Engine fuel efficiency

deveraux

Senior member
Mar 21, 2004
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While I was still in university studying Engineering, we did some simple practical tests on an old Otto cycle engine. This engine does not have any of the modern advancement in current cars such as VVT etc, it was just a simple basic Otto cycle engine.

We monitored its max power output point and max efficiency point. I understand that the numbers we found from that experiment will not relate directly to modern cars but I was curious if you could use some of the conclusions.

Note, this is an engine only experiment, so to translate into normal driving, it is pretty easy to see that you use the respective gear ratio to keep your engine at the efficient RPM (later in the post).

Max power output we found to be at around 6,000 rpm (give or take a few hundred either direction, its been a while so I don't recall the exact figure). Maximum efficiency however came at around 3k rpm.

The reason I bring this up is because I have seen a few places mention that it is better to lower RPM as the engine consumes more fuel but I would argue that if the engine is not at its peak efficient point, you may as well rev it up more to gain a better speed/fuel consump ratio.

So I guess I'm just wondering if optimum engine efficiency, and thus, fuel efficiency while driving is achieved at around 3k rpm?
 

overst33r

Diamond Member
Oct 3, 2004
5,761
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That's why best fuel efficiency, in my experience, is achieved by applying 3/4 throttle and short shifting before 3k (depending on your gearing and low end torque).

So basically your experiment found the horsepower/torque peak to be the most efficient part of the power band of an Otto cycle engine?
 

jagec

Lifer
Apr 30, 2004
24,442
6
81
Originally posted by: deveraux
While I was still in university studying Engineering, we did some simple practical tests on an old Otto cycle engine. This engine does not have any of the modern advancement in current cars such as VVT etc, it was just a simple basic Otto cycle engine.

We monitored its max power output point and max efficiency point. I understand that the numbers we found from that experiment will not relate directly to modern cars but I was curious if you could use some of the conclusions.

Note, this is an engine only experiment, so to translate into normal driving, it is pretty easy to see that you use the respective gear ratio to keep your engine at the efficient RPM (later in the post).

Max power output we found to be at around 6,000 rpm (give or take a few hundred either direction, its been a while so I don't recall the exact figure). Maximum efficiency however came at around 3k rpm.

The reason I bring this up is because I have seen a few places mention that it is better to lower RPM as the engine consumes more fuel but I would argue that if the engine is not at its peak efficient point, you may as well rev it up more to gain a better speed/fuel consump ratio.

So I guess I'm just wondering if optimum engine efficiency, and thus, fuel efficiency while driving is achieved at around 3k rpm?

It's a bit more complex than that, because when you're cruising at a constant speed, you're demanding a steady power output from your engine which is quite a bit lower than its full power. So the higher your RPMs, the lower the "percent load"--that is, the actual power output of the engine at that RPM (a constant, when travelling at a constant vehicle speed), versus the maximum power output at that RPM at WOT (scales upwards with RPM until near-redline on most cars).

Generally speaking, your engine is most efficient at 100% load (WOT), but this usually produces quite a bit more power in any gear than is necessary to keep the car moving at a constant pace. So for max efficiency, you keep the car in as high as gear as possible without lugging the engine...which is as close to 100% load as you can get.

Now obviously while 100% load is the most efficient point at that RPM, there is also a most efficient RPM. In order to achieve the maximum possible efficiency from an engine, you would have to tune the gearing so that the power output at 100% load, at the most efficient RPM, is exactly the same as the power requirements to keep the car moving at a steady speed with that particular wheel RPM.

It is actually possible to achieve this condition in practice. Given a car with a max efficiency RPM of 3000, place it in top gear with WOT, and increase the incline until the car is moving at a constant speed. The engine is now operating in its most efficient mode.

Of course, this doesn't really have much to do with MPG, because if you size and gear an engine which operates at its efficient RPM at WOT when the car is in top gear at a reasonable speed (like 60mph), the engine will be hopelessly anemic in normal driving conditions, and any small incline will cause it to slow precipitously.

The closest that we have come with actual production cars is probably the Honda Insight, which uses a much less oversized engine than most cars, with an electric motor to make up the extra power necessary for acceleration and high-speed operation. Sure enough, it gets phenomenal mileage.
 

Zenmervolt

Elite member
Oct 22, 2000
24,512
22
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Originally posted by: deveraux
While I was still in university studying Engineering, we did some simple practical tests on an old Otto cycle engine. This engine does not have any of the modern advancement in current cars such as VVT etc, it was just a simple basic Otto cycle engine.

We monitored its max power output point and max efficiency point. I understand that the numbers we found from that experiment will not relate directly to modern cars but I was curious if you could use some of the conclusions.

Note, this is an engine only experiment, so to translate into normal driving, it is pretty easy to see that you use the respective gear ratio to keep your engine at the efficient RPM (later in the post).

Max power output we found to be at around 6,000 rpm (give or take a few hundred either direction, its been a while so I don't recall the exact figure). Maximum efficiency however came at around 3k rpm.

The reason I bring this up is because I have seen a few places mention that it is better to lower RPM as the engine consumes more fuel but I would argue that if the engine is not at its peak efficient point, you may as well rev it up more to gain a better speed/fuel consump ratio.

So I guess I'm just wondering if optimum engine efficiency, and thus, fuel efficiency while driving is achieved at around 3k rpm?

An engine alone is not a car. For an engine alone, efficiency is measured in units of fuel used per unit of power produced. For a car, efficiency is measured in units of fuel used per unit of distance traveled.

Maximum engine efficiency does NOT necessarily occur at the same point at which a vehicle''s maximum efficiency occurs.

Also note that your measurements of engine efficiency were taken with the throttle wide open, not with it partially closed. This further distances results from the real world.

