Something that has bothered me for a long time is why a 600 hp engine uses more fuel per kilometer than a 80 hp engine. Let pretend I have two equal cars (same shape same weight etc) except for the engine, one is 600 hp and other is 100 hp. If I manage the throttle to accelerate both with the same acceleration, my knowledge says that will require the same energy to get to a certain speed, so why will the 600 hp will use more fuel/energy?
Also if I am right is it possible that in the future cars will be so efficient that a big engine will only consume more if pushed to the limit?
Best Answer
Unfortunately, I see a premise here that is difficult to prove true or false. Mainly because it is almost impossible to use a 600hp engine in a way similar to an 80hp engine and compare them directly.
Any car that has a 600hp engine will have other components to support it. Compared with an 80hp car, that might be a larger, more robust transmission, larger exhaust, stronger engine mounts, bigger wheels, etc. All of these make it difficult to determine how much of the efficiency difference is due to the behavior of the engine and how much is due to moving around more mass.
It is difficult to imagine a case where you have the benefit of having the power of a large engine usefully available to you without also having the penalty of increased mass.
In the realm of physics, there is no reason that a large engine in isolation couldn't be as efficient (in a power delivered per fuel consumed sense) as a smaller engine. But real engines are not developed to maximize that value at the expense of everything else, and that value doesn't immediately translate to vehicle fuel efficiency ($mpg$ or $l/100km$). Real engines are built to have reasonable fuel consumption while also holding development costs, production costs, maintenance costs, and total vehicle production costs low as well. A fuel efficient vehicle is created by not focusing only on the engine, but on many other aspects of the car as well.