FUEL METERING
THE COMBUSTION PROCESS BURNS A MIXTURE OF AIR AND FUEL. THIS AIR AND FUEL MIXTURE IS REFERRED TO AS THE A/F RATIO. AS THE RELATIVE AMOUNT OF FUEL IN THE MIXTURE DECREASES ( A LEANER MIXTURE ) THE A/F RATIO VALUE BECOMES LARGER. AS THE PROPORTION OF FUEL BECOMES GREATER ( A RICHER MIXTURE ) THE A/F RATIO VALUE BECOMES SMALLER.
THE A/F RATIO OF AN ENGINE MAY BE MEASURED IN MANY WAYS, BUT THE MOST REPRESENTATIVE AND ACCURATE METHODS USE HIGHLY SPECIALIZED EXHAUST GAS ANALYZERS. THE A/F RATIO INFORMATION IS THE KEY TO ESTABLISHING AN APPROPRIATE FUEL METERING CALIBRATION FOR A GIVEN ENGINE COMBUSTION. ANYONE ATTEMPTING TO OPTIMIZE AN ENGINE A/F RATIO SHOULD PURCHASE AND UTILIZE AN A/F RATIO MIXTURE METER. WHETHER YOU ARE TUNING A CARBURETOR, FUEL INJECTION OR FORCED INDUCTION AND USING UNLEADED FUEL, THERE ARE MANY TO CHOOSE FROM, THE "WIDE BAND" VERSION IS THE PREFERRED VERSION. DO NOT DISCOUNT THE "NARROW BAND" VERSION, EITHER VERSION IS FAR MORE ACCURATE THAN "I THINK IT'S LEAN" OR "IT FEELS RICH", WITHOUT ACCURATE INFORMATION YOU ARE WASTING TIME, MONEY AND POSSIBLY THE ENGINE.
A FULLY WARMED UP ENGINE WITH AN A/F RATIO AS RICH AS 6.0-1 AND AS LEAN AS 22.0-1, THESE ARE THE GENERAL RICH AND LEAN COMBUSTION LIMITS, BUT DURING ACTUAL DRIVING SITUATIONS THE ACTUAL A/F RATIO NEEDED DURING VARIOUS OPERATING CONDITIONS WILL BE VERY CLOSE TO THE MID-POINT OF THESE EXTREMES.
GENERALLY ENGINES OF DIFFERENT BASIC DESIGNS HAVE THE SAME A/F RATIO REQUIREMENTS. THESE CALIBRATION NEEDS ARE TYPICALLY A FUNCTION OF OPERATION MODE, ENGINE TEMPERATURE, ENGINE SPEED, AND LOAD. GENERALLY A PRODUCTION BASED HIGH PERFORMANCE ENGINE WILL HAVE A/F RATIO VALUES IN A RANGE FROM 12.0:1 TO 16.0:1.
COLD ENGINE
THE COMBUSTION PROCESS REQUIRES VAPORIZED FUEL. MOST OF THE VAPORIZATION OCCURS AFTER THE AIR AND FUEL DROPLETS HAVE MOVED PAST THE INTAKE VALVE, BUT A LARGE PORTION MUST OCCUR BEFORE THE INTAKE VALVE OPEN. IN A COLD ENGINE THE AIR, FUEL AND ALL THE COMPONENTS CONTACTED BY THE FUEL ARE AT TEMPERATURES THAT DO NOT PROMOTE VAPORIZATION, SO ADDITIONAL FUEL MUST BE ADDED SO THAT THE PERCENTAGE THAT DOES VAPORIZE WILL SUPPORT COMBUSTION. THE AMOUNT OF THIS ADDITIONAL FUEL (COLD ENRICHMENT) DEPENDS ON THE TEMPERATURE. IF THE STARTING CLIMATE IS VERY COLD (-20F) THE A/F RATIO MAY NEED TO BE 4.0:1, AS THE ENGINE WARMS UP, THE A/F RATIO MUST BE LEANED TO NORMAL VALUES.
