How does the diesel is clear to all. The heat occurs after top dead center. More rapid increase in pressure in the chamber and the most advantageous angle of the crankshaft provide good power at a reasonable expense.
What's in a petrol engine? Everything here is very simple. Too many compromises. The mixture is lit before the arrival of the piston at top dead center. For what?
Explain that for the complete combustion of fuel (it has had time to burn). But here's a nuance - a mixture at high pressures in the same percentage of burns faster. Ie, given a spark, a shock front of burning candles on the road began to distant corners of the combustion chamber, the pressure increases, unburned mixture begins to burn faster, the piston passes TDC and at some point of the crankshaft pressure reaches a maximum impact on the piston.
Now the question is - why so hard? Why not simply set the initial high pressure in the upper dead point due to the high compression ratio? There may be dusted with the assertion that it is unreasonable that there is a boundary compression ratio for gasoline moving and excess thereof, will lead to a mixture of self-ignition or detonation. I have to say - baseless allegations!)))