Let's pick this up right where the compression story leaves off — Page 72, Lesson 95

Let's pick this up right where the compression story leaves off. We've been talking about how a diesel engine compresses air so hard that it gets hot enough to ignite fuel on its own. That's the whole point of the compression-ignition cycle — the heat of compression is the ignition source, so there's no need for a spark-generating system like you'd find on a petrol engine. But here's the catch: that works beautifully once the engine is running and hot. Getting it started from cold is a different problem entirely. When the engine is cold, the air inside the cylinder isn't hot enough yet to ignite the fuel. So diesel engines use a system of glow-plugs or pre-heaters for cold starting. These are small heating elements that sit in the combustion-chamber area and provide initial localized heating — meaning they heat just that one spot, not the whole engine. They warm the air in the immediate vicinity of the fuel spray so that when you crank the engine, the first few injections actually ignite. Now, once the engine fires and starts running, the glow-plugs aren't needed anymore. Why? Because the engine is now producing its own heat. The fuel is injected into a zone where the temperatures are higher than the flash-point of the fuel. Let me define that term precisely: the flash-point is the lowest temperature at which the fuel gives off enough vapour to ignite in the presence of an ignition source. In a running diesel, the compression ratio is so high that the compressed air temperature exceeds the fuel's flash-point. So the moment fuel is injected into that hot zone, it ignites. And here's the key phrase from the text: ignition effectively by detonation becomes continuous. That's a deliberate contrast with a spark-ignition engine. In a petrol engine, you have a controlled flame front spreading from the spark plug. In a diesel, the ignition is more like a rapid, uncontrolled burn — detonation-like — but it's happening continuously, injection after injection, as long as the engine is running. That's what sustains the cycle. So to tie it all together: cold start uses glow-plugs for that initial localized heat; once running, the high compression ratio keeps the combustion-chamber temperature above the fuel's flash-point, and ignition by detonation becomes self-sustaining. No spark system needed at any point.