Why do more powerful cars consume more gas?

[brblx]

Originally posted by: Eli

Originally posted by: Skoorb
Maybe I should have said "little" to do with it? 🙂 The closest I've ever come to tearing apart an engine is taking off an intake manifold! However, if friction was really appreciable enough to literally steal multiple MPG for the work done I believe that heat and wear would transfer quickly into failing parts and bad tolerances. A well lubricated engine surely should feel extremely free in its motion.

You are correct, friction has very little to do with an engine's efficiency. Sure, an 8 cylinder engine has more drag than a 4 cylinder, that is a given.. but it is more or less negligible for purposes of this discussion. I'm not even sure if "friction" is the right term, it's more due to oil viscosity drag.

friction would be the right term, but the amount of friction in a broken in engine is very low. nothing compared to fighting compression- a used engine with the heads off is usually loose enough to keep going for a sec if you spin the crank by hand.

the important thing that the OP should realize is that horsepower and gas mileage are really not related. they may seem so, but the maximum amount of power an engine makes (at whatever arbitrary rpm) really has no direct correlation to gas mileage. it's not like if a 100hp engine gets 40mpg, you can deduce that a 400hp one gets 10mpg. just doesn't work that way, there are too many other things in play.

as i believe someone else pointed out, a big engine won't always cause a mileage drop. if the engine is able to make it's power at much lower rpm (and can cruise at that rpm) without being under too much duress, it may actually get similar highway mileage to a smaller motor. the fourth gen camaros/firebirds are a good example of that, you could get high 20's or better in the models with a somewhat ridiculous .5:1 top gear.

remember that there are two things that effect the amount of gas an engine is using- rpm and load. skoorb gave the example of a car in first gear at 25mph vs fifth. this really assumes that the amount of fuel going into an engine is only related to how fast it's spinning, which just isn't true. you could actually get worse mileage in that situation (maybe- 25mph in 1st would have the engine screaming in most cars). in a more realistic scenario, say 30mph in either 3rd or 5th, 3rd very well might afford better mileage. you have a greater mechanical advantage over the back wheels, which means that the engine doesn't have to work as hard.

 

[KIAman]

First misconception - Low speeds reduce MPG
The only reason speed/mpg graphs show low speeds (<25Mph) corresponding with low MPG is because of the limits of the transmission/gearing. Most cars either run high RPM or add load to the engine because of gear ratios. A CVT will mitigate this issue entirely. MPG will drop in direct relation to increase in speed, period.

Second misconception - Big engines = big friction
It is true that bigger engines typically mean there is more rotational mass, and internal friction losses. These are only a factor of MPG during acceleration. Once at constant speed, the inertial of the bigger rotating mass also helps to maintain the speed and internal friction losses are negligible.

Third misconception - Bigger engines (displacement) = more HP than smaller engines
Absolutely false. The OP never mentioned displacement in his scenario. Both engines could be 2.0L I-4 and all talk about displacement comparisons to MPG are automatically invalid. The only difference in making HP is how much air/fuel is burned. If that means more displacement, sure. If that means better ways to burn air/fuel, sure.

First, there are lots of missing variables in the OP.

If we assume both cars are identical in all secondary factors; drag, speed, mass, (removed the RPM) and adding my own assumptions, the engines are modern and able to scale their output exactly to load and with infinite gear ratios, they will get identical mileage.

Now if we assume both cars weigh 8000lbs, then the 120hp cannot produce enough power and will get less MPG than the 400hp engine.

If we assume both cars weigh 1000lbs and the constant speed is 5Mph, the 400hp engine cannot scale back power enough without risk of stalling.

The real reason why you never see this is because of the following.

1. No practical car with 400HP will come with CVT.
2. Modern cars are heavy (>3500lbs) and underpowered engines require higher gearing for normal driving conditions and as a result, reduce MPG.
3. MPG is severely affected by acceleration and grade of road, rather than constant velocity.

Question: If all the above were true, how come turbocharged cars don't get the same MPG as their non turbo brothers?

Most mfgrs add turbo to compensate for the lack of power of a NA of similar displacement for the given weight of a car. A 3500lbs AUDI with a 1.8L Turbo makes around 200hp. In theory, this AUDI should get kick ass MPG when the full 200hp isn't used, should get equivalent to a 1.8L NA engine. The truth is that the mass of the car and the associated gearing REQUIRES the turbos to be on the majority of the time just to be useful.

If you stuck that same engine inside a 1500lbs car, at a lot of speeds, the turbo will get similar MPG as their respective NA engine will because the mass doesn't necessitate more power than the NA engine can produce.

 

[ja1484]

Law of conservation of energy? Anyone? Anyone? Bueller?

 

[PlasmaBomb]

Engines complimicated...

The numbers associated with boundaries are brake specific fuel consumption /(lb/(hp·h))

(BSFC).

Data is for an old I4, 2v, EFI, ICE. YMMV.

 

[Tiamat]

Originally posted by: RichUK
Please tell me you are joking?

 

[Zenmervolt]

Originally posted by: fuzzybabybunny

Originally posted by: Zenmervolt
The short answer is that gasoline is stored energy. Assuming that both engines are identically efficient at turning gasoline into power, the only way to make more power is to burn more gasoline.

ZV

Right. If one unit of gasoline is X units of energy, why do two cars that are the same mass and velocity require different amounts of energy? Aren't they both using the same power? Why would the 400hp car be using more energy than the 120hp car? Isn't the 400hp basically untapped unless the car accelerates or has more resistance imposed on it like towing something?

They are using the same amount of power. The larger engine is simply less efficient because the throttle is less open than on the smaller engine.

An engine is most efficient (i.e. it makes the most power per unit of fuel burned) at wide open throttle. The closer the throttle is to being closed, the less efficient an engine is at turning fuel into power. While two cars with the same weight and total drag would require the exact same total power to go 60 mph (for typical cars this is between 15 and 25 hp on level ground), an engine that can make 400 hp at peak has to have its throttle closed much more than an engine that only makes 120 hp at peak. Therefore the 400 hp engine is operating at a smaller throttle opening and is less efficient at turning gasoline is power.

There is also the fact that larger engines have slightly more internal friction than smaller engines and adding more cylinders also increases internal friction even for the same displacement. Overcoming the additional internal friction also consumes some excess power (and therefor some excess fuel), but the chief player is the fact that the larger engine is throttled to a greater degree and "wastes" power to maintain manifold vacuum.

ZV

 

[SSSnail]

All your answers are wrong.

More powerful cars consume more gas because the people that bought them aren't some fuzzy tree hugging eco kooks, and they have a penchant for being lead footed.