I originally wrote this for my forum ecotecforum.com but I decided to share here and maybe help someone.
Fuel System 101 FAQ
Fuel System FAQ
This FAQ was made to make things more understandable for the newer and avid car enthusiast who is looking to modify and upgrade their vehicles fuel system but they have some misconceptions or misunderstandings on the fuel system.
I tried to make this as simple to understand as possible while getting some details into it.
Special Thanks to Witt & Scott @ Cobalt Performance Parts
What is a fuel systems job?
A fuel systems job is mainly to maintain a proper fuel demand throughout the system that will eventually be sprayed from the fuel injectors into the intake stream and into the combustion chamber.
What does a Fuel System Consist Of?
This depends on if it is a return or return-less fuel system. If it is a return fuel system, it has a fuel tank, fuel pump, sending fuel lines (from the pump to the fuel rail), a fuel rail, fuel injectors, fuel pressure regulator and returning fuel lines to the fuel tank. If it is a return-less fuel system, it has the same items but no fuel pressure regulator and return fuel lines.
How does a return fuel system work?
A return fuel system first starts out in the fuel tank. Everyone knows you go to the gas pump and put fuel into your gas tank, so that’s where it originates. Typically, the fuel pump is located inside the fuel tank. The fuel pump sends the fuel from inside the fuel tank through the fuel lines (sending lines). This fuel being sent is sending a high pressure to the fuel rail. As it enters the rail, fuel tries to either be sent back to the fuel tank or into the fuel injectors themselves. What keeps all the fuel from going back to the fuel tank? The fuel pressure regulator. The fuel pressure regulator will determine the amount of fuel being returned or staying inside the fuel rail. Also, by restricting the fuel line, it’s increasing the pressure inside the rail which effects the amount of pressure inside the fuel injectors and how much fuel is being sprayed out during the injectors cycle. After this, fuel that is not being used is now sent back to the fuel tank via fuel return lines (which were attached to the fuel pressure regulator). Fuel returns to the fuel tank and the cycle of fuel starts all over.
How does a return-less fuel system work?
Very similarly to a return fuel system except fuel is not sent back to the fuel tank if it’s not being used, hence, it doesn’t have an fuel pressure regulator to “regulate” fuel being sent back to the fuel tank. There are a couple of ways that fuel is maintained. On a return fuel system, fuel being sent from the fuel pump is sent at a constant rate, on some fuel systems, the fuel pump fluctuates depending on the voltage sent to the fuel pump from the computer. The computer decides when more fuel should be sent and it also determines when there should be less fuel sent. Other return-less fuel systems have a valve inside the fuel pump itself that can change the flow rate of the fuel pump.
What is every items job specifically?
1. Fuel Tank – To store fuel a certain capacity of fuel (gasoline)
2. Fuel Pump – To send the fuel towards the fuel rail via fuel lines (sending lines)
3. Sending Fuel Lines – To carry the fuel that was sent from the fuel pump to the fuel rail
4. Fuel Rail – Fuel rail is the central area where fuel comes to from the fuel sending fuel lines. The fuel that ends up here either gets pressurized into the fuel injectors or (on a return fuel system) gets sent back to the fuel tank.
5. Fuel Injectors – Fuel injectors job is to open and close at certain times on a 4 stroke engine (during the intake stroke). As it opens, it sprays fuel into the air intake stream which will get carried into the combustion chamber. Throughout the rest of the strokes, it remains closed.
6. Fuel Pressure Regulator – It is a device that is on the end of your fuel rail. On most units, there is a vacuum reference line that is connected to the intake manifold. As vacuum pressure drops (nearing towards atmospheric pressure), a spring inside the fuel pressure regulator tightens against a diaphragm which will start to restrict fuel from returning to the fuel tank. As it restricts fuel, it is also raising the pressure inside the fuel rail. Fuel pressure rises and drops according to the vehicles condition, whether it would be idle, cruising or wide open throttle. This item is only found on return fuel systems, NOT return-less fuel systems.
7. Fuel Return Lines – Carries fuel back to the fuel tank which the cycle of fuel starts over again. Once again, this item is only found on return fuel systems, NOT return-less fuel systems.
When should I change my fuel injectors?
You only change your fuel injectors when they are starting to exceed a certain duty cycle. The industry standard is 80% - 85% duty cycle maximum that a fuel injector should flow. Anything higher, you run the risk of overheating from the kinetic energy that they produce which will cause them to not open/close efficiently or just plain failure.
What is duty cycle?
Duty cycle is referring to the time that an fuel injector is open divided by the time that it can possibly be open during two complete engine revolutions.
Ok, I want to build up my car but I’m not sure what size fuel injectors to get. How do I find out what size I need?
This is a very common question asked. First, understand the point of changing your injectors is to maintain a certain demand of fuel to your engine from your fuel system as efficiently and reliably as possible.
When shopping for fuel injectors, understand that fuel injectors are also rated by fuel pressure. Fuel is being sent as high amounts of pressure which are typically measured by PSI. Lets use for example, you find fuel injectors that are 42 lbs/hr rated at 40 PSI of fuel pressure. Your car might run it’s highest at 50 PSI of fuel pressure. If you were to put these 42 lbs/hr injectors are going to be spraying more lbs/hr than the original rating just because of the increase in fuel pressure on your vehicle compared to their original rating. This might or might not what you would be aiming for depending on what you’re using to control the injectors throughout idle, cruising speeds and wide open throttle but this is what you would have to shop for.
NOTE: Industry standard for fuel injector rating is about 45 PSI of fuel pressure (3 BAR of fuel pressure)
Now, what size do you need? There is a very good calculation that you can use to give yourself a good accurate answer:
Flow Rate = (Horsepower x BSFC) / (# of Injectors x Max Duty Cycle)
Ok, you’re probably thinking “what the hell is BSFC?”. BSFC stands for brake-specific fuel consumption which are rated in pounds per hour. The average for naturally aspirated engines is about 0.45 and for turbocharged engines about 0.55 at full throttle (It could normally be anywhere from 0.4 to 0.6). Like said, these are estimations but it will give you a good idea.
Lets use an example:
Say you have a 2003 Pontiac Sunfire with a 2.2 ECOTEC and you’re looking to turbo-charge it but you want to know the proper size injectors to get. Your goal is 300 Horsepower (flywheel horsepower). It’s a 4 cylinder engine, so it has 4 injectors and the max duty cycle I’m looking to ever go to is 80%. So lets do the math…
Flow Rate = (300 HP x 0.55) / (4 x .80)
Flow Rate = (165) / (3.2)
Flow Rate = 51.5625
So I just figured out that if I wanted an estimate of 300 flywheel horsepower and don’t want to exceed 80% duty cycle, I would need fuel injectors that are about 51.5 lbs/hr and this is also understanding 51.5 lbs/hr at whatever given rating of the fuel pressure.