Fig. 6  

 

PRESSURE RELIEF VALVE

The pressure relief valve maintains the pressure in
the supply line at 100-110 lbf/in2 (689.50-758.5 kN/m2)
by returning excess fuel to the filter inlet. The fuel
pressure is increased by turning the screw slot in a
clockwise direction. If the screw slot is turned in an
anti-clockwise direction, the pressure is decreased (see Figure 6).

The pressure relief valve is normally mounted in the
supply line, between the pump and distributor.

Fig. 7  

 

METERING DISTRIBUTOR/CONTROL UNIT

The amount of fuel in each injection, and the frequency of the injections, is controlled by the metering distributor and mixture control unit (Figure 7).

The mixture control unit regulates the amount of fuel in each injection, in accordance with the requirements of the engine. The function of the metering distributor is to inject fuel into each individual inlet by a system of shuttle-metering.

The two component parts - the metering distributor and the mixture control unit - are a “matched” pair.

A faulty metering distributor or mixture control unit should be renewed, and the complete assembly recalibrated.

Operation (Fig. 8A)

When the engine is started and the rotor turns within the sleeve, the rotor port at the control stop end becomes coincident with the port in the sleeve leading to the fuel reservoir in the body casting. Fuel at high pressure enters the rotor bore and drives the shuttle to the right (i.e., towards the fixed stop end of the rotor). This movement of the shuttle displaces fuel in the rotor bore through the ports in the rotor and sleeve to the injector serving No. 1 cylinder.

A further 180 degree rotation of the rotor (Fig. 8B) causes the rotor ports at the fixed stop end to align with the sleeve port leading to the fuel reservoir. Fuel now enters at the fixed stop end of the rotor and drives the shuttle back towards the control stop end. An identical quantity of fuel is displaced, by the shuttle as it moves to the left, by way of the outlets in the rotor and sleeve to No. 2  injector.
In this way the shuttle continually moves between the two stops displacing an accurate amount of fuel to each cylinder in turn.
The quantity of fuel delivered at each injection is dependent upon the distance the shuttle travels, this distance is adjusted by the control stop.

Fig. 8 

 

PRINCIPLES OF SHUTTLE-METERING

Before considering the design features of the metering distributor, it will probably be advantageous to discuss the principles of shuttle-metering. Figure 8 shows a metering distributor for a twin cylinder engine. It consists essentially of two parts; the rotor and the sleeve. The rotor has two radial ports, which lead to a centre bore containing a shuttle - which is movable between two stops (one fixed and the other adjustable). The sleeve has fuel inlet and outlet ports. The rotor fits inside the sleeve and is connected to, and driven by, the engine.

As the rotor turns, the port at the control stop end of the rotor becomes coincident with the fuel inlet port in the sleeve (see Figure 8, top diagram).

Pressurised fuel then enters the bore and drives the shuttle towards the fixed stop. This causes fuel to be discharged through the ports in the rotor and sleeve at the fixed stop end, and hence into the first cylinder through the appropriate injector.

The lower diagram shows the result of turning the rotor through 180 degrees. Fuel now enters the bore through the port at the fixed stop end of the rotor, driving the shuttle towards the control stop. An identical amount of fuel is then delivered to the second cylinder.

As the rotor turns, the pressurised fuel drives the shuttle between the two stops, ensuring that an accurately metered quantity of fuel is injected into the appropriate cylinder at the right time. The low mass of the shuttle ensures a rapid response at high engine speeds.

The quantity of fuel injected is the product of the surface area of the shuttle end face and its length of travel. The length of travel is determined by the position of the control stop. The metering distributor shown is for two-cylinder engines. However, the same principles of shuttle-metering can be adapted for four, six and eight cylinder engines, by altering the position and the number of ports in the rotor and sleeve assemblies.