Piston Engines - Fuel — Page 85, Lesson 107

Let’s pick this up right where the combustion process leaves off. We’ve already seen the normal flame front sweep across the cylinder. Now I want to walk you through what happens when that process goes wrong — and that’s detonation. Here’s the setup. As the flame front burns across the combustion chamber, it compresses the unburned gas ahead of it — we call that the end gas. The burnt gases behind the flame have already expanded, so they push on the end gas, subjecting it to an ever-increasing pressure. Eventually, there’s enough pressure and heat to bring all of that end gas to the point of combustion at the same instant — and it doesn’t burn, it explodes. The flame rate jumps to 1000 feet per second, and the violence of that explosion depends on how much end gas is left. That explosion is detonation, and it’s destructive in four distinct ways. First, the explosion can cause the piston crown to burn — and eventually to collapse. Second, the combustion chamber overheats, which can cause the valves to split and distort, and can even burn the sparking plug electrodes. Third, there’s a sudden rise in pressure, which applies a shock loading to the engine component parts — that can cause mechanical damage. And fourth, because the maximum pressure is generated before the piston is in the correct position to use it, the piston has to overcome a high back pressure — and power is lost. Now, here’s an important contrast. Detonation in a diesel engine is quite normal. The diesel is sometimes called the ‘detonation-ignition’ engine. Diesels are constructed to withstand those additional pressures, which is why they’re generally heavier. So detonation isn’t inherently bad — it’s bad for a petrol engine that isn’t built for it. Let’s look at the causes. The key principle is this: any condition that heats the charge before combustion will aggravate matters in the end gas. Pre-heating the air before it enters the engine — like using ‘hot-air’ to overcome carburettor icing — or over-compression in the supercharger, can give rise to excessive temperatures. And once burning has started, the process should not be prolonged. There are six causes, and they can act alone or in combination. Let me go through them. a) Incorrect mixture strength. The greater the amount of fuel for a given amount of air, the greater the power obtainable without detonation. So if the power output is high, the mixture must be rich. b) High charge temperature. Anything that raises the temperature or pressure of the charge unduly before burning — for example, carburettor heating at high power, overheated cylinders, or high boost with very low rpm. c) Incorrect ignition timing. If the spark is too far advanced, the charge ignites too early, giving higher temperatures. d) Cooling. If the combustion chamber surfaces are coated with carbon — or coke, as it’s commonly called — heat from the flame won’t dissipate rapidly, resulting in high cylinder head temperatures. e) Cylinder head design. The greater the time taken for the flame front to travel through the combustion chamber, and the higher the charge temperature, the greater the risk of detonation. Design features that directly affect this include the size of the combustion chambers, the positions of the spark plugs and the valves, the compression ratio, and effective cooling. f) Use of incorrect fuel. So the whole picture is this: detonation is the explosive combustion of the end gas, driven by pressure and heat, and it’s aggravated by anything that heats the charge early or slows the flame front. The engine loses power, and the components — piston crown, valves, plug electrodes — take real damage. That’s why mixture, ignition timing, cooling, and fuel choice all matter so much in a piston engine.