Why are big engines so horribly inefficient?

[ShawnD]

If you look at any car that has 2 available engines, the big engine's gas mileage sucks compared to the small engine. Just as an example, a 2.4L honda accord uses 9.7L of fuel to go 100km while the 3.0L model needs 11.5L of fuel to go that same distance.

Since EPA fuel estimates are done under controlled conditions, that means the V6 burns almost 20% more fuel to get the same amount of power. Both cars are tested to accelerate at the same rate, drive the same speed, drive on the same track, and do everything as comparable as possible, but the V6 uses 20% more gas. Why?

 

[Andre]

I'd say internal friction, more moving parts, bigger piston / cilinder contact surfaces, more compression to generate. etc.

 

[Ivan Seeking]

Also, one might be designed primarily for efficiency and the other for performance. This would be logical given the selection of big or small. How do they compare in performance?

 

[Danger]

There is also the simple matter of a larger engine needing to breathe more because it has bigger lungs. In order to maintain the proper air/fuel ratio, that means that more fuel has to be introduced. The efficiency does change due to other factors, though. The 455 in my El Camino, for instance, gets better mileage than the 403 that was originally in it or the 305 in my old Camaro. The operating conditions also matter. A small motor will almost always get better mileage at a constant speed, but a big one might do better if a lot of acceleration is involved because it doesn't have to work so hard. With natural torque, your gear ratios can be shallower.

 

[russ_watters]

Actually, the thermodynamic efficiency of an internal combustion engine is not expressed in mpg and is strictly a function of compression ratio. Big engines tend to be inherrently [/b]more[/b] efficient than little ones thermodynamically because the friction losses are smaller proportionally (displacement is a square function of piston diameter, but surface contact area is a linear function).

There are three simple reasons why cars with bigger engines get lower mpg (assuming otherwise identical cars):

-Bigger engines mean faster acceleration. which uses more gas than slow, smooth acceleration.
-Engines of similar type have similar efficiency curves, but bigger engines will run the same car lower-down on that curve. Engines run at optimal thermodynamic efficiency near their optimal power.
-Bigger engines use more fuel at idle.

[edit - added a 3rd reason]

 

[ShawnD]

The 455 in my El Camino, for instance, gets better mileage than the 403 that was originally in it or the 305 in my old Camaro. The operating conditions also matter. A small motor will almost always get better mileage at a constant speed, but a big one might do better if a lot of acceleration is involved because it doesn't have to work so hard. With natural torque, your gear ratios can be shallower.

Interesting. The 2.4L and 3.5L Nissan Altima get the same highway gas mileage, and the car salesman said it was because the 3.5L "doesn't need to work as hard". I thought he was making stuff up, but your story agrees with his story.

Good explanation on the thermodynamics russ. That pretty much explains all of it.

 

[Aether]

There are three simple reasons why cars with bigger engines get lower mpg (assuming otherwise identical cars):

-Bigger engines mean faster acceleration. which uses more gas than slow, smooth acceleration.
-Engines of similar type have similar efficiency curves, but bigger engines will run the same car lower-down on that curve. Engines run at optimal thermodynamic efficiency near their optimal power.
-Bigger engines use more fuel at idle.

[edit - added a 3rd reason]

Reason #2 is right; reason #3 is a consequence of reason #2 and therefore it seems redundant; but reason #1 makes no sense to me...are you sure about this one?

 

[Cyrus]

More engine = more acceleration Aether.

 

[edward]

It depends a lot on the drive train and the torque vs horse power curve.

The new Toyota RAV 4 gets slightly better gas mileage with the 3.5 V6 engine than it does with the 2.4 inline 4 cyl.

The V6 also has great acceleration.

 

[Aether]

More engine = more acceleration Aether.

So what? Please explain why "faster acceleration...uses more gas than slow, smooth acceleration."

 

[Cyrus]

If the engine is turning faster, it sucks in more air. If it has more air, it needs more fuel. More power means more heat. More heat means more waste heat out the exhaust as well.

 

[Aether]

If the engine is turning faster, it sucks in more air. If it has more air, it needs more fuel. More power means more heat. More heat means more waste heat out the exhaust as well.

All true, but unfortunately this doesn't explain why "faster acceleration...uses more gas than slow, smooth acceleration" (which it doesn't).

