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I do not much about history of CRDi but would like to add here whatever I know.
First I believe that all of us understand the basic principle of CI (compression ignition) engines.
The technology is very old and I don't think anyone has got patent on it as any patent would have expired long ago.
Anyway, initially when diesel engines were developed, they had one problem, how to get the oil into the combustion chamber.
Air blast system was used initially. That is compressed air was pumped into combustion chamber along with fuel and the
compressed air helped in atomising (spraying) fuel. Something like flit gun but working principle was very much different.
When higher power was required, the amount of fuel to be injected has to be increased withing a given span of time. So
various methods of fuel injection system was developed and that included the jerk type pump (the most common for diesel
engines) and common rail type. These system was called solid injection as opposed to air blast system due the fact that
entire column of fuel after the pump behaved almost like a solid column. Each maker had his own model of fuel injection
system.
In jerk type (or barrel and plunger type) fuel pumps, each unit has its own pump and fuel ijection metering (timing
and quantity - is controlled by fuel racks) is controlled by the pumps itself. One inherent disadvantage of this system is
that at low rpm, the fuel pressure is low (as the pressure developed is affected due to amount of oil and cam speed - both is
low at low speed) and proper atomizing of fuel does not take place, which results in poor combustion. This causes narrow rpm
operational band for diesel engines. To improve upon it, various methods were developed, for example, tandem plunger or
variable inection timings (auto injection advance).
In common rail, instead of using individual pumps for individual units, all the pumps deliver the fuel into a common
pipe or common rail, the pressure inside the pipe is controlled by fuel racks of pumps and pressure control valve on common
rail. Thus the pressue developed becomes independent of metering. The fuel from this common rail is led to injectors of
individual units through individual pipes. The metering is done by seperate controls - mechanically or electronically
(solonoid valve or piezo-electric), depends on the developer and period of development and hence has more flexibility in
operation. At low rpm operation the increased pressure gave better atomising and thus faster combustion. Although the fuel
pressure at low rpm has increased but it is still not equal to value at max rpm as the air density inside the combustion
camaber is low at low rpm and higher pressure injection will result in fuel spray reaching the liner and piston which will
eventually burn out to contact of fuel.
This also resulted in some improvement in low end torque due better combustion rate. To further improve the torque or
power, direct injection system was used as it allowed the gases to act on full area of piston (Force = press x area) and
hence increased torque. But it also resulted in heavy knocking sound. As the control of metering is independent, fuel can be
injected in pulses. A pilot injection is used (a small pulse before the main pulse) to start the combustion and avoid a very
rapid rise in pressure (knocking) and then the main injection is carried out. This results in smoother running of engine. | 
1st June 2005, 18:29
