Editing Stratified charge engine (section)

==Comparison with diesel engine==
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It is worth comparing contemporary directly fueled [[petrol engine]]s with direct-injection [[diesel engine]]s. [[Petrol]] can burn faster than [[diesel fuel]], allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher [[energy density]], and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more "normal" road vehicles.

This comparison of 'burn' rates is a rather simplistic view. Although petrol and diesel engines appear similar in operation, the two types operate on entirely different principles. In earlier manufactured editions the external characteristics were obvious. Most petrol engines were carbureted, sucking the fuel/air mixture into the engine, while the diesel only sucked in air and the fuel was directly injected at high pressure into the cylinder.  In the conventional four-stroke petrol engine the spark plug commences to ignite the mixture in the cylinder at up to forty degrees before top dead centre while the piston is still travelling up the bore. Within this movement of the piston up the bore, controlled combustion of the mixture takes place and the maximum pressure occurs just after top dead centre, with the pressure diminishing as the piston travels down the bore. i.e.  the  cylinder volume in relation to the cylinder pressure-time generation remains essentially constant over the combustion cycle. Diesel engine operation on the other hand inhales and compresses air only by the motion of the piston moving to top dead centre. At this point maximum cylinder pressure has been reached. The fuel is now injected into the cylinder and the fuel ' burn' or expansion is started at this point by the high temperature of the, now compressed, air. As the fuel burns it expands exerting pressure on the piston, which in turn develops torque at the crankshaft. It can be seen that the diesel engine is operating at constant pressure. As the gas expands the piston is also moving down the cylinder. By this process the piston and subsequently the crank experiences a greater torque, which is also exerted over a longer time interval than its petrol equivalent.