Fuel injector 101

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Chongo

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Fuel injector 101



Let’s start off with some terminology, and basics.
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.
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.
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.
Weight of fuel:
Most fuel, gasoline, not diesel, weighs around 6.1 lbs per gallon as to be compared against water which is 8.35 lbs per gallon roughly. However Premium is slightly lighter than Regular. The higher the octane the lighter the fuel in gasoline.

Injector spray patterns:
This is actually very simple, most of us have or have used a garden hose, and we all have experienced the small orifice labeled Jet. When you put it on the small orifice “jet” it shoots far, then when you change to flood setting the orifice is quite large and it doesn’t shoot as far. This is where bigger isn’t really better for a spray pattern on an injector system with only 30 – 50 psi, remember most homes have a minimum water pressure of 80 psi, so 30 – 50 isn’t a whole lot, so the orifices must be small. This is where a small orifice has a much better spray pattern than a large one. This helps in the atomization in the intake manifold fuel injection. Normally the stronger the spray to a point, the better the driveability.





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Chongo

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Is Bigger better?

Calculations here are for W.O.T ( wide open throttle ) and is used for determining the best injector requirements for W.O.T. and engine requirements from idle to W.O.T. and does not have any consideration to quality of injector, driveability or mileage issues. This is a formula for picking the best injector for your hp range rating of your engine. Different spray patterns may improve or worsen driveablility issues.


Fuel requirement in lbs./hr = (Max HP x BSFC) / (number of injectors x duty cycle)

Note: to convert from lbs./hr to the Metric measurement of cc/min, use this equation: [(lbs./hr) x 60] / 6.177 = cc/min

Max HP is a realistic horsepower estimate at the crankshaft or known value from engine dyno testing. Chassis dyno horsepower figures can only be used once you factor in the drive train
losses, which can vary from vehicle to vehicle. Ask your chassis dyno operator to calculate the drive train horsepower loss for your vehicle. Add the drive train horsepower loss to the drive
wheel horsepower to closely estimate crankshaft horsepower. BSFC or brake-specific fuel consumption is the amount of fuel consumed per unit of power produced. It is an indication of the efficiency of the engine configuration and calibration. Actual BSFC is a function of compression, camshaft timing, cylinder head design, tune, ambient conditions, etc. The lower the BSFC number, the more efficiently the engine is making power. Engine dyno testing can provide exact BSFC data. To estimate the fuel requirements of your engine, use the examples below that best match your engine type. The reason we use a higher BSFC value to calculate fueling requirements for a supercharged engine is because of the parasitic losses or the power required to driving the supercharger that is never seen at the crank. In other words, a supercharged engine that dyno tests 450 hp at the crank, may actually be making 490 hp, but the supercharger and drive assembly is absorbing 40 hp, so you net out 450 hp.

Also, the heating effect of pressurizing the intake charge in a non-intercooled system also increases the fueling requirement of a super/turbocharged engine. Always remember that too lean of a mixture can result in spark knock, high combustion temperatures and engine damage. It’s smart to be slightly on the rich or safe side.

There is one other parameter involved in properly sizing fuel injectors: duty cycle. This is the percent of time that the injector is actually open (which is also referred to as pulse width) vs. total time between firing events. When an injector is open 100% of that time, the injector is in what is called a static condition. For road-racing engines that are at maximum power for extended periods of time, the desired maximum safe duty cycle is 80% to 85%. This ensures that the injector is closed a sufficient time to keep it from overheating. For a typical street engine that spends less than 1% of its time at maximum power, you could argue that a higher duty cycle could be used to calculate fueling needs. Typically we would not do this because again we want to error on the safe side. Some may ask why not just install the biggest injector you can find. Well it’s the same analogy of putting an 850cfm carburetor on a four cylinder Chevette motor, overkill at best, more like a controlled leak. So before you go putting that Holley 850 double pumper on your 5 hp Briggs and Stratton lawnmower engine, you may want to re-think your strategy. One other thing to remember is that an injector can only open and close so fast, this is called minimum dynamic flow range. If the ECM, in an attempt to lean out a rich mixture, selects a pulse width that is shorter than the injector’s minimum dynamic flow range, the injector becomes inconsistent in its ability to supply the required fuel. This results in poor engine performance, surging and stumbling. In other words bigger isn’t always better. As a matter of fact, bigger usually has a poorer spray pattern. Remember a pump only creates a flow of fluid, restrictions in the closed system in which the fluid is passed into creates the pressure.
Another consideration is always make sure the injector is tested at the same psi rating, for some give their ratings at 39 psi some at 44, and others at even lower or higher ratings. This will drastically change your #’s per hour rating. For instance, a 19# injector @ 44 psi will outflow a 24# injector @ 20 psi. Therefore how they are rated is very important.


