Percentage of non-ideal combustion is more in a diesel engine,why?

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marellasunny
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In this diagram,non-ideal combustion in a diesel engine takes-up a lot of the fuel energy. Why is this so? Also intrigues me that there is a lot more non-ideal combustion at low load than higher loads. Is this because the diesel engine operates with a lower compression ratio at lower loads? This leads me to my next question:

2.I've assumed the compression ratio to remain constant irrespective of the engine rpm,is that realistic?
LOGIC I USE: Only the mean effective pressure(measured at flywheel) varies with load ranges. The engine displacement remains the same.

http://imageshack.com/a/img571/7527/3l3n.gif
 
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A spark ignition engine runs at a high temperature. To minimise the production of air pollutants such as NOx, a stoichiometric air/fuel ratio must be used in gasoline engines. There should be little unused oxygen available in the combustion products.

A diesel takes in a fixed quantity of air with each induction stroke and runs cooler than a SI engine. The full quantity of air must be compressed in order to reach a consistent compression temperature. Only sufficient fuel is then injected to generate the power required. Non-ideal combustion therefore occurs at low power, the unused oxygen flows out with the exhaust. At full power the maximum volume of fuel is being injected and almost all the oxygen is being consumed. That is the ideal stoichiometric situation as found in spark ignition engines. Any more fuel, or less air, and the diesel will produce black sooty smoke which is again, non-ideal combustion.