Improve Engine Performance Through Ceramic Coating - Le Xia*, Ph.D.
By applying suitable ceramic coating, the efficiency of an Internal Combustion Engine can be increased. Higher efficiency of the engine is reflected in improved performance such as reduced fuel consumption, increased power output, reduced noise and vibration, reduced oil burning, and reduced air pollution. The ceramic material used for engine coating needs to have a low thermal conductivity; high temperature stability; matched thermal expansion coefficient with bonded metal; high wear- and corrosion-resistance; and the capability for form a durable bond with metal.
Internal Combustion Engine (ICE) is the workhorse in modern industry and society. At the same time, an ICE consumes energy in form of fossil fuel. To reduce energy consumption through improved engine efficiency is constant pursuit of the whole world.
A large class of modern ICE such as omnipotent gasoline engine works following the Theorem of Camot Cycle. The heart of the Carnot Cycle theorem says that an ICE works because it transfers a portion of the heat energy into mechanical work. Further, the genius of the Carnot Cycle Theorem is that it gives the specific description for the efficiency of an engine, which is:
ŋ - is the efficiency of the engine,
Tlow- is the temperature of the cold reservoir,
Thigh- is the temperature of the hot reservoir.
Interpreting the above the Theorem one can consider that Thighis roughly related to the average working temperature inside the engine cylinder and Tlowto the average temperature of the air discharged to the exhaust. For an average gasoline engine, the average working temperature is 1050°C and the exhaust temperature of 750°C. This according to Equ. 1.1 gives the engine efficiency of 28.6% (1-750/1050).
From the above equation, it can be seen that the engine efficiency is directly related to the working temperature of the engine. Under a limit and for the engine with a constant exhaust temperature, the higher the working temperature of the engine, the higher its efficiency. This is the reason why engines used in racing cars and in some airspace applications have higher cooling water temperature switching setting than ordinary cars. For ordinary road cars, the thermostat temperature is usually set at 90°C degree C while in racing cars this could be 105 °C and higher, resulting in an 50°C increase of engine working temperature to 1100°C. From Equ. 1.1, this results in a theoretical engine efficiency of 32% (1-750/1100), an increase by 12%.
Please note that the above Theorem is applicable only to an ideal engine. For a practical engine, there are many other factors that affect the performance of the engine. For example, if there is a compromise in the sealing . between the engine cylinder and piston, a certain amount of thermal energy will be lost through the escape of combustion chamber in the gap between the cylinder wall and piston. This is also accompanied by the increased burning of engine oil and/or increased noise and vibration of engine operation. At the same time, incomplete combustion will increase pollution discharge.
Therefore, to maintain a tight seal between the cylinder wall and piston; and to increase the operating temperature of the engine will have a beneficial effect to the engine performance.
II. Engine Cylinder Condition
Modern ICE cylinder consists of a steel metal insert inside the aluminum alloy body:
Engine piston moves reciprocally inside the cylinder where the seal between the two is maintained by pistons rings. As the engine operation continues, the friction between the piston ring and cylinder walls invariably cause scratches, scores, pits, and other imperfection on the cylinder walls.
These conditions on the engine cylinder cause engine performance degradation. Symptoms of performance degradation include:
- Reduced power output and increased fuel consumption. This is mainly due to the reduced compression ratio as the result of the compromised seal tightness.
- Oil burning. Part of oil enters the combustion chamber and gets burned.
- Increased noise and vibration.
- Increased pollution
III. Engine Ceramic Coating
- Lower Thermal Conductivity.
Contrary to some belief, it's better for engine cylinder wall to have low thermal conductivity with respect to cooling water rather than high conductivity. This, from Equ. 1.1., will result in higher average operating temperature, thus higher operating efficiency. This is the reason why many piston head and valve surface are coated with ceramic material with low thermal conductivity.
- High Temperature Stability.
The working temperature at metal material is usually below at 1200°C, while for ceramic material it could be at 1500°C. Again this could result in a theoretical efficiency increase by as much as 50%.
- Matched Thermal Expansion Coefficient.
