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Why are big engines so horribly inefficient? 
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#1
May2607, 05:24 AM

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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
May2607, 05:30 AM

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



#3
May2607, 06:11 AM

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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?



#4
May2607, 10:15 AM

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Why are big engines so horribly inefficient?
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.



#5
May2607, 10:27 AM

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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 lowerdown 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] 


#6
May2607, 01:08 PM

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


#7
May2607, 06:27 PM

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#9
May2607, 06:32 PM

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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. 


#10
May2607, 06:38 PM

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#11
May2607, 06:43 PM

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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.



#12
May2607, 07:10 PM

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#14
May2707, 02:15 PM

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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).



#15
May2707, 03:35 PM

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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 20062007 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. 


#16
May2707, 04:15 PM

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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%. 


#17
May2707, 04:24 PM

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#18
May2707, 06:00 PM

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