FUEL INJECTION AND FUEL INJECTION SYSTEMS

The diesel fuel injection system consists of an injection pump, delivery pipes and fuel injector nozzles. Several different types of injection pumps and nozzles are used. In this paper only the traditional cam driven mechanical injection system and the common rail system is discussed.

The task of the fuel injection system is to meter the appropriate quantity of fuel for the given engine speed and load to each cylinder, each cycle. Further, the injection system should inject that fuel at the appropriate time in the cycle at the desired rate with the spray configuration required for the particular combustion chamber employed. It is important that the injection

begins and ends cleanly, avoiding any secondary injections. The fuel is introduced to into the cylinder of a diesel engine trough a nozzle with a large pressure differential across the nozzle orifice. This large pressure differential is required so that the injected liquid fuel jet will enter the chamber at sufficiently high velocity to atomise into small-sized droplets to enable

rapid evaporation. The plume should traverse the combustion chamber in the time available and fully utilise the air charge.

Thus, the fuel injection process controls only four basic functions:

1.  Timing and duration of injection

2.  Injection rate shape

3. Injection quantity

4. Spray quality

Traditional injection system

An auxiliary cam on the engine camshaft drives a single-cylinder injection pump. Early in the stroke of the plunger, the inlet port is closed and the fuel trapped above the plunger is forced through a check valve into the injection line. The injection nozzle has several holes through which the fuel sprays into the cylinder. A spring-loaded injection needle keeps the injection valve closed until the pressure in the injector volume, acting on parts of the needle surface, overcomes the spring force and opens the valve. Thus, the phase of the pump camshaft relative to the engine crankshaft controls the start of injection, while the force given by the initial displacement of the spring gives the opening pressure. Injection is stopped when the inlet port of the pump is uncovered by a helical groove in the pump plunger, and the high pressure above the plunger and in the injector volume is released. The injection pump cam design and the position of the helical groove determine the amount of fuel injected into the cylinder. Thus for a given cam design,

the rotating plunger and its helical groove controls the load.

Fig. 1 Traditional cam-driven fuel injection system

Common rail injection system

Contrary to the traditional injection system, pressure generation and injection are decoupled in the common rail system.

The injection pressure can even be generated independently of engine revs and the injected fuel quantity can be freely selected within limits. A prerequisite for this decoupling of pressure generation and injection in common rail systems is the high-pressure accumulator, which consists of the rail and the high pressure fuel lines to the nozzles. The key component of the system is the solenoid valve controlled injector. In order to control the opening and closing time of the needle, a small chamber of pressurised fuel is present at the top of the needle. This volume is connected to the rail through a small orifice that assures that same pressure between the nozzle and the chamber when the valve is closed. At the same time, a small solenoid balance valve is in operation in this area that can open at an accurately specified time, thus creating a pressure drop. This will result in a negative net force on the valve needle, and the injection is initiated. As soon as the solenoid closes, the pressure in this chamber will increase again resulting in the closure of the needle. Shaping the injection rate,obtaining pilot injection and multiple injections are done by controlling the nozzle needle movement.

Electro magnet

Pin spring

Pressure rod

(Needle)

Fig. 2 Common rail fuel injection system

Fuel injection and combustion

The direct ignition combustion process can be divided into four phases:

1.  Ignition delay: The time between start of injection and start of combustion

2.  Premixed combustion phase: Combustion of the fuel that has mixed with air within the flammability limits during

the ignition delay

3.  Mixing controlled combustion phase: Combustion of fuel at a rate which mixture becomes available for burning

4.  The late combustion phase: Combustion of the final amounts of fuel, soot and other fuel rich areas

It is desirable to have the shortest possible ignition delay and to minimise the amount of fuel injected during this time, and thereby limit the rapid energy release in the premixed combustion phase. Obviously, it is also important to limit the energy release in the late combustion phase, meaning that as much as possible of the energy release should be mixing controlled.

Several processes controls the mixing controlled combustion phase:

•   Liquid fuel atomisation

•   Vaporisation

•   Mixing of fuel vapour and air

•   Pre-flame chemical reactions

The vapour-air mixing process is primarily controlling combustion in this phase. 

The authors believe that in a more “ideal process”, the pressure in the sac volume should rise very distinct to a sufficient high level during start of injection to secure good atomisation and rapid mixing between fuel and air. This results in a short ignition delay and equally short premixed combustion phase. The initial pressure should be limited in order to reduce the premixed heat release and thereby avoiding unnecessary large pressure gradients and local temperature peaks. During the mixing controlled combustion phase, the rate of injection should be progressive in order to maintain the cylinder pressure as the cylinder volume is increasing and less oxygen is available for combustion.  The closing of the injection valve should also be distinct to minimise the period with decreasing pressure, thereby reducing the amount of fuel injected with reduced penetration.

                                                                                         Fig. 3 Ideal injection rate (dotted) and rate of heat release (solid)   

BOND GRAPHS

Bond graphs are a unified graphical notation for the representation of physical systems. The basic concept is that power is the fundamental constraint in a physical system and is the one variable that is common to the whole system. Bond graphs consist of letters and numbers representing the components connected by lines or bonds standing for power interactions.

