Understanding Fuel Injectors!

[Redline Motorsports]

The following is a small introduction to the understanding of fuel injectors.


How does a fuel injector work?

A fuel injector is nothing more than a high-speed valve for gasoline. An engine computer or controller is used to control the fuel injector. Contrary to popular belief, this is NOT done by sending power to the injector. Fuel injectors are normally fed power whenever the ignition key is on. The computer controls the negative, or ground side, of the circuit. When the computer provides the injector with a ground, the circuit is completed and current is allowed to flow through the injector. This energizes an electromagnetic coil inside the injector, which pulls a sealing mechanism (pintle, ball, or disc) away from its seat. This makes it possible for fuel to flow through the injector and into the engine. When the computer removes the electrical ground to the injector, the electromagnetic coil becomes demagnetized and a spring forces the pintle, ball, or disc shut to cut off fuel flow. Even at an engine speed of just 1000 RPM, this is done hundreds of times per minute.


What do the terms “static” and “duty cycle” mean?

An injector in an engine turns on and off very quickly to control the amount of fuel delivered. The amount of time an injector is turned on and delivering fuel is known as the duty cycle. This is measured as a percent, so 50% duty cycle indicates that the injector is held open and held closed for an equal amount of time. When the engine needs more fuel, the time that the injector stays on (its duty cycle) increases so that more fuel can flow into the engine. If an injector stays on all the time, it is said to be static (wide open, or 100% duty cycle). INJECTORS SHOULD NOT GO STATIC IN A RUNNING ENGINE! If an injector is static in a running engine (open 100% of the time), that injector is no longer able to control fuel delivery. It is just “along for the ride”. This could be an indication that the injector is too small for the needs of the engine. Injector duty cycle should usually not exceed 80% in a running engine at any time.


What is impedance?

Impedance is the electrical resistance of the electromagnetic coil inside the injector. This is measured in ohms and can be determined with an ohmmeter. Injectors are classified as either high-impedance (also known as “saturated”) or low-impedance (known as “peak and hold”). High-impedance injectors usually range from 11 to 16 ohms of impedance, while low-impedance injectors usually range from 0.7 to 5 ohms of impedance (these impedance numbers are based on what is currently available in the consumer market and are subject to change). Most OEM engine computers are designed to control high-impedance fuel injectors. Low-impedance injectors are generally preferred for racing or ultra-high performance use because they respond more quickly, but aftermarket engine controllers are usually required to control them. Typically injectors over 75 lb/min are required to be a low-impedance injector. The magnets are so large that only a peak and hold type operation will control them. These injectors use a large current draw to open the injector (peak) and then less current to keep it open (hold). Because of this a "driver" is needed to handle the current draw.


What is an injector’s static flow rate?

Manufacturers rate fuel injectors by the maximum amount of fuel that they can flow in a given amount of time. This measurement is taken with the injector on 100% of the time (100% duty cycle, or wide open) and with the fuel at a given pressure (usually 43.5 psi). For example, a 19 pound per hour (Lb./Hr.) injector flow 19 pounds of fuel in one hour at 100% duty cycle and 43.5 psi of fuel pressure. Injectors in imported vehicles are often rated in cubic centimeters per minute (cc/min) instead of pounds per hour. This is also done at 100% duty cycle. NOTE: LS1 GEN3 engines operate at fuel pressures between 58-60 psi. Any OEM injector from these engines is rated at that pressure. Most aftermarket injectors are rated at 43.5 psi (or 3 Bar). You need to recalculate the new flow rate based upon the higher fuel pressure. For example; we use Ford Motorsport 30 lb/hr injectors with our stock displacement head/cam packages. These injectors acutally flow almost 36 lb/hr at the GEN3's 60 psi. NOTE: Ford SVO injectors are rated at 40 psi!!!!



If injectors should not exceed 80% duty cycle under operating conditions, why do manufacturers rate them at 100% duty cycle?

A test at 100% duty cycle is used to determine the maximum amount of fuel that will flow through an injector in a given time. This test is useful for determining whether an injector’s internal fuel passages were machined properly, but it does NOT check an injector’s ability to cycle on or off. It is usually NOT recommended to run an injector at more than 80% duty cycle under actual driving conditions. This 80% duty cycle operating limit is taken into account to make sure the injector will be large enough to feed the engine under ACTUAL OPERATING CONDITIONS and will not starve the engine for fuel.

I will post the calculations to determine proper sizing next!

Howard

[Redline Motorsports]

Selecting the Proper Injector

In order to select the correct size injector for your application, you might wish to use one of the following formulas.
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher.

Using these numbers as a guideline, you can select the approximate injector size in the following formula for a eight cylinder, naturally aspirated engine at 600 bhp.

Most injectors will max out at 80% duty cycle and this is the accepted industry standard.

B.S.F.C is brake specific fuel consumption - How much fuel you are using per horsepower per hour

(HP x BSFC)/(# of injectors x max. duty cycle)= lbs/per hour

(600 x .45)/(8 x .80)

(270)/(6.4)= 42 lb/hr injector

The proper injector for this application would be 42 lbs. / hr. or 441 cc / min. NOTE: the .45 BSFC was based upon a GEN style motor. These motors are extremely efficient which reduces this value.

To convert cc / min to lbs. / hr. - Divide by 10.5
To convert lbs. / hr to cc / min - multiply by 10.5


To convert lbs. / hr to gal. / hr - Divide by 6


To convert cc / min to gal. / hr. - Multiply by .015873