 

[physea]

interesting answers, but the answer is combustion instability

 

[Andy SV]

I don't know of any current piston driven ICE with stable combustion.
And of the engines that have stable combustion low power efficiency is underwhelming at best. So what are you talking about please

 

[jim hardy]

but the answer is combustion instability

Answer to which question?

I posit the answer is " 42." .

 

[poe]

"It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads."
This is false.
Consider, %100 load means? It means no work done, engine stalled. And the most efficient load is as close to %100 without stalling the engine.

"If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well."
This is false.
The efficiency of an ICE is inversely related to the load on it up to but not including %100.

"Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well."
From how your question is formulated it is obvious you have some grasp of things, but it is also obvious there is more you don't yet understand than what you do understand. A subject you are referring to without knowing, and mixing into the specific subject of engine load and efficiency is engine geometry. I'll just skip over it and not address it now.

Now, it's actually super easy to answer your question, if properly phrased: why are ICEs less efficient at lower loads?
Because the load is less than the engine is putting out and the extra is wasted.

"Thanks!"
You are welcome.*side note*
There are many many false statements in such threads. There are many confused people out there trying to help (thank you) and are creating confusion. I wish there was QC on statements in threads.
Reference https://www.physicsforums.com/threads/why-engine-efficiency-drops-for-light-loads.905999/

Reference https://www.physicsforums.com/threads/why-engine-efficiency-drops-for-light-loads.905999/

 

[Randy Beikmann]

"Consider, %100 load means? It means no work done, engine stalled. And the most efficient load is as close to %100 without stalling the engine."

This is completely wrong. 100% load means the engine is producing the maximum torque it can, at whatever speed it is turning. If it can produce 250 ft-lb at 3000 RPM, and it is operating at 250 ft-lb at 3000 RPM, it is operating at 100% load. Likewise, if it can produce 200 ft-lb at 5000 RPM, and it is operating at 200 ft-lb at 5000 RPM, it is operating at 100% load. There is no reason to discuss "stalling the engine."

"The efficiency of an ICE is inversely related to the load on it up to but not including %100."

This is also wrong. A spark-ignition ICE (like a gasoline-powered one) runs less efficiently the more the throttle is closed, due to the pumping work required to bring the air in past the throttling restriction. The further open the throttle, the more efficient it is, and the more torque it produces.

"From how your question is formulated it is obvious you have some grasp of things, but it is also obvious there is more you don't yet understand than what you do understand. A subject you are referring to without knowing, and mixing into the specific subject of engine load and efficiency is engine geometry. I'll just skip over it and not address it now."

Before you criticize others, you need to make sure you know the subject yourself. And FYI, engine geometry effects are not as fundamentally important as the throttling issue.

"Now, it's actually super easy to answer your question, if properly phrased: why are ICEs less efficient at lower loads?
Because the load is less than the engine is putting out and the extra is wasted."

This explanation makes little to no sense. I'm not even sure what quantity you are saying is wasted, nor why it would be when operating at less than full load.

 

[jim hardy]

@Randy Beikmann
@Randy Beikmann
hmmmm .. @ is not working again...

Great post, but i think you need to rearrange the "Quote" marks so your thoughts are demarcated from those you quoted.

 

[Randy Beikmann]

@Randy Beikmann
@Randy Beikmann
hmmmm .. @ is not working again...

Great post, but i think you need to rearrange the "Quote" marks so your thoughts are demarcated from those you quoted.

Thanks Jim. Duly noted.

 

[poe]

"This is completely wrong. 100% load means the engine is producing the maximum torque it can, at whatever speed it is turning. If it can produce 250 ft-lb at 3000 RPM, and it is operating at 250 ft-lb at 3000 RPM, it is operating at 100% load. Likewise, if it can produce 200 ft-lb at 5000 RPM, and it is operating at 200 ft-lb at 5000 RPM, it is operating at 100% load. There is no reason to discuss "stalling the engine.""

:)
You are one of the confused. You should not even have to refer to RPM. You should be able to talk about a single combustion event as if there isn't even a crankshaft at work.

What you are describing in the quoted text above is not relevant here.

"This is also wrong. A spark-ignition ICE (like a gasoline-powered one) runs less efficiently the more the throttle is closed, due to the pumping work required to bring the air in past the throttling restriction. The further open the throttle, the more efficient it is, and the more torque it produces."

