# Does electric fuel pumps run at constant or variable speed?

In websites I have seen that fuel pumps have a pressure range of say 30-95 psi. Why do they give a range of pressure? I had read that these pressure values refer to the unobstructed flow pressure delivered by pump.

I had read about pressure regulators having boost reference port. These ports are fed with an air line from intake manifold so that under vacuum/boost pressure generated, the boost reference pressure from manifold will control the amount of fuel bypassed by regulator to vary the fuel pressure in the delivery line (To increase the delivery line pressure we should be reducing the amount of bypass/return fuel amount right? Although this site tells that a more obstructed fuel line increase the fuel system pressure ). Thus fuel pressure regulation is done by boost referencing the pressure regulator depending on manifold pressure rather than pump varying its speed. And this is most commonly used system for delivering variable fuel demand.

But in an example calculation they mention that if the injector supplies fuel at an effective pressure of 45 psi when intake manifold pressure is at atmospheric pressure. Then if the engine is boosted to a pressure of 10 psi, without boost reference the effective pressure at which the fuel will be injected at injector will be 45-10=35 psi. When boost referenced, the by pass opening will be controlled such that the delivery line pressure is increased i.e. 45+10=55 psi. So ultimately the effective pressure at the injector will still be 45 psi. But the problem with this calculation is that even when engine is at load (since it is boosted) there is a higher fuel flow and probably higher fuel pressure requirement. So how does an effective injection pressure of 45 psi at injector will be enough under higher load?

I have also read that there are now variable speed pumps which vary the fuel pressure by varying the speed of rotation of the pump based on an electronic pressure regulator. But I think these are not still commonly used.

FPRs with a boost reference port are only used on vehicles with forced air induction (turbo/supercharger) applications to do exactly what you stated. The port tracks when boost is present in the system and increase fuel pressure to compensate. It is the easiest way to compensate for this.

There are two factors when introducing fuel into an engine through fuel injectors which matters for this conversation: fuel pressure; injector pulse width. Why do these matter?

• Fuel Pressure - As fuel pressure increases, more fuel is delivered through any given sized orifice. This is just straight physics and fluid dynamics. The converse is true as well. When you decrease the pressure. Less fuel is delivered. A simple way to make a smaller fuel injector act as a bigger fuel injector is to increase the fuel pressure. The change in fuel delivery is directly related to the change in fuel pressure. Here is a calculator you can play with to help you understand.
• Injector Pulse Width - Injector Pulse Width is just the amount of time an injector stays open. The longer the pulse width, the more fuel is delivered.

Please note that you can also change the amount of fuel injected into an engine by injector sizing. Why it is not important to this conversation is, because injector size is static. You can't make changes on the fly to an injector while it is installed. The only way you can change the amount of fuel delivered is through fuel pressure and injector pulse width.

Here is where your understanding goes wrong:

... the problem with this calculation is that even when engine is at load (since it is boosted) there is a higher fuel flow and probably higher fuel pressure requirement. So how does an effective injection pressure of 45 psi at injector will be enough under higher load?

The ECU of an engine has tables setup to know how much fuel to deliver to the engine at any given load, manifold pressure (whether it's under vacuum or boost), throttle position, etc., as well as modifiers to help it adjust for other factors which are outside the normal operating parameters. It does this by modulating the injector pulse width, not by increasing or decreasing the fuel pressure. In other words, no higher fuel pressure is required to deliver the amount of fuel needed at any given engine speed or higher load.

The reason injector pulse width is used to control the amount of fuel over fuel pressure is because the pulse width can be changed and managed much faster than can fuel pressure. The pulse width is measured in milliseconds. Then as the engine spins faster, the time between injector openings decreases as the amount of time the injector opens increases, further shortening the time the injector has to react. You could never do this as accurately with fuel pressure as you can with the injector itself. Since the injector has to open and close anyways, it makes sense to have a static fuel pressure (or near static ... it's never exactly the same) and vary the amount of time the injector stays open to compensate.

Boost referencing is an easy way to remove one of those variables so the ECU doesn't have to adjust for it and can continue to serve the engine as though there's no change at all to fuel pressure. It not only makes it easier for the ECU, but also for the tuner, whether that person is at the factory creating an OEM map or someone in the performance industry.

As for a variable speed pump I'm not aware of them being used, though that could just be my lack of knowledge. Also, the above is considering port fuel injection model and not direct fuel injection. In direct fuel injection, the lift pump provides a steady fuel pressure to the high pressure pump and the high pressure pump deals with any variance in cylinder pressure due to boost.

• A well written answer as always but a quick question, wouldn't it be good to have a higher injection pressure when engine runs at higher rpm as this helps with quicker atomization. Commented Dec 9, 2022 at 2:21
• Also, in case of Direct injection the FPR comes after the High Pressure pump right? And this also uses boost referencing? I have read that high pressure fuel pumps generate a pressure around 2500 psi which is very large compared to boost pressure generated by ordinary forced inductions system. Obviously under fine tuning boost referencing is good but with such large pressures in DI does it make a difference (If boost referencin is done)? Commented Dec 9, 2022 at 2:28
• @AbhishekPG - I really am unsure of how DI works with boost reference if at all. Commented Dec 9, 2022 at 2:40
• @AbhishekPG - Not a good idea to vary the fuel pressure at any RPM. As juhist stated, it creates too many variables which makes it a lot harder to account for ... besides, just because you speed up the engine doesn't mean it requires much more fuel per combustion event. You just have to fire the injector more often because you have a lot more combustion events per minute (or second, really). Commented Dec 9, 2022 at 2:47
• So DI doesn't use boost referencing? I thought if you inject fuel into cylinder, the fuel is still trying to overcome whatever pressure is within cylinder. So under boost you still have air being supplied at higher pressure into cylinder. So, I thought this increase in cylinder pressure due to boosted air might be overcome by boost referencing. Commented Dec 9, 2022 at 3:50

I think you are incorrect in that engine speed or load would dictate a variable fuel pressure. In fact: the opposite is true. An engine can be controlled easily only if the fuel pressure is accurately maintained at a constant value.

If you have constant fuel pressure, you can calculate amount of fuel injected from only the injector pulse width.

If the fuel pressure is variable, you have to have calibration parameters for every possible fuel pressure in the range where the fuel pressure might be. Much harder to maintain that calibration.

How fuel injection used to work is that there was a fuel pump near the fuel tank, a fuel line to the engine, a fuel pressure regulator that allowed fuel to go past it as long as the pressure increased past the setpoint, and a return line from the fuel pressure regulator back to the tank. If the engine didn't demand all fuel the pump can pump, then the regulator sent back some of the fuel to the tank. Control of injected fuel amount is by pulse width modulation: vary the injector pulse width, and you vary the injected fuel amount.

However, this arrangement had a problem. The engine bay is hot. If fuel is constantly circulating between the engine bay and the tank, heat is moved to the fuel tank. Hotter fuel increases evaporative emissions. Thus, the arrangement was changed that everything was near the fuel tank including the pressure regulator and return line, and there was only one fuel line to the engine bay.

For simplicity, I believe all fuel pumps are constant speed. They are specced to deliver the maximum amount of fuel the engine can need, plus a bit safety margin. If you see a range of pressures, I believe those indicate the setpoint pressures of the fuel pressure regulator the pump is capable of working with.

• But what is the significance of mentioning the lower pressure limit on a fuel pump. A higher pressure limit defines the capability of pump to pump up to that max pressure. A pump will be able to generate & work on any pressure below it right? Commented Dec 9, 2022 at 8:30