Diesel engines are considered "steady torque", which means that torque is same no matter the revolutions whereas torque depends on revolutions for an engine that runs on Otto cycle.
What is the reason for this?
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The diesel engines are working on diesel cycle. The heat addition(ingnition) is taking place during the constant pressure process.
After added energy is spent by the expansion of piston. The amount of expansion is fixed in terms of stroke length.
So for given fixed pressure and fixed expansion we ought to get constant work output. That's why the torque is relatively fixed in diesel engines.
In case of otto cycle/petrol/gasoline engines the heat is added during constant volume process. So you can rise the pressure as much you can for the same expansion by adding more heat. Hence you can get quick pickup/high torque in petrol/gasoline engines by adding more fuel while starting the engine.
Hope this helps.
The higher torque comes from a relatively longer stroke, which is needed to develop the higher compression for a diesel. The longer stroke gives a longer "arm" for the piston to push on the crankshaft. Or , think of the longer crankshaft throws; distance from crankshaft center-line to the throw center-line. The affect is more apparent when the throws are in the 3 O'clock and 9 O'clock positions. Diesels have an inherently narrower power range than a gasoline engine, that may give the appearance of a more steady torque output.