9.4.
Fuel Pumps
The fuel pump transfers petrol from the tank to carburettor through a fine grain filter. The pump must deliver petrol in sufficient volume and pressure to keep the carburettor bowel full of clean petrol, regardless of vehicle speed or manoeuvring. While all pumps deliver fuel through mechanical action, they are generally divided into the following two groups.
(i) Pumps mechanically driven by car engine.
(u) Pumps electrically driven by an electric motor or solenoid.
9.4.1.
Mechanical Fuel Pumps
The most common type of fuel pump used in automobiles is the single action, diaphragm-type mechanical pump. A diaphragm type fuel pump is usually mounted on the engine and is operated by an eccentric lobe on the camshaft. The sectional view of a single diaphragm pump with its drive mechanism is shown in Fig. 9.12A. A spring-loaded arm is held against the eccentric lobe, containing it at all times. In some systems a short push rod is fitted between the eccentric lobe and the spring-loaded arm. The cam-actuated lever arm pulls the diaphragm from the fuel chamber side, increasing its volume and thereby drawing fuel from the tank. A spring on the lever side of the diaphragmpushes against the diaphragm as the cam eccentric lobe movement relaxes its pull on the arm
lever and diaphragm. This spring pressure on the diaphragm moves the fuel towards carburettor as shown in Fig. 9.12B. The fuel pump is fitted with two check valves. The inlet check valve allow petrol to flow into the pump, and the outlet check valve allows petrol to leave the pump. Thus the pump lifts the fuel from the supply tank to the pump by vacuum and forces it under pressure to the carburettor. The pressure at which the fuel is delivered to the carburettor depends on the tension of the diaphragm spring that actuates the diaphragm during delivery.
Fig. 9.12. Sectional view of a typical diaphragm fuel pump.
An A.C. mechanically operated fuel pump, which is very widely used in automobiles, is shown in section in Fig. 9.13.
Fig. 9.13. A.C. fuel pump.
When the carburettor float bowl becomes full of fuel, the carburettor inlet valve closes due to which pressure in the line leading to the pump increases. This back pressure is transmitted to the pump diaphragm due to which it remains in the lower position against the pressure of the diaphragm spring, and the pull rod links are lowered so that they are disconnected from the rocker arm. The situation continues until the fuel in the float chamber falls sufficiently to open the carburettor inlet needle valve, consequently relieving the pressure in the pump chamber to restart the system. In this way the supply of fuel to the carburettor is regulated meeting the requirement.
An air dome is normally incorporated with the outlet of the pump to minimise flow variations and also to increase pump output. The dome provides an air pocket which is compressed by the outgoing fuel causing reduction in pressure and consequently this compressed air supports fuel flow.
A bowl is installed before the inlet valve of some models of the pump. This acts as a settling chamber for the collection of water and other foreign materials, which cannot pass through the fine screen of the filter. The bowl needs periodic cleaning.
The double-action mechanical pump (Fig. 9.14) is consisted of a diaphragm fuel pump and a vaccum booster pump for vacuum-operated windshields wipers. The vaccum booster is installed on the top of pump. The booster provides vacuum for operating a vacuum-driven windshield wiper when the intake manifold vacuum drops under certain engine operating conditions. Both vacuum booster (upper section) and pump (lower section) of the total assembly are connected individually through links and rocker-arm break joints to a single rocker-arm, which follows the cam by means of a rocker-arm spring. The vacuum section linkage operates the vacuum unit diaphragm in a manner similar to operation of the pump. During upstroke, the diaphragm forces the air from the vacuum chamber to the intake manifold through its outlet valve and during down stroke the diaphragm is pushed down to create a vacuum in the chamber, which opens its inlet valve to draw air from the windshield-wiper inlet connection. The diaphragm operation takes place only when manifold vacuum is insufficient for operation of windshield-wiper. When manifold vacuum is more than that created by the pump, both the booster valves are forced open to produce a direct vacuum path through the pump chamber
between the manifold and the wiper motor, and the vacuum unit diaphragm is moved up disconnecting the link from the rocker arm.
Mechanical fuel pumps are quite dependable. If they break down, it is usually due to one of the following problems.
(i) A diaphragm that leaks.
(ii) Worn out inlet or outlet check valves. (Hi) A worn or broken push rod.
(iv) A worn linkage, which reduces the pump stroke.
Occasionally, the camshaft eccentric may wear sufficiently to reduce the pump stroke, or a bolt-on eccentric may become loose from the camshaft. In these cases, the camshaft or the eccentric must be replaced. It is also possible to install an electric fuel pump to bypass a defective mechanical pump.
Fig. 9.14. Double action pump.
9.4.2.
