Fuel Injection Glossary

Fuel Injection Glossary

Atomization / Spray Pattern;

          Many technicians think that all injectors have a conical cone shaped mist spray.  This type of spray pattern is seen in the old throttle body injectors.  A multi port fuel injector may have a cone shaped spray, but many do not.  Depending on the injector’s nozzle design, the injector may spray a single straight stream or multiple streams.  Some newer injectors have an off center directional fuel discharge hole, and have a directional spray that sprays at specific angles.  How can atomization and spray patterns be seen or evaluated unless the injector is removed from the intake?


Base Fuel Pressure;

         Is the fuel delivery pressure at which the fuel pump and the fuel pressure regulator will be at start and idle.  Typical base fuel pressures are 43.5 psi (3 bars) or 58 psi (4 bars) for OEM applications.


Brake Specific Fuel Consumptions (B.S.F.C.);

         Is the amount of fuel required to produce 1 H. P. for 1 hour.  This means that an engine with a B.S.F.C. of 1 will burn 1 lb. of fuel to produce 1 H.P. for one hour.  Most naturally aspirated gas engines have a B.S.F.C. of 0.5.  Supercharged engines BSFC is 0.55 to 0.60.  Turbocharged engines BSFC is 0.60 to 0.65,



         GM’s central port injection unit.



         GM’s central sequential fuel injection.


Duty Cycle;

         Is the amount of time an injector is turned on (open) to deliver fuel.  This is measured as a percentage, so 50% duty cycle indicates that the injector is turned on 50% of the time.  When an engine needs more fuel, the engine’s computer increases the amount of time that the injector stays on (i.e. it increases its duty cycle).  If an injector stays on all the time (i.e. wide open), it is said to be static (wide open or 100% duty cycle).  Injectors have a working “duty cycle” like a welder.  Extending the duty cycle (on time) equals an eventual shortening of the life of the injector’s coil.  Injectors should not go static in a running engine.  Injectors should not exceed 80% duty cycle.



         Engine Control Module – another term for the vehicles engine computer.



         Engine Control Unit – another term for the vehicles engine computer.


Fuel Pressure Regulator;

         A mechanical fuel pressure regulator or an electronic sensor on the fuel line that controls fuel pressure.  Fuel pressure regulators raise or lower the fuel pressure from a required base fuel pressure by the same amount that the pressure in the intake manifold changes.  An example would be the fuel pressure regulator is set to a base pressure of 43.5 psi, and the intake manifold pressure goes up by 10 psi, the fuel pressure regulator will adjust the fuel pressure up to 54.5 psi to compensate for the 10 psi.  This maintains the proper fuel flow volume to the injectors.


Heat Soak;

         The term “heat soak” refers to the effects that the remaining engine heat has after the engine is turned off and the cooling properties of the intake air flow and the cooling properties of the gasoline flowing through the injector is stopped.  Heat soak causes the evaporation of the residual amount of fuel that remains inside and outside the pintle end of the fuel injectors.  After the heat evaporates any remaining fuel, it will leave behind small amounts of varnishes and waxes.  These varnishes and waxes are left to “bake on” the tip and orifice ports of the injector, and also on the inner walls and pintle valve.  This heat soak process produces a layer of baked on residue that is difficult to remove with normal gas additives.  Heat soak is more pronounced with vehicles that are started and stopped frequently with short trips.


High Impedance Injectors;

         Also known as “saturated” injectors.  High impedance injectors usually range from 11 to 16 ohms of impedance and require approximately 1 to 1.5 amps of current to open.  Most OEM engines use high impedance injectors.  The engine computer programming and circuitry for this type of injector is simpler and less expensive than the computer programming necessary for low-impedance injectors.



         The ability of a substance to attract and hold water molecules from the surrounding environment.  With gasoline fuel injection, this is an important concern when dealing with ethanol based fuels (E10 and E15), where ethanol’s affinity to attract water causes problems with the fuel system components.



         Resistance and impedance are both measured in ohms and both describe the same concept:  the opposition to the flow of electrical current.  Resistance simply is the measurement of the opposition to the flow of electrical current.  Impedance is a little more complex.  Impedance is dependent on both a component’s reactance and the current’s frequency.  The resistance of the coil changes due to the way the coil works with electricity.  A coil increases its resistance as the current’s frequency gets higher.  The coil’s resistance will continue to increase until its resistance becomes so high that no useable current will pass, and at this point they are said to be “saturated”.  In a normal basic resistor, its resistance is always the same at any given frequency.    Impedance describes a reactive component (like an electromagnetic coil) where the resistance changes with a change in the current’s frequency and the operating temperature.


Injector Balance;

         The percent difference in total flow volume between the injectors in a set.  OEM injectors are manufactured to be within 2% variation of volume flow.  A lower injector balance number is good , and there should be less than a 5% variation between the injectors in a set.  Some performance engine builders aim for 1% or less variation between injectors. 



         How can injector balance be determined or evaluated unless off the car flow testing is performed?


Injector Flow Rates;


         Are expressed in either pounds per hour (lb/hr) or cubic centimeters per minute (cc/min). 


                             1 lb/hr = 10.225 cc/min.                     1cc = 1 ml (milliliter). 


         Injectors are rated at how many lbs per hour (or cc/min) of fuel that they allow to pass when fully open at 43.5 psi fuel pressure.


