Lotus Evora, Service Notes

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Page 4
Lotus Service Notes Section PN
Compressor
The variable displacement compressor is mounted on the front side of the engine, and is driven by the
multi-rib auxiliary belt. The compressor operates to discharge refrigerant vapour at high pressure and tem-
perature into the condenser and is lubricated by a quantity of special refrigerant oil, most of which is retained
in the compressor, with the remainder being circulated with the refrigerant. The compressor contains a ring of
cylinders with axes parallel to the compressor drive shaft, and whose pistons are driven up and down the bores
by a rotating 'swash plate', the angle of which, in relation to the drive shaft, is variable. With a small swash plate
angle, a short piston stroke is produced for a low refrigerant flow; a high swash plate angle results in greater
piston stroke for a higher flow of refrigerant. The angle of the swashplate is determined by the pressure dif-
ferential between that on top of the pistons, and that within the housing, applying a force to the underside of
the pistons, in conjunction with a coil spring around the drive shaft.
This differential is controlled by a solenoid valve under ECU control, using pulse width modulation. When
the valve is open, the output from the cylinders is bled off to the compressor housing to result in no pressure
differential. The angle of the swashplate is then determined by the coil spring which pushes the plate to a
near neutral position to provide minimal flow. As the valve is progressively closed, the pressure differential
increases, with the pressure on top of the pistons pushing the swashplate to a greater angle, and producing
an increased refrigerant flow.
The engine ECU is programmed to minimise refrigerant flow until an a.c. request is made, thus allowing
the compressor to be run at all times in the interests of system lubrication, and the reduction of inactivity dam-
age.
To safeguard the drive system in the event of compressor seizure, an electromagnetic clutch in the pulley
hub is used to disengage the drive as signalled by a sensor in the compressor nose. The clutch will also be
disengaged by the ECU if a loss of refrigerant is detected by the trinary switch (see below).
Hot refrigerant vapour from the compressor is fed via flexible hoses and alloy pipwork routed through the
body RH sill, to the front mounted condenser.
Condenser
The aluminium condenser is a 'parallel flow' heat exchanger, configured with side mounted collector tanks
interconnected by 40 oval shaped, horizontal tubes, all provided with generous finning. The unit is fixed to the
front of the engine cooling radiator, sited between the longerons of the front subframe, and angled forwards at
45°. The hot vapour received from the compressor, is admitted into the top section of the RH condenser tank,
and flows through 32 tubes to the LH tank before returning to the lower section of the RH tank via the bottom
8 tubes. In so doing, heat is released to the surrounding air via the condenser finning, with airflow boosted by
two electric fans mounted on top of the engine radiator, and ram air flow caused by vehicle movement.
A union at the bottom of the RH tank directs the condensed, liquid refrigerant into pipework connecting
with the receiver-drier.
Receiver-Drier
The receiver-drier unit is fitted into the refrigerant line between the condenser and evaporator expansion
valve, and houses a screen sack filled with desiccant to absorb traces of moisture and other contaminants from
the refrigerant. The unit is mounted on the outside of the subframe RH longeron, ahead of the RHF wheel.
A trinary switch fitted into the pipe between the receiver-drier and expansion valve supplies a pressure
signal to the engine ECU, which then allows system operation only within a pressure range of 2 to 32 bar in
order to prevent system damage from too high a pressure, or from compressor oil starvation damage caused
by too low a pressure. This data is also used by the ECU to engage the two condenser fans at half speed at
pressures over 17.5 bar (see also sub-section KJ.5).
Expansion Valve
The expansion valve block is fitted into the high and low pressure pipes at the evaporator connection, and
provides a variable restriction to the flow of high pressure liquid into the evaporator, such that the consequent
pressure drop causes a change of state to a low pressure, low temperature atomised liquid.
By sensing the temperature and pressure of refrigerant leaving the evaporator, the expansion acts to
modulate the flow of refrigerant into the unit to optimise the cooling performance.