What you are describing here is irrelevant. For you to even bring up the throttle body when considering engine load versus efficiency is a sign you are mixing up concepts and can't differentiate between the different concepts at work. You are mixing things up pretty bad right now. As I said, mentioning and or considering the throttle body is a sign you don't understand how to discuss the subject.

I welcome your criticism
:)

"Before you criticize others, you need to make sure you know the subject yourself."

:)
There is nothing wrong with criticism my dear fellow. I hope to bring awareness and alertness on the subject of ICE."And FYI, engine geometry effects are not as fundamentally important as the throttling issue."
Forget about the throttle body, while in some ICEs it is a big contributor to inefficiency, that is the wrong concept to bring up and mix into the subject of load on an engine versus efficiency.

"This explanation makes little to no sense. I'm not even sure what quantity you are saying is wasted, nor why it would be when operating at less than full load."

You are welcome to ask if and when you don't understand.

I'm not here on an ego trip. I love my fellow man and strive to enlighten us all. If I criticize, it is in good intent. But that doesn't mean I am going to sugar coat things for some peoples ego. I mean no disrespect.

 

[Randy Beikmann]

"This is completely wrong. 100% load means the engine is producing the maximum torque it can, at whatever speed it is turning. If it can produce 250 ft-lb at 3000 RPM, and it is operating at 250 ft-lb at 3000 RPM, it is operating at 100% load. Likewise, if it can produce 200 ft-lb at 5000 RPM, and it is operating at 200 ft-lb at 5000 RPM, it is operating at 100% load. There is no reason to discuss "stalling the engine.""

:)
You are one of the confused. You should not even have to refer to RPM. You should be able to talk about a single combustion event as if there isn't even a crankshaft at work.

What you are describing in the quoted text above is not relevant here.

"This is also wrong. A spark-ignition ICE (like a gasoline-powered one) runs less efficiently the more the throttle is closed, due to the pumping work required to bring the air in past the throttling restriction. The further open the throttle, the more efficient it is, and the more torque it produces."

What you are describing here is irrelevant. For you to even bring up the throttle body when considering engine load versus efficiency is a sign you are mixing up concepts and can't differentiate between the different concepts at work. You are mixing things up pretty bad right now. As I said, mentioning and or considering the throttle body is a sign you don't understand how to discuss the subject.

I welcome your criticism
:)

"Before you criticize others, you need to make sure you know the subject yourself."

:)
There is nothing wrong with criticism my dear fellow. I hope to bring awareness and alertness on the subject of ICE."And FYI, engine geometry effects are not as fundamentally important as the throttling issue."
Forget about the throttle body, while in some ICEs it is a big contributor to inefficiency, that is the wrong concept to bring up and mix into the subject of load on an engine versus efficiency.

"This explanation makes little to no sense. I'm not even sure what quantity you are saying is wasted, nor why it would be when operating at less than full load."

You are welcome to ask if and when you don't understand.

I'm not here on an ego trip. I love my fellow man and strive to enlighten us all. If I criticize, it is in good intent. But that doesn't mean I am going to sugar coat things for some peoples ego. I mean no disrespect.

I speak from 33+ years of experience working and studying with some of the best IC engine experts in the industry, doing extensive tests on engines myself, and digesting the major texts on the subject matter. I'm not concerned with changing your mind, because I sense that is not possible. What concerns me is that others may read what you have said and think it is correct.

I would also say that it's more useful to explain things in scientific terms, this being a science forum. So here goes:

The most basic way to evaluate an engine's efficiency is to calculate the work it does per cycle, using a P-V diagram (pressure vs. volume) for a typical cylinder, and divide by the energy in the fuel used during that cycle. For a gasoline 4-stroke engine, you must include the intake, compression, expansion, and exhaust strokes. When you do this, you find that the area inside the compression/expansion loop is positive, but that the area inside the exhaust/intake loop is usually negative (unless operating at wide-open throttle). Torque is proportional to the work done per cycle from each cylinder, and the number of cylinders in the engine.

IF the engine is operating at wide-open throttle, then the cylinder pressure during the intake stroke, and during the exhaust stroke, are roughly at atmospheric pressure. Therefore the area inside the exhaust/intake loop (called the pumping loop) is roughly equal to zero. This means the engine is very efficient at wide-open throttle, which corresponds to full load.