 

[moose]

In some cases, the gearing is also quite different.

 

[turbo]

Years ago, my father bought a 1/2 ton Chevy 4x4 with a 305 ci engine, and he routinely got about 4mpg less than my friend's truck and my cousin's truck (both the same model, but with the 350 ci engine).

 

[rcgldr]

Please explain why "faster acceleration...uses more gas than slow, smooth acceleration."

Power is equal to force times speed. for example, horsepower = force (lbs) times speed (mph) divided by 375. To generate more acceleration, more force is required. Double the acceleration, double the force, double the power required, and more fuel consumed.

big engines versus small engines

Large engines have more internal friction and internal drag (air is contantly being moved underneath the pistons), so there's more overhead. The bottom line is how efficient an engine is making the minimum amount of power for acceptable acceleration (city driving), and cruising (highway driving). Weight and areodynamic drag are also a factor.

A general rule is that the more power an engine makes, the less efficient it is at making those "minimum" amounts of power, with engine size being a factor, but not the entire factor. Depending on the weight and drag of a car, going beyond a peak power of 150hp to 250hp, is the point where gas milage will suffer.

Engine design also is a factor. A 4 valve per cylinder engine is more efficient than a 2 valve per cylinder engine. Modern designs are more efficient. A new Volkswagen Beetle has 23/32 mpg, about the same as the 1970's VW Beetles, but with more than double the power.

Turbo and super chargers increase power, but they also consume power in the process of increasing intake pressure. For example, a 2006-2007 Corvette Z06 has a normally aspirated 7.0 liter with 505+hp, and gets slightly better gas milage than a turbo charged Porsche 911 with a turbo charged 3.6 liter engine with 480+hp. Some if this is due to the difference in weight, the Z06 is 3150 lbs, while the 911 is 3495lbs. Milage ins't great 16/26mph for the Z06, and aggresive city driving (based on magazine reports) will drop this down to 12mpg for the Z06, 11mpg for the 911.

Maximum speeds are also a factor. For the few that get to drive on the Autobahn, fuel milage at 100mph to 150mph or faster for the Z06 is very good compared to just about any other car. In the USA, legal speeds vary from 65/70mph (California), 85mph (Arizona), or 100mph (Montana, basic safe speed law with a cap of 100mph).

Adding a tidbit here, a 1000hp Bugatti Veyron, running at it's top speed of 253mph / 407kph, will empty it's tank in 12 minutes, for a range of 50miles, and fuel usage about 2.1mpg.

 

[BillJx]

If you look at any car that has 2 available engines, the big engine's gas mileage sucks compared to the small engine. Just as an example, a 2.4L honda accord uses 9.7L of fuel to go 100km while the 3.0L model needs 11.5L of fuel to go that same distance.

Since EPA fuel estimates are done under controlled conditions, that means the V6 burns almost 20% more fuel to get the same amount of power. Both cars are tested to accelerate at the same rate, drive the same speed, drive on the same track, and do everything as comparable as possible, but the V6 uses 20% more gas. Why?

More of an answer than you asked for, but I like the subject
Gasoline engines are most efficient at maximum output. So the most fuel efficient engine would be one that barely produces enough power to maintain your cruising speed on a flat road. An engine so tightly sized to the application could convert a bit more than 30% of the gasoline's chemical energy to horsepower. (Aside from the obvious shortcomings of such a "powerplant", it would wear out in a hurry so it wouldn't turn out to be so efficient after all.)

For heat engines in general, the theoretical limit of efficiency is determined by the difference in temperature between the hottest point and the coolest point in the thermodynamic cycle. In the case of a gasoline engine the compression ratio is the main limit. But running the engine "throttled" means that the entire cycle will be less efficient. (search for otto cycle) A big engine is severely throttled almost all the time, hence it's inefficient.

Even the most efficient modern large generating plants, monitored from complex control rooms, only manage 60% efficiency or less, except where they can find a use for the waste heat. The biggest marine diesels are also getting close to 60%.

 

[Aether]

Power is equal to force times speed. for example, horsepower = force (lbs) times speed (mph) divided by 375. To generate more acceleration, more force is required. Double the acceleration, double the force, double the power required, and more fuel consumed.

