Engine Cooling - TDV8 3.6L Diesel (G938526)

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The pressurized cooling system employed allows coolant to circulate around the engine and the heater circuit while the thermostat main valve is closed. The primary function of the cooling system is to maintain the engine within an optimum temperature range under changing ambient and engine operating conditions. Secondary functions are to provide:

• optimized cabin heating during the engine warm-up phase
• full automatic transmission fluid temperature control (including heating)
• low temperature fuel cooling via a dedicated low temperature sub-cooler
• Enhanced Exhaust Gas Recirculation (EGR) cooling via supply of low temperature coolant from the radiator outlet

During the initial engine warm-up, when the main thermostat is closed, warm coolant is directed to the ancillary heat exchangers:

• Cabin heater
• Engine oil heat exchanger
• Automatic transmission fluid heat exchanger (limited flow at low engine speeds)

The main engine thermostat is of a pressure relief design. The prime function of this thermostat is to use the heater and ancillary circuit as the only radiator by-pass while the thermostat is closed.

The pressure relief thermostat gradually opens the main radiator by-pass circuit at engine speeds above 1500rpm. This system ensures that the maximum available coolant flow is directed to the heater and ancillary heat exchangers during warm-up without affecting the durability of the engine. By using this type of thermostat there is enough heater flow for all conditions without the need for a dedicated electric water pump.

The coolant is circulated at up to 400 liters per minute by a coolant pump mounted on the front of the engine and driven by an ancillary drive 'polyvee' belt. The coolant pump circulates coolant through the cylinder block and cylinder heads via a chamber located in the 'vee' of the engine. Having passed through the engine the coolant returns to the thermostat housing. The coolant then progresses down the 'top hose' to the heater pipe. The heater pipes lead to the bulkhead and return to the engine side of the thermostat.

The engine contains a conventional wax thermostat, which is positioned such that the wax's temperature is controlled by the bypass coolant. The thermostat housing also contains a sprung loaded valve, which limits the amount of coolant using the bypass. At low engine speeds most coolant is diverted through the heater. At higher engine speeds the bypass opens to protect the heater matrix from to higher pressures and flow rates. This means the engine can run with minimal coolant flowing through the bypass at low engine speeds, to improve heater performance.

The radiator is a vertical flow-type with an aluminium matrix and has a drain tap on the bottom left-hand rear face. The lower radiator mountings are located part way up the module frame. The mountings are fitted with rubber bushes, which sit on plastic mounts clipped into the chassis rails. The top of the radiator is located in rubber bushes secured by brackets fitted to the bonnet locking platform.

The coolant degas tank is fitted behind the Left Hand (LH) side headlight of the LH in the engine compartment. The tank allows for system filling and any air trapped in the coolant to be removed.

The liquid cooled transmission oil cooler is mounted to the fan cowl. It is fed coolant from the sub-cooled section of the radiator, which is controlled by the oil cooler mixer valve.

The oil cooler mixer valve is a 2 stage valve, which controls the heating and cooling of the transmission oil. The operation of the first stage is to warm up oil from cold to improve emissions and driveability. The second stage is fully operational once the coolant temperature reaches 91°C (196°F) and allows cooling of the oil (starts to open at 84°C (183°F)).

For additional airflow through the radiator matrix, particularly when the vehicle is stationary or slow moving, there is an engine driven electro-viscous unit. This unit functions as a normal viscous fan, but with electronic control over the level of engagement of the viscous clutch. The Engine Control Module (ECM), which determines the required fan speed, controls the level of clutch engagement. The ECM determines engagement based on the coolant, ambient and transmission oil temperatures and Air Conditioning (AC) system pressure. The fan is mounted using a Left Hand (LH) thread.

The viscous fan unit is electronically controlled by the ECM to optimise fan speed for all operating conditions.

Due to package constraints an auxiliary radiator is required to maintain acceptable coolant temperatures under arduous driving conditions such as high-load and high ambient temperatures. This radiator is located in the in the RH wheel arch area, with cooling airflow supplied by a dedicated aperture in the front bumper. The auxiliary radiator is connected in parallel with the main radiator, with the coolant flow being controlled by the main engine thermostat.

An air to air intercooler is located behind the AC condenser, but in front of the main radiator. This reduces the charge air temperature (post turbocharger compressor) by up to 130°C (266°F) before entering the engine. The intercooler has 2 separate air paths for the LH and RH cylinder banks.

After the initial warm-up phase is complete (coolant temperature between 40°C (104°F) and 70°C (158°F), depending on ambient temperature), the EGR mixer valve begins to blend cold water from the radiator outlet. This has the effect of reducing the temperature of the coolant supplied by to the EGR coolers, thus improving their effectiveness and reducing the emissions of Nitrogen Oxides (NOx). The flow rate of this coolant is controlled by a thermostatic valve located in each EGR cooler exit and ensures that maximum EGR cooling is delivered without compromising engine warm-up or cabin heater performance.

As the temperature and pressure increases the bypass valve is forced open allowing coolant to circulate through the bypass valve. When the temperature reaches 88°C (190°F) the main engine thermostat and transmission oil cooler mixer thermostat begin to open allowing coolant to circulate through the main radiator. As the thermostat progressively opens (fully open at 95°C (203°F)), the bypass valve progressively closes forcing any coolant through the heater or radiator.

Coolant flows through the radiator from the top RH tank to the bottom LH tank and is cooled by air passing through the matrix. A small flow of coolant from the radiator and the top of the engine is directed to the degas tank where any trapped air is separated.

In arduous driving conditions such as towing and/or in high ambient temperatures the radiator sub-cooled section and dedicated fuel sub-cooler radiator supply low temperature coolant to the oil/fuel coolers. This enables sufficient heat exchange into the coolant from these systems to maintain the fluid temperatures within their required limits.

The temperature of the cooling system is monitored by the ECM via the Engine Coolant temperature (ECT) sensor located in the coolant housing. The ECM uses signals from this sensor to control the cooling fan operation.

Regulation of the coolant temperature is achieved via engagement of the electro-viscous fan assembly. This is controlled by a Pulse Width Modulated (PWM) signal with a duty cycle of between 0 and 100%, provided by the ECM and derived from inputs based on:

• Coolant temperature
• Ambient air temperature
• Engine inlet air temperature
• Air Conditioning (AC) system pressure
• AC switch operation
• Transmission oil temperature

Fan speed control is variable; however, because the fan is driven directly from the engine, the maximum fan speed available is tied to engine speed. At high engine speeds the fan is progressively disengaged to protect the clutch unit. This system provides very high levels of fan power, up to 5 kilo Watts (kW), with enhanced noise and fuel economy benefits compared to mechanically controlled viscous fans.

The speed of the cooling fan is also influenced by vehicle road speed. The ECM adjusts the speed of the cooling fans, to compensate for the ram effect of vehicle speed, using the Controller Area Network (CAN) road speed signal received from the Anti-lock Braking System (ABS) module.For additional information, refer to: Air Conditioning (412-03D, Description and Operation).