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Overview of working of commen rail injection system in cars?
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Answer Posted / karthikeyan
Solenoid or piezoelectric valves make possible fine
electronic control over the fuel injection time and
quantity, and the higher pressure that the common rail
technology makes available provides better fuel
atomisation. In order to lower engine noise the engine's
electronic control unit can inject a small amount of diesel
just before the main injection event ("pilot" injection),
thus reducing its explosiveness and vibration, as well as
optimising injection timing and quantity for variations in
fuel quality, cold starting, and so on. Some advanced
common rail fuel systems perform as many as five injections
per stroke.[7]
Common rail engines require no heating up time[citation
needed] and produce lower engine noise and emissions than
older systems.
Diesel engines have historically used various forms of fuel
injection. Two common types include the unit injection
system and the distributor/inline pump systems (See diesel
engine and unit injector for more information). While these
older systems provided accurate fuel quantity and injection
timing control they were limited by several factors:
They were cam driven and injection pressure was
proportional to engine speed. This typically meant that the
highest injection pressure could only be achieved at the
highest engine speed and the maximum achievable injection
pressure decreased as engine speed decreased. This
relationship is true with all pumps, even those used on
common rail systems; with the unit or distributor systems,
however, the injection pressure is tied to the
instantaneous pressure of a single pumping event with no
accumulator and thus the relationship is more prominent and
troublesome.
They were limited on the number of and timing of injection
events that could be commanded during a single combustion
event. While multiple injection events are possible with
these older systems, it is much more difficult and costly
to achieve.
For the typical distributor/inline system the start of
injection occurred at a pre-determined pressure (often
referred to as: pop pressure) and ended at a pre-determined
pressure. This characteristic results from "dummy"
injectors in the cylinder head which opened and closed at
pressures determined by the spring preload applied to the
plunger in the injector. Once the pressure in the injector
reached a pre-determined level, the plunger would lift and
injection would start.
In common rail systems a high pressure pump stores a
reservoir of fuel at high pressure — up to and above 2,000
bars (29,000 psi). The term "common rail" refers to the
fact that all of the fuel injectors are supplied by a
common fuel rail which is nothing more than a pressure
accumulator where the fuel is stored at high pressure. This
accumulator supplies multiple fuel injectors with high
pressure fuel. This simplifies the purpose of the high
pressure pump in that it only has to maintain a commanded
pressure at a target (either mechanically or electronically
controlled). The fuel injectors are typically ECU-
controlled. When the fuel injectors are electrically
activated a hydraulic valve (consisting of a nozzle and
plunger) is mechanically or hydraulically opened and fuel
is sprayed into the cylinders at the desired pressure.
Since the fuel pressure energy is stored remotely and the
injectors are electrically actuated the injection pressure
at the start and end of injection is very near the pressure
in the accumulator (rail), thus producing a square
injection rate. If the accumulator, pump, and plumbing are
sized properly, the injection pressure and rate will be the
same for each of the multiple injection events.
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