...double the acceleration, double the force, double the power required, yes...but for only half the amount of time, so more fuel is not consumed if everything else remains equal between these two cases.

 

[Averagesupernova]

There is also the simple matter of a larger engine needing to breathe more because it has bigger lungs. In order to maintain the proper air/fuel ratio, that means that more fuel has to be introduced.

Nope. This is not necessarily true. While a larger engine is able to displace more air, that does not mean that it will displace more air. For instance, with the throttle closed at an idle it is not taking in a full volume of air. It takes in enough to maintain a proper idle.

 

[Danger]

Nope. This is not necessarily true. While a larger engine is able to displace more air, that does not mean that it will displace more air. For instance, with the throttle closed at an idle it is not taking in a full volume of air. It takes in enough to maintain a proper idle.

The 455 in my Camino is still drawing 455 cubic inches of air/fuel mix per cycle. At idle, of course, it doesn't cycle as many times per minute as it does at 140 kph on the highway. It idles at more or less the same speed as a 273 slant six, which at idle draws 273 cubic inches of mix per cycle.
Aside from variable displacement engines, the swept volume doesn't change.

 

[Integral]

Nope. This is not necessarily true. While a larger engine is able to displace more air, that does not mean that it will displace more air. For instance, with the throttle closed at an idle it is not taking in a full volume of air. It takes in enough to maintain a proper idle.

By what mechanism does a throttle change the VOLUME of air fuel mixture a cylinder "breathes"? A throttle can can control the intake manifold pressure but it does not change the stroke of the piston.

Consider the difference in cylinder volume as the piston moves from top dead center to bottom dead center, this volume change is absolutely constant for a cylinder and is what determines the VOLUME of air fuel mixture consumed each cycle. That is why engines can be and are classified by displacement volume.

 

[rcgldr]

...double the acceleration, double the force, double the power required, yes...but for only half the amount of time, so more fuel is not consumed if everything else remains equal between these two cases.

Fuel consumed versus time yes, but not fuel consumed versus distance. If a vehicle has double the acceleration, it reaches cruise velocity in 1/2 the time. Take the simple case where acceleration is constant in both cases, and the average velocity is 1/2 the cruise velocity, which is the same in both cases. So if acceleration is doubled, then only 1/2 the distance is covered in that 1/2 of the time (with constant acceleration). Then more fuel is consumed in order to reach the same distance that the slower accelerating vechicle took to reach cruise speed.

Note I didn't state it would double fuel consumption, just increase it.

Also since a typical car has better gas milage at 45mph than it does at 10mph, there's a best minimal amount of acceleration.

The 455 in my Camino is still drawing 455 cubic inches of air/fuel mix per cycle.

By what mechanism does a throttle change the VOLUME of air fuel mixture a cylinder "breathes"?

A throttle changes the mass of the fuel / air mixture. At partial throttle, manifold (intake) air pressure is reduced substantially, so it's the same volume of air at a much lower density, and less fuel. Fuel injection systems further optimize the fuel/air ratio at various throttle conditions.

 

[Averagesupernova]

Hmmmmmmmm. We had a similar thread about six weeks ago: https://www.physicsforums.com/showthread.php?t=165455
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Integral, you still don't seem to have your mind wrapped around it. The throttle's main function is to control how much fuel and air enter the engine. Naturally more RPM means more air/fuel charge entering into the engine because there are more intake strokes per unit of time. No one will likely argue that. If you have intake manifold vacuum you are guaranteed that less than a full volume of air is entering the cylinder. I'd say that's a pretty basic concept. The throttle plate allows just enough air/fuel charge at the correct mixture of about 15:1 to enter the cylinder on each stroke to maintain correct idle speed. I don't really know any other way to explain it, maybe someone else can if it still doesn't sink in.
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Edit: Someone did explain it a bit better in the above post. The density of the air is lower at idle due to the manifold vacuum. Put another way, there are less molecules of air entering on each stroke and to maintain the correct mixture, there is also less fuel per stroke.

 

[Integral]

Average, do you have trouble with the concept of VOLUME.

It is you who is having the trouble with basic concepts. Recall from your 2nd grade science class a gas (air/fuel mixture) will expand to fill the available VOLUME. Why is it you do not understand that term?

 

[Loren Booda]

Bigger engines and the fuel needed to feed them weigh more, and so does the vehicle to carry them.

