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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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  • $\begingroup$ "Steady" is a relative term here, as SolarMike's answer explains. $\endgroup$ – Carl Witthoft Nov 27 '17 at 18:40
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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.

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  • $\begingroup$ The expansion is not fixed : the gas continues to expand once the exhaust valve is opened and less work is applied to the piston... $\endgroup$ – Solar Mike Nov 28 '17 at 9:18
  • $\begingroup$ The volume is not fixed. It is continuously expanding after heat is added. after opening the valve we cant extract work anymore. It has to fill the new air. $\endgroup$ – mustang Nov 28 '17 at 9:59
  • $\begingroup$ I was referring to your statement re the stroke length as the valve is opened before the piston gets to the end of the stroke... $\endgroup$ – Solar Mike Nov 28 '17 at 10:31
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Compared to Otto cycle engines, diesel engines have a smaller rpm range and, as they tend to have more mass (due to the higher compression ratio etc), tend to develop the torque earlier in the rev band and it tends to be a flatter curve through the range.

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  • $\begingroup$ the cam profile can be chosen in a diesel to furnish either a quasi-constant or "flat" torque curve or a peaked profile. The peaked profile is preferred in constant-speed applications, like running a pump or rock-crusher. In applications where the load speed varies (as for instance in a diesel truck), the flatter profile means less gear-shifting for the driver. $\endgroup$ – niels nielsen Nov 27 '17 at 19:41
  • $\begingroup$ @nielsnielsen many things can be achieved : I’ve seen a 6 cyl inline ford diesel put into a jensen interceptor with more power and better fuel consumption... mind you the state of tune was way above standard - but the owner ran a machine shop and knew what exactly to do... :) $\endgroup$ – Solar Mike Nov 27 '17 at 20:23
  • $\begingroup$ Cam profiles can be chosen for petrol engines as well ... $\endgroup$ – Solar Mike Nov 27 '17 at 20:24
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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.

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  • $\begingroup$ That high compression can also be achieved by the ratio of swept volume to clearance volume : as evidenced by the number of engine blocks that are common to petrol and diesel engines... $\endgroup$ – Solar Mike Nov 28 '17 at 5:19
  • $\begingroup$ When GM did this in the late 70's it was considered a failure. I remember as I was interested but our engine lab engineers advised against getting one. $\endgroup$ – blacksmith37 Nov 28 '17 at 21:52

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