Diesels the best way to get high MPG?
 

Super high MPG cars we may never see - GasSavers.org - Helping You Save at the Pump Hypermiling and Fuel Efficiency Forum

There are a few cool cars in there. A lot of them are diesel/turbodiesel. Is diesel the best way to get high MPG? Is that because of the efficiency of the much higher compression ratio?

Yes, yes, yes! At least for right now. If you look at the new gasoline/direct injected engines, it's all diesel technology using a different fuel and a spark. Mazda's Sky Activ ( gas version ) uses 14 to 1 compression, direct injection and other jiggery pokery to do the deed.

Part of it is the high compression ratio, the other part is the energy density of the fuel. Diesel fuel has more energy per gallon than gasoline. The cam timing usually trends toward making maximum torque at low engine speeds, so you don't have to rev it as high.
So the combination of all these elements yields a high mpg device.

Plus they're not throttled.

The downside is that usually a diesel engine that has the same "right foot power" (not rated horsepower, but equal driveability) will probably be heavier than a gas engine. But I think most people can overlook that.

I really like my new TDI Golf, but your driving conditions will dictate your optimum drivetrain. Unfortunately, it doesn't do anywhere near as well in stop-and-go city traffic as hybrids do. I do mostly highway miles, so it's the right tradeoff for me (especially if I can kill the karman vortex street somehow; I'm working on building up enough 70mph mpg data so I can tell whether airtabs make a difference).

Diesels also have an inherent advantage in the constant pressure expansion of the power stroke in the diesel cycle. You get more work out of the same fuel energy than you do in the Otto cycle all other things the same.

I agree with pretty much everything that's been said. The biggest factor is the cycle/compression ratio. Throttling is big too. Just to add a few things.

Diesels (typically) run at slower rpms as well, so there's less friction. Also, most modern diesels are turbocharged so it allows for the recovery of some of the exhaust heat.

I'm not so sure diesels have less friction. In spite of turning lower rpms, they typically have longer strokes; interestingly enough there are many gassers that run 1000-1200 ft/mn piston speed while the two stroke diesels anyway are running 1700! The ring pack is the largest single contributer to friction so higher piston speeds must lead to higher friction.

Plus they have heavier rotating/reciprocating parts which add friction.

Comparison of Diesel and Petrol engines (yield)
 

According to Wikipedia

Engine efficiency - Wikipedia, the free encyclopedia

thermal efficiency of a modern petrol engines is about 18 to 20%, meaning they convert 20% of total heat of petrol combustion into movement.

On the other hand, diesel engines have a thermal yield of about 40% .

From the best of my knowledge, the main reason isn´t for rpm or friction or even the fuel energy(but they count also), but for the thermodynamics curve of the diesel cycle. It works at higher pressure that needs a fuel that do not detonate at that high pressure.

Turbos are allowing constructors to make smaller and lighter engines with a lot more power, while other advances are allowing them to supress noise and vibration a lot.

It is common to see Jeeps with diesel engines that used to had 2500 cc that turned to 2200 cc producing more HP than their predecessors.

OldBeaver

The diesel motorcycles in the 2011 Vetter Challenge got better fuel economy than the gasoline bikes despite having much bigger engines. The top 3 diesel bikes got 128mpg , 110 and 90 respectively while the best gas bike ( 250 NiNja ) got 94 mpg. Diesel has a lot of advantages as stated above over gasoline engines.