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Understanding the Importance of Common Rail Injection to Today
It seems that there have been some improvements in diesel injection
technology that have allowed some “clean-up” work on the diesel exhaust
system, and I think we all can appreciate the change.
Even though some may feel the need to complain about the increased use
of electronics and how the government is cracking down by adding more
emissions components on engines that may give you trouble down the road,
things may not be all that bad.
It’s worth asking this question: if the old-school mechanical
indirect-injection technology was so good, then why did the
manufacturers convert to common rail?
Let’s face it, the use of electronics has made the same diesel engine
more powerful, more efficient and cause less pollution. The true cold
hard fact is that nothing is going to change back to the way things used
to be, and the electronics in diesel engines are only going to continue
Why is there a need for common rail injection in diesel engines? In
order to explain common rail, we need to start with a brief description
of mechanical fuel injection. The basic layout of a mechanical injection
system starts with the injection pump. The injection pump is ruggedly
durable, driven by the gear train of the engine and usually housed in
the front cover of the engine.
Fuel is generally supplied to the injection pump by a mechanical engine
driven fuel pump actuated by the camshaft or an electric pump mounted
somewhere on the vehicle. As the injection pump is turned by the
engine’s gear train, which is very similar to the distributor of a
gasoline engine, fuel enters a delivery chamber inside the pump at a
The fuel is then compressed and sent out through a hard metal line that
leads to the injector at the cylinder head. So the injection pump not
only compresses fuel to a specific pressure, but also serves as a timing
device for when the fuel will be compressed inside the pump.
When the pressurized fuel reaches the injector, the pressure of the
incoming fuel opens the pintle inside the injector and fuel is delivered
through the injector’s nozzle and enters the combustion chamber in a
very fine mist.
The pistons inside the engine are unique in design by having what is
generally known as a pre-ignition chamber, a small bowl-shaped area
inside the piston dish which helps compress the fuel to help ignite the
mixture as the piston moves close to the top of the bore.
Fuel pressure in a mechanical system will vary by the manufacturer of
the pump, but usually will be in the neighborhood of around 1,800 to
3,000 psi. This amount of pressure is needed to lift the injector’s
pintle off the seat to allow fuel to be sent through the injector’s tip.
Most injection pumps are sealed and cannot be tampered with, so
whatever the injection pump delivers to the injector is what the
cylinder is going to receive.
Even though these systems worked well and were typically trouble-free,
they were very inefficient. This predetermined, metered amount of fuel
may not always be enough or sometimes was too much. Either way, these
diesel engines did run and often performed flawlessly and went many
miles before servicing.
However, a better system that seemed to change the way we looked at
diesel engines was developed. You have to admit that today’s diesel
engines are much quieter than the older ones. A friend of mine recently
drove up in a 2011 Ford F250 with a 6.7L diesel engine and I thought
that the truck was outfitted with a gasoline engine. It was so quiet and
smooth that I could not hear the engine run.
This quiet and smooth operation is thanks primarily to common rail
injection. How does common rail differ from the old mechanical injection
system? To give a basic understanding of common-rail, think of it this
way: the higher the pressure at which diesel can be injected, the more
efficient it will burn.
The common rail system consists of a high-pressure pump, high-pressure
fuel rail, fuel lines, and injectors. The high-pressure pump is driven
by the engine much the same way as the mechanical injection pump. Fuel
is generally supplied to the high-pressure pump; in the case of the
Duramax, the high-pressure pump is also the supply pump.
As the engine turns the high-pressure pump, fuel inside is pressurized
to around 5,000 psi, the amount of pressure that most common rail system
manufacturers select for proper idle. This 5,000 psi of pressure is
considered to be the “threshold” at which the engine will fire – if a
high-pressure common rail pump does not reach the 5,000 psi threshold it
indicates that there may be a problem in the system and the engine will
As the fuel is being pressurized, it is then delivered to a
high-pressure fuel rail, often called a fuel log. The high-pressure fuel
rail also has the high-pressure fuel lines connected to it, which
deliver fuel to the injectors. This fuel rail is the storage center for
the pressurized fuel and the delivery unit that supplies the injectors.
Once pressurized fuel reaches the injectors, the injectors are commanded
by inputs to the computer that tell the injector when to open.
Pressurized fuel can then be sent to the cylinder at a precise time and
with the precise amount.
Now, for the common rail to function properly, there are various sensors
and actuators that must be utilized in the electronics outlay of the
vehicle by the manufacturer for the high pressure fuel actuation. The
electronics help dictate how and when fuel is delivered, based on engine
load and operation.
Great caution must be taken when servicing common rail systems because
pressures can reach as high as 28,000 psi under wide-open throttle. In
the next Diesel Dialogue, the common rail components will be explained
and detailed with the operation of the engine.