More Degrees of Freedom in Thermal Management

Reducing thermal losses in the combustion engine is an expedient way of further decreasing emissions. Phasechange cooling developed by IAV permits adjustment of the wall temperature in the combustion chamber across a broad range, depending on the load point. This reduces fuel consumption in gasoline engines by seven percent in the WLTC.

Conventional convection cooling systems have an adjustable coolant temperature range of about 80 to 130 degrees Celsius, which limits the extent to which they can be adapted to the requirements of the combustion process. “Particularly in part-load operation, it is desirable to adjust the wall temperature of the combustion chamber as closely as possible to about 220 to 230 degrees Celsius in order to minimize wall heat losses. At full load, it is advantageous for the combustion chamber wall temperature to be brought below 150 degrees Celsius in order to increase the supercharging rate or the compression ratio”, explains Thomas Arnold, head of the Design and Testing Future Combustion Engines team at IAV. “But normal liquid cooling currently achieves exactly the opposite: high component temperatures at full load and low component temperatures at part load.”

IAV has therefore developed a two-phase cooling system in the framework of a proprietary development project, where the coolant temperature can be adjusted between 80 and 260 degrees Celsius regardless of the operating point. The coolant used in the closed circuit consists of a mixture of water and ethanol. It evaporates in the engine, pressure is released by a valve, and a cooler condenses it again. A high-pressure pump brings the medium into the engine at pressures between one and 25 bar. “The higher the pressure, the higher the temperature in the circuit”, says Arnold. “And so we can adjust the ideal pressure for the specific engine operating point, thus achieving the best possible wall temperature for the combustion process.” With great precision: the difference in temperature between wall and coolant is between 5 and 20 Kelvin maximum.

While the wall temperature should be as high as possible at part load, by contrast it should be as low as possible at full load to protect components. The new two-phase cooling system does this too: “Reducing the pressure brings about far lower temperatures at full load than with conventional cooling systems”, says Arnold. “This also extends the service life of the engine.”

To demonstrate the potential of two-phase cooling, IAV has put the new technology into a 1.4-liter turbocharged spark-ignition engine. Tests indicate reductions of up to seven percent in fuel consumption in the WLTC. One contributory factor is the lower coolant mass flow. Depending on operating point, the mass flow is only one to two percent that of conventional liquid cooling, so that far less drive power is needed for the coolant pump.

“Although two-phase cooling offers more benefits for gasoline engines, it is still worth putting them in a diesel engine as well. They increase the exhaust temperatures at part load, which makes the exhaust gas aftertreatment system more efficient”, reports Arnold. “The new technology can also be used to cool components in electric vehicles.” IAV is currently working on several customer projects that deal with two-phase cooling, for future passenger cars but also for diesel engines in trucks.