While most discussions regarding petrol VS diesels revolve around the actual price of each fuel and the mileage they deliver, this one’s a basic write up on why diesels have more torque. I won’t get into the technicalities of what torque is, but it’s basically the down-force of the piston movement. Just imagine a basketball being bounced. If you bounce it from your ankle level, you have to constantly bounce the ball to keep it in motion. The ball moves up and down faster, but if you were to stop applying effort the ball would stop bouncing almost instantly. Now bounce the ball from your waist level. You put in a little effort periodically.
The ball bounces up and down, but takes more time from floor to palm. The ball requires less effort to bounce, but if you stop applying any effort the ball will keep bouncing for a few more seconds. The latter is an example of high torque at low rpm. The bouncing ball is the piston’s stroke and with less energy required, it delivers the required power. A long stroke equates to more punch delivered per stroke, thereby delivering the required power at relatively lower rpms. Diesels are usually designed as long-stroke engines specifically to generate torque. This is why commercial vehicles like bulldozers and trucks run on diesel engines.
Some heavy-duty truck diesels operate at a maximum of only 2200 RPM! Lighter-duty truck diesels may redline at 3000 to 3500 RPM, and 4000 RPM is considered a "high-speed" diesel. Let's just summarize by saying that diesel engines are usually designed with as long a stroke as practical for the desired peak engine speed. Diesel fuel has about 11% more energy per litre than gasoline too, this is a contributing factor to why diesel is also more efficient a fuel than a gasoline.
Diesel fuel by itself has a higher flash point that gasoline, which is why diesel isn’t as easily flammable. Diesel engines use glow-plug as opposed to spark plugs. The glow plug heats up the air in the engine chamber when the diesel fuel is injected into the compressed air which is what causes the explosion that would be brought about by a spark plug in a petrol engine. The longer stroke delivers torque at lower rpms, but because of this each stroke takes longer to complete as well. So a diesel will manage 60 kph at a lower rpm level as compared to a petrol engine, but will take longer to reach that speed as well. This is why more and more diesels are offered with turbochargers as naturally aspirated diesels tend to be sluggish. Long stroke engines, a higher energy, less volatility fuel result in vehicles with more torque that are lower on emissions and manage a better fuel efficiency.