Understanding HCCI Characteristics in Mini HCCI Engines 2008-01-1662

This paper examines the successful use of HCCI combustion in a standard issue model-aero “diesel” engine. This two-stroke engine, unlike the more common glow-plug versions, operates without any form of combustion initiator. The fuel and air are premixed using a simple carburettor and ignited by piston compression only. The engine therefore operates in HCCI mode even though it is referred to as a “model diesel engine”. Of particular interest is the fact that the engine is easily started from cold, warm and hot conditions. It runs stably from idle to over 11000rpm and is shown to run at high load points across the speed range with extremely conservative pressure rise rates. Furthermore, this engine is shown not to exhibit any knocking (high pressure oscillations) within its normal range of operation.

The speed-load operational envelope of the engine is mapped out using a range of propellers and a propeller speed-load calibration rig. Air/fuel ratio, inlet air and exhaust gas temperatures are examined within this operational envelope. Areas of unstable operation and extremities in stable operation are also mapped out and discussed. The engine's use of inlet throttling, crankcase pumping and residual exhaust gas metering is modelled and discussed.

The standard model-aero engine fuel (a mixture of paraffin, ether, castor oil and ignition improver) is tested in an Ignition Quality Tester™ and in a larger 4-stroke variable compression ratio HCCI engine. These results are then compared with the HCCI operating characteristics of n-heptane in this larger engine.