[OpenFlash]

Hi guys,
For quite some time now, we’ve been quietly determining the limitations and weaknesses of the n54’s fuel system. Since it’s very different from the conventional fuel systems found in other cars, there was a lot to test and even more to learn. And much of it was surprising! Looking at things from a tuning perspective, the need to evaluate the limitations of the factory fuel system were even more of a priority considering that even the mildest of tunes results in 60hp gains with no associated hardware/component changes.

Understanding BMW’s Direct Injection Fuel System

Anyone familiar with conventional fuel injection systems will find some comfort in knowing that our BMW fuel system is actually quite traditional. At least the first half of it is. It consists of 6 components that play key roles:

Low Pressure Fuel Pump
Low Pressure Fuel Pump Controller
Fuel Pressure Regulator
Fuel Pressure Sensor
High Pressure Fuel Pump
Fuel Injectors





Low Pressure Fuel Pump (LPFP)


As with other conventional fuel injected applications, this assembly sits submerged in the fuel tank. It is accessible through an airtight lid which is located under the rear passenger seat cushion. The main function of this device is to pump fuel from the fuel tank, under the car, and to the High Pressure fuel pump which is then used to pressurize the fuel rail which distributes fuel to the fuel injections. But there’s a bit more going on behind the scenes. First, the pump sits in a bucket. And this bucket is spring loaded to sit firmly pressed against the bottom of the tank. This bucket performs the roll of ensuring that the fuel pump’s pick-up stays submerged in fuel. Even when the fuel level is low or even during hard cornering where fuel sloshes to the side of the tank. In fact, even when the fuel tank is nearly empty, you will find that the bucket is often completely full. Without this function, the fuel pump would suck in air bubbles which can be stressful for pump internals and even cause engine running issues such as misfire.


Those familiar with more motorsport focused fuel systems know that such system often employ two (or more) fuel pumps. One main pump and one (or more) smaller “lifter” or “transfer” pump(s) which simply move fuel from the corners of the fuel tank into the bucket houses the main pump. Miraculously, the low pressure fuel pump assembly in our BMW accomplishes all these important functions with just 1 pump. This eliminates the need for additional wiring, heavier duty electronics, more plumbing and more pump hardware. This is cleverly accomplished by a small secondary pressure port in the low pressure fuel pump. This secondary port is located at the bottom of the bump body. It’s internally restricted so only a relatively small amount of fuel is pushed out of it. This pressurized stream of fuel is routed through a fancy-looking plastic fitting which is mounted at the bottom of the bucket, right over a one-way flapper door. In this fancy fitting is a Venturi jet (Suction jet pump in BMW speak) which increases the velocity of the fuel (by gradually reducing its diameter). Through the wonderful world of physics, this higher velocity stream of fuel is used to create a localized low pressure zone which is used to pull fuel upwards (against gravity) through the one way flapper door and into the bucket. Without this mechanically generated low pressure zone, the fuel level in the buck would be equal to the fuel level of the fuel tank which would be unfortunate when the fuel level of the tank is low.

With these systems in action, the level in the bucket is high enough to keep the fuel pump pick-up fully submerged. Once the fuel is pushed out of the bucket by the low pressure fuel pump, it is routed towards the engine bay. But not before it passes through the fuel pressure regulator.

Fuel Pressure Regulator (FPR)

This device is located on the other side of the tank, under the driver’s side rear seat cushion. It’s job is to ensures that the entire low pressure fuel system (from the outlet of the low pressure pump to the inlet of the high pressure fuel pump is pressure regulated to a steady 5 bar (72psi) of pressure. The excess fuel that is bypassed by the regulator is then sent back to the other side of the fuel tank and dumped back to where it is needed most-- back into the bucket. But during its return trip it is pushed through another Venturi jet fitting, this time dragging fuel from the other side of the saddlebag-shaped, over the hump in them middle, and back into the fuel pump bucket. Which means that not only does the bucket stay filled, its side of the fuel tank is always at a higher level than the other side.

Low Pressure Fuel Pump Controller (EKP)


As you would expect, turning on the fuel pump in the BMW is a bit more complicated than using a simple $5 fuel pump relay to activate when the engine is running. Instead of turning the pump on or off, the Low Pressure Fuel Pump Controller (EKP in BMW-speak) constantly changes how hard it is driving the pump based upon engine demands. It does this by constantly adjusting the pump voltage between 3 and 11 volts. The higher the voltage, the faster the turbine in the fuel pump spins and the more fuel volume it pumps.

