The choice of metal as a material from which internal combustion engines are constructed is an unfortunate one. When internal combustion engines were first being developed, metals were the only suitable material available at that time.
Metallic internal combustion engines are inefficient because:
1. their combustion efficiency is low (metals are relatively low temperature materials for engines) and
2. they readily loose valuable heat, generated during the combustion of fuel, to their surroundings (metals are good thermal conductors).
Metallic internal combustion engines operate at temperatures too low* (less than 600 C) for fuel to be burnt completely. The maximum service temperature of cast iron is less than 600 C, that of steel is less than 500 C and that of aluminium alloys are less than 300 C. Metal engines thus suffer from relatively low combustion efficiencies.
Also, the heat generated¶ within a metallic combustion chamber of an engine is readily lost to its surroundings due to a metal’s high thermal conductivity. That is, metals are good conductors of heat (e.g. the thermal conductivity of cast iron is 18-34 W/m.K, that of carbon steels are 28-70 W/m.K and that of the aluminium alloys are about 205 W/m.K. Typically about a third of the heat generated during combustion is lost to the coolant or radiator water.
Unburnt fuel and partial combustion products are discarded into the exhaust adding to airborne pollution.
A suitable engine material should have twin capabilities; namely be able to operate at high temperatures and also contain the heat generated within the combustion chamber. Zirconia ceramic is such a material. It is a high temperature material and it also possesses a low thermal conductivity.
[A] are able to operate at higher temperatures (possibly > 800 C) resulting in more complete combustion of fuel and increased thermal and combustion efficiency (and could lead to decreased fuel consumption)
[B] are relatively good thermal insulators and are thus able to reduce the heat lost to the surroundings (e.g. the thermal conductivity of zirconia ceramics is about 2 W/m.K).
Therefore making the combustion chamber of an internal combustion engine entirely out of zirconia ceramic or else insulating the surface of the combustion chamber with zirconia ceramic tiles should improve the engine's combustion characteristics and reduce heat loss. This could lead to higher engine performance and lower exhaust emissions.
¶Peak gas temperatures during a combustion cycle can be as high as 2300 C and the heat generated is readily lost through the walls of the metallic combustion chamber.
[*¶See Wilson, T., Bryanston-Cross, P., Chana, K., et al. (2002) in Combustion & Flow Diagnostics; Vol. 2002-01-0747. Presented at SAE 2002 World Congress & Exhibition, Society of Automotive Engineers, Detroit, MI, USA].
[*See also Smith et al, Metallurgical & Materials Transactions A, Volume 30A, pp 133, January 1999]
The family of zirconia ceramics was adopted as an engineering material only during the 1970s when the transformation toughening of zirconia ceramic improved its mechanical strength for use in structural applications. Because of this feature, the family of zirconia ceramics has since been dubbed “ceramic steel” and has found many important engineering applications [From: An Introduction to Zirconia by Dr Ronald Stevens of Leeds University, UK; Magnesium-Elektron Publication number 113, July 1986].
Yttria (Y-TZP) and Magnesia (Mg-TZP) zirconia ceramics are prone to hydrothermal degradation and thus have limited life when used as engine materials.
Ceria (Ce-TZP) zirconia ceramic does not appear to undergo hydrothermal degradation and should be more suitable for internal combustion engines. Indeed, one of our customers had UHM’s CeTZP sleeve bearings completely immersed in a hot (~240 F) aqueous alkaline bath (pH ~ 13.3) for over three months without degradation and without any indication of wear.
Previous use of zirconia ceramics in engines:
During the 1980s, in a development program at Ford Motor Company, zirconia-based ceramic components (e.g. a short ceramic cylinder liner) have been successfully tested in reciprocating internal combustion engines for over 500 hours of operation before failure. It was believed that the failure of zirconia ceramic components in internal combustion engines was due to the use of Yttria (Y-TZP) and Magnesia (Mg-TZP) zirconias. These zirconia ceramics are prone to hydrothermal degradation due partly to the moisture present in the fuels (see W.Bunk and H.Hausner "Ceramic Materials and Components for Engines" Proceedings of the Second International Symposium; 14-17 April 1986, Lubeck-Travemunde, FRG). It is believed that these programs were abandoned mainly because of the hydrothermal degradation of the zirconias they used.
Using Silicon Nitride/Carbide in Engines:
Silicon nitride and silicon carbide ceramics have thermal conductivities that are higher than zirconia ceramics and therefore are less likely to prevent heat loss if engines are made from silicon nitride and silicon carbide ceramics.
Silicon nitride and silicon carbide ceramics in a humid oxidising atmosphere (such as during fuel-air combustion in internal combustion engines) are prone to converting to the more thermodynamically stable oxide state (ultimately silicon dioxide which is glass or sand) and likely to loose their exceptional properties. Please also see, for example, a NASA-Glenn Research Center publication on Silicon Carbide degradation (Linus Ogbuji 2000 Science Reviews: Materials at High Temperatures; Volume 17, pages 369 - 372).