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Why do diesels produce more torque than petrol engines?

by on December 10, 2013


Lots of reasons, and in different scenarios. Assuming a 4-stroke turbo.

  • Diesel piston rods usually connect to a crank shaft further from center than in a gasoline engine. This provides more leverage when pushing down against the crank and higher torque. (Imagine using a short or long wrench to tighten a bolt. Longer the wrench, easier the job.)
  • The larger radius around the crank shaft usually yields more movement of the piston yielding a longer stroke. Thus, higher compression ratio of the fuel and more energy exerted back on the piston during combustion.
  • The higher compression and longer stroke means there is both more energy and time for other systems to assist; 1. The higher energy is recycled in a feedback loop (in the form of a turbo) which creates higher pressure on the fuel intake for the next cycle, 2. The longer time means more efficient use of low RPM. Lower RPM means all air flow, part movement, and other system calibration processes can be measured and controlled more accurately without worrying too much about extreme cases of fluid dynamics
  • With the assisting pressure of a turbo, fuel inlets can be safely timed/programmed to push additional fuel into the piston chamber, increasing the length of the fuel intake stroke well beyond one quarter of the total 4-stroke cycle.

One of our lab minions asked:giz-gad-shipping

What’s the difference?

Especially when compared two of the same cars…

BMW 335d Is 234bhp and 400torque

BMW 335i Is 300 bhp and around 330 torque

Why does the petrol rev higher though? I think it goes up to 8000?

However in the same car but diesel it goes up to only 5000.

Why is this?

The simple answer is that diesel is a slower burning fuel. Diesel engines often have longer strokes to take advantage of the longer power stroke then gasoline engines as well. More time to move a given distance + longer distance to travel = lower rpm. The trucks we have at the lab are 12 liter diesels. They max out at 2200 rpm. The container ships have house sized diesels. Most of them top out around 130 rpm’s, but push 8-10,000 hp.