Engine efficiency
 

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.

You've missed out expansion ratio - a major reason why diesels are more efficient than gasoline engines.

how does expansion ratio relate to anything????

I agree. I think a lot of this has to do with stroke and RPM. If you look at any of the super-efficient industrial diesels out there, they run at extremely low RPM and have an insane stroke. They are getting absolutely every last ounce of expansion energy out of the gas before the exhaust stroke begins. Running a short stroke and/or high RPM will allow you to make more power, but at the expense of efficiency. The other downside of the low-RPM long-stroke engine is that its specific output (HP/LB) is extremely low.

Here's an article on the most powerful engine in the world... over 50% thermodynamic efficiency!

Most powerful diesel engine in the world

compression ratio and expansion ratio are pretty much unrelated other then they have the word "ratio" in both.

Unless I am missing something? Which is why I posted this thread in the first place.

A higher compression ratio results in an increase of thermal efficiency.

Don't be fooled about "engine speed" by only looking at crankshaft rpms:

Most powerful diesel in the world: 98" stroke at 102 rpm = 1,666 ft/mn piston speed

Honda 50cc Super Cub at max fuel efficiency speed: 1.63" stroke at 4300 rpm = 1,168 ft/mn piston speed.

Hmmmm.

Yes one is a supercharged two-stroke diesel and one is a four stroke gasser. Different engine types seem to have different piston speed ranges for max fuel efficiency.

You can easily drop off the bottom end of the engine's efficiency zone by doing that.

Language issue. Apparently in some places Expansion Ratio is used instead of Compression Ratio.

I have always thought of Expansion Ratio as the ratio things expand as they go from Liquid to Gas - like Water to Steam.

Wrong. Ex: Atkinson cycle - Wikipedia, the free encyclopedia