Introduction

A CFD study was performed to investigate the impact of film cooling an internal combustion engine. The injection of a thin layer of cool air into the combustion chamber to create an insulating layer between the combusting mixture and the walls was studied. Assumptions about the design of the film cooling system were based on experience from gas turbine design. Transient film cooling evaluation methodologies were developed and applied to an in-cylinder CFD model using Fluent’s dynamic mesh tools. Combustion, viscosity, swirl and heat transfer were modeled. The impact of reduced heat transfer on engine performance and the potential for film cooling to reduce knock were investigated.

Robert Goddard developed film cooling to protect his liquid rockets from the high temperatures of combustion. Film cooling is injecting a thin layer of cool air between hot gases and metal surfaces to insulate those surfaces from high heat transfer from the gases. Since that invention, film cooling has been developed in rockets and gas turbines to enable higher combustion temperatures to make possible fighter aircraft like the F15 and F22.

Automobile inefficiencies lead to heat transfer to the cylinder, exhaust energy, and incomplete combustion. Modern automobile engines use liquid cooling to discharge up to ½ the energy in the system through the radiator. It is theorized that film cooling can be used to reduce each of these losses. Film cooling reduces heat transfer to the walls to retain the heat in the cylinder, increasing the available work to be extracted by the piston. Energy that was previously lost to the radiator is converted to work and leaves in the exhaust as higher exhaust temperature. Film cooling Changes the composition and temperature of the gases at the edges of the cylinder, the ‘end-gases’, allowing control of knock. Controlling or eliminating knock allows engines to be designed at higher compression ratios, leading to higher efficiency. Film air used to purge the piston ring grooves of fuel reduces unburned hydrocarbon emissions. Computational Fluid Dynamics was employed to verify the potential of film cooling to reach the theorized objectives.

Engine Operation

Next are 2-d CFD simulations through the center of the engine. The left model is temperature without film, and the right is temperatures in the cylinder with film.

2-D Animation of Film in Cylinder

(Move Cursor Over Image to Show Animation)

No Film Film

After the film injection is stopped at 10 degrees BTDC, the film mixes with the combustion gases. The mixing rate is slow enough as to maintain film effectiveness on the cylinder walls throughout combustion and expansion. The film remains against the walls due to two effects. The combusting gases expand, pressing the film air against the chamber walls. The high tangential velocity of the film air when it is injected causes the centripetal acceleration of the air to force it to the walls. The mixing of the film with the combustion gases is slow enough to allow the film to persist through the expansion stroke, when film is most necessary.

Heat Transfer Reduction

In order to maximize the potential efficiency improvement for film cooling, the reduced gas temperature near the walls must be used to reduce heat transfer and not reduce wall temperature. In order to achieve this in an engine, the coolant side water flow level can be reduced. In the model, the fixed wall temperature boundary condition is used in both film and no film runs and the wall temperatures are set equal in both.

Figure (5) shows the instantaneous heat transfer rates through the different surfaces of the combustion chamber. Integrating the instantaneous heat transfer shown in figure (5) results in a 15% reduction in heat transfer to the cylinder head, 1% reduction in heat transfer to the piston, and a 53% reduction in heat transfer to the cylinder walls. These reductions amount to a 27% total reduction in heat transfer to the cylinder.

Figure (5) Heat Transfer is reduced on Cylinder and Cylinder Head with Film Cooling

Knock Reduction

The CFD models were run using Fluent’s Knock model. The model uses the Douaud and Eyzat equation. The equation predicts the time to onset of knock and is shown in equation (3)

(3)

Where τ is in milliseconds, p is that absolute pressure in atmospheres, and T is in Kevin, and ON is the octane number of the fuel.

The base case model was modified to have higher compression ratio by extending the stroke. Compression ratios of 8.9, 10, 12, 14, and 17 were run with and without film cooling. The models were base lined to knock at a compression ratio of 12:1 by setting the wall temperature on the cylinder head in the clearance volume hot enough to cause knock. This higher wall temperature is to model build-ups in the engine that are known to increase an engine’s propensity to knock.

Fluent reports the ratio of τ over τ_knock. τ_knock is defined as the point where knock will begin. Figure (6) shows the results of several models where a y-axis value of 1 indicates knock. The plot shows that film cooling changes the position of the dramatic increase in τ toward knock. The increase is approximately 4 compression ratios.

Figure (6) propensity to knock with and without film cooling