Traditionally, such fuel pumps only had one speed which was ON (full voltage). Which means that it was always pumping at full speed. Which meant that the fuel pressure regulator played a bigger role in regulating fuel system pressure. This mean that during conditions of low fuel demands (ie, low load, cruise, idle, etc,.), most of the fuel that was pumped by the fuel pump was bypassed by the FPR and routed back to the fuel tank. The downside of this is that you had a bunch of extra fuel flowing needlessly. Not only does this heat up the fuel, it also results in excessive vaporization which contributes to increased fuel tank emissions. So over time, most manufactures implemented a two stage fuel pump control circuit. So that at periods of low fuel demand, the pump would be driven at just 5-6v. And then at a full 11-14v when the engine is under load. Of course, leave it to BMW to take this one big step further by constantly varying the fuel pump voltage (by PWM control) based upon feedback it gets from the DME which is constantly monitoring the fuel pressure through a fuel pressure sensor mounted at the end of the low pressure fuel system (just upstream of the HPFP). In fact, the EKP modules acts much like a conventional boost control system. But instead of PWM controlling a boost control solenoid based upon boost pressure, it PWM controls the pump based upon fuel pressure. Clever! Which means that the pump only flows close to what the engine needs, reducing the amount of fuel that is returned to the fuel tank by the FPR.

High Pressure Fuel Pump (HPFP)


The most criticized component in our car. For something so finicky, it’s role is actually very straight-forward: To accept the fuel that is being fed to it’s inlet (regulated to 72psi) and to “step it up” to 1000-4000psi at its outlet in the fuel rail. As you would expect, this takes quite a bit of power to do this (probably a few HP). Which is why the HPFP is driven directly off of the engine and not fully powered by an electrical control system. Unlike the low pressure fuel pump, this pump's speed cannot be controlled based upon demand as it’s always spinning at a fixed ratio with engine RPM. Which means that another device must be used to regulate fuel rail pressure. And this is where the fuel supply control valve comes into play. Unlike an FPR which regulates fuel system pressure by controlling the rate of fuel return, the supply control valve regulates fuel rail pressure by controlling the amount of fuel going into the HPFP inlet. This valve is controlled by the DME which monitors fuel pressure through a sensor mounted in the fuel rail. The DME establishes the fuel pressure target from internal calculations based upon calculated cylinder pressure, engine speed, etc. All things equal, the higher the cylinder pressure, the higher the fuel rail pressure. This direct relationship ensures that the Fuel Injector “squirts” travels far enough into the combustion chamber during the compression stroke to initiate the desired optimal burn. The more boost pressure, the higher the cylinder pressures. The higher the cylinder pressures, the higher the desired fuel rail pressures. Pretty simple. Mapped properly, this results in better fuel economy, lower emissions and more engine torque.

More Thoughts
So all this begs a few question which we hope to answer in depth in Part 2 of this tech series: What is up with all these failed HPFPs? Are they really that poorly designed/built? Or are other issues contributing to their failure? Or are some of these HPFP failures are being misdiagnosed?

BMW has had many attempted fixes for this ongoing problem. First, they just replaced HPFP units. After this didn’t address the problem, they redesigned the pump. A few times. Then, they reprogrammed the DME so that it would “prime” the low pressure fuel pump system to ensure full pressure (72psi) at the HPFP inlet (before it started to spin) which suggests that they believed that pump cavitation may have been a potential cause of failure/wear.


Next in Part 2.... Bench testing results are in... Stock vs UPGRADED Fuel System Show Down

[Dr. G]

[topspin]

Great work!

[themyst]

Upgraded fuel system? Please do tell!

[possible335i2008]

Finally!!!! This is going to rock...don't leave us in suspense to long

[creaminz]

Team Vishnu rocks!!!

[francisomar]

You guys are the best, that's why I am committed to buy your products as soon as I end the exterior mods! You Rock!

[Slidewayz]

I'm waiting for Part 2 with bated breath !

[EDM92]

First page!

[TMR]

So are you saying, modify the voltage to the rear pump, put in a higher volume pump in the rear tank and away we go?

[GQ335i]

Great work and write-up Shiv! Excited to see what's to come!

[Brey335i]

I called it!

Anyways, way to tease us and now we have to wait until part 2 to see the effects of your work. Sometimes I do hate you

[KyLu II]

good read and good to know.
now about that meth kit....

[Brey335i]

Originally Posted by KyLu II

good read and good to know.
now about that meth kit....

He already said. Meth kit won't be released until after "something else"...

I believe you just read about the "something else"

[OpenFlash]

Originally Posted by KyLu II

good read and good to know.
now about that meth kit....

We really had to develop a proper fuel system upgrade before we released our PWM meth kit. We didn't want people to use our meth kit as a means to "fix" an inadequate fuel system. But rather as a method to raise octane and reduce IATs.

Shiv

[Brey335i]

Originally Posted by shiv@vishnu

We really had to develop a proper fuel system upgrade before we released our PWM meth kit. We didn't want people to use our meth kit as a means to "fix" an inadequate fuel system. But rather as a method to raise octane and reduce IATs.

Shiv

Any estimated ETA on when we will receive part 2?

[135forthewin]

Props to Shiv and Vishnu for taking the initiative and effort to give us an upgraded fuel system!

[gian335]

this looks good...great job shiv, hope theres much more to come

[Mahhvinn]

Great work Vishnu Crew! Definitely subscribing to this!

[enrita]

Now this is something to look forward to especially for upgraded turbos guys!
Great work Shiv!

[EastBayE90]

Great work can't wait for part 2

[McMuffin]

Much appreciated