Originally Posted by
Zate
i dunno i think the problem is that fuel only has so much energy per unit in it. X amount of fuel when burnt perfectly produces Y amount of HP. Doesnt matter if you have a 6.0L V8 or a turbo 2.5L, 500 HP requires a crapload of fuel.
Same on the smaller scale... doesnt matter if its a 900cc single cylinder with a Turbo, or a 1.8L, 110 HP requires about the same amount of fuel, its the efficiencies in how its burnt, and how the energy produced is transmitted to the ground that is important.
With more efficient burning process and a more efficient method of getting the energy from the combustion to the wheels you could use less fuel to produce the same effect.
Wrong on every point.
Engine vacuum is the enemy: The more vacuum created, the less efficient the motor is. One reason is because the dynamic compression ratio is low, another is because vacuum requires energy to create. The most efficient parts of the motor are in the higher load points. How many 18 wheelers do you see cruising in high vacuum? Semi's and the like make the motor just barely large enough to cruise, and use a turbo when acceleration is needed. In a car with a smaller engine, turbo or not, will be more efficient since the motor will spend most of its life in a more efficient powerband (less vacuum).
Smaller engines have less internal friction: One of the sources of friction in the engine is piston rings / wall contact. Basically we want the maximum amount of piston surface area, with the least bit of circumference, since the circumference is the friction surface. The area / radius (a/r) ratio increases linearly. An 80mm piston as an a/r of ~20mm2, and a 160mm piston has an a/r of ~ 40mm2. The larger piston will be more fuel efficient because it has less friction. Larger pistons also require lower pressures to make the same amount of torque. So inherently with no other changes, a single cylinder engine is more fuel efficient than a comparable twin. Smaller engines also tend to have smaller main and rod journals (less friction).
Also worth mentioning, when in boost, a motor is generally less efficient than a naturally aspirated motor at the same power level, because the motor is usually run richer in OEM applications (except with some of the new gas direct port injection stuff). This is usually offset by the fact that 99% of the motors life is spent in vacuum. On the flip side, with forced induction, you are increasing the power output, with almost no increase in internal friction. So it varies from motor to motor. However still saying 500hp requires x amount of fuel regardless on the engine is ignorant to say the least. You're pretty much ignoring every aspect of engine dynamics at that point.