Fuel Management Units (also called Rising Rate Fuel Pressure Regulators)


Summary:  Fuel pressure is a factor in fuel flow rate.

A fuel injected engine needs to manage the rate at which fuel flows into the cylinders, so that it can control the air/fuel mixture. This is critical because the ratio of air to fuel determines how well the fuel burns, or if it burns at all. The rate at which fuel flows through an open fuel injector depends on the size of the opening in the injector, and the difference in pressure on either side of the injector. If, for example, there was 50 psi of air pressure on one side of the injector and 50 psi of fuel pressure on the other side, then nothing would flow through the injector. Fuel injectors are rated according to their flow at a standard pressure differential (~39 psi), and they are designed to have the best spray pattern and fuel atomization (both important for good mixing with the air) at this pressure differential. Note that at idle the air pressure is typically around 3-4 psi, so the fuel pressure will be around 42-43 psi.

The rate at which fuel will flow through a given injector at different pressures is related to the standard rate by the square root of the ratio of the pressure differential. Thus if you double the fuel pressure differential the flow rate will increase by about a factor of 1.414. Conversely, cutting the pressure in half will cause the flow rate to decrease by a factor of 0.707. As the pressure increases or decreases to much and the spray pattern is degraded the effective flow rate will decrease because the fuel will form droplets and not mix with the air properly. This graph shows the theoretical flow rates due to this basic relationship.

Summary:  How to calculate the fuel pressure that an FMU will deliver.

A Fuel Management Unit (FMU for short) is typically described by a ratio like 10:1. This means that for every 1 psi of air pressure in the intake manifold, the device will increase the fuel pressure by 10 psi. It does this by restricting fuel flow back to the fuel tank, causing the fuel pump to build up pressure against the restriction. The FMU's pressure rises from zero psi at 1 atmosphere of air pressure in the intake manifold. Zero psi of fuel pressure is not very useful, but remember that the fuel system also has a normal Fuel Pressure Regulator (FPR, all cars have one stock). An FPR is a similar device, but it raises the fuel pressure by 1 psi for each 1 psi of air pressure in the manifold -- starting at about 38 psi. The overall fuel pressure in the system is determined by the greatest restriction, so at low (or no) boost levels the fuel pressure is determined by the FPR since the FMU will be causing only a small restriction. At some point of increasing boost, however, the FMU will become more of a restriction than the FPR, and it will cause the fuel pressure to increase above the level that the FPR would normally cause. The result is shown in the graph below where FMUs of various ratios are overlayed on a stock FPR's normal pressure curve.

Summary:  Fuel Management Units (also called Rising Rate Fuel Pressure Regulators) are not a good mechanism to precisely control a boosted engine's A/F mixture.

The FMU attempts to control the injector flow rate independently of the engine management computer by adjusting the fuel pressure. It does this by varying the fuel pressure in a linear fashion based on the positive manifold pressure. Unfortunately the flow rate is related to the pressure according to the square root of the pressure ratio, so the FMU is trying to control a linear variable using a non-linear mechanism. Further, boost is not the proper value to measure because the A/F mixture is based on the air and fuel mass, not pressure. As air temperature increases, its density decreases which means that for a given boost level at a higher temperature, the air mass is lower. Since compressors can dramatically heat the intake air this can be a significant effect (albeit one which can be somewhat controlled by an efficient intercooler). There are other, more subtle effects at work as well, like the heating of the fuel due to being pumped repeatedly through the system at higher pressures.

As mentioned above, the high fuel pressures caused by an FMU can cause problems with the spray pattern of injectors. This can cause hot spots in cylinders where fuel didn't mix well enough, resulting in lean conditions within the cylinder. As a practical matter, running with very high fuel pressures can be dangerous as it increases the chance of blowing off or bursting fuel lines. Maintaining the higher fuel pressures puts a strain on the fuel pump, and these pumps (typically designed to flow at 40-50 psi) lose their flow rate ability as the pressure increases. Typical pumps can lose half of their flow rate by 75 psi, and all of it by around 110-120 psi (according to data from Walbro). This gradual loss of flow ability can lead to bad lean conditions when you need it most -- at maximum boost.

The first graph shows approximately the theoretical relationship between boost and A/F mixture (fuel temperature and other more subtle factors have been ignored). As you can see it wanders significantly. The second graph shows the relationship between boost and fuel pressure, with the ideal relationship overlayed. You can see how the actual pressure varies significantly from the ideal (12.5:1 A/F ratio).