Electric Fuel Pumps
There are four basic kinds of electric fuel pumps; plunger, diaphragm, bellows and impeller (turbine). The first three types are driven by solenoids. The turbine pump is driven by a small electric motor. Electrically actuated pumps are at an advantage over the engine-driven mechanical pumps, in that their outputs are high under starting conditions, when a steady supply of fuel is desirable, and during idling when a pulsating supply of fuel could cause unstable running.
The plunger type pump (Fig. 9.15) has a stainless steel plunger in a brass cylinder. Valves installed at the bottom of the pump and above the plunger work like the check valve in a diaphragm pump. No rings or seals are used in
this design. The fuel constantly leaks between the plunger and its cylinder wall to provide lubrication. The pump maintains pressure in the fuel line even when the engine is not running.
A solenoid controls the plunger.
A magnet senses plunger position through the brass cylinder wall and makes and breaks the electrical contact, causing the plunger to move up and down inside the solenoid. Wear of the piston and cylinder wall surfaces may cause the pump to operate faster. A sediment chamber, a strainer, and removal valves help cleaning the pump. AC Universal electronic solenoid pump described in section 10.6.3 is also suitable for operation with petrol.
A diaphragm type electric fuel pump differs from a mechanical diaphragm pump in that it uses a solenoid to operate the diaphragm instead of the camshaft. When the solenoid is
Fig. 9.15. Plunger type electric fuel pump.
energised, the diaphragm is pulled against its spring and produces vacuum, which draws petrol from the tank. The return or pressure stroke is produced when the solenoid circuit to the electromagnet is automatically broken and the spring pushes the diaphragm due to which petrol flows into the carburettor. A widely used electrically operated diaphragm pump is the S.U. horizontal pump shown in Fig. 9.16 in sectional view.
Fig. 9.16. S.U. horizontal pump in sectional view.
Electric in-line pumps are normally installed near the petrol tank and are usually kept away from the engine. The electric pump starts working once the ignition circuit is switched on. The operation of the pump is indicated by the characteristic tickling sound, which is rapid initially and slows down gradually as the float chamber fills. The contact points remain open when the float chamber is full. Figure 9.17 represents the circuits for operation of in-line electrical fuel pumps. During cranking (Fig. 9.17A) the fuel pump receives current through the starter relay (solenoid) and the normally closed contacts of the oil pressure switch. When the engine starts the oil pressure switch opens one set of contacts and closes another. Current then flows through the ignition switch (Fig. 9.17B), through the oil pressure switch, and to the fuel pump as long as the oil pressure is above a minimum level. A bellows type electric pump (Fig. 9.18) is similar in design and operation to a diaphragm pump.
Fig. 9.17. Circuit diagram for in-line electrical pump.
Fig. 9.18. Bellows type electric pump.
Fig. 9.19. Impeller-type electric pump.
The impeller type pump (Fig. 9.19) is called a turbine or a rotary vane pump and is installed inside the fuel tank. It sucks fuel and pushes it out through the fuel line to the carburettor. Since this type of pump uses no valves, the fuel is moved in a steady flow rather than pulsating motion of all other electric and mechanical pumps.
The pump consists of an electric motor coupled with a vane type impeller. As there is no valve between the pump and the carburettor the fuel draws back into the supply tank when the pump is stopped. This prevents pressure built up and overflows in the carburettor when engine is stopped facilitating restarting. Figure 9.20 shows the electric wiring circuit for the fuel pump located in the fuel tank.
9.4.3.
Pump Performance Tests
Capacity Test. The fuel line is disconnected from the carburettor and directed into a measuring container. The engine is started and allowed to idle for 30 seconds while the fuel is being collected in the container. The engine is turned off at the end of 30 seconds and the amount of petrol delivered by the pump is measured and compared with the specification of the pump. If the capacity is low, the test should be repeated on the fuel pump side of the filter. The filter is plugged if the test on the fuel pump side of the filter is normal while test on the carburettor side is low. If both tests are low, the problem is with the fuel pump operation or with the fuel lines.
Fig. 9.20. Electric circuit diagram for in-tank fuel pump.
Static Pressure Test.
Fuel pump pressure is checked by placing a pressure gauge on the outlet side of the fuel pump, usually at the carburettor end of the fuel line, and measuring the fuel pressure as the engine idles. Fuel pressure ranges from 34.5 to 69 kPa, depending upon the engine. If the fuel pump eccentric is normal, low fuel pressure indicates that a new pump is needed.
Vacuum Test.
To conduct this test a vacuum gauge is installed on the tank side of the fuel pump. The fuel lines from the tank are plugged during the test to keep petrol from leaking out of the line. A normal fuel pump shows about 254 mm of vacuum at idle. If the vacuum is less, the pump or pump-actuating cam is faulty.
