Q: I understand how petrol engines stop, by stopping the spark, inhibiting the ignition of the fuel. But diesel engines are run by compression of the diesel at a high temperature, so they don't require an ignition source other than compression. So how does turning the ignition key stop the engine when its running? Does it stop the injectors from injecting fuel into the cylinders? If so, how were pre-injector diesel engines stopped?

A: It's just a matter of cutting off the fuel supply. These days there's a spring-loaded valve which is held open electrically when the "ignition" is on, when you switch off the valve shuts itself.

Previously you shut the valve yourself by pulling a knob, wire or lever which shut the valve.

I used to wind up a colleague by leaving the cut-off knob pulled out, which he never noticed and wondered why the old van wouldn't start . . .

Q: I live in Ohio and I have a 1985 Chevy Diesel pickup. The problem is that I need to leave it sit for about 10 hours while I am at work and I will not have the option to plug it in during this time (like I do when it sits at home over night). I've heard that starting a diesel in cold weather can damage the engine if it is not heated. Do I need to be concerned about doing damage to the engine when I start it after work? At what temperature might I have to worry about this? At what temperature does diesel fuel start to jell?

A: There is no reason to be concerned about starting a diesel engine in the cold. It may be a little more difficult to light it off, but once it is running, it is really no different than a spark ignited engine.
It is true that standard number two diesel fuel will gel at very cold temperatures. However, most diesel fuels today that you buy at your local gas station are blended to match the local weather conditions. As long as you keep th fuel in your tank current, it should not gel. If in doubt, ask at the counter before filling up or add an anti gel agent.

Doctor Ratz
Engineering manager, Engine development
For a major engine manufacturer