Why are big engines so horribly inefficient?

  1. ShawnD

    ShawnD 953
    Science Advisor

    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?
  2. jcsd
  3. I'd say internal friction, more moving parts, bigger piston / cilinder contact surfaces, more compression to generate. etc.
  4. Ivan Seeking

    Ivan Seeking 12,122
    Staff Emeritus
    Science Advisor
    Gold Member

    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?
  5. Danger

    Danger 9,663
    Gold Member

    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.
  6. russ_watters

    Staff: Mentor

    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]
    Last edited: May 26, 2007
  7. ShawnD

    ShawnD 953
    Science Advisor

    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.
  8. Aether

    Aether 712
    Gold Member

    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?
  9. More engine = more acceleration Aether.
  10. 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.
    Last edited: May 26, 2007
  11. Aether

    Aether 712
    Gold Member

    So what? Please explain why "faster acceleration...uses more gas than slow, smooth acceleration."
  12. 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.
  13. Aether

    Aether 712
    Gold Member

    All true, but unfortunately this doesn't explain why "faster acceleration...uses more gas than slow, smooth acceleration" (which it doesn't).
  14. In some cases, the gearing is also quite different.
  15. turbo

    turbo 7,063
    Gold Member

    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).
  16. rcgldr

    rcgldr 7,695
    Homework Helper

    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.

    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.
  17. More of an answer than you asked for, but I like the subject:biggrin:
    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%.
  18. Aether

    Aether 712
    Gold Member

    ...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.
  19. 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.
  20. Danger

    Danger 9,663
    Gold Member

    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.
    Last edited: May 27, 2007
  21. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
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    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.
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