INTRODUCTION

Combustion temperatures are in the neighborhood of 2,500 K in a spark-ignition engine, and the exhaust gas temperature is about 1,300 K. On the other hand, the melting point of aluminum is about 933 K, and the melting point of iron is about 1,808 K. It is clear that some provision has to be made to keep the piston, valves, and cylinder walls cool, or they will melt. Even considerably short of melting, at high temperatures metals begin to lose their strength, and this must also be avoided. Any heat removed represents a loss of energy, so we want to cool only as much as is necessary to maintain the strength of the materials, maintain clearances, and prevent the lubricant from breaking down.

The problem areas are the exhaust valve and the piston crown. The exhaust valve head loses most of its heat to the valve seat (the amount lost to the valve guide is relatively small, because the path is long and the conduction area is small). Unfortunately, when the valve opens, it is exposed to the exhaust gases, which flow past it at high velocities (making for good heat transfer), and while this is happening the valve head is not in contact with its seat.

In water-cooled engines the water returning from the radiator, which therefore has the lowest temperature, usually flows around the cylinders and then up into the cylinder head. This is not the most effective order because the cylinder heads, and particularly the valve seats, require cooling much more than the cylinder walls. However, it is adequate, and cheap.