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There are still good reasons to persist with spark ignited engines. For a start the constant volume (on a PV diagram) combustion process that approximates how they burn the fuel is fundamentally more efficient than the constant pressure process that approximates compression ignition.
Lose the pumping loss and bump up the compression (ethanol, methanol or CNG?) and the SI engine will be more efficient than CI.
SI are lighter and smaller for the same power output. CI always run lean so can't ever use all of the air they pump through them. That means they have to be bigger and heavier to compensate. CI are inherently heavier anyway because they, in effect, always operate under knock. The slower combustion rate required, limited by the rate the fuel and air can mix, also limits the power they can make. (Anticipating "What about turbo's?: at the equivalent level of development; SI can also be turbo.)
The fuel systems and emission controls are inherently more complex and expensive with CI over SI.
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