|COMBUSTION CHAMBER THERMAL BARRIER
HEAT TRANSFER IN A COMBUSTION CHAMBER
When we examine the combustion process, we find that temperatures may exceed 1650 °C for very short periods of time, and exhaust temperatures can exceed 870 °C. Combustion surfaces rarely exceed 315 °C, though this temperature differential with the cooling water allows lots of heat to be dissipated into the cooling system, and through the piston into the oil and the rings. This means that more energy is lost through the engine water cooling system, and that the top ring has a hard time to survive with very hot and thin oil to lubricate the cylinder bore. These losses translate into a lower thermal efficiency of the engine.
A thermal barrier helps to reduce the heat transfer into the cooling system and into the piston. It results in an increase in power output and overall efficiency of the engine. This is achieved by improved expansion of the gases in the combustion chamber, resulting in an increased thrust on the piston.
There are a number of important requirements for a successful thermal barrier coating in this "boiler hot box" of an internal combustion engine.
In the past, insulation of the cylinder head, the valve heads and the piston crowns was previously achieved by an old generation ceramic coating. Our experience together with the CSIR was a giant leap in technology in order to improve the reliability and performance of racing engines. The latest developments make use of a barrier coating consisting of a ceramic like material topped by a polished metallic layer with various advantages on the original system.
During the induction process a cool charge of air and fuel is drawn into the engine and this charge is heated by heat transfer from the inlet ports, the hot combustion chamber and piston surfaces. If this charge is heated, it becomes less dense resulting in reduced performance. An effective surface coating retards the transfer of heat to the cool gases. Thus improving the volumetric efficiency of an engine.
When the surfaces of the piston crown and combustion chamber are polished, it reflects the heat instead of absorbing it. If this could be achieved together with a thermal barrier, then the result provides even better combustion. This means the heat generated by the combustion would be more evenly dispersed within the chamber, thus increasing the efficiency of the engine, producing more power and reducing fresh charge losses through the exhaust. This has been demonstrated by increased power output and reduction in carburettor jet sizes, or leaner fuel injection settings. Traditional ceramics might have been the source of detonation due to overheated surfaces, which ignited the gases prematurely.
While traditional ceramics are prone to flaking due to the nature of these materials, this is not the case with this modern material. The ceramic like coating is not achieved through the bonding or fusion of zirconia ceramics, as traditionally has been done. The resin when cured takes on some of the characteristics of a ceramic. This material has a degree of flexibility and can expand and contract with the surface. The melting point of the coating is far above the temperature that would be generated by combustion, and in testing has maintained adhesion when a piston was exposed to sufficient heat to actually melt the piston. Neither thermal shock nor physical impact has damaged the coating, even when sufficient impact was used to dent the substrate. The coating maintained adhesion and followed the deformation.
A ceramic coated exhaust manifold and exhaust pipe reduces the heat radiation from the hot exhaust gases, and results in lower under bonnet temperatures. This is an important aspect for a DKW where some of the heated air reaches the water radiator. An insulated exhaust also results in higher gas speeds in the exhaust manifold which means improved exhaust gas extraction from the engine.
The black ceramic coating for the exhaust and the heat deflector shield on the manifold gives a durable rust free surface for improved engine appearance.
MOLYBDENUM DISULPHDE (MoS2) PISTON SKIRT COATINGSPiston skirt coatings are excellent for reduced friction between piston and cylinder wall. MoS2 is a high performance lubricant and it reduces the possibility for piston seizures. The MoS2 coating is less than 0.005 mm thick and it is not necessary to change piston-cylinder clearances.
CERAMETALLIC COATINGS IN TWO STROKE ENGINESThe piston in a two-stroke engine is under a much heavier thermal load than in a four-stroke engine, because the piston receives a heat blaze during every engine rotation. In a four-stroke engine this happens only every second rotation, which provides twice the piston cooling time between the sucessive combustion processes.
The cerametallic coating shields the piston from the heat flux, resulting in lower piston temperatures which replects in increased reliability: lower ring temperatures, which means better lubrication. This means less piston expansion which could prevent piston seizures under adverse conditions.