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Originally Posted by redpoint5
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I have read before that increasing the compression ratio increases fuel efficiency, but what is the reason behind this?
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This is the most interesting aspect of a college level course on thermodynamics. The 'dirt simple' answer is the expansion ratio is effectively increased as the fuel-air charge at ignition is increased. But the detailed answer is a little longer ... you might check the Wiki or ask Mr. Google. I'd have to reconstruct my college course notes (from 40 years ago!)
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Originally Posted by redpoint5
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If increasing cylinder pressure increases fuel efficiency, why not direct-inject gasoline into the cylinder at just the right moment and not worry about detonation (like a diesel engine)?
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That is the theory of direct injection engines and promises to make a significant improvement in ordinary ICE engine operation. Everyone is working on their versions.
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Originally Posted by redpoint5
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This leads me to the question of why it's important to have a stoichiometric air/fuel ratio in a gasoline car when it doesn't matter with a diesel?
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It has to do with the 3-way, catalytic converter that needs an oscillation between lean and rich. A balance is needed so the oxides and hydrocarbons combine to complete the hydrocarbon combustion and convert the oxides from reactive NO{x} to N{2} and CO{2}.
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Originally Posted by redpoint5
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I imagine a turbo increases the effective compression ratio, and this explains the increase in fuel efficiency. Why not just engineer a higher ratio into the design of the engine?
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Weight as a turbo-charger lets a physically smaller engine perform at power levels a substantially heavier engine would require. It breathes like a big bore and makes power like a big bore but it is much lighter so there is less rolling drag and inertial losses (aka., when the brakes have to slow or stop the car.)
Bob Wilson