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:

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[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.