why do diesel engines develop more torque than petrol engines?

Answer 1

Because of their higher compression ratio, the stroke of the piston is longer to raise compression and makes for more leverage in the cycle of combustion. Some gas engines are more torquey than others because of this feature in engine design...longer stroke=more torque.
http://robotics.caltech.edu/~mason/ramblings/dieselTorque.html

Answer 2

Everybody above is wrong! Much of what they said is true, but the real reason is due to the amount of air entering the engine. With a gasoline or LP engine, the air to fuel ratio must be kept in the14-15:1 area regardless of how fast the engine is turning. Doesn't matter whether you're going fast or slow, it's the same. When you step down on the gas pedal, you open up the throttle butterfly which then allows more air to enter the engine. Let up and you restrict the amount of air entering and the engine slows down and less power is produced.

With diesel engines, the pedal (or lever depending upon the application) controls the amount of fuel injected into the combustion chamber only. There is no air control whatsoever on a diesel engine, no throttle butterfly, no anything. At idle, the engine is sucking in as much air as it can but the pump is injecting only a small amount of fuel which is what limits the engine speed. The air to fuel ratio of a diesel at idle can be in the neighborhood of 50:1 while maximum power production can bring them down to the 6-10:1 ratio.

Regardless of the fuel type, power and torque are the same thing and are the result of expanding pressure in the combustion chamber. Fuel itself won't burn and it needs air to do so. The pressure developed is more dependant upon the amount of air contained than the amount of fuel.

Answer 3

Very simple:

- the knocking resistivity of diesel is higher
- as a consequence you can have a higher compression of diesel without not controlled explosion instead of combustion
- with higher pressures, you get more force which increases the torque.

IF you increase the compression in gasoline engines at the same level, the brutal explosion (1000x faster than a normal combustion) would create strong explosions. This would damage the motor on the long run.
This knocking is nowadays electronically controlled and the motor torque can be increased until close to the maximum (through the ratio gas/air, the time, pressure and speed of injection, but also the amount of exhaust gas being recirculated).

Answer 4

Diesel engines develop more power at LOW revs than petrol engines of equivalent size.

I have always understood that this is due to the slower, (and therefore longer) period of combustion that takes place in a diesel engine, as aganst the very rapid (and consequently shorter) period of burning in a petrol engine.

Answer 5

this is a simple answer but true
they need more pressure to burn the fuel than petrol!