People who by bigger engines are often looking to "race" their engine or pull heavy loads and buy accordingly - inefficient in normal use.

 

[Ki Man]

Real question is why do we need 1.5 ton cars to carry our 150 lb ***es

 

[Averagesupernova]

Integral:
That's Averagesupernova to you buster. I think my above post explains it well enough. Yes I remember 2nd grade science quite well.

For instance, with the throttle closed at an idle it is not taking in a full volume of air.

What I meant and should have specified was that it won't take in a full volume of atmospheric pressure air and that the pressure is reduced so the actual quantity of air molecules (as I mentioned previously) is reduced. I suspect you are too stuborn to let it go at that and would rather nitpick about volume and volume at a specified pressure. According to your thinking, the whole damn thing could be under a high vacuum and it would still be taking in a full volume of air right? Never mind the fact that their is no air to be taken into start with. SO there, have a taste of my nitpicking.

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I think my original argument was concerning Danger's post #4. Danger said that a bigger displacement engine has to take in more fuel to maintain a correct air/fuel ratio since it is taking in more air. I challenged that statement and made the OMG! mistake of misusing the word volume.
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Incidentally, how would you have liked me to state it?

 

[ShawnD]

Real question is why do we need 1.5 ton cars to carry our 150 lb ***es

Many Americans are extremely overweight, so your argument is invalid

 

[rcgldr]

Bigger engines and the fuel needed to feed them weigh more, and so does the vehicle to carry them.

Not always, referreing once again to the 2006/2007 Corvette Z06, it's 7.0 liter V8 engine weighs less than the Porsche's 3.6 liter engine. Some of this is due to using push rods instead of dual overhead cams, and some due to using very light materials in the engine (titanium rods in the Z06 engine). The Z06 weighs 3150 lbs, less than a Subaru WRX STI, again due to materials (magnesium engine frame, aluminum main frame, composite body panels). The Z06 engine has a relatively high redline of 7100rpm, impressive for a 7.0 liter V8.

As an extreme example, the 2005 F1 race car 3.0 liter V10 engines were making around 950hp. The current 2.4 liter V8 engines are making around 820hp, and F1 cars with a high coeffiecient of drag around 1.0 (all the downforce related stuff), and get about 3.1mpg during a race. The rules now limit the engines to 19,000 rpm.

http://en.wikipedia.org/wiki/Formula_One_car

Why do we need 1.5 ton cars ...

Motorcycles are an option, even the most powerful ones get 35mpg, and smaller bikes get even better milage.

 

[Danger]

Danger said that a bigger displacement engine has to take in more fuel to maintain a correct air/fuel ratio since it is taking in more air. I challenged that statement and made the OMG! mistake of misusing the word volume.

You do realize, I hope, that the same vacuum conditions apply to small engines?

 

[rcgldr]

... the same vacuum conditions apply to small engines?

True, but in order to produce the same power, the smaller engines need to spin faster.

A quote from this Wiki article would seem to indicate that size doesn't make that much difference, as most of the power losses aren't friction related:

"Most internal combustion engines waste about 36% of the energy in gasoline as heat lost to the cooling system and another 38% through the exhaust. The rest, about 6%, is lost to friction."

http://en.wikipedia.org/wiki/Internal_combustion_engine

I'm not sure if this article is taking into account the amount of aerodynamic drag that occurs in an engine. A significant amount of this drag is related to movement of air in the crankcase underneath the cylinders. Pro-stock drag racing motorcycles use a vacuum pump on the crankcase to reduce this aerodynamic drag. As an experiment, go down a hill in a car in a lower gear with the engine off. One surprise is that throttle position doesn't make any perceptible difference, although this doesn't imply that the power consumed above the pistons isn't significant, just not significantly affected by the throttle position.

 

[brewnog]

Russ got this one covered in post 5. In thermodynamic terms, bigger engines are more efficient. But if too large an engine is specified for the average duty cycle being asked of it, you won't be running as close to peak efficiency as a smaller engine, and a smaller engine will outperform in fuel economy.

The 455 in my Camino is still drawing 455 cubic inches of air/fuel mix per cycle. At idle, of course, it doesn't cycle as many times per minute as it does at 140 kph on the highway. It idles at more or less the same speed as a 273 slant six, which at idle draws 273 cubic inches of mix per cycle.
Aside from variable displacement engines, the swept volume doesn't change.

