ELECTRONIC CONTROL OF DIESEL INJECTION (COMMON RAIL SYSTEMS)
Advantages of the electronic diesel injection system
i) Direct injection is a more efficient method of fuel delivery: it develops more power and
has a lower fuel consumption than indirect injection (where the fuel is delivered into a pre-
combustion chamber).
ii) Another advantage of using a common delivery rail is the location of high-pressure
pumping element in or close to the injector, does not pass through a long delivery pipe,
hence shortening the distance from the injectors. With a long delivery pipe carrying high
pressure, when the pump delivers the fuel it causes a pressure wave to travel along the
delivery pipe; the time delay in the high pressure wave reaching the injector causes timing
inaccuracies of injector opening and closing. With the short or non-existent high pressure
delivery pipe on common rail systems, this delayed pressure wave problem is eliminated
or reduced.
iii) The objective of modern direct injection diesel fuel system is to control the initial
injection quantity in what is termed the ‘pilot injection phase’, which is when a small
quantity of fuel is injected, ahead of the main injection period. This small quantity of fuel
causes a small rise in pressure during the pilot phase, thus reducing the rapid speed of
combustion, which in turn reduces combustion knock.
Disadvantages of the electronic diesel injection system
i. Direct injection does, however, have one major disadvantage: the combustion noise is higher
than that of indirect injection, which is undesirable in passenger motor vehicles. The
combustion noise is generally referred to as combustion knock and is caused by ignition of the
fuel after injection has initially started. The short delay between ‘start of injection’ and the
ignition of the fuel means that there is a relatively large quantity of fuel that initially ignites;
this causes a rapid combustion and pressure rise in the early phases of the combustion process,
which causes an audible knock.
ii. Diesel fuel injection systems relying on mechanical pumps were relatively difficult to
precisely control the fuel quantity delivered during the early injection phase. If the initial
quantity of fuel injected is too large, ignition is rapid and initial gas expansion is rapid, thus
causing the combustion knock.
Common rail
Common rail refers to a system whereby the injectors receive fuel from a common supply rail, which
is fed fuel under pressure by an engine driven pump. The early common rail systems, the pump
provided fuel at a relatively low pressure; the fuel was then passed to the injectors through a pipe.
The injectors contained a pumping element, which produce the high pressure necessary for injecting
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diesel fuel. Some common rail systems delivered fuel to a separate pumping element which then
passed the fuel at high pressure to the injector.
One advantage of using a common delivery rail is that the high pressure pumping element is located
in or close to the injector, so the high pressure created by the pumping element does not have to pass
through a long delivery pipe, which is the case for traditional diesel fuel pump systems, where the
pump is a considerable distance from the injectors. With a long delivery pipe carrying high pressure,
when the pump delivers the fuel it causes a pressure wave to travel along the delivery pipe; the time
delay in the high pressure wave reaching the injector causes timing inaccuracies of injector opening
and closing. With the short or non-existent high pressure delivery pipe on common rail systems, this
delayed pressure wave problem is eliminated or reduced.
Electronically controlled common rail injection systems
The logical evolution of the common rail system is to use a single high pressure pump feeding the
common supply rail. The injectors will therefore not contain a pumping element but can still be
controlled using a solenoid which regulates the outlet port of the injector. The fuel at high pressure
will pass from the common fuel rail to the injectors, the opening and closing of the injectors is not
dependent on pressure waves passing through the pipe: it is totally dependent on the solenoid action,
which causes the injector outlet port to open or close (as on the unit injector). With this type of
common rail system, as well as controlling the injector timing and injector duration (fuel quantity
control), the pressure at which the fuel is injected into the combustion chamber can also be altered to
suit the engine operating conditions and cylinder pressure. The fuel delivered to the common fuel
supply rail (by the engine driven high-pressure pump) can be monitored by a pressure sensor and
controlled using a pressure regulator. The pressure of the fuel delivered to the injectors can therefore
be controlled so that it is always at the desired value. Typical injection pressures are around 1600 bar.
With electronic control, the injection timing can be accurately controlled to allow the fuel to ignite
and burn correctly within the combustion chamber.
For this type of common rail system with higher fuel pressure, the volume of fuel can be injected into
the combustion chamber in stages: a pilot injection, main injection and sometimes a post injection.
The pilot injection period can be controlled, so that only very small quantities of fuel are injected,
thus reducing combustion knock. Post injection can be used to aid the control of emissions: a small
quantity of fuel is injected at the end of the power stroke or even on the start of the exhaust stroke, the
fuel vaporises and passes through to NOx catalyst which then reduces NOx emissions. Using the
information from the various system sensors, the ECU determines the volume of fuel and the point in
time at which the fuel is to be injected to provide the required power from the engine.
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1. Fuel rail; 2. Pressure control valve; 3. Return line to fuel tank; 4. Inlet from high pressure pump;
5. Rail pressure sensor; 6. Fuel line to injector.
Fig. 3: High pressure fuel system- common fuel rail.
1. High pressure pump; 2. Rail (high pressure accumulator); 3. High pressure solenoid valve;
4. Injector fuel; 5. Injection nozzle.
Figure 4: Electronically controlled common rail system using a single high pressure pump.
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1. Fuel tank; 2. Pre-filter; 3. Pre-supply pump; 4. Fuel filter; 5. Low-pressure fuel lines;
6. High-pressure pump; 7. High-pressure fuel lines; 8. Fuel rail; 9. Injector; 10. Fuel return line;
11. Fuel-temperature sensor; 12. ECU; 13. Sheathed-element glow plug.
Figure 20: Common rail injection system (single high pressure pump system)
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Figure 20: A complete common rail injection system (single high pressure pump system)
Key to Figure 20
A Sensors and set point generators
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1. Pedal-travel sensor; 2. Clutch switch; 3. Brake contacts (2); 4 Operator unit for vehicle speed
controller (cruise control); 5. Glow-plug and starter switch (‘ignition switch’); 6. Road speed sensor;
7. Crankshaft speed sensor (inductive); 8. Camshaft speed sensor (inductive or Hall sensor);
9. Engine temperature sensor; 10. Intake air temperature sensor; 11. Boost pressure sensor;
12. Hot-film air mass meter (intake air);
B Interfaces
13. Instrument cluster with displays for fuel consumption, engine speed; 14. Air-conditioner
compressor with operator unit; 15. Diagnosis interface; 16. Glow control unit Controller Area
Network
C Fuel-supply system (low-pressure stage)
17. High pressure pump; 18. Metering unit; 19. Fuel filter with overflow valve; 20. Fuel tank with
pre-filter and Electric Fuel Pump, EFP; 21. Fuel-level sensor.
D Additive system
22. Additive metering unit; 23. Additive control unit; 24. Additive tank.
Engine, engine management, and high pressure fuel injection components
25 Engine ECU; 26 Fuel rail; 27. Rail pressure sensor; 28 Pressure control valve (DRV 2); 29.
Injector 30. Glow plug; 31. Diesel engine (01)M Torque
E Air supply
32. Exhaust gas recirculation cooler; 33. Boost pressure actuator; 34. Turbocharger (in this case
with variable turbine geometry (VTG)); 35. Control flap; 36. Exhaust gas recirculation actuator;
37. Vacuum pump.
F Exhaust-gas treatment
38. Broadband lambda oxygen sensor, type LSU; 39. Exhaust gas temperature sensor;
40. Oxidation type catalytic converter; 41. Particulate filter; 42. Differential pressure sensor;
43 NOx accumulator type catalytic converter; 44 Broadband lambda oxygen sensor, optional NOx
sensor.
Figure 4.16 Unit injector with combined pumping element for a common rail system