But most of the time, a vehicle doesn't need all the torque an engine can produce. So we use the throttle to restrict the flow of air into the engine, and thereby reduce the work done per cycle (and thus, the torque). This reduces the positive work done during compression/expansion, reducing torque. But the pressure during the intake stroke is now below atmospheric, so there is a substantial amount of negative work in the pumping loop. The throttling process is thermodynamically irreversible, and represents a mechanical loss.

As the throttle is closed more and more, an increasing amount of the positive work from the cycle is used just to bring air in during the pumping loop. Therefore the efficiency of the engine steadily declines. At a certain throttle opening, the work done in compression/expansion is just equal to that of exhaust/intake, and the work output is zero. So is the efficiency.

So the main factor in efficiency dropping as load decreases is the throttle.

 

[jim hardy]

What you are describing here is irrelevant. For you to even bring up the throttle body when considering engine load versus efficiency is a sign you are mixing up concepts and can't differentiate between the different concepts at work. You are mixing things up pretty bad right now. As I said, mentioning and or considering the throttle body is a sign you don't understand how to discuss the subject.

I think you are the one who is deluded.

https://www.mechadyne-int.com/refer...ion/part-load-pumping-losses-in-an-si-engine/

As @Randy Beikmann said.

 

[poe]

"I speak from 33+ years of experience working and studying with some of the best IC engine experts in the industry, doing extensive tests on engines myself, and digesting the major texts on the subject matter."
Great to hear. I believe it. Maybe you can help me out then. But first let's hash things out.

"I'm not concerned with changing your mind"
I thought a mind change was the whole point.

"because I sense that is not possible."
I wouldn't bet on that, if you're right. It's just that I'm saying you're not.

"What concerns me is that others may read what you have said and think it is correct."
Admirable, thank you.

"I would also say that it's more useful to explain things in scientific terms, this being a science forum. So here goes:"
Yes, let me know where I deviate.

"The most basic way to evaluate an engine's efficiency is to calculate the work it does per cycle, using a P-V diagram (pressure vs. volume) for a typical cylinder, and divide by the energy in the fuel used during that cycle."
This refers to thermodynamic efficiency.

"For a gasoline 4-stroke engine, you must include the intake, compression, expansion, and exhaust strokes. When you do this, you find that the area inside the compression/expansion loop is positive, but that the area inside the exhaust/intake loop is usually negative (unless operating at wide-open throttle)."
This refers to design efficiency.
There is also operation efficiency, which is what this thread is about, and is not what you're talking about.

"Torque is proportional to the work done per cycle from each cylinder, and the number of cylinders in the engine."
True, but irrelevant here.

"IF the engine is operating at wide-open throttle, then the cylinder pressure during the intake stroke, and during the exhaust stroke, are roughly at atmospheric pressure. Therefore the area inside the exhaust/intake loop (called the pumping loop) is roughly equal to zero. This means the engine is very efficient at wide-open throttle, which corresponds to full load."
What you have not gotten yet is that an automotive engine which lacks a throttle drops in efficiency under light load also, which again, is what this thread is about. This is specifically operational efficiency and you are mixing thermodynamic and design efficiency in.

"But most of the time, a vehicle doesn't need all the torque an engine can produce. So we use the throttle to restrict the flow of air into the engine, and thereby reduce the work done per cycle (and thus, the torque). This reduces the positive work done during compression/expansion, reducing torque. But the pressure during the intake stroke is now below atmospheric, so there is a substantial amount of negative work in the pumping loop. The throttling process is thermodynamically irreversible, and represents a mechanical loss."
This statement is accurate. Irrelevant, however.

"As the throttle is closed more and more, an increasing amount of the positive work from the cycle is used just to bring air in during the pumping loop. Therefore the efficiency of the engine steadily declines. At a certain throttle opening, the work done in compression/expansion is just equal to that of exhaust/intake, and the work output is zero. So is the efficiency."
This statement is also accurate, and also irrelevant.
You obviously know what you yourself are talking about. Maybe just not this thread.

"So the main factor in efficiency dropping as load decreases is the throttle."
Not all automotive engines have throttles.
And it is still true that "engine efficiency drops for light loads."
Your statement doesn't hold water here.

 

[poe]

I think you are the one who is deluded.

https://www.mechadyne-int.com/refer...ion/part-load-pumping-losses-in-an-si-engine/
View attachment 210443As @Randy Beikmann said.

Hi Jim,

I am familiar with the diagram. Thank you for taking time to post it though.

:)

 

[poe]

I am new here.

I was checking out your profile.

