Automotive Why engine efficiency drops for light loads?

[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!

 

[poe]

Jack, good call on English not being my first language. My usage of language is somewhat eccentric even in my mother tongue. Though no less effective at conveying my thoughts, I hope.

My "less than what the engine is putting out" statement was a poorly thought out and incomplete one. After Randy pointed it out the first time, I realized it made no sense. I want to go over the statement again in another post. Because while it is an incoherent statement, it is a good way to see how individuals construe the statement and where their focus isn't.

On another note, I spend a lot of time examining patents. Mechanical designs with specific attention to internal combustion engines. I'm going to start bringing some overlooked concepts to the foreground in this post. And I'm not fluent in math, although I understand it more than I can speak it. I would love and appreciate any work done on the mathematics.

Where I'm coming from is, I think most people who are interested in the subject of efficiency are interested from a design/form perspective, intuitively. They want to understand the structure/map of efficiency; at what points efficiency comes into play and what the causes of drops in efficiency are.

The experts of the field, mostly academics, talk from the narrower perspective of the given forms/designs with no regard to the form itself. It's a limited perspective that seems to me to come from being focused on the same form/design for too long and or with limited knowledge. These people have an excellent grasp of function, overwhelmingly function and poor grasp of form/design and its role. So formulas are brought into topics that have to do with form/design and the topic is hijacked by function and formulas. Form defines function.

So let's get into the nitty-gritty.
Plot me the graph that shows: of the constant and even pressure, say by way of hydraulics, exerted on the crown of a piston, what percentage of the constant and even pressure is translated to load output of the cranckshaft through what could be a power stroke? Don't think RPM, think single event. The graph should have TDC at origin on the X axis and BDC at other end of X axis. And load output is on the Y axis. This graph partially points to where focus is lacking.

Plot me another graph. One that shows the load/pressure on the piston crown during the power/combustion stroke of a diesel cycle from TDC to BDC. Again, TDC and BDC go on the X axis. Pressure, in your preferred unit obviously goes on Y axis.

Once we have these two graphs, let's look at how they relate. And there are other more important and more overlooked concepts I want to go over after this one.

I am still going to list the main categories of automotive engine efficiency and populate them, just waiting to see if anyone will. There are 4 main categories. Anything and everything that has anything to do with engine efficiency will fall into one or more of these categories.

 

[poe]

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 some misunderstanding. I'll respond later.

 

[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.

I do have a problem here. I know, I know, it's how it's defined in the books. I use thermal efficiency in a different way, which will be illustrated soon in my future post regarding the categories of efficiency. And I will try to make the case that what is described as thermal efficiency above is better described as energy efficiency or something else, because we need to save thermal efficiency for the appropriate use. The way it is described now is confusing.

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.

I think we have big disagreements here. I think this is an area where there is confusion. Give me time to think and I'll respond in a meaningful way.

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

Interesting Heywood is brought up. I see him as lacking structure in his understanding of internal combustion engines, as detailed and precisely as he understands the subject. But it wouldn't be right for me to say what I just said without backing it up. I have already started putting together the post that will illustrate my claim.

 

[poe]

You seem to call that «thermodynamic efficiency»

I'm sorry, but this is a fine example of I myself using the wrong word. My next post will clarify. And typing such long replies on this little phone screen causes me to error more than usual since I can only see so little of it all.

And I got to say, it is nice to be here. You guys are definitely tightening my usage of language regarding the very subject I'm talking about.

 

[poe]

That which effects the efficiency of the device but is outside the device and does not effect how the device is operated is a limitation factor, such as the efficiency drop that would occur the closer the temperature outside the device gets to the temperature inside the combustion chamber. This effects how the device is operating and not how it's operated. This is a thermodynamic limit, it is not thermodynamic efficiency.

Efficiency is the measure of an amount usable divided by total amount. Usable is the key word. Of the total amount, fuel in this case, some portion is used, some portion is wasted due to inefficiencies, and some portion is wasted due to thermodynamic limitation. That's why the term "thermodynamic efficiency" doesn't make sense when taking about engine efficiency. It's a limit and should not be combined with efficiency. When combined, which is too often, it is confusing and isn't meaningful.

That which effects the efficiency of the device and is from within the device is an efficiency factor, such as heat lost to the surfaces of the combustion chamber, which is a factor of thermal efficiency.

An automotive engine is a runaway device, and the most efficient way to regulate such a device is by loading it. This type of regulation falls in operational efficiency. As opposed to throttle regulating or fuel regulating, both of which fall under design efficiency.

Design efficiency is the most difficult category to deal with and is what largely determines the efficiency factor of mechanical and thermal efficiency. It is the base. Mechanical efficiency is next in line in terms of its effect on engine efficiency. And operational efficiency is separate from the engine, while still effecting engine efficiency because it effects the insides of the engine, how the engine is operated.

There are 4 categories of efficiency: design efficiency, mechanical efficiency, thermal efficiency, and operational efficiency. And now I have given some examples too, without having drawn a map yet. Who can populate the categories? How does what fit where?

It's not proper to haphazardly dip into different categories of efficiency and grab disparate concepts and mush them together and call it, for example, thermal efficiency or thermodynamic efficiency or whatever. There is a structure, and it's fixed. And efficiency must be understood under this structure. It is so confusing right now. Not in terms of mathematical formulations, but in terms of concepts of form/design.

I'll wait still before providing the map/picture of the categories and populating them. Just because thinking is good for us.

 

[jim hardy]

tightening my usage of language

reasoning is but language well arranged - Laviosier

 

[Randy Beikmann]

... I know, I know, it's how it's defined in the books. I use thermal efficiency in a different way, which will be illustrated soon in my future post regarding the categories of efficiency. And I will try to make the case that what is described as thermal efficiency above is better described as energy efficiency or something else, because we need to save thermal efficiency for the appropriate use. The way it is described now is confusing.

I don't necessarily agree with the way some quantities are defined in thermodynamics, but we need a common language in order to communicate outside this small group.

Richard Feynman actually derived many relationships in trigonometry and calculus before he saw them used in any book, and he thought that his notation was much more meaningful and appropriate. But even he had to adapt to the standard symbols for anyone else to understand his work.

 

[poe]

But even he had to adapt to the standard symbols for anyone else to understand his work.

I concede to this point. I'm going to need help adapting to standard symbols and terminology. But the language aside, I'll post the structure I've been referring to soon. I thought I could type it up, but I think I'll have to draw it, take a picture, and post it.

Please bear with me.

 

[jim hardy]

but I think I'll have to draw it, take a picture, and post it.

paint is primitive and painful

but it's great for annotating other pictures. Microsoft snipping tool is real handy.

 

[poe]

I don't necessarily agree with the way some quantities are defined in thermodynamics, but we need a common language in order to communicate outside this small group.

Something to start getting the idea across as I try to put more information together.