The half arrow shows the direction in which the positive power is flowing. Two other types of variables that are important are momentum, p, and displacement, q, in generalised notation. Momentum is defined as the time integral of an effort and the displacement variable is the time integral of a flow variable. Momentum and displacement variables can be used to

represent system energy and thus state variables.

Using the classification of power and energy variables presented previously, it turns out that only nine basic types of multi-port elements are required in order to represent models in a variety of energy domains. These multi-ports function as components of subsystem and system models. They are in many cases idealised mathematical versions of real components,in other cases they are used to model physical effects. The nine basic multi-ports are classified in Table 1, according to the

way they process energy. Roughly speaking, these elements account for energy dissipation, storage, supply and conversion of energy from one form to another.

Spring, volume, capacitor

Mass, inertia, inductance

Source of force, pressure, voltage

Source of speed, flow, current

1-junction

Using these basic elements, a graphical model can be constructed of a system. It is a remarkable fact that models based on apparently diverse branches of engineering science all can be expressed using the notation of bond graphs. This allows one to study the structure of a system model. The nature of the parts of the model and the manner in which the parts interact can be made evident in a graphical format.

MODELLING

From a modellers point of view, fuel injection systems consist of more or less the same components, enabling the modeller

to use and re-use basic sub models. This re-usability of models is crucial in an effective modelling environment. We will

now present the sub-models required for assembling overall models for both the traditional injection system⁴ and the

common rail injection system.

Nozzles

Hydraulic nozzle flow is modelled using the general equation:

Hydraulic flexibility of volumes

Fig. 4 Resistor element

Hydraulic flexibility must be included in a model where high frequency dynamic behaviour is to be studied. The hydraulic flexibility is given by the bulk modulus of the fluid at constant temperature, defined as:

Fuel pumps

The fuel flows from the lower pressure system (Se-element) through the inlet port (R-element) into the pump cylinder (C-field). The fuel then flows through the discharge valve (R-element) and into the fuel line. The volume of the pump cylinder is given by the displacement of the cam (Sf-element). Between the cam and the pump cylinder transformers converts power from the mechanical to the hydraulic energy domain. The flow through ports are dependent upon the position of the

plunger, Z, and also the fuel rack position, X.

For the common rail system, the high-pressure pump is modelled as a flow source. These pumps are mostly multi-cylinder displacement pumps where the flow is governed in very much the same manner as for the traditional pumps by controlling the effective pump stroke. The effective pump stroke is given by a governor with the purpose of achieving a given rail pressure. In this case there is little to be gained with respect to accuracy by modelling the individual pump cylinders.

Fig. 8 Bond graph model of mechanical parts of the injectors (Common rail injector bottom)

The common rail injector is modelled much in the same manner as the traditional injector. The only difference is the additional force originating from the pressure in the volume above the pressure rod.

In order to determine the motion of the valve needle, the pressure distribution along the needle must be determined accurately since it varies due to the acceleration of the fluid as it enters the sac volume. 

The flow is considered as steady state without friction with known pressure in the

injector volume. The local pressure can then be found by using Bernoulli’s law. 

 

The mechanical part of the injector model will now be connected to the hydraulic part. The fuel

flows trough the bores (Pipe) and into the variable volume surrounding the valve needle (C-

field). The pressure in this volume acts on the injector needle. The fuel then flows through the

flow area given by the position of the needle (R-element) and in to the sac volume (C-element).

The pressure differential between the sac volume and the engine cylinder (Se-element) determines

the flow into the cylinder through the nozzle bores (R-element). 

Fig.Bond graph model of conventional injector

As we can see to the right, the bond graph model of the common rail injector is a bit more comprehensive than the model of the traditional injector. The model of the mechanical part of the

solenoid valve is quite similar to the model of the mechanical part of the valve needle. The major difference is the force from the electromagnet (Se-element).

At the top of the internal bores (Pipe), the fuel flows through a small orifice (R-element) into a variable volume on top of the pressure rod (C-field). The pressure in this volume acts on both the

valve needle and the valve shutter. From this volume, the fuel flows through a valve (R-element) where the flow area is determined by the position of the valve shutter (Solenoid controlled) and into an another variable volume (C-field). This volume is connected through an orifice (R-element) to the low-pressure fuel system (Se-element). 

Input to the injection valve models is the upstream pressure.

Complete system model

The sub models presented above are modelled using the modelling tool MS1. The sub-models are used to assemble total models of a traditional injection system and a common rail injection system much in the same manner as building the physical system. Using bond graphs in combination with matching software enables the engineer to make changes and modifications to a model in a straightforward manner without reformulation of equations. The figure below shows the two models as they appear in MS1.

SIMULATION

The plot shows the pressure in the sac volume for both the traditional injection system and the common rail injection system. The pressure in the rail is 1000 bar.

Characteristically, the common rail system gives a distinct opening and closing of the injection valve. This secures rapid atomisation and mixing between fuel and air in the initial phase, and minimises the end-phase of the injection period with rapidly dropping pressure. The pressure in the sac volume is fairly constant throughout the injection. For the traditional injection system, the pressure in the sac volume rises more slowly in the early stages of injection. However, throughout the injection the pressure in the sac volume continues to increase due to the cam profile and is exceeding the pressure obtained by the common rail system. But in the final stage, the pressure in the sac volume decreases slower as the pressure is relieved at the pump side of the system.