Today’s diesel engines are cleaner, more economical and more
fuel efficient than ever before. Common Rail Direct Injection (CRDI) is the
most advanced technology in diesel fuel injection. The name implies that the
injection takes place directly inside the combustion chamber.
Difference between
CRDI and an ordinary diesel injection system:
The difference between an ordinary diesel injection system
and CRDI is that in an ordinary system, the fuel has to be pressurized for each
injection cycle. In the case of CRDI, a common rail is installed which
maintains the pressure of the fuel throughout its entire length. The Electronic
Control Unit (ECU) regulates the fuel pressure at different speeds and also
modulates the fuel supply required based on the operating conditions.
Therefore, with the help of an ECU, fuel compression and injection are made
independent of each other. This feature helps in lowering emissions and
increasing fuel economy.
History of CRDI:
Robert Huber of Switzerland developed the first prototype of
CRDI in 1960. Denso Corporation of Japan developed the common rail system for
heavy duty vehicles and it was successfully installed in Hino Rising truck in
1995.
The modern CRDI which is controlled extensively by ECU have
electrically controlled injectors replacing the mechanical ones. This system
was prototyped in 1990s by Fiat, Magneti Marelli and Elasis. But the German
company Bosch GmbH acquired the design for completion and mass production.
Bosch ultimately bought the license from Fiat. The first passenger car to have
a CRDI was Alfa Romeo 156 2.4 JTD in 1997.
Working of CRDI:
The common rail system is equipped with a high pressure pump
which can pressurize the fuel up to 2000 bar. The pressurized fuel is then sent
to a common rail which maintains the pressure throughout its entire length. The
common rail is fitted with a rail pressure sensor to detect the pressure inside
the rail. A pressure limiter valve is also provided in common rail. Controlled
via the ECU, the pressure limiter valve limits the pressure in the rail, while
at the same time suppressing pressure fluctuations.
In a CRDI, the fuel quantity and timing of injection is
governed by the ECU or microprocessor. The common rail supplies fuel to each
injector based on the firing order of the engine. The piezoelectric injectors
are controlled by the ECU.
To lower the engine noise and vibration, the injector can be
controlled to inject a small amount of fuel just before the main injection.
This is known as ‘pilot’ injection.
The pilot injection is followed by main injection. Some advanced CRDI injectors
can achieve multiple injections (5 to 10) during main injection. This helps in
lowering noise and emissions. The main injection is usually followed by a post
injection.
The start of injection occurs at a pre-determined pressure
and also ends at the same pre-determined pressure. Therefore, the pressure is
maintained throughout the injection process, thus forming a square injection
rate.
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