Crown coatings

With the push for ever-improved engine efficiency in general, and a specific drive toward increased torque and power output from race engines, there has been much work done on coating piston skirts to reduce frictional losses. There are various options available, from resin-bonded polymers to DLC. However, other piston coatings can also help us achieve our goals, although by very different means.

Piston crown coatings are used for various reasons but, as we shall find out, there are generally other positive side-effects that can help us improve efficiency and increase engine output. There are two main types of crown coating - ceramics and metallic.

Ceramic piston crown coatings generally have very low thermal conductivity; this acts as a thermal barrier, reducing the transfer of heat through conduction from the crown to the rest of the piston and engine structure. The heat rejected to any oil used for cooling the piston is also reduced, and if the flows are re-optimised for a ceramic-coated crown then the squirt jet flows can be reduced or perhaps even done away with.

This re-optimisation of squirt jet flows has benefits. The lower the heat rejection then the smaller the cooler needs to be to reject the heat to atmosphere. Not only is this a lighter component, but aerodynamic drag is also reduced. Lower piston cooling flows generally mean lower frictional losses from the bottom end of the engine; there is less oil entrained in the general airflow and less oil hanging around which can generate heat through constant shearing due to solid components passing close by.

Reduction of squirt jet flows can mean that smaller pumps are required, which may also lead to a very slight decrease in frictional losses. With a lower thermal conductivity piston crown, overall heat rejection is reduced and the engine, for a given trapped mass of air and fuel, should be more efficient. Increasing the thickness of the thermal barrier coating minimises the amount of heat rejected through the piston crown.

So why do we not coat all pistons with a generous layer of thermal barrier coating? The result of coating the crown is that the combustion chamber temperatures are increased, and the wall temperatures are also higher. This means that any fresh charge suffers from higher heat transfer from the wall of the combustion chamber, and this leads to a premature increase in pressure, reducing the pressure difference across the inlet valve and thus reducing inlet charge mass flow rates. The result can be a drop in volumetric efficiency. However, the overall fuel conversion efficiency is generally increased. The net effect may be to reduce engine performance, although fuel economy may be improved
Metallic coatings are used widely in turbocharged and supercharged engines. Such forced-induction engines, with high combustion chamber pressures and temperatures, are prone to engine knock, a kind of uncontrolled combustion event. Although many engines continue to increase output as knock ensues, the downside is that rapid engine damage follows, especially where pistons are concerned. In general, it is necessary to change ignition timing to bring the engine out of knock, but performance suffers. By using a hard metallic coating, it is possible to run an engine into knock without suffering the mechanical damage and engine failure often seen with uncoated pistons.

Written by Wayne Ward