There has been a lot of talk about diesels on this forum, and the fact that they give better economy because they don't have to fight the vacuum of the
engine. Trying the following experiement, on a petrol engine, however, gives some interesting results:
Driving downhill, in 1st gear, at about 15mph, or whatever gives you 75% of
maximum revs, let go of the accelerator, and switch off the engine. You will
get quite a lot of engine braking so make sure no-one is behind you!.
Now, if you floor the accelerator pedal, you will notice that there is no
perceptible reduction in engine braking. However, the engine is now running
like a diesel, at least in terms of no-throttle-plate. I have always thought that, with the throttle plate closed, there will be high vacuum, but the compression stroke will be easy, whereas, with the throttle plate open, there will be low vacuum, but the compression stroke will be more difficult - therefore they cancel each other out!
Therefore, the difference (apart from the higher calorific energy of diesel) must be due to the fact that diesels run leaner than stoichometric.
Has anyone tried running extreme lean-burn, like a diesel?. At high load, or
high RPM, this is likely to cause detonation, and destroy the engine, but at
low revs, it may well be possible. Environmentalists would of course want to add a cat system capable of reducing the NOx emissions this would generate.
This may also be an option with LPG, or hydrogen, etc...
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