Knock Resistance in Turbocharged IC Petroleum Engines

By Sean Kelly

The resistance of a fuel to detonate is a function of its self-ignition temperature. This is directly related to the component referred to as Octane number. The higher the octane number, the higher the self-ignition temperature is; these properties are related but not intrinsically equal. Additives like tetraethyl lead make the self-ignition temperature higher.

To eliminate detonation, you must first understand the nature of the phenomenon as well as the exact factors that contribute to it. Detonation occurs when the temperature of the unburnt air-fuel mixture inside the combustion chamber exceeds the self-ignition temperature of the fuel. This mixture is referred to as the "end gas". Several factors contribute to this self-ignition. Firstly is the temperature of the stagnant intake charge, which can be caused by many things but mainly the compression due to the work the compressor performs on the air. Another factor is energy transferred to the charge from the engine itself via the intake manifold and intake valves. Thirdly is the convection from the spark-ignited charge during the combustion process. Self-ignition happens away from the spark source, where the end gas has time to absorb energy from the surroundings during compression; to the point that the end gas is hot enough to self-ignite.

There are several primary ways to reduce heat transfer to the end gas, or rather transfer energy from it. The easiest and most efficient is to reduce the temperature of the air being pressurized by the compressor. This can be done mechanically with aftercooling, whether by liquid heat transfer or by air; or chemically with additives like methanol or water injection. Next is to increase the cylinder wall to end gas volume ratio; this will maximize the heat transfer from the end gas to the engine block and cylinder heads. The surface area that this factor refers to is called the quench area, because it quenches the eng gas. Lastly is to reduce the distance from the spark source from the extents of the unburnt fuel-air mixture.

Another way to reduce the occurrence of detonation is to delay ignition timing. If the ignition timing is retarded towards the piston's top-dead-center (TDC), the work done by the piston to compress the fuel-air charge is less, and thus in-cylinder temperatures are reduced which allows less heat transfer to end gas. However, this energy does not get lost, retarding ignition timing increases exhaust temperatures. This is due to the combustion/expansion process being delayed; this can cause a problem with the materials used in the exhaust system, and increases wear on a turbine as well as increased oxidation levels in the post-engine boost-control systems.

Using a fuel with additives or a higher intensive self-ignition temperature allows ignition timing to be advanced, which brings the combustion event closer to TDC, which increases the work done on the piston. The limit of the advance of ignition timing is called the "knock threshold" and is a factor of many different conditions in the engine as well as the fuel used. Cooling the engine to maximize heat transfer from the end gas is a good way to reduce knock, so therefore the cooling system plays a part in detonation resistance. However, cooling systems do not change state pending detonation conditions, so this factor is not as easily tuned as ignition timing. The same steady-state nature applies to the aftercooling systems, quench are, and fuel temperatures, which also contribute to energy transfer into or from the end gas. Hence, it makes sense to utilize control of the ignition timing based on knock conditions assuming a set status of these steady-state systems. One should experimentally establish the running conditions of these systems, and change ignition timing once they are set. However, too much cooling causes reductions in engine efficiency, and too little cooling causes problems with crossing the temperature limits of the materials used. Thus, once a datum for acceptable heat transfer limits of these systems is established, they are tuned and set so that the more easily controllable factors can be utilized naturally.

Undesirable mechanical features that may contribute to the self-ignition of the end gas should be addressed before increasing the engine's output is considered. This includes combustion chamber design, spark plug heat range, valve material and turbocharger matching. Allowing these factors to lower the knock threshold is anti-progressive and too controllable to allow. Manufacturers allow a certain margin in the design between the operating conditions and the knock threshold, breaching this margin without raising the threshold is asking for engine damage.