[drmiller100]

What makes one engine more efficient then the next?

First, lets define efficiency. For my purposes I define efficiency as making the most miles per gallon of fuel.

Given that definition, Diesel will win, always. Why? First, because there is more energy in a gallon of diesel then a gallon of gasoline. 10 to 20 percent more depending on how you define diesel, gasoline, and the ethanol content.

Second, even given the exact same amount of BTU's of fuel, a diesel is inherently more efficient then a Gasoline engine. Diesels do not have pumping losses (no throttle plate restriction). And diesels have PLENTY of air to burn all the diesel making them slightly more efficient in burning.

Diesel engines are usually heavier then gasoline engines, and we all agree less weight makes a more efficient vehicle.

From there, picking on a gasoline engine, some engines are more efficient then others.

Pumping losses are a function of size of engine (displacement), RPM, and throttle position.

The bigger the engine, the more pumping losses, and to some extent, the more friction.

The more RPM, the more losses due to pumping and friction.

Throttle position closed means higher vacuum, which means more pumping losses.

So, in a perfect world, at cruising speed, we would have a small engine running at low RPM, at WOT. Again, the diesel wins this war over the gasoline engine as they have no throttle.

Which is more efficient - a 500 cc engine running at 4000 rpm or a 2 liter engine running at 1000 rpm)? If both vehicles have the same intake vacuum (measure of throttle and engine efficiency), then I would bet on the 2 liter.

I believe the trick for a gasoline engine is to have the manifold vacuum to be as minimal as possible, and the engine displacement times the RPM to be as small as possible.