**3. Effects of ceramic coatings to internal combustion engine performance**

To reduce damages occurring from high cycle temperatures, high cycle forces, sliding, erosion and corrosion on engine parts, several special techniques have been developed. Water cooling and thick combustion chamber walls had been utilized up to the end of Second World War to transfer excessive heat which material properties of combustion chamber construction materials such as cast iron can't bear. Later on, using low thermal conductivity materials such as glass and its' derivatives were considered. Despite low thermal conductivity, cost and low expansion rate, glass couldn't be used in internal combustion engines due to its' lacking strength. Using glass ceramic materials in engine parts was first seen at 1950s. In those days, ceramics used in spark plugs although low

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 205

Thermal barrier coatings used for reducing heat loss from cylinders and converting engines to low heat rejection engines also prevent coated materials from decomposing under high temperatures. ZrO2 is the most preferred material in thermal barrier coated internal combustion engines due to its' low thermal conductivity and high thermal expansion rate. To avoid negative effects of phase changes of ZrO2 at higher temperatures, it should be partially or fully stabilized with a stabilizer material. By this procedure, whole structure is formed with one phase, generally cubic phase. As stabilizer, usually MgO, CaO, CeO2 and

There are a vast number of studies investigating effects of thermal barrier coatings and especially ceramic coatings to internal engine performance and exhaust emission behaviours. Investigated parameters can be summarized as coating material, coated material, coating thickness, engine types and operational conditions such as engine load and speed. Obtained results can be different in dimensions and magnitudes such as volumetric efficiency, thermal efficiency, engine torque, engine power, specific fuel consumption, heat rejection from cylinders, exhaust temperature, exhaust energy and exhaust emissions. Investigations of thermal barrier coating in internal combustion engines are mostly focused on diesel engines because of detonation and knocking problems of spark ignition engines at higher in cylinder temperatures. For diesel engines, studies can be divided into two main categories; turbocharged engines and non-turbocharged engines. For non-turbocharged engines, thermal barrier coating application and thus ceramic coatings of internal combustion engine cylinders generally results negatively due to decreasing volumetric efficiency. In the other hand, turbocharged diesel engines exhibit better performance and exhaust emissions according to improved volumetric efficiency and in cylinder temperatures. This phenomenon's main reason is the increased exhaust gas energy which is converted to mechanical energy and later on to air mass flow rate increase in turbocharger. For instance, Leising and Prohit (1978) suggested that desired results by heat rejection insulation could only be achieved by the utilization of turbocharger and intercooler. They also reported that a diesel engine performance could be increased up to 20% by the addition of a turbocharger. When studies about thermal barrier coated engines without turbochargers are considered, it was observed that most of the studies were conducted on a single cylinder, four stroke diesel engines. Miyairi et. al. (1989), Dickey (1989) and Alkidas (1989) are some of these researchers. Prasad et. al. (2000), Charlton et. al. (1991), Chang et. al. (1983) can be given as examples for researchers that studied on natural aspirated multicylinder diesel engines. In the other hand, multi-cylinder diesel engines types were mostly preferred for turbocharged thermal barrier coated engine researches. For instance Woods et. al. (1992), Kimura et. al. (1992), Woschni and Spindler (1988), Hay et. al. (1986) and Ciniviz (2005) performed parametric studies on thermal barrier coated turbocharged multi-cylinder diesel engines. Parlak (2000) and Kamo et. al. (1997) are two studies among limited

turbocharged single cylinder thermal barrier coated engine investigations.

Coating materials and methods can be divided into two categories for this book; ceramics and non-ceramics. Coating thickness is usually changes between 100-500 μm. A typical thickness for coating materials is 0,15 mm binding layer and 0,35 mm coating material. Parlak et. al. (2003) and Taymaz et. al. (2003) are two of these studies which used the typical coating thickness. For the researchers that preferred ceramic materials, zirconia is the most seen material among other ceramics. NiCrAl is frequently used as binding materials for

Y2O3 oxides are used.

application numbers. Requirements for ceramic coatings for high temperature applications had been started to increase at 1960s. Especially developing gas turbines leaded that requirement because of metals and various alloys that couldn't resist high temperatures. Ceramic coating technology was initially applied to space and aviation areas and then at 1970s it had been started to apply to internal combustion engines, especially diesel engines. Performance increase and specific fuel consumption decrease of aforementioned ceramic coated systems created an interest to the topic.


