Fuel system layouts:
The basic EFI fuel system has a pump in the tank that feed the injectors that sit in a common fuel rail. After the fuelrail there is a fuel pressure regulator that regulate the pressure in the fuel rail by releasing fuel back to the tank. 3 bar bar over the intake pressure is the standard but the pressures on stock cars range from 2.5-4.5 bar.
A more complicated fuel system can be made with a Catch Tank (or swirl pot, surge tank or whatever they are called where you live ;) and if you drive on the right or wrong side of the road).
The Catch Tank is feed from the main tank with a low pressure pump.
Why would it be more effective than the intank pump? I see people applying it rather as a workaround. If one uses the orignal tank this is for keeping fuel supply to the motor during hard cornering, also it is cheaper to have low pressure hoses to the catch tank located in the engine bay and then use high pressure hose. But if one make a proper fuel tank the catch tank may not be needed
The pressure in the catch tank can be regulated in a few different ways:
- A deadhead pump (regulates it's own pressure) feed's the catch tank from the main tank. No return. The fuel temperature can get pretty high in a tank like this and it's a good idea to bleed the air from it at first startup.
- A deadhead pump (regulates it's own pressure) feed's the catch tank from the main tank. Return to main tank through small orifice.
- A pump (deadhead or normal) feed's the catch tank from the main tank. Unrestricted return to the main tank.
- A pump (deadhead or normal) feed's the catch tank from the main tank. A low pressure fuel pressure regulator on the return to the main tank.
The high pressure fuel pump takes it's fuel from the catch tank and normally the return line is also connected to the catch tank.
- The return from fuel rail should be connected directly to the main tank to avoid high temperatures in fuel (that happens when the fuel goes from catch tank -> pump -> fuel rail -> fuel pressure regulator -> catch tank) Not needed and not working good!!! the heated fuel is feed back to the catch tank, the fuel in the catch tank is circulated by the low pressure feed pump. The high pressure pumps often have MUCH more flow then the low pressure pump feeding the catch tank! (Jörgen)
Bosch in-tank pumps from CIS cars is always good and can handle really high fuel pressures. They are usually rated at 5bar. Walbro also has very good in-tank pumps. But always remember to check at which pressure the rated flow is specified.
The american race type pumps is really tricky and it's usually best to stay far away from them. Some are good and can run forever and some fail all the time. Some have very crappy flow ratings at high pressures too. I have seen an Aeromotive efi pump rated at 700hp chicken out at less then 500hp, when finally finding the flow rating it was 360lbs/hr@45psi. The stock pump in most CIS turbo audis is rated at 169l/h@5bar, that's something like email@example.com. If IRC the Walbro 255l/h flow like the bosch 169l/h pump.
Fuel pressure regulators:
The stock fuel pregulator usually work fine as long as it has a vacuum port (which means it is MAP referenced: constant pressure across injectors). The american race type regulators are usually easier to install if a custom fuelrail is used. Some of them fail once in a while and you should get one that can be rebuilt and get the rebuild kit too if you aren't located in the US.
Fuel pressure regulator can be
- atmosphere referenced: pressure across injector dicreases with higher MAP (not recommended)
- MAP referenced: pressure across injector remains constant across the MAP range (most common, and best in most cases)
- Rising Rate: pressure across injector increases as MAP increases (so the fuel pressure increases more than MAP !). Read the above 2 types carefully as well: the rising rate setup does not keep the flow the same (for a given injector pulsewidth and RPM) through the injectors when the manifold pressure changes.
TODO: curves: fuel pressure in function of MAP; pressure across injectors and flowrate (which is proportional to squareroot of pressure across injectors) for the 3 types
Rising Rate Fuel pressure regulators:
Normally RR-FPR-s are used by tuners who choose not to install a proper(ly configured) ECM, but stay with the factory ECM that cannot add extra fuel at boost.
Among users of easily configurable ECMs (like GenBoard) constant pressure across injectors and larger injectors are common.
But RR-FPR can also help maintaining a smoother idle for very high performance engines, where idle injector pulsewidth would be very small without lowering the fuel pressure. Injector staging would probably be a better way. Careful configuration is needed in any case, compared to the simple case.
As the RR setup should maintain (does it do fast?) a consistent (but not constant!) pressure across the injectors in function of MAP, the injection control table (j, which is appr. ~VE) can be tuned to consider this (the higher MAP values will be set smaller to compensate for the higher pressure across injectors).
What can make an RR-FPR setup even more sophisticated?
- measure fuel pressure: GenBoard/VerThree has input channel
- change fuelcalc to consider measured fuel pressure (minor firmware mod)
- optionally PWM the pump to control fuel pressure. Normally the pump is not PWM-ed but run at full voltage, and the regulator controls pressure; but there are tricky setups (GenBoard/VerThree has the output but firmware mod is needed)
Fuel temperature sensor:
My Range Rover has this sensor on the fuel rail. What is it use for? Taking the density into fuel account, or maybe monitoring the possibility of vapour locks?
