Detonation Theory - Cause and Effect
Advanced detonation damage.
Basic understanding of detonation.
Detonation has been around since the birth of the combustion engine and is described as auto ignition of the end gas in the combustion chamber. In the early days of mechanical inspections people thought detonation was impossible and that damage observed was actually pre-ignition. It took until the 1940s for the theory of detonation to be proven. We will look at how to detect and eliminate detonation as well as looking at the damage it can do to an engine.
When an engine goes into Detonation it makes a loud “pinging” noise. This is also referred to as engine knock. When detonation starts in an engine it is vitally important to identify what is causing it. Engine operation may need to be changed to eliminate the problem. Personally I work with large industrial gas engines and detonation can wreck an engine in a very short period of time. In some cases an engine can go into detonation and not be damaged and in other cases even small amounts of detonation can cause serious damage. Therefore it is best to determine the cause of detonation and eliminate it as quickly as possible.
The characteristic pinging sound from an engine in detonation is caused by pressure waves in the combustion event making the cylinder walls vibrate. The gasses and the shape, size and thickness of the combustion chamber and cylinder walls determine the pitch of the sound wave. The sound is metallic almost like banging 2 hammers together. Early detonation damage is commonly but not always found at the edge of the piston crown. Some of the engines I work on have puller holes machined into the piston crown and if detonation occurs typically you will see the first signs of damage there.
Advanced damage occurs on the crown and all the way down the side of the piston. It looks badly scuffed and scratched. Small pieces of aluminium from the piston can break off. The melted aluminium that breaks off the crown can stick to the cylinder walls which are typically cooler. As the piston moves up and down in the cylinder the aluminium is rubbed flat on the liner wall and also on the side of the piston. This is the scoring seen in the attached pictures. Any aluminium build up on the liner wall can lead to piston ring damage which in turn can lead to massive engine failure. Gasses could escape past the piston rings into the crankcase called blow by or oil could end up in the combustion chamber. This situation is obviously bad and the route cause is detonation.
Signs of Detonation
The next pictures you see are from total piston seizure from detonation. The engine was suffering from detonation and large amounts of aluminium built up on the liner wall. Eventually the piston seized from the aluminium build up. Remember even though one piston seizes the engine is still running. The connecting rod which connects the piston with the crankshaft pulled the piston apart and the connecting rod which is now free came through the engine block. Luckily no one was injured in this case but the engine was scrapped.
What is detonation?
OK, so what is detonation? I described it earlier as auto ignition of the end gas. If you can imagine the spark plug in the middle of the combustion chamber the end gas will be furthest away from the spark plug during ignition. The effects of heat and pressure on the end gas can cause it to ignite before ignition from the spark plug reaches it. This means we now have two flame fronts in the combustion chamber. When the two flame fronts collide this is called detonation. The resulting explosion from the flame fronts colliding causes a huge pressure spike which can damage the engine and can also be heard. Detonation occurs after the spark plug has ignited the air/fuel mixture.
Normal combustion is a controlled event in which the spark plug ignites the air and fuel in the combustion chamber and spreads out across the combustion chamber at the same rate. Much like throwing a stone into a pond on a calm day and watching the ripples move away at a constant speed.
Let’s now look at some of the causes of detonation:
The timing of our spark in the combustion chamber is critical. Advancing the timing allows the air and fuel longer time to react. Every degree the timing is advanced the pressure increases and peak pressure occurs earlier. Due to this increase in pressure, the fuels volume decreases and its temperature increases, thus increasing the internal energy (Adiabatic Pressure). The increase of pressure and temperature on the end gas can lead to detonation.
The compression ratio also can cause detonation. The compression ratio measures how much the air/fuel mixture is compressed when the piston reaches the top of its travel.A ratio of 10:1 means that the volume of the air/fuel mixture is compressed to 1/10th of its original volume when the piston reaches the top of its travel. When you increase the compression ratio, pressure increases as well as temperature, so the end gas exposed to high pressure and temperature is more likely to auto ignite. Deposits building up on the piston crown can also cause the compression ratio to change.
As combustion pressure increases heat also increases. The fuels ability to resist detonation decreases, so we increase the risk of detonation. Reasons combustion pressure can increase are form high compression ratios or too advanced timing or maybe even turbo Boost.
Increased temperature in the combustion chamber can lead to detonation. With higher coolant temperature and air intake temperatures you can run the risk of detonation. Heat transfer will not be so effective if coolant is too hot and the fuel mix will be hotter leading to higher temperatures in the combustion chamber. Engines in buildings with poor ventilation will also be at risk. Having hot exhaust gasses blowing into and engine air intake could cause this. I have seen engine installations where this happens and detonation is a problem.
Some causes of detonation.
Higher engine speed helps reduce the likelihood of detonation because you actually reduce compression time and volumetric efficiency. Peak pressure is therefore reduced and the air/fuel mixture will not be exposed to high temperatures for as long. An example of that is when you drive your car along a straight flat road then encounter a hill. As you accelerate up the hill you start to loose power and you can sometimes hear your engine detonate. So to get the power back you shift down one or two gears and accelerate again. The power is back and the detonation is gone.
Increased humidity actually reduces the risk of detonation. The higher of content of water in the air helps to reduce combustion temperatures and it also absorbs some of the energy in the charge. In some cases increased humidity can be a pest. I have a friend who works in South America and he went to site out in the rainforests of Venezuela where his customer couldn’t start his industrial gas engine. The humidity was so high the air fuel mixture wouldn’t ignite.
Changes in air/fuel ratio for example can change combustion temperatures and pressures. Changing the AFR can also affect timing, possibly exposing fuel to high heat and pressure for longer again causing detonation.
Detonation is not a new problem and designers have been trying to eliminate or reduce the problem for years and years. It is complicated and involves many different factors but to truly understand how an engine operates you need to understand how detonation works and the knock on steps that promote it.
All these factors and more can lead to detonation occurring in an engine, spark timing, compression ratio, inlet fuel / air temperature, coolant temperature, engine speed, atmospheric humidity, combustion chamber deposits, fuel air ratio, engine load or misfire.