Getting rid of the heat

Air-cooled vs. Water Cooled

A simple fact of the internal combustion engine is that it generates heat in order to produce power. As it turns out only a small percentage of the heat generated actually gets converted to mechanical force and the rest is discarded either through the exhaust, the cylinder walls, cylinder head, crankcase walls or the engine oil. The challenge is to remove the waste heat in the most efficient and reliable manner possible.

Air-cooled engines dissipate waste heat directly through the cylinder head and walls to the outside air and also through the engine oil. In fact the engine oil plays a very significant role in heat dissipation in an air-cooled engine. The problem is that a healthy percentage of heat must be dissipated directly from the cylinder head and walls to the outside air. Air is not a good conductor of heat because it is a gas - in fact it is often used for its insulation properties. Fluids however have much better thermal conductivity because by their nature they are more dense than a gas. A few drops of water or oil will carry away several times more heat than several cubic feet of air and the same is true for water. So in order to transfer heat into air there needs to be a large amount of exposed surface area from which to radiate the heat and a substantial volume of air is required to pass over that surface.

Fins are added to the cylinder heads and walls in order to increase the surface area that can radiate the heat to the air passing by. The problem is that there is only so much surface area physically available on a cylinder head in which to have fins and the fins must be of a practical size in order for the engine components to fit together and for the entire engine to fit comfortably in an aircraft. So we arrive at the major compromise of an air-cooled engine. Either have exceedingly large fins on the cylinder head and walls and a lower air volume or have smaller fins and a high volume of air to remove the same amount of heat.

For lower powered air cooled engines - that is engines that have a relatively low power to cubic inch displacement ratio - air cooling works reasonably well. The problem of heat dissipation gets compounded significantly as the power per cubic inch of displacement increases. As more power is made from the same number of cubic inches (whether that additional power is created by the use of a turbo charger or higher compression ratios) the heat dissipation requirements increase significantly simply because there isn't anymore surface area radiating the excess heat away. The only solution is to increase the volume of airflow past the cooling fins and attempt to restrict that airflow so as to both compress it (increasing its density and thus thermal conductivity) and slow it down giving it more time to absorb the heat from the surfaces it is passing by.

The heat that is generated during the combustion cycle must be expelled during the exhaust cycle. The hot gases pass by the exhaust valve, valve seat, valve stem and guide on the way out the exhaust port. Exhaust gas temperatures in a typical aircraft engine operating at 60 to 90% power are typically between 1200 and 1600F. With each exhaust cycle the exhaust valve face, seat, stem, guide and port are exposed to the blast furnace temperatures of the exhaust gases. The heat that is absorbed by these components of the cylinder head must be dissipated as quickly as possible. The problem is that these components are deep on the inside of the cylinder head. Before the heat can be dissipated to the air it must essentially soak through a sizable amount of metal before radiating away on the cooling fins on the exterior. The thicker the material the greater the thermal gradient will be between the source of the heat and the point at which it is being conducted away.

Since air conducts heat way from the fins relatively slowly the transfer of heat through the cylinder head to the fins is relatively slow and the internal cylinder head/exhaust port/valve seat temperatures rise to very high levels potentially as high as 900F or higher. Exhaust valves face the worst of the heat since the face and lower stem of the valve are completely bathed in the full fury of the exhaust gases on each exhaust stroke. In order to function with any sort of reliability an exhaust valve needs to dissipate heat through the valve face when it is closed and is in contact with the valve seat. Unfortunately since the rate of thermal conduction is relatively low in an air-cooled cylinder head and the valve seat is buried deep within the mass of the cylinder head it is especially difficult to remove the extreme heat that the exhaust valve face and seat build up. Without a path through the valve face and seat the heat from the valve face conducts up the valve stem to the next best location of thermal conductivity - the valve guide. This is one of the key reasons for poor exhaust valve lifespan and exhaust valve failure (either in terms of stem failure or stuck valves). We will discuss this further in the next section, Heat - the top end killer

Water (or water and antifreeze) offers 1000 times greater thermal conductivity than air and it can be routed much more closely to high temperature areas such as the exhaust valve seat and guide. As a result the heat has far less metal to go through substantially reducing the thermal gradient and due to the high thermal conductivity of water it literally sucks the heat right out of the metal keeping the internal temperatures of the exhaust valve face and seat much lower than any air-cooled cylinder head could possibly achieve. The overall result is that the exhaust valve and seat are now able to dissipate the vast majority of heat that is absorbed during the exhaust stroke through the valve face thereby preventing any significant amount of heat from being conducted up the stem of the valve and into the valve guide.

Unlike air, the flow of water through a liquid cooled cylinder head can be manipulated much more easily. The flow pattern, rate, temperature and pressure can all be adjusted in order to provide maximum cooling effectiveness without increasing the surface area of the cylinder head or walls. Suddenly it becomes possible to start increasing the power per cubic inch without any detrimental thermal effects on the cylinder head and valve train. When water cooling is compared to air-cooling this effect is almost magical in that it solves a myriad of thermally related problems all of which have a positive effect on the reliability of the components used in the top end of the cylinder head including the cylinder head itself.