NICE!

I am very interested to see your patents. I am searching for them. I would love a ink or anything.

 

[jim hardy]

What you have not gotten yet is that an automotive engine which lacks a throttle drops in efficiency under light load also, ...

That would be in the US only a handful of hybrid vehicle engines and diesels.
Mazda recently announced a HCCI gasoline engine for 2019 but details are sparse.
This source describes it as "Throttle-less"
https://www.thoughtco.com/hcci-homogeneous-charge-compression-ignition-85588

Throttleless induction system eliminates frictional pumping losses incurred in traditional (throttle body) spark engines.

- perhaps that's why the electric supercharger which is presumably for light load cruising.

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well.

Do we know the reasons for that? I am particularly interested in the low load efficiency decrease, but high load would be interesting as well.

Thanks!

Tone of original question suggests physea is not a 'subject matter expert' . One tries to provide answers not too distant from questioner's apparent level .

Regards -

old jim

 

[poe]

.
Tone of original question suggests physea is not a 'subject matter expert' . One tries to provide answers not too distant from questioner's apparent level .

You are right

"That would be in the US only a handful of hybrid vehicle engines and diesels. Mazda recently announced a HCCI gasoline engine for 2019 but details are sparse"
I am very much interested in this and it is a current subject of research for me.

 

[jim hardy]

I added to post #36 a link to one article i found in the popular literature. Surely there are ASME and SAE papers out there - sadly i belong to neither.

old jim

 

[poe]

"This explanation makes little to no sense. I'm not even sure what quantity you are saying is wasted, nor why it would be when operating at less than full load."
I will explain the claim technically. My day is over though. Tomorrow.

 

[Randy Beikmann]

"I speak from 33+ years of experience working and studying with some of the best IC engine experts in the industry, doing extensive tests on engines myself, and digesting the major texts on the subject matter."
Great to hear. I believe it. Maybe you can help me out then. But first let's hash things out.

"I'm not concerned with changing your mind"
I thought a mind change was the whole point.

"because I sense that is not possible."
I wouldn't bet on that, if you're right. It's just that I'm saying you're not.

"What concerns me is that others may read what you have said and think it is correct."
Admirable, thank you.

"I would also say that it's more useful to explain things in scientific terms, this being a science forum. So here goes:"
Yes, let me know where I deviate.

"The most basic way to evaluate an engine's efficiency is to calculate the work it does per cycle, using a P-V diagram (pressure vs. volume) for a typical cylinder, and divide by the energy in the fuel used during that cycle."
This refers to thermodynamic efficiency.

"For a gasoline 4-stroke engine, you must include the intake, compression, expansion, and exhaust strokes. When you do this, you find that the area inside the compression/expansion loop is positive, but that the area inside the exhaust/intake loop is usually negative (unless operating at wide-open throttle)."
This refers to design efficiency.
There is also operation efficiency, which is what this thread is about, and is not what you're talking about.

"Torque is proportional to the work done per cycle from each cylinder, and the number of cylinders in the engine."
True, but irrelevant here.

"IF the engine is operating at wide-open throttle, then the cylinder pressure during the intake stroke, and during the exhaust stroke, are roughly at atmospheric pressure. Therefore the area inside the exhaust/intake loop (called the pumping loop) is roughly equal to zero. This means the engine is very efficient at wide-open throttle, which corresponds to full load."
What you have not gotten yet is that an automotive engine which lacks a throttle drops in efficiency under light load also, which again, is what this thread is about. This is specifically operational efficiency and you are mixing thermodynamic and design efficiency in.

"But most of the time, a vehicle doesn't need all the torque an engine can produce. So we use the throttle to restrict the flow of air into the engine, and thereby reduce the work done per cycle (and thus, the torque). This reduces the positive work done during compression/expansion, reducing torque. But the pressure during the intake stroke is now below atmospheric, so there is a substantial amount of negative work in the pumping loop. The throttling process is thermodynamically irreversible, and represents a mechanical loss."
This statement is accurate. Irrelevant, however.

"As the throttle is closed more and more, an increasing amount of the positive work from the cycle is used just to bring air in during the pumping loop. Therefore the efficiency of the engine steadily declines. At a certain throttle opening, the work done in compression/expansion is just equal to that of exhaust/intake, and the work output is zero. So is the efficiency."
This statement is also accurate, and also irrelevant.
You obviously know what you yourself are talking about. Maybe just not this thread.

