Changing to a diesel

Rallydriver

thescouselander wrote: »

I would say its just the opposite. You have to change gear more often in a diesel because the power is delivered over a very narrow rev range. You'll really notice this when the red line quickly approaches in the middle of an overtaking manouvre.

This is true in certain situations. Given the average driver (vast majority) who drives a petrol never takes it into the top of the rev range then it probably will be quicker for them, bear in mind these drivers don't usually overtake anything going quicker than a tractor.

For a driver used to using the full rev range of a petrol engine then it will require planning of the manouvre which is not natural to them and possibly an extra gearchange.

The science bit. (simplified alot)
With a petrol engine, the fuel is ignited by a spark. That spark is timed to ignite the fuel so it burns at the correct time to push the piston down. As the revs of the engine increase, the spark needs to happen earlier in order for the fuel to be burning at the correct time to push the piston down. This is known as ignition advance.
Essentially this means that at higher revs the power from each stroke stays more constant but because there are more power strokes the engine creates more power.

With a diesel, the fuel is ignited by the diesel/air mixture being squashed by the rising piston (compression ignition). So ignition happens at a set time in the cycle no matter what the engine speed is. Therefore, as the engine speed rises, each power stroke produces less power. Hence a diesel engine runs out of steam as the revs rise.

Simplified alot to be understood, I know there are flaws in the explanation but they make it easier to understand the basic concepts.

No doubt one day someone will manage to advance the ignition on a diesel, lets hope they manage it before the oil runs out. lol.