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World's Only Diesel-Electric Honda Insight
Jake Staub writes "Just replaced the gasoline engine in a Honda Insight with a Diesel engine. On a 3,000 mile cross-country shakedown journey the car averaged 92mpg over 1,800 miles. Around a very hilly town in Northwest Washington, the car is averaging 78mpg. These mileage averages are without the electric side of the vehicle fully functional. With a bit more tinkering on the electric side and through a slight gearing change through tire size, it is anticipated that the car will likely average 100mpg. The build for the car has been documented on the web site and is as close to open source as my time allows. The car was built by two guys in a garage in Southern Maryland. If we can do it I don't see any reason why major auto manufacturers can't do it since we used their parts."
The main reason gasoline hybrids get better mileage than direct-coupled engines is that the gasoline engine is not forced to operate at inefficient points on its' BSFC map (near closed throttle). The engine only runs when needed, and then it runs near its' BEP (Best efficiency point), or occasionally at maximum power which also has decent efficiency. It is not forced to idle and off-idle conditions where the pumping losses are horrible and efficiency s#x (5x fuel for same marginal power).
Diesel engines have entirely different BSFC maps, and do not suffer the same pumping losses (vacuum across throttle plate). Their drop off at idle is _much_ lower than for gasoline engines, so they're great in city-wide European traffic jams. Diesel fuel also is ~15% denser (more heat per gallon) and the higher compression ratio is about 5% more theoretically efficient.
But a diesel hybrid does not have much to gain by hybridization. The BSFC map is much flatter, and the engine restarting power & wear is considerably higher.