For steady-state driving (i.e. a fixed, non-variable speed, such as a long interstate trip) the most efficient situation is where the engine is spinning as slowly as possible while still avoiding lugging.

ZV
 

Auric

Diamond Member
Oct 11, 1999
9,591
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Originally posted by: Zenmervolt
For steady-state driving (i.e. a fixed, non-variable speed, such as a long interstate trip) the most efficient situation is where the engine is spinning as slowly as possible while still avoiding lugging.

I concur... and that RPM varies greatly with various designs. Mine is at 1900.
 

deveraux

Senior member
Mar 21, 2004
284
0
71
Originally posted by: Zenmervolt
An engine alone is not a car. For an engine alone, efficiency is measured in units of fuel used per unit of power produced. For a car, efficiency is measured in units of fuel used per unit of distance traveled.

I completely agree with you hence why I put effort into specifically mentioning that I am talking only of the engine. A lot of other factors creep into the equation that is well beyond the control of a car's engine.

However, would it not follow logically that even for a car, the maximum efficiency will be close to the engine's max efficiency? I of course should also take care to mention that I am specifically assuming you do not coast since obviously that will be more efficient for that short duration. Is it incorrect to assume this?

Originally posted by: Zenmervolt
Also note that your measurements of engine efficiency were taken with the throttle wide open, not with it partially closed. This further distances results from the real world.

Agreed.

Originally posted by: Zenmervolt
For steady-state driving (i.e. a fixed, non-variable speed, such as a long interstate trip) the most efficient situation is where the engine is spinning as slowly as possible while still avoiding lugging.

I do not quite understand the term 'lugging'. Could you (or anyone else for that matter) elaborate? Thanks.

Originally posted by: jagec
Generally speaking, your engine is most efficient at 100% load (WOT), but this usually produces quite a bit more power in any gear than is necessary to keep the car moving at a constant pace. So for max efficiency, you keep the car in as high as gear as possible without lugging the engine...which is as close to 100% load as you can get.

When we ran the test, the engine was definitely not most efficient at 100% load (unless I am misunderstanding the term of 100% load). After the maximum power point, higher RPM produced lower power (presumably because internal friction was now scaling better than power output from increased RPM).

Similarly, the maximum efficiency point was well below the maximum power point just before the roll off. Max efficiency in fact, could only be found by plotting a completely different graph to the power/torque vs RPM (obviously). We had to consider our throttle and fuel rate relative to power after which more fuel is consumed for the same increase in power (past max efficiency point).

Originally posted by: jagec
It is actually possible to achieve this condition in practice. Given a car with a max efficiency RPM of 3000, place it in top gear with WOT, and increase the incline until the car is moving at a constant speed. The engine is now operating in its most efficient mode.

Of course, this doesn't really have much to do with MPG, because if you size and gear an engine which operates at its efficient RPM at WOT when the car is in top gear at a reasonable speed (like 60mph), the engine will be hopelessly anemic in normal driving conditions, and any small incline will cause it to slow precipitously.

Interesting points.

PS: Just wanted to say that the purpose of this post is to know if there is such a thing as an optimum range of RPM to operate the car for "most" situations. And if possible, I was trying to tie in some of the things I learned into this since I never cared much about cars until recently.
 

jagec

Lifer
Apr 30, 2004
24,442
6
81
Originally posted by: deveraux
When we ran the test, the engine was definitely not most efficient at 100% load (unless I am misunderstanding the term of 100% load). After the maximum power point, higher RPM produced lower power (presumably because internal friction was now scaling better than power output from increased RPM).

Similarly, the maximum efficiency point was well below the maximum power point just before the roll off. Max efficiency in fact, could only be found by plotting a completely different graph to the power/torque vs RPM (obviously). We had to consider our throttle and fuel rate relative to power after which more fuel is consumed for the same increase in power (past max efficiency point).

By "100% load", I meant that the engine was at WOT, but the power demands of the transmission were equal to the power production, so that the RPM stays constant. So basically you'd be at, say, 2000RPM, WOT, but the RPMs aren't budging.

Actually, the reason why engine power tends to drop off once you're getting near redline is often due to breathing problems, not increased friction. Widen and polish the intake and exhaust, and increase valve lift and duration, and you'll produce more power at the top end, plus peak power output will occur at a higher RPM. Unless you also have issues with valve float or something. Of course, it will run like crap at lower RPMs, and if you don't beef up the internals a bit it won't be able to rev past the old redline to take advantage of this extra power.
 

Zenmervolt

Elite member
Oct 22, 2000
24,512
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Originally posted by: deveraux
However, would it not follow logically that even for a car, the maximum efficiency will be close to the engine's max efficiency? I of course should also take care to mention that I am specifically assuming you do not coast since obviously that will be more efficient for that short duration. Is it incorrect to assume this?

Yes, it is. Any time a gasoline engine is throttled back, it is terribly inefficient from a power-producing point of view (this is the single largest reason that Diesel engines are more efficient than gasoline engines, bigger even than the fact that Diesel fuel has more energy and that Diesel engines have higher compression). My car, for example, has about 15 inches of manifold vacuum at a steady 65 mph. That means the throttle is almost closed and the engine is wasting relatively huge amounts of power to suck in air.

Very inefficient from the engine's point of view, but from the overall vehicle's point of view, it's the most efficient in terms of mpg.

Originally posted by: deveraux
I do not quite understand the term 'lugging'. Could you (or anyone else for that matter) elaborate? Thanks.

"Lugging" is when the engine's RPM drops to the point where there is insufficient power produced to accelerate smoothly. As an exaggerated example, think of shifting directly from 1st to 5th. Typically lugging results in "groaning" or some sort of bucking/vibration from the engine as well.

ZV