The diesel engine actually has longer stroke length. For a gasoline engine, the bore diameter is more but the piston does not travel much up and down while in a diesel engine the bore diameter is not much wide but the piston moves quite more up and down. Now torque is the force multiplied by the distance. So in a diesel engine, the force is more and the distance of the stroke is more.
Diesel Engines are TurboCharged:
Diesel engines use heavy-duty pistons, iron blocks, connecting rods, crankshaft all of which add together to make a heavy engine that can handle the increased torque that is produced by the diesel engine. Also, the air-fuel ratio for diesel is 18:1 to 70:1. They burn lean and thus have more air to compress and produce more work. Pumping loss is also not there during the intake making the engine more efficient.
The diesel fuel has slightly lower calorific value than petrol. So more heat is stored in the petrol for the same volume of given diesel. But diesel is much denser than petrol and can store 15% more energy. So each time diesel is combusted it produces more energy to pressure the piston and more torque to the crankshaft.
We Hope that now you know Why a diesel engine produces more Torque than a Gasoline.
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