Groover wrote:
I suspect that it would be more accurate to say that high performance engines don't do as well in the real world as lower performance engines. For the purpose of the test they use a specially trained driver that adheres to rigid rules for acceleration and decceleration. In the real world it sure is fun to punch the accelerator and feel the power but it is horrible for fuel economy.
It mainly has to do with the fact that engine load has a greater effect on gasoline engines than diesel. Increasing speed going from the 60 mph speed limit the EPA tests are done at to 70 mph in the real world will have a worse effect on a gasoline engines mpg than it would a diesels.
Since the air/fuel ratio can vary widely(even greater than 80 air to 1 fuel) in a diesel and depending on where the turbo is mostly efficient will allow the diesel to to increase power without injecting more fuel to an extent. This is even greater with modern VG turbos since they can adjust for a greater efficiency range. You cannot do this with a gas engine since they have to stay around a 14.7 air to 1 fuel ratio. If you add more air, then you have to add more fuel to stay within that range.
This is why a diesel can be at 5 psi of boost getting good mileage cruising down the highway since the turbo is leaning out the mixture making it more efficient at making power per drop of fuel, but any time you are in the boost in a gas engine then are burning more fuel.
