Heating system – water-cooled engine Figure 9.50 Circuit diagram of a three-speed control system
9.13.2 Heating system – water-cooled engine Figure 9.50 Circuit diagram of a three-speed control system
Heat from the engine is utilised to increase the temperature of the car interior. This is achieved by
recirculated air. The main reason for this is to use of a heat exchanger, called the heater matrix.
decrease the time it takes to demist or defrost the Due to the action of the thermostat in the engine
vehicle windows, and simply to heat the car inte- cooling system the water temperature remains
rior more quickly to a higher temperature. The broadly constant. This allows for the air being
other reason is that, for example in heavy congested passed over the heater matrix to be heated by a set
traffic, the outside air may not be very clean. amount depending on the outside air temperature and the rate of airflow. A source of hot air is there- fore available for heating the vehicle interior.
9.13.3 Heater blower motors
However, some form of control is required over The motors used to increase airflow are simple how much heat (if any) is required. The method
permanent magnet two brush motors. The blower used on most modern vehicles is the blending
fan is often the centrifugal type and in many technique. This is simply a control flap, which
cases the blades are positioned asymmetrically to determines how much of the air being passed into
reduce resonant noise. Figure 9.49 shows a typ- the vehicle is directed over the heater matrix. The
ical motor and fan arrangement. Varying the main drawback of this system is the change in
voltage supplied controls motor speed. This is air-flow with vehicle speed. Some systems use a
achieved by using dropping resistors. The speed valve to control the hot coolant flowing to the
in some cases is made ‘infinitely’ variable, by the heater matrix.
use of a variable resistor. In most cases the motor By a suitable arrangement of flaps it is possi-
is controlled to three or four set speeds. ble to direct air of the chosen temperature to
Figure 9.50 shows a circuit diagram typical of selected areas of the vehicle interior. In general,
a three-speed control system. The resistors are basic systems allow the warm air to be adjusted
usually wire wound and are placed in the air between the inside of the windscreen and the
stream to prevent overheating. These resistors driver and passenger footwells. Most vehicles also will have low values in the region of 1 ⍀ or less. have small vents directing warm air at the dri-
ver’s and front passenger’s side windows. Fresh cool air outlets with directional nozzles are also
9.13.4 Electronic heating control
fitted. One final facility, which is available on
Most vehicles that have electronic control of the many vehicles, is the choice between fresh or
heating system also include air conditioning,
234 Advanced automotive fault diagnosis To understand the principle of refrigeration
Blower
the following terms and definitions will be useful.
motor
Temperature control switch
● Heat is a form of energy.
Blend flap or
Temperature means the degree of heat of an
Temperature
water valve
sensors ECU
object.
Direction
● Heat will only flow from a higher to a lower
● Heat quantity is measured in ‘calories’ (more
Fresh air or
often kcal).
recirculate
1 kcal heat quantity changes the temperature of 1 kg of liquid water by 1°C.
Figure 9.51 An electronically controlled vehicle heating
● Change of state is a term used to describe the
system
changing of a solid to liquid, a liquid to a gas,
a gas to a liquid or a liquid to a solid. ● Evaporation is used to describe the change of
which is covered in the next section. However, a state from a liquid to a gas. short description at this stage will help to lead
● Condensation is used to describe the change into the more complex systems. Figure 9.51
of state from gas to liquid. shows a block diagram representing an electron-
● Latent heat describes the energy required to ically controlled vehicle heating system.
evaporate a liquid without changing its tem- This system requires control of the blower
perature (breaking of molecular bonds), or the motor, blend flap, direction flaps and the fresh
amount of heat given off when a gas condenses or recirculated air flap. The technique involves
back into a liquid without changing tempera- one or a number of temperature sensors suitably
ture (making of molecular bonds). positioned in the vehicle interior, to provide information for the ECU. The ECU responds
Latent heat in the change of state of a refrigerant to information received from these sensors and
is the key to air conditioning. A simple example sets the controls to their optimum positions.
of this is that if you put a liquid such as methy- The whole arrangement is in fact a simple closed
lated spirits on your hand it feels cold. This is loop feedback system with the air temperature
because it evaporates and the change of state (liq- closing the loop. The ECU has to compare the
uid to gas) uses heat from your body. This is why position of the temperature control switch with
the process is often thought of as ‘unheating’ the information that is supplied by the sensors
rather than cooling.
and either cool or heat the care interior as The refrigerant used in many air conditioning required.
systems is known as R134A. This substance changes state from liquid to gas at ⫺26.3°C. R134A is HFC based rather than CFC, due to the
9.13.5 Air conditioning
problems with atmospheric ozone depletion
introduction associated with CFC based refrigerants. A key to
understanding refrigeration is to remember that
A vehicle fitted with air conditioning allows the low pressure refrigerant will have low tempera- temperature of the cabin to be controlled to the
ture, and high-pressure refrigerant will have a ideal or most comfortable value determined by
high temperature.
the ambient conditions. The system as a whole Figure 9.52 shows the basic principle of an still utilises the standard heating and ventilation
air conditioning – or refrigeration – system. The components, but with the important addition of
basic components are the evaporator, condenser an evaporator, which both cools and dehumidi-
and pump or compressor. The evaporator is situ- fies the air.
ated in the car; the condenser is outside the car, Air conditioning can be manually controlled
usually in the air stream; and the compressor is or, as is not often the case, combined with some
driven by the engine.
form of electronic control. The system as a whole As the pump operates it will cause the pres- can be thought of as a type of refrigerator or heat
sure on its intake side to fall, which will allow the exchanger. Heat is removed from the car interior
refrigerant in the evaporator to evaporate and and dispersed to the outside air.
draw heat from the vehicle interior. The high
Electrical systems 235
laden vapour from the evaporator, compresses it and pumps it as a super heated vapour under high pressure to the condenser. The temperature of the refrigerant at this stage is much higher than the outside air temperature hence it gives up its heat via the fins on the condenser as it changes state back to a liquid.