"So the main factor in efficiency dropping as load decreases is the throttle."
Not all automotive engines have throttles.
And it is still true that "engine efficiency drops for light loads."
Your statement doesn't hold water here.

I saw this presentation posted earlier:

http://www.iitg.ernet.in/scifac/qip...n_engine/qip-ice-06-valve timing diagrams.pdf

Please read Slide 20, and compare it to my previous post (# 31). You'll see that my description explains that slide exactly. The diagram evens shows a partially-closed throttle reducing the cylinder pressure during the intake stroke, and increasing the area of the pumping loop. As Jim's post and link said, throttling is the main reason spark-ignition engines are less efficient than diesels at low load.

Please let me know if you need any further clarification. It may take me a while to respond though, since I'm grading a thermodynamics exam; and preparing a lecture on ideal engine cycles, and what reduces their thermal efficiency (which is exactly the main point of this thread).

 

[jack action]

Consider, %100 load means? It means no work done, engine stalled. And the most efficient load is as close to %100 without stalling the engine.

For my part - and I think this is true for everybody on this thread, including the OP - «load» means the torque as measured at the crankshaft output. It is usually presented as a percentage of the maximum torque that can be produced at a given rpm. Mathematically:
$$load_{@x} = \frac{T_{@x}}{T_{max\ @x}}$$
Where:

• $T$ is the crankshaft torque;
• $T_{max}$ is the maximum possible torque;
• $x$ is the reference rpm.

Could you define (mathematically, if possible) what is your definition of «load»?

The efficiency of an ICE is inversely related to the load on it up to but not including %100.

This refers to design efficiency.
There is also operation efficiency, which is what this thread is about, and is not what you're talking about.

The efficiency stated in the OP is:

It is known that automotive engine efficiency (ie. fuel efficiency) [...]

For my part - and I think this is true for everybody on this thread, including the OP - «fuel efficiency» means the mass flow rate of fuel used per unit of power. It shouldn't be a percentage.

Could you define (mathematically, if possible) what are your definitions of «fuel efficiency», «thermal efficiency», «design efficiency» and «operation efficiency», and how they relate to each other?

why are ICEs less efficient at lower loads?
Because the load is less than the engine is putting out and the extra is wasted.

With this quote, you can understand how your statement doesn't fit with my definition of «load». I'm reading that the load [output torque] is less than the engine is putting out [output torque again] and the extra is wasted [How do you waste torque?].

Even if I use «output energy» or «output power» as a definition of «load», it still doesn't make sense to say «the load is less than the engine is putting out», as the load is what the engine is putting out.

I really need your definition of «load».

 

[poe]

For my part - and I think this is true for everybody on this thread, including the OP - «load» means the torque as measured at the crankshaft output. It is usually presented as a percentage of the maximum torque that can be produced at a given rpm. Mathematically:
$$load_{@x} = \frac{T_{@x}}{T_{max\ @x}}$$
Where:

• $T$ is the crankshaft torque;
• $T_{max}$ is the maximum possible torque;
• $x$ is the reference rpm.

Could you define (mathematically, if possible) what is your definition of «load»?The efficiency stated in the OP is:

For my part - and I think this is true for everybody on this thread, including the OP - «fuel efficiency» means the mass flow rate of fuel used per unit of power. It shouldn't be a percentage.

Could you define (mathematically, if possible) what are your definitions of «fuel efficiency», «thermal efficiency», «design efficiency» and «operation efficiency», and how they relate to each other?

With this quote, you can understand how your statement doesn't fit with my definition of «load». I'm reading that the load [output torque] is less than the engine is putting out [output torque again] and the extra is wasted [How do you waste torque?].

Even if I use «output energy» or «output power» as a definition of «load», it still doesn't make sense to say «the load is less than the engine is putting out», as the load is what the engine is putting out.

I really need your definition of «load».

Great post.

It is true we are not talking about load the same way.

Things are getting interesting! I will respond as soon as I find a big enough window to sit down.

 

[poe]

Firstly, with or without a throttle, it is still true that engine efficiency drops with light load out of the device and on the device .

However it would not be true to say engine efficiency drops with higher load on the device, while it is true for out of the device. And the throttle is still not the fundamental reason why such is the case.

Also, while I may be wrong, I do not believe the OP was thinking about load output, hence the reason why I have replied multiple times that the torque formula does not apply here.