Table 3. Plasma spray coating technology; Components and parameters

application numbers. Requirements for ceramic coatings for high temperature applications had been started to increase at 1960s. Especially developing gas turbines leaded that requirement because of metals and various alloys that couldn't resist high temperatures. Ceramic coating technology was initially applied to space and aviation areas and then at 1970s it had been started to apply to internal combustion engines, especially diesel engines. Performance increase and specific fuel consumption decrease of aforementioned ceramic

**PERAMETERS COATING PARAMETERS PROCESS PARAMETERS** 

Chemical composition Mechanical properties Atmospheric plasma spray Phase stability Thermal expansion rate Inert gas plasma spray Thermal expansion Oxidation resistance Vacuum plasma spray Melting characteristics Work piece dimensions Under water plasma spray Grain size distribution Surface quality Sprayed powder

> SERVICE CONDITIONS AT OPERATION CONDITIONS

Specific surface area Wear Plasma temperature

Chemical components Adhesion strength Metallurgical reaction Mechanical properties Physical properties Coating thickness

Residual stresses Coating properties under load QUALITY CONTROL

Porosity

 TEST PRODUCTION

Table 3. Plasma spray coating technology; Components and parameters

Fluidity Wear-wet corrosion Speeds of sprayed powders

Wear-oxidation Powder supply speed Wear- gas corrosion Pre heating and cooling of

Wear-erosion Surface cleaning COMPOSITE COATING Spraying environment

Plasma gases

work piece

COATING MATERIAL COATED MATERIAL PROCESS

coated systems created an interest to the topic.

**SPRAYED POWDER** 

Grain morphology

Thermal barrier coatings used for reducing heat loss from cylinders and converting engines to low heat rejection engines also prevent coated materials from decomposing under high temperatures. ZrO2 is the most preferred material in thermal barrier coated internal combustion engines due to its' low thermal conductivity and high thermal expansion rate. To avoid negative effects of phase changes of ZrO2 at higher temperatures, it should be partially or fully stabilized with a stabilizer material. By this procedure, whole structure is formed with one phase, generally cubic phase. As stabilizer, usually MgO, CaO, CeO2 and Y2O3 oxides are used.

There are a vast number of studies investigating effects of thermal barrier coatings and especially ceramic coatings to internal engine performance and exhaust emission behaviours. Investigated parameters can be summarized as coating material, coated material, coating thickness, engine types and operational conditions such as engine load and speed. Obtained results can be different in dimensions and magnitudes such as volumetric efficiency, thermal efficiency, engine torque, engine power, specific fuel consumption, heat rejection from cylinders, exhaust temperature, exhaust energy and exhaust emissions. Investigations of thermal barrier coating in internal combustion engines are mostly focused on diesel engines because of detonation and knocking problems of spark ignition engines at higher in cylinder temperatures. For diesel engines, studies can be divided into two main categories; turbocharged engines and non-turbocharged engines. For non-turbocharged engines, thermal barrier coating application and thus ceramic coatings of internal combustion engine cylinders generally results negatively due to decreasing volumetric efficiency. In the other hand, turbocharged diesel engines exhibit better performance and exhaust emissions according to improved volumetric efficiency and in cylinder temperatures. This phenomenon's main reason is the increased exhaust gas energy which is converted to mechanical energy and later on to air mass flow rate increase in turbocharger. For instance, Leising and Prohit (1978) suggested that desired results by heat rejection insulation could only be achieved by the utilization of turbocharger and intercooler. They also reported that a diesel engine performance could be increased up to 20% by the addition of a turbocharger. When studies about thermal barrier coated engines without turbochargers are considered, it was observed that most of the studies were conducted on a single cylinder, four stroke diesel engines. Miyairi et. al. (1989), Dickey (1989) and Alkidas (1989) are some of these researchers. Prasad et. al. (2000), Charlton et. al. (1991), Chang et. al. (1983) can be given as examples for researchers that studied on natural aspirated multicylinder diesel engines. In the other hand, multi-cylinder diesel engines types were mostly preferred for turbocharged thermal barrier coated engine researches. For instance Woods et. al. (1992), Kimura et. al. (1992), Woschni and Spindler (1988), Hay et. al. (1986) and Ciniviz (2005) performed parametric studies on thermal barrier coated turbocharged multi-cylinder diesel engines. Parlak (2000) and Kamo et. al. (1997) are two studies among limited turbocharged single cylinder thermal barrier coated engine investigations.

