Fuel Pump

Fuel Pump

Is this an electrical part or a mechanical part? You be the judge. It is on the chassis, so I put it under that category.

The fuel pump assembly (with an integral pressure regulator) resides inside the fuel tank. If you power the electrical system from a 12-volt battery in place of the capacitor, you can hear the fuel pump run for a second or two prior to a starting attempt. During this time, the fuel pump draws a current of approximately 1.5 amperes.

With the engine running, K-Scan reports the fuel pump voltage to be almost exactly 15 volts, whereas it may be only about 11 volts when powered via a battery. This “additional voltage” comes from a separate stator winding (discussed in the Stator section) and a DC-DC boost converter inside the ECU.

I have been told by a reliable source that the pump is a KTM unit and very similar to those used in the EFI Vertigo trials bikes. The pump itself is marked Made in China.

Fuel Pump Assembly, partially removed from bike. Note 3 small holes to admit fuel to the housing. There are 3 more on the opposite side.

Stator output from fuel pump winding (loaded).

Mahle KL97 fuel filter as used by OSSA

Fuel Pump Operation

There is a fine screen on the suction side of the pump to prevent debris from getting into the pump.

Note the position of the fuel filter. Fuel flows through it in the direction of the arrow and is constantly circulating through it (not just once through). That way, the outlet pressure is not affected by dirt in the filter. I am told that the dirt in the filter is primarily carbon residue from the DC motor's brushes.

There are two quick-disconnect fittings inside the fuel tank. A special tool (available at auto parts stores) is required to separate them. In the photo, an electrical connector is partially obscured from view by the male side of a quick-disconnect fitting.

The pressure regulator is not adjustable.

Fuel pump assembly with parts annotated


I was aghast when I first learned that fuel flows through the brushed DC motor inside an electric fuel pump. If you have ever seen arcing in a brushed motor, you would probably ask, “Can that be safe? What if air gets in there?” Apparently, it is safe. With bazillions of miles on automotive systems, I have never heard of a single fire starting in a fuel pump. My hypothesis is that, even if air did get inside, the mixture would be far too rich to burn.

Any flammable material has a minimum and maximum amount that can be mixed with air and still burn. These amounts are called the Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) respectively.

The stoichiometric air-fuel ratio for gasoline is about 14.6:1 by mass. Gasoline will only burn within a fairly narrow range of air-fuel ratios (from memory it is about 8:1 on the rich end and 22:1 on the lean end).

With very lean mixtures, there is just not sufficient heat produced to raise the temperature of the adjacent mixture to the ignition point. There is a lot of non-combustible gas in air (mostly nitrogen) that must be heated in order to raise the local temperature to the ignition point.

With very rich mixtures, the fuel itself must be elevated in temperature sufficiently to continue burning.

The pump is designed this way because the fuel cools the motor. That is why it is usually frowned upon to run an EFI vehicle completely dry, or even on very little fuel.

Credit: agcoauto.com Electric fuel pump cutaway view