This guide captures ‘bygone’ advice on HOW DO FUEL PUMPS WORK ON A CLASSIC CAR.

Useful information for Classic Car and Retro Car enthusiasts.

The fuel pump is a vital link in the fuel system – it gets petrol from the tank to the carburettor. It is also a common cause of engine trouble, so it is as well to know how it should work in case you find that some­thing is wrong. 

For safety, the petrol tank is normally located as far from the engine as possible. In a front-engined car, for example, the tank is generally found underneath the boot. The reason for this is clear; if the car is involved in an accident and the tank splits it is vital that the petrol should be well away from the hottest part of the exhaust pipe and the live electrical wiring, as either could set the fuel alight.

With the tank well away from the engine it is necessary to pump the fuel up from the tank to the carburettor, and this is what the fuel pump does. The pump can be located almost anywhere, but the three common positions are on the engine itself, next to the fuel tank and inside the boot. 

There are many different types of pump, though they can be divided into two categories, mechanical and electrical.

Mechanical fuel pumps 

Mechanical pumps are always bolted to the side of the engine and are driven by the camshaft. Although they are of several makes-AC Delco, Autolite, Carter and so on­ all operate on similar principles.

The AC Delco pump, for example, is made in two halves, with the top part containing the inlet and outlet valves for the petrol. The lower half holds the pumping gear and clamped between the two is a diaphragm. 

A cut-away view of an AC Delco mechanical fuel pump
A cut-away view of an AC Delco mechanical fuel pump

The pump is operated by a lever in the base. This bears directly on an eccentric (pear-shaped) lobe on the camshaft, so that as the camshaft turns it rocks the lever. When the camshaft lifts the lever, this in turn pulls down on the rod that links it to the diaphragm (shown below).

AC Delco Fuel Pump Diaphragm Diagram
A simplified drawing showing how the mechanical pump works.
The arm has risen on the cam. so the diaphragm is pulled down

The diaphragm flexes and draws petrol into the main chamber of the pump. As the lever runs on to the back of the lobe, the pull on the diaphragm is relaxed. The spring underneath the diaphragm then pushes the diaphragm back into the chamber, and forces the petrol into the pipe that leads to the carburettor (see below). 

AC Delco Fuel Pump Cam Action

To make sure that the diaphragm movements pump the petrol in one direction only, there are one-way valves in the inlet and outlet pipes. These open and close in response to the pressure of the fuel in the pump. When the diaphragm descends, its sucking action closes the outlet valve, but opens the inlet, thus allowing fuel to enter. As the diaph­ragm rises again the pressure of the fuel closes the inlet, but forces open the outlet.

If the pump were simply driven by the engine, the pumping rate would depend on the engine speed. But there are times when this would produce unwanted fuel pressure, which could damage the carburettor. For example, when the car is descending a steep hill in low gear, the engine is revving hard but very little petrol is needed. To get over this problem, the lever driven by the engine only pulls down the diaphragm-raising it again, thereby pumping the petrol, is left to the spring. 

The spring beneath the diaphragm in the AC Delco pump is strong enough to raise the diaphragm only when the pressure of fuel in the pipe to the carburettor is re­latively low. Therefore, when the float chamber in the carburettor fills and the needle valve closes to prevent fuel entering, the pressure in this pipe rises and becomes too great for the spring. The lever is then able to rise and fall, leaving the diaphragm down until the float falls again.

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Electric fuel pumps – diaphragm type 

There are several types of electric fuel pumps, the first of which is in some respects similar to the mechanical pump in that it also pumps fuel by means of a diaphragm. The major difference, however, is in the way the diaphragm is flexed. The most common electrical diaphragm mechanism includes a solenoid, a pair of points (not unlike those in the distributor), and a spring whose job is similar to the one described above. 

SU Butec Electric Fuel Pump Cut Away
A cut-away drawing of an SU Butec electric fuel pump

The solenoid consists of a steel plunger inside copper wire windings. When electric current flows in the windings, the magnetic field created draws the plunger downwards. The diaphragm is attached to the plunger, so as the plunger descends the diaphragm flexes and sucks in fuel from the tank (see below). 

A simplified view of an electric fuel pump.
A simplified view of an electric pump. Here the plunger
has descended, flexing the diaphragm and opening the points

When the plunger reaches the bottom of its stroke a rocker attached to it pushes the points apart (as can be seen above), thus breaking the circuit.

The spring then takes over, and forces the diaphragm and plunger back up. The movement of the diaphragm pushes petrol out of the pump body and along the pipe to the carburettor. As the spring raises the plunger the points come back together and the process begins again. 

SU Electric Fuel Pump simplified sectional diagram
The plunger then rises again, the diaphragm is flexed and
petrol forced through the valve and into the carburettor

As in the mechanical pump design, the spring is relatively weak and cannot raise the diaphragm against the pressure of the petrol in the fuel line when the carburettor is full. The diaphragm therefore remains down and the points apart until the engine uses the surplus fuel and relieves the pressure.

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su fuel pump


Electric pumps – centrifugal type

The centrifugal pump is a second, though less common, example of an electric fuel pump, found in a few older cars. 

