RE-OPENING OF FUEL VALVE AFTER MAIN INJECTION IS KNOWN AS SECONDARY INJECTION. THE SUDDEN CLOSING OF INJECTOR NEEDLE CAUSES A PRESSURE WAVE TO BE SET UP WHICH REFLECTS FROM DELIVERY VALVE BACK TO THE INJECTOR NEEDLE. IF SUMMATION OF RESIDUAL PRESSURE AND REFLECTED PRESSURE EXCEED INJECTOR LIFTING PRESSURE, SECONDARY INJECTION WILL TAKE PLACE.
PREVENTION OF SECONDARY INJECTION:-
- LENGTH OF FUEL PIPE SHOULD BE SMALL.
- PRESSURE UNLOADING :- THIS SYSTEM BY WHICH THE RESIDUAL PRESSURE IS DROPPED DOWN TO A FIXED VALUE BY A VALVE USUALLY INCORPORATED IN DISCHARGE VALVE ITSELF.
- VOLUME UNLOADING :- NON RETURN VALVE OF FUEL PUMP IS FITTED WITH A SMALL COLLAR JUST BELOW METER SEAT. S THE VALVE CLOSES, THE COLLAR ACTING AS A RELIEF PISTON ENTERS THE VALVE SEAT BORE TO EXTRACT FUEL FROM HP PIPE BEFORE VALVE IS COMPLETELY CLOSED. THIS CAUSES THE PRESSURE IN HP PIPE TO REDUCE SLIGHTLY AND PREVENTS SECONDARY INJECTION.