Fuel rail stock can be bought from www.force-efi.com (I can only say good things about him.) and a few other places.
If you build fuelrails for V engines or for use with staged injection you should not connect the fuelrails in series. It's better to use a T to feed both rails at one end and use a three port regulator at the other end. This helps fuel distribution and minimize the pressure pulses in the fuel rail.
- Coated steel. Heavy and hard to work with. Cheap.
- Stainless steel. Lighter and easier to work with. Relatively expensive.
- Aluminium. Very light and very easy to work with. Similar cost to stainless. Some folks don't think it's suitable for EFI use due to the connections not being as robust. Not sure if this is accurate or not.
Most of the fuel system should be hard line where possible, as it's less likely to be damaged. It's also cheaper per meter than hose. Sizes can vary depending on application, but 3/8" is fairly common for street use.
- Generic fuel hose - DONT USE THIS!
- EFI fuel hose
- "Name brand" performance hose
- Stainless braided hose
Rubber "EFI" hose can seem insanely expensive but don't try to use regular carburettor hose. It's normally used around 5 psi, with perhaps a 500% margin on it? We will be running around 50 psi. If you need serious abrasion resistance, stainless steel braided hose can work out reasonably priced, or a rubber "performance" hose if you don't need that strength. Check your options, because I found that Russell -6 stainless braided hose was cheaper than Gates basic automotive 5/16 EFI fuel line. You should use flexible hose to connect your hard line to things that may move, like the engine and possibly fuel tank.
Fuel system connectors:
When creating a fuel system all the above parts needs to be connected to each other in a secure way. You probably don't want to your car burned beacuse of fuel leaks from bad connectios. Some commonly used types with some pros/cons are listed here.
- Change sealing washers once opened to avoid minor leaks
JIC / AN connectors:
- Easy to remove, no need to change sealing washers
- Uses a 37° sealing angle - don't try to use a 45° flare with these fittings unless you like leaks. Summit Racing has a cheap 37° flare tool http://store.summitracing.com/partdetail.asp?part=SUM%2D900311
- Sizes are in inches for JIC or "dash" sizes for AN. The dash size indicates the number of 16ths OD it suits. Eg, -6 AN is equivilant to 3/8" (6/16ths)
- Widely available in aluminium and stainless, also in nice colours :) that scratches off easily :(
- Unprofessional feel & look
- Cracks, leaks..
If you want to use them anyway, make sure you use EFI clamps that don't bite into the rubber hose. You can get clamps with a hose-end style cover too, but aside from looking pretty they are no better.
Other available types:
- Metric sized connectors. Adapters are available for AN style if required.
- Quick disconnect connectors. Expensive and probably better suited to racing.
- Others ?
See the PortInjected/FuelInjectors page
Fuel pressure accumulator:
The fuel pressure accumulator was used to allow the old CIS injection systems to maintain the system pressure until the engine has cooled down. Their deadhead injector layout made them very hard to start if the fuel vaporized in the injector feed lines. The fuel pressure accumulator is an pressurized air volume separated from the fuel with a membrane. Fuel pressure accumulators isn't normally used in a modern EFI fuel system but a few people I know who run batch fire has seen huge improvement when attaching a CIS fuel pressure accumulator to their fuel rails.
There are usually two filters in a modern EFI fuel system. One ~100 micron screen in the tank / before the pump, and a finer 10 micron filter between the pump and the fuel rail. Using a regular 10 micron filter before an inline pump is generally considered a bad thing. A fine filter is more likely to clog up and cause the pump to overheat or cavitate... YMMV.
There are a couple of expensive ($70 USD) EFI rated aluminium filters with replacable elements and AN fittings available, but keep in mind that most OEMs successfully use a clamp-on disposable filter.
Fuel pressure gauges:
These can be mounted directly to the fuel inlet with a 1/8 NPT T-fitting. Make sure it's got the range you need, with most EFI suitable gauges reading either 60 or 100 psi full scale. NOS has a nice red 100 psi unit :)
It's advisable not to route these mechanical gauges into your cabin. If something goes wrong, you've potentially got a 50 psi fuel leak behind your dashboard. If you want to monitor fuel pressure on the dash, get a gauge with an electronic sender.
Bosch motorsports sells high pressure fuel pumps and injectors as used in direct gasoline injected engines. Those fuel systems run @ +200bar. The injectors are controlled at 90v and 20Amps.
- 20A is beyond the limit of EC36 (14A). 2 pins would have to be used per channel. Some airwires needed to protect onboard traces from the extremely high current
- 90V is beyond the limit of the FET we currently use (easy to use other FET, since FETdrivers are there)
- there is no SMPS supply to make the 90V. SMPS is planned on an ARM based CDI board though.
Someone any experiants with building a high pressure fuel system?
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