Let’s calculate the fueling requirements of a few engines to illustrate what we have been talking about.

For the first example let’s take a stock Ford 5.0L Mustang motor that makes an advertised 215 hp and look a the very conservative approach Ford used to calculate the injector size for the factory engine by using the O.E. typically safe .80% duty cycle limit.

Fuel injector size = (215 hp x 0.55) / (8 x 0.80) = 18.5 lbs./hr
or the ACCEL/DFI p/n 150119 injector

Now let’s upgraded the engine with more efficient GT-40 type components that will lower the BSFC and use a more realistic 0.85 duty cycle limit. Ford says this combination of GT-40 parts will produce about 275 hp. What injector size is required to support this?

Fuel injector size = (275 hp x.50) / (8 x 0.85) = 20.1 lbs./hr
or the ACCEL/DFI p/n 150121 injector

Until now your only choice would have been to go with a 24 lbs./hr unit, which would be fine if the engine was making about325 hp, but not ideal for 275 hp. Remember the comment about realistic horsepower; don’t kid yourself! Now let’s factor in an adjustable fuel pressure regulator as a tuning tool for this setup. By adjusting fuel pressure you can change the flow rating of a given injector. The calculation is simple, as long as you know the static flow rating of an injector at a specific pressure. For example ACCEL/DFI p/n 150121 flows 20.0 lbs./hr at 2.7 BAR or 39.6 PSI, which just happens to be where the stock Ford nonadjustable fuel pressure regulators are preset. As a point of reference, most GM factory fuel pressure regulators are preset at 3.0 BAR or 44.1 PSI. If we were to increase the fuel pressure from 39.6 PSI to 45 PSI, what will be the new flow rating of the ACCEL/DFI p/n 150121 injector?

New flow rating = [square root of (new pressure /old pressure)] x old flow rating

New flow rating = [square root of (45 PSI / 39.6 PSI)] x 20.0 lbs./hr = 21.3 lbs./hr

This increase in flow rating would support about 15 additional horsepower on our GT-40 engine. An adjustable fuel pressure regulator is an excellent tuning tool as long as the fuel pressure does not exceed 55 PSI, which is the limit that the stock fuel line fittings are designed to handle. So let’s say we increase the fuel pressure up to 55 PSI, then the ACCEL/DFI p/n 150121 injector would be flowing 23.6 lbs./hr. But because ACCEL/DFI offers p/n 150123 that flows 23.1 lbs./hr at 39.6 PSI and 150124 that flows 24.3 lbs./hr at 39.6 PSI, radical increases in fuel pressure are not required to find the perfect match for your engine. The key is to make power efficiently, choosing the correct injector for your intended needs and using the adjustable pressure regulator as a fine tuning tool.

For the third example let’s use Ford’s new 392 crate motor p/n M-6007-A392. Out of the crate, using a 750cfm carburetor, this engine dyno tested at 453 hp with a .454 BSFC. Let’s calculate the injector size you would need if the 392 were to be fuel injected.

Fuel injector size = (453 hp x 0.454) / (8 x 0.85) = 30.2 lbs./hr units
or the ACCEL/DFI p/n 150130 injector

As a point of reference, this same 392 crate engine has made over 530 hp on a dyno with Air Flow Research 185cc heads vs. stock GT-40X heads. To support this new-found power, using the same equation, larger 35.2 lbs./hr units or the ACCEL/DFI p/n 150136 would be needed. So when calculating injector size, if you are planning on large power adders in the future, keep in mind that you may have to upgrade your injector size. Just like if you might have had to put a bigger carburetor on a modified motor in the past.

Best wishes…………… Chongo
:bandit:
 
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