MisterMMM.com: Petrol or Diesel?

March 8

How deeply do you want to look at the differences between Petrol and Diesel cars?

Petrol engines are high revving, light and powerful. Everything you need for a racing engine. They are inherently inefficient because the speed control of the engine is done by using a butterfly valve. This restricts the airflow and makes the engine work. So at tick-over, the engine is being restricted by lack of air getting in. A bit like having an Asthma attack. This is because the fuel/air mixture has to be relatively fixed at 1 part petrol vapour to 15 parts air. The mixture is compressed to about 1/10th the volume and then a spark is used to explode it at just the right time to push the piston back down, and turn the engine. At tick-over the engergy from the fuel is used to turn the engine over, but mostly to suck the air laboriously past the butterfly valve. No wonder they actually cough sometimes, especially the older ones.

Diesel engines on the other hand, do not have any real constriction on the flow of air into the cylinders. This means that they are always free breathing and very little energy is lost sucking the air in. The air is then compressed to about 1/20th the volume, and gets very hot, something like 500-600C. At the right time, the injectors squirt a tiny drop of Diesel fuel into the cylinder which then starts to burn and pushes the piston back down, turning the engine.

The main difference between driving them, is that at tick-over the Diesel is breathing freely and has plenty of oomph, what Jeremy Clarkson calls "torques"; whereas the Petrol doesn't because it is having an induced Asthma attack. This means that to move off in a Petrol engine, you must give it some gas first, and give the car some time to get moving. In the Diesel you don't even need to think about it!