11MP E MECHANICS Cooling systems 2
Page 57
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)NE of the disadvantages of rater cooling is that the boiling oint of water is lower than the -lost efficient engine operating amperature.
It is essential to prevent the r mat i o n of steam pockets round those parts of the water ]ckets subject to very high „mperatures, eg around the xhaust ports.
As the pressure on water inreases, so does the tempera_Ire at which it boils, and it has ecome a common practice to ubject the cooling water in lotor vehicle engines to a iressure slightly higher than tmospheric pressure.
This can be easily accom,lished by sealing off the water rom the atmosphere, so that the team formed by the boiling 'ater will raise the pressure and top further boiling until the iressure has risen even higher.
Obviously, there are limits to he pressure which the radiator ore and ,rubber hoses can tand, so tome form of safety alve which will release the iressure when it becomes exessive must be provided.
This valve is incorporated in he radiator filler cap, shown in liagramatic form in Figure 1. It vill be seen that there are, in act, two valves, one to release he pressure from the system nd the other to relieve the vauum which forms in the system vhen the engine cools. When he pressure valve opens, the water and/ or steam escapes down the overflow pipe.
Care must be taken when removing this type of radiator cap from a very hot engine, since the release of pressure will allow the water to boil violently, resulting in a jet of steam and water squirting out causing serious scalds.
When the engine temperature is below 100 C, the cap may be released slowly. It is so designed that the pressure escapes through the overflow pipe before it can come through the main opening.
To control the temperature of the engine, and to permit a rapid warm up from cold, a thermostat is fitted in the cooling system.
The thermostat consists of a valve which, when closed, cuts off the water circulation to the radiator when the engine is cold, and opens when the engine reaches its working temperature, so permitting normal circulation of the coolant.
The bellows type of thermostat, illustrated in Figure 2, con
sists of a concertina or bellows type vessel of thin metal, which contains a fluid such as alcohol, ether or acetone, which has a boiling point lowerthan water.
As the water temperature in the engine increases, the press ure inside the bellows increases, and the bellows expands and opens the poppet valve to which it is attached, allowing the coolant to circulate freely. A small hole in the valve acts as a vent to prevent air being trapped underneath the valve when the system is being filled. A jiggle pin, which isa loose fit in the hole, prevents it from becoming choked.
A second type of thermostat, the wax element type, illustrated in Figure 3, is more often used in modern engines, particularly as it is not affected by pressure in the cooling system.
In this type, a tapered rod is enclosed in a synthetic rubber boot filled with wax. The upper end of the tapered rod is screwed into a bridge spanning the component.
As the wax expands on being heated, it forces the capsule downwards against the return spring, carrying with it the valve which opens, allowing complete circulation to take place.
When a thermostat valve is closed, limited circulation is possible through a by-pass system which eliminates the radiator.
Further engine temperature control can be obtained by a hand-controlled radiator blind, which restricts the flow of air through the radiator, or by radiator shutters. The latter may be hand controlled or operated automatically by a thermostatically controlled device.
Fans on water cooled engines may well be unnecessary, ex cept when the engine is working hard. Much of the time, for example on motorway journeys when driving into the wind, t forward motion of the vehic will provide enough air passi over the radiator to maintain t engine at its correct workit temperature.
Fans which are driven cc tinuously absorb power, and design where the fan operal only when high engine temp ature requires it is attractive.
This can be achieved in tv ways:
(1) by an electrically driv, fan with a thermostatic switc and
(2) by a special couplii which can automatically d 'connect the fan drive when t fan is not needed.
Water has its maximu density, that is the minimL volume for a given mass, at C; when cooled to 0C it ( pands. If the water in a cooli system is allowed to freeze, will often crack the cylind block casting.
An anti-freeze solution, ni mally ethylene-glycol with anti-corrosion additive, ‘4, lower the freezing point to bel( 0' C by an amount dependi on the concentration used.
A well-known anti-freeze n nufacturer states that 25 r cent anti-freeze by volume, one quarter of the cooling cal city, will give complete prat' tion down to —12.2' C(10 331/2 per cent anti-freeze gi% protection down to —20' (-4 F).