IDLE
THE A/F RATIO FOR A STABLE IDLE IS DETERMINED PRIMARILY BY THE CAMSHAFT PROFILE. A LONG DURATION CAMSHAFT WITH BIG VALVE OVERLAP CAUSES THE INLET CHARGE TO BECOME DILUTED BY EXHAUST GASES, THIS DILUTED CHARGE BURNS VERY SLOWLY AND MAY REQUIRE A LOT OF SPARK ADVANCE. ALSO THE COMBUSTION BECOMES ERRATIC, SO A RICH A/F RATIO IS REQUIRED TO REDUCE THE CYCLIC VARIATION (LOPING IDLE) WHEN TUNING. THE A/F RATIO MAY NEED TO BE 11.5:1 OR RICHER WITH A REALLY SERIOUS RACE CAMSHAFT PROFILE. WITH A SHORT DURATION CAMSHAFT PROFILE THE A/F RATIO DOES NOT NEED TO BE AS RICH FOR A STABLE IDLE AND MAY BE AS LEAN AS 14.7:1 WHERE EMISSIONS NEED TO BE A MINIMUM.
LOW SPEED AND LIGHT THROTTLE
THE CONDITIONS THAT AFFECT A/F RATIOS AT IDLE ALSO AFFECT THE FUEL CALIBRATIONS AT OFF-IDLE OPERATING CONDITIONS WHERE ENGINE RPM IS LOW AND THE LOAD IS LIGHT (LOW INLET DENSITY AND HIGH MANIFOLD VACUUM). AGAIN THE LONGER VALVE DURATION / OVERLAP, THE RICHER THE A/F RATIO WILL NEED TO BE FOR SURGE-FREE OPERATION. IN THE IMMEDIATE OFF-IDLE RANGE THE A/F RATIO MAY NEED TO BE AS RICH AS THE IDLE, SOMEWHERE AROUND 12.5:1 TO 13.0:1 IS COMMON, THEN GRADUALLY BECOMING LEANER WITH AN INCREASE IN SPEED OR LOAD. WITH A VERY MILD CAMSHAFT PROFILE, THE ENGINE MAY TOLERATE A/F RATIOS IN THE 14.0 TO 15.0:1 RANGE FOR THE SAME OPERATING CONDITIONS.
MINIMUM SPEEDS AND LOADS
AS ENGINE RPM INCREASES THE THROTTLE IS OPENED, THE EFFECT VALVE DURATION AND OVERLAP BEGIN TO DIMINISH. THERE IS LESS INLET CHARGE DILUTION, SO A LEANER A/F RATIO MAY BE USED WITHOUT ANY SURGES OR DRIVABILITY PROBLEMS. A/F RATIOS FROM 14.0 TO 15.5:1 ARE THE NORM. TYPICALLY THE BEST ECONOMY A/F RATIO IS FROM 15.5 TO 16.5 WITH A "STREETABLE" HIGH PERFORMANCE ENGINE COMBINATION, BUT WILL REQUIRE ADDITIONAL SPARK ADVANCE TO COMPENSATE FOR THE SLOW BURN RATES OF LEAN MIXTURES.
HEAVY LOAD AT PART THROTTLE
AS LOAD ON THE ENGINE INCREASES FROM ADDING MORE THROTTLE OPENING (HIGH INLET CHARGE DENSITY / LOW MANIFOLD VACUUM) THE A/F RATIO NEEDS TO BE ENRICHED TO PRODUCE MORE POWER AND REDUCE THE DRIVABILITY ISSUES BECAUSE OF LEAN A/F RATIOS AT HIGH LOADS. THE A/F RATIO SHOULD BE SOMEWHERE BETWEEN THE CRUISE AND WOT A/F RATIO VALUES, GENERALLY AROUND 14.5 TO 13.0:1 DEPENDING ON LOAD AND SPEED.
WIDE OPEN THROTTLE
ALL 4-CYLE GASOLINE ENGINES HAVE AROUND THE SAME A/F RATIO NEEDS AT WOT, WHERE THE GOAL IS TO PRODUCE THE MAXIMUM TORQUE / POWER FROM A GIVEN ENGINE COMBINATION. THE LEANEST A/F RATION THAT PRODUCES MAXIMUM TORQUE / POWER IS REFERRED TO AS "LEAN BEST TORQUE" WHICH IS USUALLY AROUND 13.3:1 A/F RATIO. THE RICHEST IS A A/F RATIO OF 11.5:1 "RICH BEST TORQUE". THE DIFFERENCE BETWEEN "LEAN BEST TORQUE" AND "RICH BEST TORQUE" CAN BE CLOSER AT HIGH ENGINE SPEEDS, SO THE BEST TARGET A/F RATIO FOR WOT USAGE IS BETWEEN 12.0 TO 12.5:1 A/F RATIO, THIS INSURES THE BEST PERFORMANCE AT WOT POWER UNDER ALL CIRCUMSTANCES.
