DEAR TOM AND RAY: I have a diesel engine. If I turn off the car while it is moving, then engage fourth or fifth gear and pop the clutch, will the engine jump-start? – Bobby
RAY: As long as you’re moving fast enough.
TOM: Diesel engines don’t use spark plugs. They use high compression in the cylinders to create enough heat in there to combust the air and diesel fuel.
RAY: So then all you need is fuel delivery – which you have when the key is in the run position – and something to get the engine turning.
TOM: That’s what the jump-start is for. Normally, when you put the car in gear (which connects the engine to the wheels), it’s because the engine is already turning, and you want to use it to turn the wheels.
RAY: A jump-start (or roll-start, most accurately) turns that equation around. Your wheels are already turning, and you want to use them to turn the engine.
TOM: So it should work, Bobby. The exact speed and exact gear combination will be different for different vehicles. I mean, if you have a big V-8 diesel engine with really high compression (which means it takes more force to make the engine turn), and you try to start it in too high a gear or at too low a speed, the engine could actually win that battle with the wheels and bring the wheels to a stop.
RAY: So, for instance, if you’re rolling at 15 mph in a big, honkin’ 6-liter diesel V-8, and you pop the clutch in fifth gear, the car may come to a halt, without starting the engine. So you would have to experiment with different speeds and gears to figure out where the limits are.
TOM: When you do that, wear your seat belt, a crash helmet and a mouth guard.
RAY: So theoretically, yes, you certainly can roll-start a diesel engine using fourth or fifth gear. But keep in mind that we do have a recent invention that makes this completely unnecessary in most cases, Bobby. It’s called the ignition key.
Got a question about cars? Email Click and Clack by visiting the Car Talk website at cartalk.com.