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EFFECTS OF KEY FACTORS ON THE ENGINE COMBUSTION MODE SWITCHING BETWEEN HCCI AND SI
INTRODUCTION
 HCCI IS A PROMISING ENGINE COMBUSTION CONCEPT
 ADVANTAGES OF HIGH FUEL ECONOMY AND ULTRA LOW NOx EMISSIONS
 LIMITED APPLICATION-DIFFICULTY IN CONTROLLING AUTO-IGNITION
 SOLUTION:A HYBRID COMBUSTION SYSTEM IN GASOLINE DIRECT INJECTION ENGINES
HYBRID COMBUSTION SYSTEM
 COMBINATION OF SI AND HCCI MODE IN DIFFERENT OPERATING REGIONS
 HCCI MODE IN MIDDLE/LOW LOADS
 SI MODE IN HIGH SPEED/LOADS
 MAJOR CHALLENGE-PERFORM SMOOTH MODE SWITCHING BETWEEN SI-HCCI
VARYING PARAMETERS
 TWO STAGE CAM PROFILE-CONVENTIONAL POSITIVE OVERLAP AND NEGATIVE OVERLAP VALVES
 THROTTLE POSITIONING
 INJECTION STRATEGY
 A STEPPED SWITCHING STRATEGY IS PUT FORWARD DUE TO SLOW RESPONSE OF THROTTLE
STEPPED SWITCHING
 THROTTLE MOVEMENT IS ISOLATED
 DURING TRANSITION THE THROTTLE POSITION REMAINS IN THE INITIAL POSITION
 ENSURES SMOOTH SWITCHING
 LOOSES THE BENEFIT OF HIGH FUEL ECONOMY
MODE SWITCHING FROM HCCI TO SI
 INJECTION SWITCHED TO TWO STAGE
 SPLIT INJECTION ENSURES CHARGE STRATIFICATION
 DELAY IN ESTABLISHING OIL PRESSURE TO TRIGGER THE CAM SWITCHING MECHANISM LED TO DESYNCHRONISATION
 SWITCHING DONE IN THE SAME MANNER BUT THE LAST TWO SI CYCLES FAILED
 DELAY IN CAM SWITCH CAUSED DESYNCHRONISATION
 NO IGNITABLE CHARGE PRESENT NEAR THE SPARK PLUG
 PEAK RISE IN THC EMISSIONS AND SHARP DROP OF IMEP
 SYNCHRONISATION CAN BE ACHIEVED BY RETARDING THE INJECTION SWITCHING COMMAND
 FIRST HCCI CYCLE-VERY HIGH PEAK PRESSURE AND PRESSURE RISE RATE
 THIS ABNORMAL FIRST HCCI CYCLE AFFECTED THE FOLLOWING CYCLES
 THEREFORE FIRST CYCLE IS THE KEY STEP FOR THE OPTIMIZATION
OPTIMIZATION OF SWITCHING FROM SI TO HCCI
 OPTIMIZATION PARAMETERS INCLUDE INJECTION MASS, INJECTION TIMING AND SPARK TIMING
 OPTIMIZING OBJECTIVES ARE RELATIVELY LATE COMBUSTION, LOW PEAK PRESSURE, LOW PRESSURE RISE RATE AND LOW IMEP FLUCTUATIONS
INJECTION MASS
 FOR THE FIRST HCCI CYCLE A SINGLE STAGE INJECTION IS ADOPTED
 TO ACHIEVE LOWER IMEP FOR FIRST HCCI CYCLE VARIED INJECTION MASSES WERE TESTED
 OPTIMAL INJECTION MASS-80% OF NORMAL MASS
 THIS CAN ONLY BE ACHIEVED BY SELECTING CORRECT INJECTION MASS AND SYNCHRONIZING THE THROTTLE
INJECTION TIMING
 INJECTION TIMING VARIED BETWEEN 260deg BTDC AND 40deg ATDC
 PREFERRED FIRST HCCI CYCLE SHOULD HAVE RELATIVELY LATE COMBUSTION,LOWER PEAK PRESSURE
 PRESSURE RISE RATE CAN BE DECREASED BY RETARTED INJECTION
 MISFIRE OCCURRED WITH INJECTION AT 40deg ATDC
 PREFERABLE INJECTION TIMING BETWEEN 80deg BTDC AND 40deg BTDC
SPARK TIMING
 OVEREARLY OR OVERLATE SPARK WOULD RESULT IN UNSTABLE COMBUSTION
 SPARK TIMINGS VARIED FROM 27deg BTDC TO 10deg ATDC
 CONTINOUS PRESSURE TRACE SHOWS STABLE COMBUSTION WHEN OPERATED BETWEEN 27deg-17deg BTDC
CONCLUSIONS
 MODE SWITCHING FROM HCCI TO SI IS RELATIVELY STRAIGHT-FORWARD,BUT THE PROCESS IN THE OPPOSITE DIRECTION IS MORE CHALLENGING AND MUST BE OPTIMIZED.
 OPTIMIZATION SHOULD BE FOCUSSED ON THE FIRST HCCI CYCLE
 THE NVO INJECTION IN THE FIRST HCCI HAS TO BE REDUCED IN QUANTITY IN ORDER TO AVOID AN ADVANCED COMBUSTION
 THE INJECTION TIMING IN THE FIRST HCCI CYCLE AFFECTS THE SWITCHING PROCESS.
 RETARDING THE INJECTION TIMING TO A CERTAIN EXTENT MAKES SWITCHING SMOOTHER
 SPARK TIMING HAS LITTLE INFLUENCE ON THE MODE SWITCHING.