The cooling system

Thursday, January 21, 2010

Tags :  engine-structure

heads-blocksA major consideration of any engine designer right from the outset is that of the cooling system. Even in the most efficient of gasoline race engines, only 35% of the fuel energy available is converted into mechanical power. The rest, something like 850kW from an engine delivering 450kW, is converted into heat which will need to be dissipated into the surrounding atmosphere. Putting it into perspective that's something like 280+, 3kW electric fires worth of heat, approximately half of which will go out through the exhaust system with the remainder - around 400kW rejected into the engine coolant/oil systems. And since the lube oil system will only disperse a small proportion of this, it will be left to the main cooling circuit with its water jackets and flow channels to cope with the rest.

A simple analysis of the rate of heat released of combustion plotted against the piston position will give an indication as to where and when this heat is released. With peak combustion pressures around 7-10 degrees after top dead centre much of this heat is created next to the combustion chamber (hence the name). Later in the expansion stroke the exhaust valve will open and the exhaust 'blow-down' thus created will divert part of this heat into the exhaust port. And while the cylinder bores and piston crown will take some of the heat away, by far the vast majority is conducted through the cylinder head combustion chamber walls. In the attempt to maintain a constant temperature across and around the whole engine the cooling system to minimize thermal stresses, all this has to be taken into account.

From that stated above the cylinder block and liners needs relatively little cooling. A small water jacket towards the top of the liner perhaps may be all that is required. However since there is generally a lot of space available down and behind the bore, designers tend not to skimp in this area and water jackets here are invariable larger than they truly need be. With the water pump positioned generally towards the front of the end, the coolant will be normally pumped from this position and along the cylinder block. Since water jackets are large and heat flux low, the coolant temperature will normally not increase much at all. It is only when it enters the cylinder head that things can get quite critical. If all the flow goes to the back of the block and then into the cylinder head for its return trip, the heat picked up along the return may cause combustion issues - detonation in the front cylinder. Likewise the water flowing down the intake side will be considerably cooler than that travelling down the side of the exhaust. To get round this, manufacturers tend to bleed off coolant from the cylinder block up into the cylinder head at various points down the block and feed the coolant from the cool inlet side of the head across and between the chambers and into a water rail across to the other, exhaust side. In this way temperatures along the cylinder head will be more uniform with the minimum of distortion. With competition engines, where the added complexity and costs can be accommodated, this exhaust side water rail may be external to the cylinder head itself.

Designing an efficient cooling system has never been easy but with modern computational methods it is so much easier to get it nearer the optimum if not perhaps quite so much fun.

Fig. 1 - Cylinder head water rail.

Written by John Coxon

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