Quote:
Originally Posted by catch2otwo  a diesel does not get better mileage because of the lack of a "throttle plate" whatever that is. a diesel is a totally different engine. |
From wikipedia-
While higher compression ratio is helpful in raising efficiency, diesel engines are much more economical than gasoline (petrol) engines when at low power and at engine idle.
Unlike the petrol engine, diesels lack a butterfly valve (throttle) in the inlet system, which closes at idle. This creates parasitic loss and destruction of availability on the incoming air, reducing the efficiency of petrol/gasoline engines at idle. In many applications, such as marine, agriculture, and railways, diesels are left idling unattended for many hours or sometimes days. These advantages are especially attractive in locomotives
I would add that its just not at idle. Any time the butterfly is choking the engine and creating a vacuum in the manifold you are losing efficiency. Its pumping losses really. Engines use less energy to turn over when they are compressing a cylinder full of air. Some motorcycle engine braking systems work off this principal.