Recent Developments And Trends

Fuel injection systems have a significant influence on the combustion process and hence a key role to play in improving engine fuel consumption and reducing noxious exhaust emissions. The following characteristics of an injection system are desirable in achieving these goals:

• Injection pressures during the whole process should be above 1000-1200 bar for a good spray formation and air-fuel mixture; a tendency in practice to 1600-1800 bar and higher is noted.

• Total nozzle area should be as small as possible in relation to cylinder diameter for good combustion, particularly at part load.

• Total injection duration should be 20 degrees of crank angle or less for achieving a minimum burning time in order to exploit retarded combustion for reduced NOx emissions without loss in efficiency. A high compression ratio is desirable.

• High pressures at the beginning of injection promote reduced ignition delay, while increased mass flow can result in an overcompensation and increased pressure gradients. Consequently, rate shaping is necessary in some cases, particularly with high speed engines.

• High aromaticity fuels cause increased ignition delay in some cases. Pre-injection with high injection pressures is necessary and can achieve non-sensitivity to fuel quality.

• Electronically-controlled adjustment of injection timing should be applied for optimised NOx emissions at all loads, speeds and other parameters.

• The load from the torque of the injection equipment on the camshaft and/or the gear train should be as low as possible in order to prevent unwanted additional stresses and noise.

• For safety reasons, even a total breakdown of electrical and other power supplies should not result in the engine stopping.

To understand fully the complex hydraulic events during the fuel injection process, the Swiss-based specialist Duap says it is essential to appreciate the function of all the elements (pump, pressure pipe, fuel valve and nozzle) forming the injection system.

It is commonly believed by non-specialists that the plunger within the pump pushes the fuel upwards like a pillar, thus effecting the immediate injection of fuel into the combustion chamber. The reality is, however, that the plunger moves with such a high speed that the fuel likely to be conducted is highly compressed locally (elasticity of fuel). The compressed fuel now generates a pressure wave which runs through the pipe and valve, causing the nozzle to open and inject. The pressure wave of the fuel is forced through the system at a speed of around 1300 m/s. In the same way that sound in the air is reflected by houses and hills, the wave is reflected between the fuel valve and the pump. It may therefore easily run back and forth between those components several times before the nozzle is actually forced to open and inject.

This process, Duap warns, underlines the importance of maintaining the injection system in an excellent condition and ensuring that the fuel is properly treated and free of dirt. If, for example, the nozzle spray holes are partially blocked by extraneous elements or carbon particles the pressure wave may not be sufficiently reduced within the system. This eventually results in the destruction of the fuel pump cam or other vital parts of the injection system upon the next stroke.

The task of the injection system is to feed fuel consecutively to each cylinder within a very short period of time (0.004-0.010s, depending on the engine rev/min). It is also essential that the same amount of fuel is delivered with each stroke: deviations in the quantity supplied to different cylinders will adversely affect the performance of the engine and may result in crankshaft damage due to resonance.

All the key elements of the system must be manufactured to very small tolerances; the clearance between plunger and barrel, for example, is not larger than 4-16 microns (depending on the plunger diameter). Such a high precision clearance also dictates an adequate surface quality (within 0.2-0.5 microns), a finish which can only be achieved by careful grinding and lapping.

Before the fuel is pushed into the pressure pipe (linking the pump and fuel valve) it has to pass the pressure valve, which has several tasks. The first task is to separate hydraulically the pump from the pressure pipe after the fuel has passed the valve; the second is to smooth the pressure wave running back and forth within the pipe: this calming down is necessary to secure the proper closing of the fuel valve without having additional and uncontrolled injection; and the third is to maintain a certain pressure within the pressure pipe for the next injection stroke (rest pressure). The fulfilment of these tasks can only be guaranteed by using pressure valves manufactured to very tight tolerances (bearing in mind that the valves play an important role in ensuring that identical amounts of fuel are delivered to each engine cylinder).

Another highly underestimated component of the fuel injection system is the pressure pipe. Despite its small size and apparent simplicity, Duap notes, the pipe has to endure pressure waves of up to 1800 bar; even the smallest mark may thus lead to fracture.

The last link in the injection system chain is the fuel valve (nozzle holder assembly and nozzle) which is designed to inject and atomize the fuel into the combustion chamber when the pressure wave has reached a pre-determined strength.

UNIT INJECTOR VERSUS PUMP/PIPE/INJECTOR

A unit injector is more likely to be considered for engines today than in the past but the choice between unit injector (Figure 8.11) and pump/pipe/injector (Figure 8.12) systems can be complex. The UK-based specialist Delphi Bryce (formerly Lucas Bryce) cites the following merits and demerits of the two configurations:

Unit injector positive features: hydraulic volumetric efficiency (smaller plunger); longer nozzle life; fewer components; no high pressure pipe; and fast end of injection. More complex cylinder head designs and higher cylinder head loads are negative features.

Pump/pipe/injector system positive features: less space required by injector; facilitates use of residual fuels and water injection; simple pushrod mechanism; generally easier servicing; and higher low load operating

Figure 8.11 Outline of electronic unit injector (Delphi Bryce)

pressure. Large trapped fuel volume and parasitic power loss are negative features.

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