Low Impedance Injectors 

         Also known as “peak and hold” injectors.  Low Impedance Injectors typically have a coil resistance of 0.5 to 6 ohms.  Computer controlled drive circuitry for “peak and hold” low impedance injectors is more complex and costs more to manufacture than simpler computer programming for the “saturated” type of high impedance injectors.  In a peak and hold system, the computer sends an initially high current for a short time to initially turn on the injector.  After the injector is fully open, the computer then reduces the current to a lesser value to “just enough” current to keep the injector held open.  The peak current may be up to 4 amps to initially open the injectors.  Once open, the hold current might be reduced to as little as 1 amp or less.  This design puts less total current on the coil, and therefore causes less heat production which give these injectors more longevity and reliability.



         Octane is nothing more than a measure of the “anti-knock” quality of fuel, i.e. the fuel’s ability to resist self-igniting (in one or more spots) due to heat and/or pressure, and therefore only igniting due to the spark plugs igniting spark.  High octane fuel results in only one ignition source with a resulting controlled smooth burning of the fuel.  There is a widespread perception that the greater the octane, then the better the engine power and performance.  However, once enough octane is supplied to prevent pre-ignition (engine knock/pinging) there is little if any performance improvement.  Another misperception is that using a higher octane fuel will results in improved fuel economy (increased miles per gallon).  Octane is nothing more than a measure of a fuel’s anti knock characteristics.    


         The bottom line is, buy the octane level fuel that your vehicle requires and no more.  A higher octane fuel won’t make your car run better or get better mpg.


         A fuel’s energy content is determined by the different gasoline blend components which determine the BTU’s generated.  Two fuels of identical octane could have different energy contents due to compositional differences.  An example is E10 ethanol blend fuel which has less BTU’s energy  per gallon when compared to a gallon of ethanol free gasoline. 

      100% Ethanol is 130 Octane !!!

Composition Energy Content (BTU/gallon)
100% Ethanol 77,300
100% Gasoline 114,000
E10 Blend 110,300



         Powertrain Control Module – another term for the vehicles’ computer.


Pulse Width;

         Is the injector’s open time. Pulse Width is measured in milliseconds (1000’s of a second).  We flow test injectors at various pulse widths and at various RPMs during the flow testing.  We test all injectors at 3 ms, 6 ms, and 12 ms pulse widths and from 750 rpm (idle simulation) up to 3000 rpm (high speed simulation).  Additionally, we also flow test injectors in static mode (wide open).  This ensures that we will know if the injector is performing properly under all real world conditions.



         If an injector stays on all the time, it is at 100% duty cycle and is said to be “static”.  Static injectors indicate that the injector is too small for the engine. The injector’s duty cycle should not exceed 80%


         During flow testing, the injectors are tested in static mode (wide open) only for a short time in order to test flow rates.


Static Flow Rate;

         Manufacturers rate a fuel injector’s flow rate by the maximum amount of fuel that the injector can flow in a certain amount of time at a specific fuel pressure.  The static flow rate is normally considered 100% duty cycle (wide open) at a fuel pressure of 43.5 psi (3 Bar).


         An example – At a 100% duty cycle, a 19.5 lb injector at 43.5 psi of fuel pressure will flow 19.5 lbs of fuel per hour.



         This deals with the relative quantities of air and fuel necessary for proper and complete internal combustion.  A stoichiometric amount or ratio is the optimum amount or ratio where:


                                          1. All of the fuel is consumed and all of the oxygen is consumed

                                          2. There is no shortage of fuel or oxygen (O2)

                                          3. There is no excess of fuel or oxygen (O2) left over.


         Clean emissions and maximum engine efficiency are dependent on correct cylinder combustion.  Correct cylinder combustion is reliant on having the correct air to fuel ratio which is known as the stoichiometric ratio.  The stoichiometric ratio for gasoline is 14.7 parts air to 1 part fuel (14.7 to 1).


         Different fuels require different stoichiometric air/fuel ratios:

                                           – Gasoline – 14.7 to 1

                                           – Natural Gas – 17.2 to 1

                                           – E10 Ethanol – 14.1 to 1

                                           – E85 – 9.765 to 1


         With gasoline:

              – Any mixture less than 14.7 to 1 is considered a rich mixture

              – Any mixture more than 14.7 to 1 is considered a lean mixture


         Vehicles use oxygen sensors to measure the exhaust gas composition and this allows the ECU to automatically compensate for too little or too much O2 in the exhaust after combustion.  The ECU will adjust the air/fuel ratio by controlling the fuel volume via the fuel injector’s ‘on’ time.


         To maintain correct emissions with minimum pollution, and to achieve maximum performance, all of the injectors must be working correctly with both:

                                             1. Flow Volume (amount of gas)

                                             2. Spray Pattern/Atomization of Fuel


         With computer controlled engines, the O2 sensor(s) and the ECU (engine control unit) can only average out the air/fuel mixture of all the cylinders.  If the injectors are not flowing equally, this will cause some cylinders to run rich and others to run lean!  The cylinder(s) running lean will be due to the clogged injector(s).  If the O2 sensor reads lean, then the computer will compensate by increasing the on time of the next injector(s) to fire, resulting in a new overly rich fuel condition.  Now, hydrocarbon emissions (due to unburned fuel) and carbon monoxide emissions will rise in the cylinders that are now getting too much fuel.  This is one cause of carbon and debris deposits on the injector, the valves, the piston and even the throttle body!! 


         These carbon deposits due to some cylinders running overly rich, will also cause your oil to turn BLACK much faster than normal.


         The only fix for this problem is for the injectors to all have the same fuel flow rate and same spray pattern!!


Send ‘em in and get ‘em fixed!!!


Weight of Fuel;

         1 Gallon of premium fuel weighs 6.34 lbs/gallon.