EFFECT OF ENGINE VARIABLES ON FLAME PROPAGATION

EFFECT OF ENGINE VARIABLES ON FLAME PROPAGATION

RATE OF FLAME PROPAGATION AFFECTS THE COMBUSTION PROCESS IN SI ENGINES. HIGHER COMBUSTION EFFICIENCY AND FUEL ECONOMY CAN BE ACHIEVED BY HIGHER FLAME PROPAGATION VELOCITIES. UNFORTUNATELY, FLAME VELOCITIES FOR MOST OF FUEL RANGE BETWEEN 10 TO 30 M/SECOND. 

THE FACTORS WHICH AFFECT THE FLAME PROPAGATION's ARE 

• AIR FUEL RATIO
• COMPRESSION RATIO
• LOAD ON ENGINE
• TURBULENCE AND ENGINE SPEED 5. OTHER FACTORS

• A : F RATIO: THE MIXTURE STRENGTH INFLUENCES THE RATE OF COMBUSTION AND AMOUNT OF HEAT GENERATED. THE MAXIMUM FLAME SPEED FOR ALL HYDROCARBON FUELS OCCURS AT NEARLY 10% RICH MIXTURE. FLAME SPEED IS REDUCED BOTH FOR LEAN AND AS WELL AS FOR VERY RICH MIXTURE. LEAN MIXTURE RELEASES LESS HEAT RESULTING LOWER FLAME TEMPERATURE AND LOWER FLAME SPEED. VERY RICH MIXTURE RESULTS INCOMPLETE COMBUSTION AND RESULTS IN PRODUCTION OF LESS HEAT AND FLAME SPEED REMAINS LOW. 

• COMPRESSION RATIO: THE HIGHER COMPRESSION RATIO INCREASES THE PRESSURE AND TEMPERATURE OF THE MIXTURE AND DECREASES THE CONCENTRATION OF RESIDUAL GASES. ALL THESE FACTORS REDUCE THE IGNITION LAG AND HELP TO SPEED UP THE SECOND PHASE OF COMBUSTION. THE MAXIMUM PRESSURE OF THE CYCLE AS WELL AS MEAN EFFECTIVE PRESSURE OF THE CYCLE WITH INCREASE IN COMPRESSION RATIO. FIGURE ABOVE SHOWS THE EFFECT OF COMPRESSION RATIO ON PRESSURE (INDIRECTLY ON THE SPEED OF COMBUSTION) WITH RESPECT TO CRANK ANGLE FOR SAME A: F RATIO AND SAME ANGLE OF ADVANCE. HIGHER COMPRESSION RATIO INCREASES THE SURFACE TO VOLUME RATIO AND THEREBY INCREASES THE PART OF THE MIXTURE WHICH AFTER-BURNS IN THE THIRD PHASE.

• LOAD ON ENGINE: WITH INCREASE IN LOAD, THE CYCLE PRESSURES INCREASE AND THE FLAME SPEED ALSO INCREASES. IN S.I. ENGINE, THE POWER DEVELOPED BY AN ENGINE IS CONTROLLED BY THROTTLING. AT LOWER LOAD AND HIGHER THROTTLE, THE INITIAL AND FINAL PRESSURE OF THE MIXTURE AFTER COMPRESSION DECREASE AND MIXTURE IS ALSO DILUTED BY THE MORE RESIDUAL GASES. THIS REDUCES THE FLAME PROPAGATION AND PROLONGS THE IGNITION LAG. THIS IS THE REASON, THE ADVANCE MECHANISM IS ALSO PROVIDED WITH CHANGE IN LOAD ON THE ENGINE. THIS DIFFICULTY CAN BE PARTLY OVERCOME BY PROVIDING RICH MIXTURE AT PART LOADS BUT THIS INCREASES THE CHANCES OF AFTER BURNING. THE AFTER BURNING IS PROLONGED WITH RICHER MIXTURE. IN FACT, POOR COMBUSTION AT PART LOADS AND NECESSITY OF PROVIDING RICHER MIXTURE ARE THE MAIN DISADVANTAGES OF SI ENGINES WHICH CAUSES WASTAGE OF FUEL AND DISCHARGE OF LARGE AMOUNT OF CO WITH EXHAUST GASES.