Coated ceramic needs to have a thermal expansion coefficient that is closely matched with that of the underlying metal; otherwise during the thermal cycling of operation, the bonding integrity between the metal and ceramic coating will be compromised. Usually the thermal expansion coefficient of metal is higher than that of ceramics.
- Wear- and Corrosion-Resistance.
In many cases there is mechanical movement occurring on ceramic coating surface such as piston movement inside an engine cylinder. The ceramic coating in these cases needs to be wear- and corrosion-resistant in order to have long operating life.
A critical drawback of most ceramic materials is its brittleness that causes low impact strength. Much effort has been put into increasing the toughness of ceramic material. This includes reducing the ceramic particle size to nano level; change material crystal structure; and by compositing ceramic material. Toughened ceramic material has been used in piston rings.
- Ceramic Coating to Metal.
Ceramic coating is applied to metal surface usually through two approaches:
- high temperature deposition by melting the ceramic material which is then sprayed onto the base metal surface; and
- using a ceramic slurry which is then sprayed, brushed, dipped or by other methods to the metal surface.
IV. Zirconium Coating on Engine Cylinder
Zirconium, or ZrO2, is a ceramic material that has been proved to be the most appropriate ceramic material for engine surface coating. It meets virtually all requirements mentioned above for ideal coating material. ZrO2has a much lower thermal conductivity than other ceramic materials. Dedicated engine research work has shown that with Zirconium coating on the engine components, the average temperature of the engine increases by 75°C. Using Equ. 1.1, this theoretically can improve the engine efficiency by 16.4%.
The melting point of Zirconium is 2715°C and its creep deformation temperature is between 2400 °C to 2500 °C, which is higher than working temperature of virtually any ICEs. Its wear-resistibility is 15 times higher than that of Al2O3(Alumina). The thermal expansion coefficient of ZrO2is among ceramic materials the closest to that of steel (about 11/x10-4K-1). Also much of research work has been done for Toughened Zirconium Ceramic (TZC) , which makes ZrO2have a relative higher ductility.
Despite of these characteristics for ZrO2that are highly desirable for metal surface coating, its uses are have been limited to piston rings, journal bearing surfaces, the coating of the valve tops. It has not been widely used for engine cylinder wall coating - the most important application deemed - at production level by engine makers. The main reason is that the ceramic coating is not entirely compatible with traditional engine making process. After the coating is done, the cylinder bore has to be re-grinded before piston assembly can be put in. This is a time consuming process; and as a result, the added cost of ceramic coating is deemed difficult to be justified by engine makers.
However, there is an elegant solution of engine Zirconium ceramic coating offered by some auto aftermarket product manufacturers. A notable example is a product called NanoEnergizer produced by Nanotec International (Korea) Company. The product uses precious meal coated nano-size Zirconium powder emulsified in mineral oil and when applied, simply add it to the engine through oil filler port.
With NanoEnergizer, the Zirconium nano powder will be brought by piston movement to all working surface of the engine during its operation. The precious metal coating on the nano Zirconium powder facilitates the bonding of ceramic powder to the metal surface as well as helping with a cleaner combustion of air-fuel mixture. Thanks for the heat generated during normal operation of the engine, the ceramic powder attached to the engine surface is gradually cured. When the ceramic coating is completely cured, it will form a hard and smooth surface on the working components of the engine. Because nano ceramic powder is in liquid form, it will be brought to any working surface of an engine including cylinder walls, piston rings, piston top, valve tops, bearing surfaces, etc.
The elegance of this solution is that it is achieved during the normal operation of the engine; there is no need for additional separate process to apply the coating; and there is no need to disassembly the engine.
The effect of the NanoEnergizer coating has been tremendous. The following photos show the engine cylinder walls after the engine has run for a period of time following the use of NanoEnergizer. It can be seen that the cylinder walls now have a smooth and hard ceramic surface. And the piston rings also have matching smooth surface of Zirconium.
- improved fuel economy;
- increased power output;
- reduced oil burning;
- reduced noise and vibration;
- reduced pollution discharge.
Ceramic coating, especially with Zirconium, has a beneficial effects on engine efficiency and performance. Regular use of aftermarket products based on this principle is encouraged to maintain a healthy and high-performance running engine.