Technical function modes of the Common Rail (CRD) System
The common rail system prototype was developed in the late 1960s by Robert Huber of Switzerland. After that, the technology was further
developed by Dr. Marco Ganser at the Swiss Federal Institute of Technology in Zurich, later of Ganser-Hydromag AG (estb. 1995) in Oberägeri.
In the mid-nineties, Dr. Shohei Itoh and Masahiko Miyaki, of the Denso Corporation, a Japanese automotive parts manufacturer, developed the
Common Rail Fuel System for Heavy Duty Vehicles and finally turned into its first practical use on their ECD-U2 Common Rail system, which was
mounted on the Hino Rising Ranger truck and sold for general use in 1995.
Modern common rail systems, whilst working on the same principle, are governed by an engine control unit (ECU) which opens each injector
electronically rather than mechanically. This was extensively prototyped in the 1990s, with collaboration between Magneti Marelli, Centro
Ricerche Fiat and Elasis. After research and development by the Fiat Group, the design was acquired by the German company Robert Bosch GmbH
for completion of development and making suitable for mass-production. In 1997 they extended its use for passenger cars. The first passenger
car that used the common rail system was the 1997 model Alfa Romeo 156 1.9 Jtd and later on that same year Mercedes-Benz E 320 CDI.
Common rail engines have been used in marine and locomotive applications for some time. The Cooper-Bessemer GN-8 (circa 1942) is an example of
a hydraulically operated common rail diesel engine, also known as a modified common rail.
The high-pressure pump, the Rail and the injectors |
Pressure Comparison diagram
blue = CR, red = PD, green = VE |
In conventional diesel engines injection pressure is generated for each injector individually. A direct injection engine based on the common rail principle separates the two functions pressure generation and injection by first storing the fuel under high pressure in a central container ("common rail") and delivering it to the individual injection valves (injectors) only on demand. This way an injection pressure of up to 1,500 bar (in the future up to 1,600 bar) is available at all times, even at low engine speeds. The high pressure produces a very fine atomisation of the fuel leading to better and cleaner combustion. Moreover, the fuel supply is not dependent on the engine revolutions but can be optimised independently. The time and duration of injection is not fixed (as in older conventional engines) but can be chosen independently for every operation point in order to optimise combustion and emissions. In modern common rail systems injection is split into several individual injections: pre-injection, main injection and post-injection.
Benefits of the common rail principle compared to conventional engines are lower engine noise levels, stronger performance and greater combustion efficiency leading to lower emissions and enhanced fuel economy.
Four basic components of a common rail system are:
- A high pressure pump with pressure regulator and inlet metering valve.
- A rail which contains a pressurised reserve of fuel.
- Injectors which inject precise amounts of fuel into the combustion chamber as required.
- A diesel control unit – the ‘brain’ of the system, which precisely controls injector flow and timing as well as rail pressure while continuously monitoring the operating conditions of the engine.
As an example for a common rail engine for railway applications the following table gives the technical data of the MTU 4000 engine.
| Technical data of the MTU 4000 common rail engine |
| Power range |
760-2720 kW |
| Power weight |
2,7 – 3,5 kg/kW |
| Power per volume |
250 – 310 kW/m3 |
| Maximum revs per min |
2100/min |
| Specific fuel consumption |
195 g / kWh |
Pump Duse Nozzle Injection System
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Pumpe düse (PD) is a quite rare beast, because in the world of diesel cars, it's only the Volkswagen Group that uses this technology. The name Pumpe Düse is German for Pump Metering. This refers to the PD unit being an all-in-one assembly comprising the high pressure pump (that pressurises fuel to injection pressure) and the injector (which injects the pressurised fuel into the engine cylinder).
There is one complete pump düse assembly (otherwise known as a unit injector) on the cylinder head, serving each cylinger, and the pump part of the PD is driven by a camshaft in the cylinder head (usually the same camshaft that operates the engine's own cylinder valves).
PD is mechanically more fiddly than common rail, and it needs a specially designed cylinder head, so it's costly too. But its big advantage is that is can generate considerably higher injection pressure than even common rail can, and VW's PD units are good for 2,050 bar (against common rail's typical 1600 bar). That's great for producing more torque than would otherwise be possible, and it's also very useful for reducing polluting exhaust emmisions. Like common rail, PD has pilot-injection built into it to hush combustion rattle, and Volkswagen has said that it will soon be possible to build multiple-injection technology (such as that incorporated into the very latest CR systems) to further clean-up emmisions and reduce noise.
But the Volkswagen Group is unique in adopting this technology, and some say that other's shun it because its scope for further refinement - particularly in terms of the number of multiple injections - is limited.
Proponents of the system though, point out that it's safer, in that it doesn't store fuel in a rail at incredibly high pressure, but simply generates that pressure as and when needed.
Of course, as with CR, electronic management constantly monitors what the engine and driver are up to, and continually adjusts injection timing and fuel dosage.
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