To be frank, I have to call out, for the sake of the subject at the very least, that formulas are being thrown around without understanding their underlying concepts and the relations between the different concepts at work. But let's see if me calling out others on the subject is only going to make an ass out of myself. So I pose the following challenge.

In an effort to stay as true to the thread as possible, I will not generalize and stay specific to automotive engines.

And let's do talk about fuel efficiency, thermal efficiency and what not. This does happen to be the area where there is a lot of confusion going on. And I see a lot of posts lacking the insight required for this specific subject and honestly, really, even if the OP meant load output there is still not the answer given for that in this thread.

So what are the main categories of engine efficiency? Can anyone list and populate the categories? Can anyone say in what category the throttle falls in? Can anyone say how thermal efficiency falls in? I will anyway. It's just that I don't think those who have been trying to impart knowledge on the subject can.

The synonym I use for load is pressure. And I feel the thread has been narrowed only to pressure out of the device not minding pressure on the device, which I believe is what the OP was referring to unknowingly.

Me:
why are ICEs less efficient at lower loads?
Because the load is less than the engine is putting out and the extra is wasted.

With this quote, you can understand how your statement doesn't fit with my definition of «load». I'm reading that the load [output torque] is less than the engine is putting out [output torque again] and the extra is wasted [How do you waste torque?].

Thank you for pointing the problem with this statement. In trying to keep it simple I have made an incoherent statement. Randy also pointed this out.

What I should have typed is: Because the load is less than the engine is putting out for the amount of fuel processed and the extra is wasted.

I will wait a day or two to see if anyone is willing to put themselves out and list and populate the categories of engine efficiency. And then I will do so myself anyway.

I have posted this from my phone and so it's not as well put together as I would like for the sake of clarity. When I finally get on a PC, I will try to do a good job on the subject of efficiency.

 

[jim hardy]

I will try to do a good job on the subject of efficiency.

A picture is needed.

If an engine is connected to a load, and "the load is less than the engine is putting out" ,

will not both the engine and its load be accelerated by the excess power until equilibrium is re-established ?

Excess power isn't wasted it turns into kinetic energy of the rotating parts. Or of the automobile.

Straight thinking begins with unambiguous definition of terms.

What do you mean by "what the engine is putting out" ? Shaft torque X RPM , which is power?

 

[Randy Beikmann]

And let's do talk about fuel efficiency, thermal efficiency and what not...

You can look up the definition of thermal efficiency of an engine, and it is simply a ratio: the engine's work output divided by the energy of the fuel consumed in producing it. Usually the fuel's energy is quantified by its LHV (lower heating value). If the engine converts all of the fuel's energy to work (impossible of course), then the efficiency is 1.

The efficiency can be defined and measured at several points of "output." If you use the work done by cylinder pressure on the pistons, it is called indicated thermal efficiency. If you use the work measured at the crankshaft (torque times rotation angle), it is called brake thermal efficiency (so named because an engine's output used to be measured on a Prony brake).

Indicated thermal efficiency is the most basic, measuring the effectiveness of the engine to utilize the air and fuel in the engine to push the pistons. It ignores the friction that is involved in the pistons' sliding up and down the cylinders, parts like the crankshaft rotating in the bearings, and whatever is required to turn the camshafts, water pump, oil pump, etc. The sum of these friction components is the difference between indicated thermal efficiency and brake thermal efficiency.

It's true that friction contributes to the drop in efficiency at low loads, but it's not the only cause. The work the engine must do in simply drawing air in and expelling the exhaust products is another factor, especially at low loads. This is the "pumping work" I referred to previously. In spark-ignition engines like gasoline engines (as opposed to diesels, which use compression-ignition), a throttle is used to restrict the air inducted, which reduces the amount of work done (and torque produced). The pumping work thus increases as the load decreases. This means diesels are much more efficient than spark-ignition engines at light load.

Much of the work done in recent years to reduce fuel consumption in gasoline engines has gone into reducing pumping work, such as 1) using smaller engines, 2) running engines slower, 3) changing valve overlap to reduce torque without reducing the throttle opening, and 4) deactivating some cylinders by keeping their valves closed. All of these operating strategies involve running at larger throttle openings, which is no coincidence.

I think you can find all of this in John Heywood's book on IC engines, if you hunt around.

 

[poe]

A picture is needed.

Yes, you are right. It's on me to paint it, with all the talking from my end.

