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What is Common Rail Fuel Injection

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The Difference Between Gas and Common Rail Fuel Injection

The greatest difference between common rail fuel injection and gasoline injection systems is the method used to ignite the fuel once it is injected into the cylinder. Gasoline engines ignite the fuel using a spark plug, while diesel engines use compression to cause ignition. Since diesel combustion relies on compression, it stands to reason that combustion must be much higher in the diesel engine; air is compressed in the cylinder to such a level that the heat caused by the compression causes ignition of the fuel. Diesel fuel is injected directly into the cylinder, while gasoline is injected and mixed with air in the manifold. Diesel injection systems do not use a throttle to regulate air used during combustion. The RPM and power of the diesel engine is controlled by the amount of fuel injected into the cylinder. More diesel fuel – more energy and power.

Diesel Injection Pressure

Common Rail diesel injection is used by most modern diesel engines. A fuel pump delivers diesel fuel to the injectors at very high pressure. That fuel is sent to the cylinder at pressures measuring up to 27,000 psi. This high pressure is needed to completely atomize the diesel fuel, which increases the surface volume of the fuel as well as the amount that can be injected. The suction control valve is responsible for maintaining and controlling the correct pressure as it gets to the rail. This is a relatively inexpensive, but critical part to the proper operation of the common rail fuel injection system, and here is a quick vid to explain what it does and why it is so critical.

Diesel Injection Timing

With common rail fuel injection, an engine Control Unit (ECU) controls the fuel injection and combustion process. The ECU may deliver multiple pulses of fuel through the diesel fuel injectors, depending on operating conditions. The exact timing and synchronization of the fuel pulses, the injectors and the air/fuel mixture directly affects the emissions, power and noise of the diesel engine. The more precisely all of these events can be timed, the more efficient the engine runs.

The first injection is the “pilot injection”, and prepares the cylinder for the main pulse as well as helping to reduce emissions. The pilot pulse pre-heats the cylinder for the following fuel pulses and reduces engine noise. The main pulse is the “power pulse” and produces most of the energy during combustion. The secondary pulse helps to reduce particulate emissions. All fuel pulses are modified in real-time by the Engine Control Unit for optimum performance of the engine.

Diesel Injector Fuel Distribution and Design

Diesel fuel distribution and the design of the injector is critical to maximum fuel economy and minimum emissions. Optimizing fuel atomization and distribution throughout the cylinder is critical to proper operation of the diesel engine. In common rail fuel injection, each injector is designed to work specifically with the cylinder and piston design of the engine.

Engine Control Unit

The complete diesel fuel injection system is controlled by the Engine Control Unit. The ECU monitors and controls all relevant engine information to control and maintain optimum engine operating conditions. Temperature, air volume, pedal angle and RPM are just some of the factors the ECU uses to control injection and ignition.

As you can see, diesel engines with modern Common Rail diesel injection are as advanced as gasoline injected engines, but produce much higher fuel economy, torque and reliability.