I think the others have just about sorted this out with semantics about volume, but at idle, your engine will be drawing in less fuel and air than when it's running balls-to-the-wall. This isn't just because of the engine speed; your Camino could be doing (say) 50 down the motorway in top gear, still at engine idle speed, but it will be using more fuel (and air) than it was when idling at the traffic lights because the throttle will be further open. (I don't think you were disputing this, Danger, but I thought I'd make it clear for some others.)

By the way, the same goes for a Diesel engine guys; it doesn't have a throttle but the engine definitely isn't using as much fuel and air at idle as it is on load. Interestingly though, the air mass flow rate for a naturally aspirated Diesel engine is reasonably proportional with engine speed, regardless of load, unlike a gasoline/petrol engine, because it doesn't need to maintain an overall air/fuel ratio in the cylinder.

A quote from this Wiki article would seem to indicate that size doesn't make that much difference, as most of the power losses aren't friction related:

"Most internal combustion engines waste about 36% of the energy in gasoline as heat lost to the cooling system and another 38% through the exhaust. The rest, about 6%, is lost to friction."

The heat lost to the cooling system is highly dependant on engine size; largely because the surface area to volume ratio decreases with increasing cylinder displacement. The heat loss is (nominally) a factor of surface area, power (nominally) of displacement.

 

[Averagesupernova]

Thank you Brewnog!

 

[Aether]

Fuel consumed versus time yes, but not fuel consumed versus distance. If a vehicle has double the acceleration, it reaches cruise velocity in 1/2 the time. Take the simple case where acceleration is constant in both cases, and the average velocity is 1/2 the cruise velocity, which is the same in both cases. So if acceleration is doubled, then only 1/2 the distance is covered in that 1/2 of the time (with constant acceleration). Then more fuel is consumed in order to reach the same distance that the slower accelerating vechicle took to reach cruise speed.

I see your point, but this doesn't have anything to do with engine efficiency; it has to do with assumptions that you are making about what the drivers of these vehicles will do after the initial acceleration. For example, if in your example above both engines are switched off after attaining cruising speed, and then both vehicles coast to a simultaneous stop at the same finish line, then the vehicle with the faster acceleration will have used less fuel.

 

[Aether]

The heat lost to the cooling system is highly dependant on engine size; largely because the surface area to volume ratio decreases with increasing cylinder displacement. The heat loss is (nominally) a factor of surface area, power (nominally) of displacement.

At any given speed, a higher surface area to volume ratio for a lower displacement engine leads directly to more cylinder heat loss. However, if this lower displacement engine is operated at a proportionally higher speed (e.g., maintaining the same output power) then it isn't necessarily true that a higher fraction of the fuel energy will be lost to the cooling system.

 

[Danger]

I think the others have just about sorted this out with semantics about volume, but at idle, your engine will be drawing in less fuel and air than when it's running balls-to-the-wall. This isn't just because of the engine speed; your Camino could be doing (say) 50 down the motorway in top gear, still at engine idle speed, but it will be using more fuel (and air) than it was when idling at the traffic lights because the throttle will be further open. (I don't think you were disputing this, Danger, but I thought I'd make it clear for some others.)

Yeah, I think that there's a major communications failure here. Thanks for helping to sort it out. In the very simplest terms, I'm going to point out what I was talking about.
1) The swept-volume of an engine does not change unless it's a variable displacement unit such as some of the new hemis and other such-like; if it's 350 ci at idle or not running at all, it will still be 350 ci at 8,000 rpm. (This is assuming no rod stretch, for the nitpickers.)
2) The volumetric effectiveness does change with speed. If the valve train, intake runners, and exhaust system can't breathe as much as the cylinders want, you have to add more throttle to try and compensate. That's why the 60's hemis were blowing everything away--those man-hole size ports. Likewise why so many engines now have multiple valves per cylinder. Where I misinterpreted Average and Jeff was in thinking that they were trying to imply that small engines are not subject to that whereas large ones are. My fault for not reading more closely.
As for the thermodynamics information that Russ put up, I really don't understand anything with that many numbers in it. I know that he knows what he's talking about, though, so I'll just take it at face value.