I am currently designing my fuel system and have some thoughts that I haven't seen discussed in the copious amount of previous FUEL and AFPR threads. This also is the reason I am hoping some type of Sticky can come out of this eventually to prevent future threads and because I know I am lazy and hate digging through subscriptions for info I found previously.......so
There are 3 major EFI fuel system designs
1. Returnless
The Fuel Pump output is controlled (usually by an ecu and pwm) and a sensor at or near the rail provides feedback. Fuel pressure is maintained fully by the fuel pumps duty cycle and adjusted to maintain the desired value observed by the sensor at the rail.
Pros: Simple system in terms of fuel lines/returns and regulator. No excess fuel heating or excess power wasted by the pump: fuel is supplied on Demand. Also less fuel evaporation for same reason. Claims as good or better fuel pressure consistency than a manual regulator.
Cons: Potential for Lag: While the sensor can sample fuel pressure extremely fast there is a theoretical potential for lag between when the PWM's duty cycle % is increased > pump speed reacting > pressure increased through the line to the rail. Corvette's and other OEM performance lines are using this technology but I am unsure of current forced induction applications.
2. Return Regulated
Factory Configuration*
The Fuel pump supplies fuel directly to the rail at one end. At the opposite end is a return line which goes to the regulator which effectively acts as a dam. The regulator maintains the desired pressure and bleeds off any excess via a return line which routes back to the tank. Can be used in conjunction with pump control to minimize the amount of work the pump is doing and consequently the amount of fuel being returned at idle and low load conditions. (Factory system uses a hi low load relay triggered by the ecu which forces the current through a .73 ohm resistor to limit pump speed.)
Pros: Max fuel delivery from the pump is provided straight to the rail. This design has the lowest potential for inadequate fuel supply due to a sudden increase such as low load > WOT & high boost.
Cons: Excessive fuel heating occurs as pump is running at a high output (epecially if supplied constant 12V) and heat generated is transferred directly to fuel in tank. Excess fuel supplied is further heated as it passes through the hot rail just off the top of the engine and back to the tank.
3. Supply Regulated
The Fuel Pump supplies fuel via supply line to the regulator which then bleeds off pressure above the desired setpoint. This set pressure is delivered to the rail. This design can also be used with some type of pump control to limit over supply conditions and excessive fuel being returned at idle and low load situations. Used in some OEM applications and sometimes with the regulator at the rear next to or mounted in the tank for a near returnless system.
Pros: No electronic lag issues like a returnless system due to the pump supplying a higher than needed volume to maintain the desired pressure. Reduced fuel heating compared to a return regulated system as the excess fuel that is returned does not pass through the rail.
Cons: Potentially less reactive design compared to return regulated as fuel is limited before reaching the rail. Rely's on regulator reaction to rapid increases in fuel demand.
Other Considerations
Reaction: Obviously this is very important, if your fuel pressure dips even slightly due to not being able to adjust to demand a lean condition can occur and destroy an engine.
Fuel Heating Something I don't see discussed much on this forum compared to others. Warmer fuel = less dense = leaner for given quantity. This is not so much a concern for a dedicated drag car with short runtimes but for a street car or autocross car which encounter long run periods it can be a serious issue. IJ for one has commented on encountering AFR's shifting leaner after periods of run time and not being able to pinpoint or explain it until he felt his tank after a long drive one day and it was HOT. AFRs shifted lean across the board could also lead to a lean condition under high load and if severe enough be just as dangerous as inadequate reaction.
FPD Fuel Pressure Dampener* This little part is often chucked but is included on virtually all OEM systems and for good reason. The best discussion I have seen can be found HERE
If built out for a sticky I would also include common info I have compiled including: fuel rail fitting sizes, return and supply line sizes, common regulators and controllers ect which comes up quite frequently
There are 3 major EFI fuel system designs
1. Returnless
The Fuel Pump output is controlled (usually by an ecu and pwm) and a sensor at or near the rail provides feedback. Fuel pressure is maintained fully by the fuel pumps duty cycle and adjusted to maintain the desired value observed by the sensor at the rail.
Pros: Simple system in terms of fuel lines/returns and regulator. No excess fuel heating or excess power wasted by the pump: fuel is supplied on Demand. Also less fuel evaporation for same reason. Claims as good or better fuel pressure consistency than a manual regulator.
Cons: Potential for Lag: While the sensor can sample fuel pressure extremely fast there is a theoretical potential for lag between when the PWM's duty cycle % is increased > pump speed reacting > pressure increased through the line to the rail. Corvette's and other OEM performance lines are using this technology but I am unsure of current forced induction applications.
2. Return Regulated
Factory Configuration*
The Fuel pump supplies fuel directly to the rail at one end. At the opposite end is a return line which goes to the regulator which effectively acts as a dam. The regulator maintains the desired pressure and bleeds off any excess via a return line which routes back to the tank. Can be used in conjunction with pump control to minimize the amount of work the pump is doing and consequently the amount of fuel being returned at idle and low load conditions. (Factory system uses a hi low load relay triggered by the ecu which forces the current through a .73 ohm resistor to limit pump speed.)
Pros: Max fuel delivery from the pump is provided straight to the rail. This design has the lowest potential for inadequate fuel supply due to a sudden increase such as low load > WOT & high boost.
Cons: Excessive fuel heating occurs as pump is running at a high output (epecially if supplied constant 12V) and heat generated is transferred directly to fuel in tank. Excess fuel supplied is further heated as it passes through the hot rail just off the top of the engine and back to the tank.
3. Supply Regulated
The Fuel Pump supplies fuel via supply line to the regulator which then bleeds off pressure above the desired setpoint. This set pressure is delivered to the rail. This design can also be used with some type of pump control to limit over supply conditions and excessive fuel being returned at idle and low load situations. Used in some OEM applications and sometimes with the regulator at the rear next to or mounted in the tank for a near returnless system.
Pros: No electronic lag issues like a returnless system due to the pump supplying a higher than needed volume to maintain the desired pressure. Reduced fuel heating compared to a return regulated system as the excess fuel that is returned does not pass through the rail.
Cons: Potentially less reactive design compared to return regulated as fuel is limited before reaching the rail. Rely's on regulator reaction to rapid increases in fuel demand.
Other Considerations
Reaction: Obviously this is very important, if your fuel pressure dips even slightly due to not being able to adjust to demand a lean condition can occur and destroy an engine.
Fuel Heating Something I don't see discussed much on this forum compared to others. Warmer fuel = less dense = leaner for given quantity. This is not so much a concern for a dedicated drag car with short runtimes but for a street car or autocross car which encounter long run periods it can be a serious issue. IJ for one has commented on encountering AFR's shifting leaner after periods of run time and not being able to pinpoint or explain it until he felt his tank after a long drive one day and it was HOT. AFRs shifted lean across the board could also lead to a lean condition under high load and if severe enough be just as dangerous as inadequate reaction.
FPD Fuel Pressure Dampener* This little part is often chucked but is included on virtually all OEM systems and for good reason. The best discussion I have seen can be found HERE
If built out for a sticky I would also include common info I have compiled including: fuel rail fitting sizes, return and supply line sizes, common regulators and controllers ect which comes up quite frequently
Cheers for the good info.