A. Why do Diesel engines have to prime for several secs before starting?
B. What are glow plugs exactly, what do they have to do with A
C. Are all diesels like A?
They don’t prime, the glow plugs have to heat up. Glow plugs assist in the combustion on the compression stroke while the engine is cold. A cold engine may not generate enough heat on compression to ignite the fuel and the glow plugs help. I believe there are diesels without glow plugs that alter the injection timing on startup or have higher compression so they don’t need the glow plugs.
Neat, thanks
“Prime” here may mean having the fuel pump get up to pressure. In any case the instruction to turn the key on until the light goes out (or whatever) before turning the key to the start position is the way to go. Sometimes it helps to turn the key off and back on for a second few seconds when it is very cold.
Indirect injection (IDI) diesels use glow plugs for starting, while modern direct injection (DI) diesels do not. All modern (at least since 2007) automotive diesels are direct injection due to the stringent emissions standards and increase in power density and efficiency. DI engines generally have a lower compression ratio than IDI engines.