• TURBULENCE: TURBULENCE PLAYS VERY IMPORTANT ROLE IN COMBUSTION OF FUEL AS THE FLAME SPEED IS DIRECTLY PROPORTIONAL TO THE TURBULENCE OF THE MIXTURE. THIS IS BECAUSE, THE TURBULENCE INCREASES THE MIXING AND HEAT TRANSFER COEFFICIENT OR HEAT TRANSFER RATE BETWEEN THE BURNED AND UNBURNED MIXTURE. THE TURBULENCE OF THE MIXTURE CAN BE INCREASED AT THE END OF COMPRESSION BY SUITABLE DESIGN OF THE COMBUSTION CHAMBER (GEOMETRY OF CYLINDER HEAD AND PISTON CROWN). INSUFFICIENT TURBULENCE PROVIDES LOW FLAME VELOCITY AND INCOMPLETE COMBUSTION AND REDUCES THE POWER OUTPUT. BUT EXCESSIVE TURBULENCE IS ALSO NOT DESIRABLE AS IT INCREASES THE COMBUSTION RAPIDLY AND LEADS TO DETONATION. EXCESSIVE TURBULENCE CAUSES TO COOL THE FLAME GENERATED AND FLAME PROPAGATION IS REDUCED. MODERATE TURBULENCE IS ALWAYS DESIRABLE AS IT ACCELERATES THE CHEMICAL REACTION, REDUCES IGNITION LAG, INCREASES FLAME PROPAGATION AND EVEN ALLOWS WEAK MIXTURE TO BURN EFFICIENTLY. 

• ENGINE SPEED THE TURBULENCE OF THE MIXTURE INCREASES WITH AN INCREASE IN ENGINE SPEED. FOR THIS REASON, THE FLAME SPEED ALMOST INCREASES LINEARLY WITH ENGINE SPEED. IF THE ENGINE SPEED IS DOUBLED, FLAME TO TRAVERSE THE COMBUSTION CHAMBER IS HALVED. DOUBLE THE ORIGINAL SPEED AND HALF THE ORIGINAL TIME GIVE THE SAME NUMBER OF CRANK DEGREES FOR FLAME PROPAGATION. THE CRANK ANGLE REQUIRED FOR THE FLAME PROPAGATION, WHICH IS MAIN PHASE OF COMBUSTION WILL REMAIN ALMOST CONSTANT AT ALL SPEEDS. THIS IS AN IMPORTANT CHARACTERISTIC OF ALL PETROL ENGINES. 
• ENGINE SIZE ENGINES OF SIMILAR DESIGN GENERALLY RUN AT THE SAME PISTON SPEED. THIS IS ACHIEVED BY USING SMALL ENGINES HAVING LARGER RPM AND LARGER ENGINES HAVING SMALLER RPM. DUE TO SAME PISTON SPEED, THE INLET VELOCITY, DEGREE OF TURBULENCE AND FLAME SPEED ARE NEARLY SAME IN SIMILAR ENGINES REGARDLESS OF THE SIZE. HOWEVER, IN SMALL ENGINES THE FLAME TRAVEL IS SMALL AND IN LARGE ENGINES LARGE. THEREFORE, IF THE ENGINE SIZE IS DOUBLED THE TIME REQUIRED FOR PROPAGATION OF FLAME THROUGH COMBUSTION SPACE IS ALSO DOUBLED. BUT WITH LOWER RPM OF LARGE ENGINES THE TIME FOR FLAME PROPAGATION IN TERMS OF CRANK WOULD BE NEARLY SAME AS IN SMALL ENGINES. IN OTHER WORDS, THE NUMBER OF CRANK DEGREES REQUIRED FOR FLAME TRAVEL WILL BE ABOUT THE SAME IRRESPECTIVE OF ENGINE SIZE PROVIDED THE ENGINES ARE SIMILAR. 

• OTHER FACTORS: AMONG THE OTHER FACTORS, THE FACTORS WHICH INCREASE THE FLAME SPEED ARE SUPERCHARGING OF THE ENGINE, SPARK TIMING AND RESIDUAL GASES LEFT IN THE ENGINE AT THE END OF EXHAUST STROKE. THE AIR HUMIDITY ALSO AFFECTS THE FLAME VELOCITY BUT ITS EXACT EFFECT IS NOT KNOWN. ANYHOW, ITS EFFECT IS NOT LARGE COMPARED WITH A : F RATIO AND TURBULENCE.