If an engine is connected to a load, and "the load is less than the engine is putting out" ,

will not both the engine and its load be accelerated by the excess power until equilibrium is re-established

Yes sir, precisely. And I admit I did a bad job

 

[poe]

Sorry guys, I accidentally hit post reply.

...Yes sir, precisely. And I admit I did a bad job with my statement regarding "less than what the engine is putting out."

Excess power isn't wasted it turns into kinetic energy of the rotating parts. Or of the automobile.

This is where there is some disagreement. And I acknowledge I can't leave it at that. Only allow me some time before I address this, so there is time for others who might want to chime in.

Straight thinking begins with unambiguous definition of terms.

What do you mean by "what the engine is putting out" ? Shaft torque X RPM , which is power?

Yes sir, you, Jack, and Randy have all now called me out on this statement! I love it, thank you! And I will respond to all this here in the next couple of days.

 

[poe]

You can look up the definition of thermal efficiency of an engine, and it is simply a ratio: the engine's work output divided by the energy of the fuel consumed in producing it. Usually the fuel's energy is quantified by its LHV (lower heating value). If the engine converts all of the fuel's energy to work (impossible of course), then the efficiency is 1.

The efficiency can be defined and measured at several points of "output." If you use the work done by cylinder pressure on the pistons, it is called indicated thermal efficiency. If you use the work measured at the crankshaft (torque times rotation angle), it is called brake thermal efficiency (so named because an engine's output used to be measured on a Prony brake).

Indicated thermal efficiency is the most basic, measuring the effectiveness of the engine to utilize the air and fuel in the engine to push the pistons. It ignores the friction that is involved in the pistons' sliding up and down the cylinders, parts like the crankshaft rotating in the bearings, and whatever is required to turn the camshafts, water pump, oil pump, etc. The sum of these friction components is the difference between indicated thermal efficiency and brake thermal efficiency.

It's true that friction contributes to the drop in efficiency at low loads, but it's not the only cause. The work the engine must do in simply drawing air in and expelling the exhaust products is another factor, especially at low loads. This is the "pumping work" I referred to previously. In spark-ignition engines like gasoline engines (as opposed to diesels, which use compression-ignition), a throttle is used to restrict the air inducted, which reduces the amount of work done (and torque produced). The pumping work thus increases as the load decreases. This means diesels are much more efficient than spark-ignition engines at light load.

Much of the work done in recent years to reduce fuel consumption in gasoline engines has gone into reducing pumping work, such as 1) using smaller engines, 2) running engines slower, 3) changing valve overlap to reduce torque without reducing the throttle opening, and 4) deactivating some cylinders by keeping their valves closed. All of these operating strategies involve running at larger throttle openings, which is no coincidence.

I think you can find all of this in John Heywood's book on IC engines, if you hunt around.

Thank you for taking the time to post, Randy. I'd like to read over your post carefully, which I won't get to for a little bit. And I'd also like to talk about the same subject from a different perspective here in the next couple of days.

 

[jack action]

The synonym I use for load is pressure. And I feel the thread has been narrowed only to pressure out of the device not minding pressure on the device, which I believe is what the OP was referring to unknowingly.

A load is a resistance you put on a system. With mechanical systems, it usually refers to a force/torque imposed on the system (ex.: a water turbine on an engine crankshaft) and with electrical systems, it usually refers to a power imposed on the system (ex.: a heater on a generator).

why are ICEs less efficient at lower loads?
Because the load is less than the engine is putting out and the extra is wasted.

What I should have typed is: Because the load is less than the engine is putting out for the amount of fuel processed and the extra is wasted.

If I understand you correctly (english doesn't seem to be your mother tongue), you're talking about the force acting on the piston (the pressure inside the cylinder times the area of the cylinder) - what you call «the load» - versus the torque output - what you call «the engine is putting out». Of course there are losses in between, which are essentially going through friction and to power engine accessories, like the oil and water pumps. It is called «mechanical efficiency». It seems to be what you call «operation[al] efficiency» in post #33:

There is also operation efficiency, which is what this thread is about, and is not what you're talking about.

What you have not gotten yet is that an automotive engine which lacks a throttle drops in efficiency under light load also, which again, is what this thread is about. This is specifically operational efficiency and you are mixing thermodynamic and design efficiency in.

The friction losses are mostly independent of the cylinder pressure. So if the cylinder pressure (what you call the «load») decreases, the losses won't and therefore the mechanical efficiency will drop accordingly. As you stated, if the mechanical efficiency drops to zero (the pressure is just high enough to fight the friction losses), the engine stalls.