It differs from the other design in two ways. First, it is essentially an electric motor coupled to a small centrifugal pump. Fuel is allowed into the pump through an inlet port, and a constantly running impeller forces it out through a delivery tube. 

The second major difference lies in the fact that the pump is located inside the petrol tank, below the fuel level. This is obviously a dangerous place for any electrical equipment, and the pump is heavily insulated. The wires carrying power from the battery are enclosed in thick plastic tubes and the point where they enter the body of the motor is sealed by a heavy rubber gland. There is also a gauze flame trap between the motor and the pump.

Pumps for petrol injection systems 

Remember folks this is a ‘bygone advice guide’ so not a lot to say on Fuel Injection in those days.

Petrol injection systems are usually fitted with electric fuel pumps. The immediate difference between these and the other pumps described is that those designed for use with PI must be very much more powerful. The Lucas Mark II system pump, for example, delivers fuel at lOO psi (just under 7 bar), while most other pumps operate at only l/50th of this pressure. 

Bosch PI pump 

The Bosch petrol injection fuel pump is one type which uses centrifugal force. In fact it has a remarkable design for its day, with the electric motor itself totally immersed in petrol­ as a safety feature! 

Inside the casing are the working parts of the electric motor and a rotary pump. This pump consists of a rotating disc with rollers at its periphery. It is eccentrically mounted in its casing, but the rollers are free to move outwards to run along the inside of the casing. As the motor spins, centrifugal force flings the rollers outwards so that they sweep up the petrol trapped in the space between the roller and the casing. 

Simplified cross section of a Bosch centrifugal fuel pump.
A cut-away view of the Bosch centrifugal petrol injection
pump. The roller disc forces fuel through the casing, the
petrol also acting as an effective insulator for the motor

Since the motor has to be close to the pump, it would be possible to use a heavily insulated motor separate from it. Even so, there would be a risk from sparks igniting a fuel leak. What the system does is to use petrol itself-which does not conduct electricity-as the insulating medium. The motor, including brushes and commutator, spins inside the fuel enclosure. As long as the casing remains full of fuel there is no danger, since petrol needs a supply of oxygen before it will ignite. The pump itself is inefficient at pumping air, so there is little chance of an ignitable mixture being present.

It is important that the fuel is kept at the correct pressure, and on occasions the pump raises the pressure too high. The pressure regulator is therefore an essential extension of the pump. There are· three pipes connected to the re­gulator. Fuel flows in from the pump through one, out to the metering unit (see page 402) through another, while the third carries excess fuel back to the petrol tank. The base of the third pipe is sealed, inside the regulator, by a diaphragm beneath which is a spring.

When fuel is passing through the regulator at normal pressure the diaphragm is undisturbed, but when it exceeds the desired pressure the diaphragm will flex and the excess fuel will escape up the third pipe. This will reduce the pressure and the diaphragm will reseal the pipe. The normal operating pressure of the regulator diaphragm is 32psi (2.2 bar), but this can be raised or lowered by the adjuster at the base of the unit.

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Lucas Mark II PI fuel pump 

The pump fitted to the Lucas Mark II PI system fulfils the same purpose as the Bosch design, but differs in some details. The pump itself is of the twin gear type and is driven by an electric motor-in this case, heavily insulated. As the gears mesh they force the petrol through to the metering unit at the required pressure. The Lucas pump also has a pressure relief valve similar in construction to the Bosch model above. 

A further feature of the Lucas pump is the large petrol filter. The pressure of the fuel throughout the PI system makes it extremely important that all traces of dirt be removed from the fuel, so a large filter is added to . the system. It is an unusually complex filter, the petrol being forced through a series of paper coils before reaching the main pump.

The LUCAS Petrol Injection Pump
The Lucas petrol injection pump. A section has been removed
right across the pump to show the position of the twin gears.
The fuel outlet is behind the gears, on the pump’s far side

Racing fuel pumps – often achieved with two or more pumps

The requirements of racing make special demands on fuel pumps, and the equipment found on many racing cars is a good deal more sophisticated than that on the family saloon. 

Most Formula One cars, for example ( in those days) were equipped with two pumps, one electrical, the other mechanical. The electrical pump (which is often a modified Lucas PI unit) supplies fuel in the lower range of engine speeds and the mechanical pump takes over as speeds rise. The reason for this is that Formula One cars have the ultimate in power to weight ratios, and the extra weight of a battery needed to drive an electric pump throughout a Grand Prix race would make a significant difference to the overall load on the car. 

A mechanical pump needs no battery and the size of the battery needed for low speeds only can, as a result, be kept down. The reason a mechanical pump is not used to feed the engine all the time is that it would over-pump in the lower speed range and cause additional complications in the fuel injection system.

Below is an interesting photograph by Robert M Cooper showing and describing a once well known set up.

Twin fuel pumps on Rob Turnbulls Formula 2 hillclimb car.
Two of the three fuel pumps used on Rob Turnbull’s Formula 2
hillclimb car. The pumps in view are a Lucas high pressure unit
and a low pressure electric pump. Both are located beneath the
driver’s seat. The third pump is mechanically driven and is
mounted on the engine block. This car had a Hart engine of only
2 litre capacity, but was capable of producing well over 300 bhp