SPARK ADVANCE REQUIREMENTS
THE CHARGE OF THE AIR / FUEL IS BURNED BY A FLAME-FRONT BEGINNING AT THE SPARK PLUG. THE FLAME STARTS A KERNEL WITH A RATHER SLOW RATE OF EXPANSION, BUT ONCE A SMALL PERCENTAGE OF THE CHARGE IS IGNITED, THE COMBUSTION PROCESS ACCELERATES AT A FASTER RATE. DUE TO THE VERY SLOW INITIAL REACTION RATES, IGNITION MUST OCCUR "BEFORE TOP DEAD CENTER" IF MAXIMUM EFFECT IS TO BE UTILIZED. THIS IS THE "ADVANCE" IN IGNITION AND IS MEASURED IN DEGREES OF CRANKSHAFT ROTATION. THE BEST ADVANCE USUALLY PRODUCES THE BEST TORQUE WHEN MAXIMUM CYLINDER PRESSURE IS ACHIEVED AT AROUND 15 DEGREES "AFTER TOP DEAD CENTER". DEPENDING ON DESIGN AND OPERATING CONDITIONS, THE SPARK ADVANCE CAN BE FROM LESS THAN 5 DEGREES UP TO MORE THAN 30 DEGREES FOR A HIGH PERFORMANCE / RACE ENGINE, AND 0 DEGREES UP TO MORE THAN 50 DEGREES FOR A STOCK PRODUCTION ENGINE WITH EMISSION COMPONENTS IN PLACE AND FUNCTIONAL.
THOSE OF US IN ENGINE DEVELOPMENT STRIVE TO CALIBRATE SPARK ADVANCE TO VALUES REFERRED TO AS " MINIMUM BEST TORQUE", THIS IS THE MINIMUM SPARK ADVANCE THAT WILL PRODUCE THE MAXIMUM TORQUE AT A GIVEN OPERATING CONDITION OF SPEED AND LOAD OF A GIVEN ENGINE COMBINATION. IN MOST CASES THE SPARK ADVANCE CURVE CAN BE ADVANCED SEVERAL ADDITIONAL DEGREES BEYOND "MINIMUM BEST TORQUE" BEFORE TORQUE BEGINS TO DROP OFF. IF "KNOCK" OCCURS BEFORE "MINIMUM BEST TORQUE" ADVANCE CAN BE DETERMINED. THE ADVANCE IS REFERRED TO AS "KNOCK" LIMITED. THE FUEL OCTANE, CAMSHAFT PROFILE AND OR THE ENGINE'S STATIC COMPRESSION RATIO WILL NEED TO BE ADDRESSED BEFORE MAXIMUM OUT-PUT CAN BE ACHIEVED.
THE VARIABLES THAT INFLUENCE THE SPARK ADVANCE REQUIREMENT INCLUDE THE BASE ENGINE DESIGN, THE SPECIFIC COMPONENTS OF THE PARTICULAR ENGINE ( CAMSHAFT, COMPRESSION, PISTON AND CYLINDER HEAD CONFIGURATION), THE INTENDED FUEL TO BE USED AND THE OPERATING CONDITIONS (RPM / LOAD / TEMPERATURES). THE SPARK ADVANCE GENERALLY INCREASES WITH ENGINE RPM UP TO A POINT WHERE IT WILL "PEAK" AND IN SOME CASES WILL DECREASE SLIGHTLY WITH FURTHER INCREASE OF RPM. ADVANCE REQUIREMENTS DECREASES WITH LOAD AND THE MINIMUM ADVANCE AT ANY GIVEN ENGINE SPEED IS AT WOT.
THE ADVANCE REQUIREMENTS OF AN ENGINE OF THE SAME DESIGN BUT DIFFERENT COMPONENTS ARE DICTATED MORE BY CAMSHAFT PROFILE, INCLUDING COMPRESSION. WITH A RADICAL CAMSHAFT PROFILE, THE WOT ADVANCE CURVE CAN BE VERY AGGRESSIVE AND REACH MAXIMUM ADVANCE AT A LOWER RPM BECAUSE OF THE POOR VOLUMETRIC EFFICIENCY AT LOW RPM, AND A VERY SLOW COMBUSTION RATE AND ITS HIGH RESISTANCE TO "KNOCK". PART THROTTLE ADVANCE ON ENGINES WITH LARGE CAMS CAN ALSO BE AGGRESSIVE BECAUSE OF THE REDUCED FLAME SPEED CAUSED FROM A SIGNIFICANT EXHAUST DILUTION OF THE INLET CHARGE FROM A CAMSHAFT WITH A LOT OF OVERLAP.