Thermal efficiency is basically the mechanical energy output based on the Pressure-Volume diagram (indicated power stated by @Randy Beikmann and well illustrated in @jim hardy 's post #32) vs the energy that the fuel can produce by heat alone. The throttle position will affect the PV diagram (pumping losses and less air-fuel mixture in will produce smaller peak pressures), therefore it will affect the thermal efficiency. With a diesel engine (no throttle), less fuel in will lower the peak pressure of the PV diagram. But lower pressures due to less fuel doesn't affect thermal efficiency as much since less fuel means it also produces less heat.

You seem to call that «thermodynamic efficiency» or «design efficiency» in post #33:

"The most basic way to evaluate an engine's efficiency is to calculate the work it does per cycle, using a P-V diagram (pressure vs. volume) for a typical cylinder, and divide by the energy in the fuel used during that cycle."
This refers to thermodynamic efficiency.

"For a gasoline 4-stroke engine, you must include the intake, compression, expansion, and exhaust strokes. When you do this, you find that the area inside the compression/expansion loop is positive, but that the area inside the exhaust/intake loop is usually negative (unless operating at wide-open throttle)."
This refers to design efficiency.

Now, the fuel efficiency (as stated in the OP) relies directly on both thermal and mechanical efficiencies. So throttle position (if any) will affect the fuel efficiency.

When the OP says:

It is known that automotive engine efficiency (ie. fuel efficiency) is maximum at a specific range of medium loads. If we go higher than this range, the efficiency drops. If we go lower than these loads, the efficiency drops as well.

Assuming «load» means torque, it make perfect sense. At high loads (high torque) the fuel efficiency decreases because to obtain them we must use a rich mixture where some fuel will not burn, hence the decrease in fuel efficiency. At low loads (small torque), the mechanical efficiency drops radically and most of the fuel is used to fight friction. As you stated, the closer you get to zero mechanical efficiency, the closer we get to zero torque output and, obviously, the fuel efficiency gets close to zero as well (if we consider power output over fuel consumption).

 

[poe]

A load is a resistance you put on a system. With mechanical systems, it usually refers to a force/torque imposed on the system (ex.: a water turbine on an engine crankshaft) and with electrical systems, it usually refers to a power imposed on the system (ex.: a heater on a generator).If I understand you correctly (english doesn't seem to be your mother tongue), you're talking about the force acting on the piston (the pressure inside the cylinder times the area of the cylinder) - what you call «the load» - versus the torque output - what you call «the engine is putting out». Of course there are losses in between, which are essentially going through friction and to power engine accessories, like the oil and water pumps. It is called «mechanical efficiency». It seems to be what you call «operation[al] efficiency» in post #33:The friction losses are mostly independent of the cylinder pressure. So if the cylinder pressure (what you call the «load») decreases, the losses won't and therefore the mechanical efficiency will drop accordingly. As you stated, if the mechanical efficiency drops to zero (the pressure is just high enough to fight the friction losses), the engine stalls.

Thermal efficiency is basically the mechanical energy output based on the Pressure-Volume diagram (indicated power stated by @Randy Beikmann and well illustrated in @jim hardy 's post #32) vs the energy that the fuel can produce by heat alone. The throttle position will affect the PV diagram (pumping losses and less air-fuel mixture in will produce smaller peak pressures), therefore it will affect the thermal efficiency. With a diesel engine (no throttle), less fuel in will lower the peak pressure of the PV diagram. But lower pressures due to less fuel doesn't affect thermal efficiency as much since less fuel means it also produces less heat.

You seem to call that «thermodynamic efficiency» or «design efficiency» in post #33:

Now, the fuel efficiency (as stated in the OP) relies directly on both thermal and mechanical efficiencies. So throttle position (if any) will affect the fuel efficiency.

When the OP says:

Assuming «load» means torque, it make perfect sense. At high loads (high torque) the fuel efficiency decreases because to obtain them we must use a rich mixture where some fuel will not burn, hence the decrease in fuel efficiency. At low loads (small torque), the mechanical efficiency drops radically and most of the fuel is used to fight friction. As you stated, the closer you get to zero mechanical efficiency, the closer we get to zero torque output and, obviously, the fuel efficiency gets close to zero as well (if we consider power output over fuel consumption).

Man, you guys are quick!