Tuesday, 29 May 2012

Fuel Pumps

Fuel Pumps
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Bosch Scroll pump
The Bosch scroll pump consists of a plunger running in a barrel. The plunger is shaped as per the diagram and is rotated in the barrel by the fuel rack.

Bosch scroll pump plunger.

Bosch scroll pump spill.

Position one-The plunger is travelling down the barrel and the suction and spill ports are uncovered. A charge of oil enters the chamber

Position two-The suction and spill ports are covered and the barrel is travelling up the barrel. Pressure builds up until the fuel valve opens and injection commences

Position three-the spill port is uncovered, pressure above the plunger rapidly drops as the oil spills out. End of injection

It can be seen that by rotating the plunger the bottom edge uncovering the spill port can be moved. In this way the amount of fuel delivered is varied. On this only the end of injection timing is varied. Start of injection is constant. Some adjustm =481>

A standard bosch fuel pump can be fited wih a profiled plunger. The avantage of this is that the combustion process can be controlled to suit load conditions thereby improving efficiency.
Variable beginning and end-Variable Injection Timing (VIT) control
Profiled plunger

This allows for ideal matching of load to injection timing for various qualities of fuel. The Barrel insert can be moved up and down by action of the Nut. This has the effect of altering the position of the spill port relative to the plunger stroke. Therefore the beginning of injection is altered. The end of injection is varied by its normal way of rotating the plunger.

The Nut, which moves linearly, is controlled by the VIT rack, this is altered- continuously by the engine management.

Pump adjustment-Individual pumps may be adjusted in order to account for wear in the pump itself or the entire range of pumps can be adjusted to suit particular loads or fuel ignition quality. Individual pumps are adjusted by means of the screwed links from the auxiliary rack to the nut, just as the main rack adjustment is carried out. Adjustment of all pumps is simply by movement of the auxiliary fuel rack.

Problems associated with jerk pumps-the main problem with pumps of this type is that sharp edges on the plunger and around the spill port become rounded. As injection commences when the spill port is covered by the plunger, this means that later injection takes place. With the variable injection pump this can be accounted for by lowering the barrel insert and hence the spill port, so that it is covered at the required point. In older type pumps, adjustment required washes and shims to be placed between the plunger foot and cam follower or shims removed from below the pump body in order to lower it and the spill port relative to the plunger. Wear also causes leakage between the plunger and barrel but the only solution is replacement. Original timings must be restored.
The period of fuel injection
Typical fuel pressure curve at outlet from pump

Pressure curve in fuel delivery pipe

A-Pump spill closes (approx. 8o)
B-Fuel injector opens (approx. -4o)
C-Spill opens (approx. 12o)
D-Fuel injector closes (approx. 16o)
E-Reflected pressure wave
F-Period of partial equilibrium i.e. the rate of delivery from the rising plunger in the barrel equals the flow out of the injector, therefore no pressure rise. Instability of the wave form can indicate too low viscosity fuel supplied.
G- Injection period (approx. 20o)

It can be seen that the maximum pressure generated by the pump is far higher than the opening pressure by the injector ( 650 against 350 Kg/cm2). Engine monitoring equipment can be used to generate this graph allowing diagnosis of the fuel supply equipment. For example, the rate of rise of pressure before the fuel injector first opens indicates wear in the fuel pump.

Period of equilibrium

    This is the period between the beginning and end of stroke and can be divided into three periods.
        Delivery with no injection- being subject to high pressure the fuel reduces in volume, about 1 %. This causes a loss of effective plunger stroke and hence delays the start of injection. The main factor in this is the length of fuel pipe. The effect must be considered when advancing the fuel cam in relation to engine speed.
        Main injection period-This is directly related to the effective stroke of the fuel pump plunger and consequent engine load. The engine speed can alter the resilient pressure fluctuations in the fuel pipe and so alter the fuel delivery curve and cause irregular discharge from the injector.
        Secondary injection period-This is referred to as 'dribbling' and is due entirely to the resilient pressure fluctuations in the fuel piping and related to engine speed. The fuel oil passing to the injector has kinetic energy. At end of injection a low pressure wave passes through the fuel closing the needle valve in the injector. The kinetic energy in the fuel is converted to pressure energy and a pressure wave is formed. This can be seen below as the 'reflected pressure wave'. Avoided by fitting short, large diameter rigid fuel lines and having a sharp cut off at the fuel pump or an anti dribble device.

Effects of high speed;
a, start of injection can be delayed 3 to 10o - counteracted by advancing fuel cam by appropriate amount.

b, fuel pressure can be reduced by half maximum desired
Anti-dribble Non-return valve-fitted to fuel pump discharge
Anti-dribble valve

Variable rate injection
In an effort to improve the combustion characteristics of the burn period profiled cams have been used which reduce the initial rate /*of delivery smoothing out the process.

Anti-dribble valve
Sulzer Type Fuel pump
Anti-dribble valve

The Sulzer differs from the Bosch scroll pump in that it operates with a plain plunger, timing being effected by operation of valves.

The cam, which is driven via gears by the crankshaft forces the plunger up the barrel thereby delivering fuel to the injectors during the period that both suction valve and discharge valve is shut.

The eccentric cam which alters the timing of spill is rotated via the fuel rack driven from the governor. The eccentric cam altering the opening and closing of the suction port, may be altered manually or driven off an engine management system to change the beginning of injection.
Common Rail System
When compared to the jerk system the common rail system is said */to be quieter, gives more accurate control of fuel pressure, has no high torque's or sudden loads transmitted to the camshaft.

High pressure fuel (300 bar) I delivered from a crank driven constant output pump to the fuel main, which supplies all the cylinders. The pump drive is chain driven from the crankshaft. The cam operated timing valves control the start and the duration of fuel injection to each cylinder. The pressure can be controlled by air operated relief or spill valves. The air pressure is controlled by a cam operated reducing valve. The excess fuel is spilled from the HP main and passes to the buffer. An overspeed trip collapses fuel pressure to a drain tank.

Modern common rail system. Modern requirements for very precise fuel injection timing and delivery, varying fuel quality and load/speed variations has led large slow speed engine designers to the common rail system.

An electric driven high capacity pump supplies fuel to electric operated solenoid valves. One solenoid is fitted for each fuel valve. By computerised control the requirements can be met
Modern Hydraulically driven pump
Development of the slow speed engine has lead to the 'camshaftless' design. Here the motive force for the fuel pump has changed from mechanical cam and follower to hydraulic.

Hydraulic oil is suppled via either a dediciated supply or more 8*normally common rail system. Accummulators are fitted on the pumps to smooth the motive oil pressure at the pump.

Hydraulic oil is diverted from the system to the pump actuation piston via an electrically controlled solenoid valve. This valve has three positions the middle being neutral.
The Control of the solenoid valve is carried out by the engine management system and is affected by such parameters as engine loading, engine revs, fuel quality and exhaust gas condition fuel oil flows under 8 bar boost pressure through a non-return suction valve and the piston falls to start of stroke position.

The solenoid valve may be proportioning in that it may control the flow rate to the power piston thereby changing the rate of fuel injection flow. For example, at lower loads a higher rate on injection may be allowed for. This has the effect of increasing Pmax, gives better heat release and thereby improving fuel economy.

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