The Effect of Water Addition on HCCI Diesel Combustion

The application of homogeneous charge compression ignition in engines requires the limitation of high pressure rise rates at high loads and ignition timing control. Both effects can be influenced by the addition of water: The cooling effect based on its high latent heat of vaporization can be used to control ignition timing. Too early heat release at high compression ratios can be retarded towards TDC. Pressure rise rates can be reduced by inhomogeneous cooling and the dilution of the charge by steam.

The paper discusses the effects of Diesel-water emulsions. The results are compared to the separate direct injection of fuel and water. The experiments were carried out on a rapid compression and expansion machine. Optical measurements were conducted using the shadowgraph technique in order to visualize spray penetration as well as combustion in the same cycle. A heat release analysis was performed on basis of the pressure indication. Gaseous emissions were electrochemically measured.

It is shown that fuel-water emulsions are more effective than separate injections to retard ignition. Problems of soot and NO_x emissions caused by diffusive combustion of wall adhering fuel can be reduced by emulsions with a sufficient water content. For the same amount of water the pressure rise rates can be substantially reduced with separate water injection.