Keep it cool or you'll fail
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OVER the past few years, the complexity of cooling systems has increased, as has the amount of heat to be dissipated — especially on the heavier trucks with their 200plus bhp engines. To put these changes into perspective, this new series of articles will cover the main principles of cooling systems, and the main causes of failure.
Although the modern diesel is a most efficient internal combustion engine it converts only about 35 per cent of the combustion energy into work, leaving some 30 per cent to be dissipated in the exhaust, and about 30 per cent in the cooling system. The heat can get away quite easily in the exhaust, but to reach the cooling system, it must pass through the cylinder walls and cylinder head first. Temperatures of 2,000° 2,500° C are reached in the combustion process, so without an efficient cooling system, the cylinder head, valves and pistons would soon get so hot that the stresses would lead to distortion and cracking—and if a cooling system is neglected, this sort of problem can crop up all too easily.
One of the difficulties with a cooling system is that the temperatures reached by the components vary widely, and as a general rule, it is far easier to cool the components that run at fairly low temperatures than those that inevitably get very hot. For example, it is easy enough to cool the cylinder walls, which may run at 100°150° C, but it is difficult to cool the exhaust valves adequately.
The exhaust valves can run at over 700° C, so the mos critical area in a diesel is th( region between the exhaus valve seat and the injector.
Although air cooling is use on some diesels and smal petrol engines, we are normall: involved with the mon efficient but more complicate( water cooling. In either case, o course, coolant must be force( across or around the cylinder and through passages in th head to take the heat away With air cooling, there ar numerous fins to give the maxi mum surface area for the ai to cool, and, of course, th warmed air is discharged int, the atmosphere, more air bein drawn in to continue the prc cess.
With liquid cooling, it i easier to transfer the heat t the liquid, so fins are nc needed. The warmed water i passed to the radiator, wher it is cooled by air, and is the returned to the engine.
The basic elements in liquid cooling system, then, at a jacket around the cylinder and head to allow a generou flow of liquid, a coolar radiator, some hoses and pump. A fan is also needed t draw air through the radiato although on many rnoder vehicles this does not operat all the time. Water is the basi liquid used, but these days is mixed with anti-freeze a the year round.
In fact, a cooling systei with those components onl would work, but the Moder systems are far more comple with thermostats, vent pipe built in oil coolers, head( tanks and so on. We shall s( why all these are needed the next article.