Piston Cooling

Hannu Jääskeläinen

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• Piston Temperature
• Critical Areas of the Piston
• Piston Cooling Options
• Heat Transfer Considerations

Piston Temperature

The maximum temperature of the piston—or, more specifically, certain key areas of the piston such as the top ring groove and the bowl rim—must be controlled to prevent premature wear and failure of the piston and subsequent engine damage. About 3-5% of the fuel energy in quiescent combustion chambers and 6-8% in swirl-type chambers is transferred to the piston. If the piston is uncooled, up to 60% of this heat can pass through the piston ring region and into the cooling jacket. Additional heat is passed through the skirt into the coolant jacket and from the underside of the piston via oil splash/mist to the crankcase oil [371]. If the piston is cooled with oil, a significant portion of this heat is carried away by the oil, reducing the relative amount that passes through the ring region. Figure 1 illustrates the effect of this heat transfer in a gasoline engine piston and an oil cooled diesel engine piston [3466].

Gasoline engine piston temperature is highest at the center of the piston crown and drops toward the top land. For diesel engine pistons and DI gasoline engines with a bowl shaped piston, the maximum temperature occurs at the bowl rim and from there, drops toward the center of the bowl and toward the top land. In diesel engines, the temperature profile around the circumference of the bowl rim is largely determined by the number and orientation of the injection holes, injection pressure, injection time and duration, and combustion bowl geometry. The highest temperatures around the bowl rim occur at locations that coincide with the center of the burning diesel fuel jets. As a result of the non-uniform introduction of heat through these “combustion lobes”, a wave-shaped temperature profile is typical. The difference between maximum and minimum temperature around the bowl rim circumference can exceed 40°C in some applications.

The thermal load on the piston and the resulting temperature profile affect the piston function and, if maximum temperature limits are exceeded, could result in component failure and engine damage. Three critical effects are [3466]:

• Piston fatigue strength. Increased piston temperature decreases the fatigue resistance of the piston. In some aluminum piston alloys, the fatigue resistance loss can be as high as 80% compared to room temperature properties. Ferrous materials are less sensitive at temperatures up to 400°C.
• If the temperatures in the piston ring area becomes too high, plastic deformation and increased wear can result, especially in the first piston ring groove. Also, coking of the lubricant can lead to carbon deposits in the ring groove that can act as an insulator or cause ring sticking.
• Radial deformation of the piston. This affects noise, frictional losses and clearances between the piston and other components. If sufficient clearances are not maintained during all possible engine operating conditions, piston seizure or contact with the valves can result.

Some typical temperature values for passenger car pistons are [3466]:

• Center of piston crown (gasoline engine, port injection) 270–310°C
• Piston crown bowl (gasoline engine, direct injection) 270–350°C
• Bowl rim (diesel engine, direct injection) 350–400°C
• Support area 200–250°C
• Pin bore (zenith) 200–250°C
• Top ring groove (spray-jet cooling, salt-core cooling channel) 200–280°C
• Top ring groove (cooled ring carrier) 180–230°C
• Cooling channel (zenith) 250–300°C

The primary reason to cool a piston is to control the temperature in several of the above key areas. Piston temperatures scale with engine power output so that in order to avoid excessive piston temperatures the power output of the engine can be limited by piston temperature considerations, Figure 2. This figure shows power rating per unit piston area (π · bore²/4) for engines surveyed in the 1990s [371].

In lower power density applications fitted with aluminum pistons, the conductivity of the material is high and surface areas in contact with the liner large enough that the piston can be operated uncooled or with an oil spray directed at the bottom of the piston without exceeding maximum piston temperatures. With ferrous pistons, this is not generally possible because of smaller surface areas in contact with the liner and low thermal conductivity of the material; oil cooling is essential [371].

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