Coating materials and methods can be divided into two categories for this book; ceramics and non-ceramics. Coating thickness is usually changes between 100-500 μm. A typical thickness for coating materials is 0,15 mm binding layer and 0,35 mm coating material. Parlak et. al. (2003) and Taymaz et. al. (2003) are two of these studies which used the typical coating thickness. For the researchers that preferred ceramic materials, zirconia is the most seen material among other ceramics. NiCrAl is frequently used as binding materials for

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 207

specific fuel consumption was decreased 5-9 percent, carbon monoxide emission was decreased 5 percent, and soot was decreased 28 percent for a specific power output value. Considering these positive results nitrogen oxide however was increased about 10 percent. By the development of exhaust catalysers, increase in nitrogen oxide becomes no more a problem for present day. When results are generally investigated, it was concluded that engine

Appropriate measurement equipment, their calibration and operational conditions have an important effect on experimental results. Engine specifications are given in Table 4. Experiments were conducted in internal combustion engines workshop in Gazi University Technical Education Faculty Mechanical Education Department Turkey. Cross section view of the engine is shown in Fig. 6 and solid model view of experimental setup is illustrated in

performance was clearly improved by zirconia ceramic coating.

**ENGINE SPECIFICATIONS** 

Cylinder number/diameter/stroke 4/97.5 mm/133 mm

Engine power 66 kW (2800 rev/min) Maximum torque 266 Nm (1400 rev/min) Operation principle 4 stroke diesel engine Injection sequence 1-3-4-2 (cylinder numbers)

Total cylinder volume (Combustion room +

cylinder) 3972 cm3 Compression rate 17.25

Table 4. Specifications of engine used in experiments

Fig. 6. Cross sectional view of test engine

Nominal revolution rate 2800 rev/min

Brand, type and model Mercedes-Benz/OM364A/1985

**4.1 Experimental setup** 

Fig. 7.

those studies. Uzun et. al. (1999), Beg et. al. (1997), Taymaz et. al. (2003), Marks and Boehman (1997), Schwarz et. al. (1993) and Hejwowski (2002) can be referred for these studies. Alternatively, Sun et. al. (1994), proposed silicon nitride (HPSN) piston materials and thick coating layers of plasma sprayed zirconia between 2-7 mm for cylinders. Matsuoka and Kawamura (1993) used Si3N4 instead of zirconia.

Specific literature survey was resulted that specific fuel consumption, heat rejection from cylinders and NOx emissions are the most reported results of experimental and numerical studies for ceramic coated engines. Depending on rising in cylinder temperatures, almost all studies expressed an increase in NOx emissions. This event can be named as the main side effect of ceramic coating or thermal barrier coating of internal combustion engines. The increase in NOx emissions is observed between 10-40% from the literature. Gataowski (1990), Osawa et. al. (1991) and Kamo et. al. (1999) some of the papers in which these aforementioned results can be found. However there are some suggestions for reducing this increase by changing injection timing or decreasing advance angle. Winkler and Parker (1993) reported 26% decrease in NOx emissions of thermal barrier coated engine by changing injection timing. Similarly Afify and Klett (1996), stated that 30% decrease in NOx emissions was achieved by advance adjustment. When specific fuel consumption is considered, results are varying both negatively and positively. This is particularly the result of volumetric and combustion efficiency. Specific fuel consumption decrease can be observed from the literature between 1- 30%. Ramaswamy et. al. (2000), reported 1-2% specific fuel consumption decrease while Bruns et. al. (1989), stated specific fuel consumption decrease between 16-37% by means of ceramic thermal barrier coating. On the contrary, Sun et. al. (1993) and Beg et. al. (1997) expressed 8% increase in specific fuel consumption by the utilization of ceramic thermal barrier coating. Similarly Kimura et. al. (1992), specified that thermal barrier coating resulted 10% increase in specific fuel consumption. As desired, ceramic thermal barrier coatings were resulted as a decrease between 5-70% in heat rejection from cylinders to engine block and cooling system. Vittal et. al. (1997) reported 12% decrease in transferred heat from cylinders and Rasihhan and Wallace (1991) informed that heat rejection rate was decreased between 49,2-66,5% after ceramic coating.

There are several more indicators that show effectiveness of ceramic thermal barrier coatings. Further search can be conducted for specific parameters.
