Low Temperature Testing Facility

One of the main objectives of the project was to investigate the effect of the transmission lubricant temperature on the losses. Therefore, the test rig was developed to facilitate low temperature testing of the transmission. This required some form of temperature control system.

From simple preliminary tests carried out with an earlier transmission test rig used by Chan [48] and Valenta [49], some basic values for the rate of heat rejection by the transmission could be calculated. Initial costs of environmental control systems for the test cell were too high and therefore an alternative and more economic solution to the problem had to be found.

Initial design calculations were based on the heat rejection values and the specific heat capacities of a number of fluids. From these values it was possible to calculate the volumes of fluid required to act as a heat sink for the transmission, based upon a minimum time period to perform the tests. Once a fluid had been chosen a temperature control system was developed to transfer the heat from the transmission fluid to the heat sink fluid efficiently and with some means of control.

It was planned that if a large heat sink capacity could be cooled over a long period of time it would allow testing to be carried out for a comparatively short period. Initial aims were to chill the heat sink overnight to allow for 30 minutes testing each morning. The temperature control circuit, see Figure 4-8, was based upon a standard domestic deep freezer unit, acting as an insulated reservoir, filled with approximately 100 litres of a 50% water and 50% ethylene glycol mixture. The ethylene glycol mixture acts as the heat sink of the system. The mixture was chosen since it has both a high specific heat capacity and a very good heat transfer coefficient.

A Flowcool industrial water chiller unit, rated at 0.4 kW, is used to chill the fluid. The chiller uses R404a refrigerant, which is pumped on the low pressure side through a plate heat exchanger to extract heat from the ethylene glycol mixture. On the high pressure side of

the compressor the refrigerant enters an air cooled condenser with a fan unit to force convection.

An electrically driven centrifugal pump draws the ethylene glycol mixture from the reservoir and pumps it around the chiller circuit. A three way valve in the ethylene glycol circuit allows the circuit to be used in two configurations. In one position the circuit simply circulates the flow though the chiller and back to the reservoir for cooling. In the second position, the fluid is pumped through a further oil to water heat exchanger in which the glycol/water mixture extracts heat from the transmission fluid. At this point the chilled water glycol mixture meets the transmission fluid.

The internal transmission pump, assisted by an independent hydraulic pump driven by an electric motor circulates the transmission fluid. With the two pumps running simultaneously a flow rate of up to 20 L/min can be achieved, thus circulating the 5 litre volume of the transmission sump four times per minute. The independent pump also allows the facility of pre-cooling the transmission prior to a test, without any heat generation from the transmission.

Within the oil circuit there is also an additional oil to water heat exchanger, which can be bought online using the 3 way valve. This heat exchanger is connected to the departmental cooling water supply and can be used when testing at higher temperatures or as a preliminary form of heat removal prior to the oil passing through the chilling circuit. The heat exchanger can therefore be bypassed if the cooling water temperature is above the temperature of the oil exiting the transmission. In effect the oil to water cooler system represents the oil to air cooler that is used for in-vehicle applications.

The amount of heat rejected to the chiller system is controlled by modulating the flow of transmission fluid through the main heat exchanger. This is achieved by controlling two proportional electro-hydraulic throttle valves. To reject more heat the oil flow through the heat exchanger is increased, by opening the inlet valve (A) and restricting the bypass valve (B). Conversely to slow the rate of heat rejection valve A is restricted and valve B is opened.

A temperature control module monitors the transmission return line oil temperature and varies the flow through the proportional throttle valves A and B. Each valve is driven by an individual Vickers amplifier card which is in turn controlled by an off the shelf temperature

controller incorporating a PID controller module, which was tuned to achieve accurate temperature control.

In addition to the low temperature test apparatus, the transmission also needed a facility to warm the oil. During normal testing the transmission warmed up due to the inherent losses. However, during the strip down tests the number of loss mechanism were reduced and thus an oil heater had to be fitted to warm the fluid for the higher temperature work. The oil heater was a simple electrical immersion heater unit with a built in thermostatic control.