This high-pressure liquid is then passed to the receiver drier where any vapour which has not yet turned back to a liquid is stored, and a desic- cant bag removes any moisture (water) that is contaminating the refrigerant. The high-pressure liquid is now passed through the thermostatic
expansion valve and is converted back to a low- pressure liquid as it passes through a restriction
Figure 9.52 Basic principles of air conditioning
pressure or output of the pump is connected to in the valve into the evaporator. This valve is the the condenser. The pressure causes the refriger-
element of the system that controls the refriger- ant to condense (in the condenser), thus giving
ant flow and hence the amount of cooling pro- off heat outside the vehicle as it changes state.
vided. As the liquid changes state to a gas in the Figure 9.53 shows some typical components of
evaporator, it takes up heat from its surroundings, an air conditioning system.
thus cooling or ‘unheating’ the air that is forced over the fins. The low-pressure vapour leaves the evaporator returning to the pump thus completing
9.13.6 Air conditioning
the cycle.
If the temperature of the refrigerant increases
overview
beyond certain limits, condenser cooling fans can The operation of the system is continuous cycle.
be switched in to supplement the ram air effect. The compressor pumps low pressure but heat
A safety switch is fitted in the high-pressure side
Figure 9.53 Heating, ventilation and air conditioning HVAC components
236 Advanced automotive fault diagnosis of most systems. It is often known as a high-low
ranging from ⫺10 to ⫹35°C even in extreme pressure switch, as it will switch off the com-
sunlight.
pressor if the pressure is too high due to a com- ponent fault, or if the pressure is too low due to a leakage, thus protecting the compressor.
9.13.8 Seat heating
The concept of seat heating is very simple. A heating element is placed in the seat, together
9.13.7 Automatic temperature
with an on-off switch and a control to regulate
control
the heat. However the design of these heaters is more complex than first appears. The heater
Full temperature control systems provide a must meet the following criteria. comfortable interior temperature in line with
the passenger controlled input. The ECU has ● The heater must only supply the heat loss full control of fan speed, air distribution, air tem-
experienced by the person’s body. perature, fresh or recirculated air and the air
● Heat is to be supplied only at the major con- conditioning pump. These systems will soon be
tact points.
able to control automatic demist or defrost when ● Leather and fabric seats require different sys- reliable sensors are available. A single button
tems due to their different thermal properties. currently will set the system to full defrost or
● Heating elements must fit the design of the demist.
seat.
A number of sensors are used to provide input ● The elements must pass the same rigorous to the ECU.
tests as the seat, such as squirm, jounce and bump tests.
● Ambient temperature sensor mounted outside the vehicle to allow compensation for extreme
Figure 9.54 shows a seat containing heating temperature variation. This device is usually a
elements.
thermistor. In order for the passengers (including the ● Solar light sensor mounted on the fascia
driver) to be comfortable rigorous tests have been panel. This device is a photodiode and allows
a measurement of direct sunlight from which the ECU can determine whether to increase the air to the face vents.
● The in car temperature sensors are simple thermistors but to allow for an accurate read-
ing a small motor and fan can be used to take
a sample of interior air and direct it over the sensing elements.
A coolant temperature sensor is used to monitor the temperature of the coolant sup- plied to the heater matrix. This sensor is used to prevent operation of the system until coolant temperature is high enough to heat the vehicle interior.
● Driver input control switches. The ECU takes information from all of the
above sources and will set the system in the most appropriate manner as determined by the soft- ware. Control of the flaps can be either by solen- oid controlled vacuum actuators or by small motors. The main blower motor is controlled by
a heavy-duty power transistor and is constantly variable. These systems are able to provide a com- fortable interior temperature in exterior conditions
Figure 9.54 Heated seat – great on a cold morning!
Electrical systems 237
carried out to find the optimum heat settings and prevent the heater being left on too long. The timer the best position for the heating elements. Many
will switch off after 10 to 15 minutes. The elem- tests are carried out on new designs, using manikin
ents are usually positioned to defrost the main area with sensors attached, to measure the temperature
of the screen and the rest position of the rear and heat flow.
wiper blade if fitted.
The cable used for most heating elements Front windscreen heating is being introduced consists of multi-strand alloyed copper. This
on many vehicles. This of course presents more cable may be coated with tin or insulated as the
problems than the rear screen, as vision must not application demands. The heating element is lam-
be obscured. The technology, drawn from the air- inated and bonded between layers of polyurethane
craft industry, involves very thin wires cast in to foam.
the glass. As with the heated rear window this device can consume a large current and is oper- ated by timer relay.