Air-cooled system Coolant from the engine water jacket passes
7.20.1 Air-cooled system Coolant from the engine water jacket passes
through a hose to the radiator at the top. It then Air-cooled engines with multi-cylinders, espe-
passes through thin pipes called the radiator cially under a bonnet, must have some form of fan
matrix to the lower tank and then back to the cooling and ducting. This is to make sure all cylin-
lower part of the engine.
ders are cooled evenly. The cylinders and cylinder Many water passages between the top and heads are finned. Hotter areas, such as near the
bottom tanks of the radiator are used, to increase exhaust ports on the cylinders, have bigger fins.
the surface area. Fins further increase the surface Fan-blown air is directed by a metal cowling so
area to make the radiator even more efficient. A it stays close to the finned areas. A thermostatic-
cooling fan assists air flow. The heat from the ally controlled flap will control airflow. When the
coolant passes to the pipes and fins and then to engine is warming up, the flap will be closed
the air as it is blown by a fan over the fins.
Engine systems 149
Many modern radiators are made from alu-
7.20.3 Sealed and semi-sealed
minium pipes and fins with plastic tanks top and
systems
bottom (down flow), or at each end (cross flow). The cross flow radiators with tanks at each end
Cooling systems on most vehicles today are sealed are becoming the most popular. The more trad-
or semi-sealed. This allows them to operate at itional method was to use copper and brass.
pressures as much as 100 N/m 2 (100 Pascal) over
A thermostat is a temperature controlled valve. atmospheric pressure, raising the boiling point of Its purpose is to allow coolant to heat up more
the coolant to as much as 126.6°C (remember quickly and then be kept at a constant temperature.
water boils at 100°C at atmospheric pressure). The The total coolant volume in an engine takes time to
system can therefore operate at a higher tempera- heat up. Modern engines run more efficiently when
ture and with greater efficiency. at the correct operating temperature. The action of
The pressure buildup is made possible by the the thermostat is such as to prevent water circula-
radiator pressure cap. The cap contains a pressure tion from the engine to the radiator, until a set tem-
valve which opens at a set pressure, and a vacuum perature is reached. When the valve opens there is
valve which opens at a set vacuum. On a semi-
a full circuit for the coolant and a good cooling sealed system, air is pushed out to atmosphere action occurs because of full flow through the radi-
through the pressure valve as the coolant expands. ator. The constant action of the thermostat ensures
Air is then drawn back into the radiator through that the engine temperature remains at a constant
the vacuum valve as the coolant cools and con- level. The thermostat used by almost all modern
tracts. A sealed system has an expansion tank into engine is a wax capsule type. If the thermostat is
which coolant is forced as it expands, and when faulty ensure that the correct type for the engine is
the engine cools, coolant can flow from the tank fitted as some work at different temperatures.
back into the cooling system. Figure 7.50 shows a The water pump is driven by a V-belt or multi
semi-sealed type cooling system. V-belt from the crankshaft pulley or by the cam
Correct levels in the expansion tank or in an belt. The pump is a simple impeller type and is
unsealed radiator are very important. If too much usually fitted at the front of the engine (where the
coolant is used it will be expelled on to the floor pulleys are). It assists with the thermo-siphon
when the engine gets hot. If not enough is used action of the cooling system, forcing water around
then the level could become low and overheating the engine block and radiator.
could take place.
The engine fan, which maintains the flow of air through the radiator, is mounted on the water-pump
Warning
pulley on older systems. Most cooling fans now are If a pressure cap is removed from a hot sys- electric. These are more efficient because they only
tem, hot water under pressure will boil the work when needed. The forward motion of the car
instant pressure is released. This can be very also helps the air movement through the radiator.
dangerous.
Figure 7.50 Semi-sealed cooling system
150 Advanced automotive fault diagnosis Heat from the engine can be used to increase
7.21.2 Test equipment
the temperature of the car interior. This is achieved by use of a heat exchanger, often called
Note: You should always refer to the manufac- the heater matrix. Due to the action of the thermo-
turer’s instructions appropriate to the equip- stat in the engine cooling system the water tem-
ment you are using.
perature remains nearly constant. The air being passed over the heater matrix is therefore heated
Cooling system pressure tester
to a set level.
(Figure 7.51)
A source of hot air is now available for heat- ing the vehicle interior. Some form of control is
This is a pump with a pressure gauge built in, required over how much heat is required. The
together with suitable adapters for fitting to the method used on most modern vehicles is blend-
header tank or radiator filler. The system can then ing. This is a control flap that determines how
be pressurized to check for leaks. The pressure much of the air being passed into the vehicle is
can be looked up or it is often stamped on the filler directed over the heater matrix. Some systems
cap. A good way of doing this test is to pressurise use a valve to control the hot coolant flowing to
the system when cold and then start the engine the heater matrix.
and allow it to warm up. You can be looking for By a suitable arrangement of flaps it is possible
leaks but beware of rotating components. to direct air of the chosen temperature to selected
areas of the vehicle interior. In general, basic sys-
Antifreeze tester
tems allow the warm air to be adjusted between the inside of the windscreen and the driver and
This piece of equipment is a hydrometer used to passenger footwells. Fresh cool air outlets with
measure the relative density of the coolant. The directional nozzles are also fitted.
relative density of coolant varies with the amount One final facility, which is available on many
of antifreeze. A table can be used to determine vehicles, is the choice between fresh or recir-
how much more antifreeze should be added to give culated air. The primary reason for this is to
the required protection.
decrease the time taken to demist or defrost the vehicle windows and simply to heat the car inter-
Temperature meter/thermometer
ior more quickly, and to a higher temperature. The other reason is that for example, in heavy
Sometimes the dashboard temperature gauge read- congested traffic, the outside air may not be very
ing too high can create the symptoms of an over- clean.
heating problem. A suitable meter or thermometer can be used to check the temperature. Note though