Automotive Technical Articles - Common Rail Diesel

Electronic Diesel Control

Beyond the realisation of the majority of the motoring public, we in the motor industry are doing our bit and conforming to legislation through new technologies, to reduce the impact that burning carbon based fuels has on the environment.

Electronic control on compression ignition engines was developed for exactly the same reason that spark ignition engines are now managed electronically, primarily to reduce emissions through increasing the efficiency of the combustion process. Subsequent benefits in terms of noise reduction, power, torque and driveability improvements were a positive, and well engineered ‘side effect’.

The holy grail that is diesel-utopia consists of injection timed to perfection for the entire engine operating range, from start-up to full speed and load; and rate of delivery, which is optimum fuel delivery from the start of the injection phase right until the end. This will result in a clean burning, quiet, efficient power unit.

Mechanical Systems

In early mechanical systems, the injection timing is designed to be the best compromise for the majority of engine operating speeds and loads. There are ‘blind spots’ in the range that cannot be managed adequately. The same is true for rate of delivery. The optimum fuel flow can only be achieved at maximum nozzle lift, which happens at maximum pressure. There is gradual increase in the rate of delivery during the opening and a reduction during the closing phase of the mechanical injector due to the pressure characteristics of the pump, the way the system is designed to cope with pressure and vacuum oscillations within the pipe-work and the dynamics of the needle and spring set-up within the injector. This translates into the belief held by many people that diesel engines are smoky, dirty and noisy.

With the later injection systems, injection quantity, timing, duration and rate can be controlled more efficiently over a wide range of engine loads and speeds, from cranking and start-up, right the way through to full power. The biggest challenge that the electronic system faces is to achieve the very high pressure needed for injection (maximum pressure of approximately 2000 bar in unit injection systems), and still be able to optimise the rate of flow and timing with compact, precise low voltage electronic solenoid valves.

There are two other generic types of diesel fuel injection systems besides the mechanical pump, line and nozzle. One is typified by the VAG Pumpe Duse (PD) system which uses unit injectors, and the other by the Common Rail System as implemented by Bosch, Seimens, Delphi and Denso.

The PD system has unit injectors where the high fuel pressure can be generated inside the injector itself. This system has both a low supply  and a high injection (2000 bar) pressure.

In this type of system, a central pump supplies fuel to unit injectors. The injectors are actuated by the lobes of an engine driven camshaft, typically shared with engine valves. Injection pressure is not generated by the pump, but by the movement between this injector camshaft and the pumping element (multiplier pistons) inside each injector.

Due to the injector being operated by a camshaft, the rate of injection is influenced and limited by the cam profile. The rail pressure is determined mostly by the speed of the fuel supply pump, which is driven by the engine. This system is a distinct improvement over the pump, line and nozzle types, as it reduces the influences of pressure build up and oscillation. However, it is not a perfect solution because the issues surrounding rate of delivery and injection timing still exist, although to a lesser extent.

Electronic Control.

Bosch is attributed with inventing fuel injection for diesel engines and therefore has the knowledge and skill to develop a near perfect solution. 

Prior to electronic control, fuel injection systems utilized hydraulic logic to achieve fuel control. The heart of the electronic system, the pump and injectors, are a product of all those years of diesel experience, and hydraulic logic still plays a major role in this fuel injection system.

The rail pressure is used to control rate of delivery and therefore the pressure needed for injection must exist in the rail at all times. The pressure is generated by a three element radial pump, driven at engine camshaft speed. The power needed to drive this pump is extremely low, slightly more than 10% of a comparable distributor-type pump.

The inlet and outlet of each pumping element is controlled with simple, but very precise check valves. The outlets are channelled together in the pump body before reaching the rail connection. This enables pressure in the rail to reach a maximum of approximately 1500 bar. The fuel rail pressure is controlled by a fuel rail pressure control valve or a variable restriction in the high pressure pumps' fuel inlet.

The injector design is quite complicated because each of the passages, chamber volumes, piston sizes and helper springs must be 'tuned' to attain the desired movement of the needle. The injectors are operated with a solenoid valve which opens and closes a vent in the control chamber above the servo piston.

With injection pressure available at all times, and with full electronic control of the rail pressure and injector solenoids, the injection event is separated completely from the control of any mechanical device. Timing, duration, quantity and rate are no longer dictated solely by mechanical means, but programmed into a computer. This not only provides rate control, but also the capability for multiple injections.

Injection.

This system uses pilot-injection (pre-injection), in which a small amount of fuel is injected into the combustion chamber a few crank-degrees early, to get combustion started more gradually. Once the fire is started, the nozzle is opened a second time to add a full charge of fuel. The net effect is a more gradual pressure rise in the cylinder. This reduces noise, improves cold-starting, fuel consumption and most importantly, emissions.

Electronics are the true enabler within this system. The injector solenoids are operated by the electronic driver unit, which supplies a direct current in a peak-and-hold cycle. The ECU uses inputs from a familiar group of sensors, including the camshaft position, crankshaft position, intake air temperature, engine coolant temperature, boost pressure and a hot-wire air mass sensor. There is also an accelerator pedal position sensor which makes this a drive-by-wire system.

Who said being good was easy?

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© James Dillon.  Date of article MMII.