The Ultimate Guide: Common Rail Diesel Technology

Diesel technology continues to gain market share and as a result opens up a significant market for repair facilities and their parts suppliers. This warrants a discussion surrounding Diesel Injection Technology and if you are unfamiliar with diesel injection, this is a great place to get started.


Diagram of a common rail injection system

The major difference between gasoline and diesel engines is their method to ignite the fuel that is in the combustion chamber. Gasoline engines use a spark plug; the spark ignites the fuel to start the combustion process. Diesel engines use compression to ignite the fuel. The compression ratio is much higher on a diesel engine. So high, that it compresses the air in the combustion chamber to the point where the fuel ignites as a result of the heat created during compression. Fuel is injected directly into the combustion chamber on the compression stroke. Another important difference is that diesels don't have a throttle that regulates the air entering the engine. Speed and power output of the engine is controlled by the amount of fuel injected into the combustion chamber – adding more fuel creates more speed and power.


High Injection Pressure

Almost all modern diesel engines use a “common rail” fuel injection system. Fuel is supplied to the injectors at extremely high pressure via a mechanically driven fuel injection pump. Fuel is injected into the combustion chamber at pressures of up to 26,000 psi. High fuel pressure is needed to help atomize the fuel as it is injected and increases the volume of fuel that can be injected in the short amount of time available for the injection event.

Diagram of a Common Rail Fuel System
Photo: Bosch - Common Rail Fuel System

Injection Timing

On modern diesel engines using common rail injection, the injectors are electronically controlled by the Engine Control Unit (ECU). Each combustion cycle may actually receive the required fuel via multiple injection pulses. Some manufacturers use up to 5 or more injections depending on the operating conditions. Each injection pulse has a specific purpose and affects engine power output, emissions and engine noise (knock). Injection pulses are timed precisely and the time in between the injection events is in the micro-second time frame.


The pilot injection pulse is the first injection event and helps reduce NOx emissions. The pilot pulse pre-conditions the combustion chamber for the main injection pulse. This reduces ignition lag of the main pulse because the pilot pulse pre-heats the combustion chamber. Another significant benefit of the pilot pulse is that it reduces engine noise significantly. New diesel engines are nearly as quiet as their gasoline counterparts.


The main injection pulse is where the majority of the fuel is injected and is responsible for producing most of the power during the combustion event.


The secondary injection pulse occurs directly after main injection while combustion is still taking place. This additional pulse aids in burning soot particles and reduces particulate emissions significantly.


Since all injection pulses are controlled by the ECU they may be modified on a real time basis determined by prevailing operating conditions.


Injector Spray Pattern and Design

The spray pattern and design of the injector nozzle is critical in reducing fuel consumption and emissions. Diesel injectors have multiple orifices from which the fuel exits the tip of the nozzle. More orifices provide better fuel atomization, which is beneficial in reducing emissions. However, the number of orifices utilized is limited by the geometry of the nozzle tip and the extreme pressures present. Most injectors utilize five to seven orifices. The injection angle is much different than that of a gasoline injector and is optimized to the combustion chamber and piston bowl design.


The pressure at which the fuel is injected also plays a critical role in fuel atomization - the higher the pressure, the better the atomization. Proper fuel atomization is critical in diesel applications because the fuel is injected directly into the combustion chamber during the compression stroke. There is very little time for the fuel to mix with the air prior to combustion, whereas on a gasoline application the fuel is injected into the intake manifold where it has time to mix with incoming air prior to being drawn into the combustion chamber.

Two different types of common rail injectors are used; solenoid operated and piezo operated. First generation common rail injectors are solenoid operated. The injector is activated by a magnetic field which is created when the ECU energizes the solenoid. These injectors are being replaced by piezo injectors which are capable of opening much faster than solenoid operated injectors. For an in-depth look at these types of injectors, see our article on solenoid and piezo diesel injectors here.


Electronics

The injection system is controlled by an on-board computer system or Engine Control Unit (ECU). The ECU monitors numerous engine parameters to determine the proper injection timing and volume. This includes engine boost, engine temperature, mass air flow, accelerator pedal position and engine speed.

Since diesels do not have a throttle plate, the ECU interprets the accelerator pedal position sensor to determine driver demand. When increased power demand is sensed, the ECU increases fuel flow by increasing injection pressure and increasing the injector pulse width. As more fuel is added the engine speed and power output increases. The entire process is controlled electronically.


Modern diesel engines are as sophisticated as gasoline injected engines while offering better fuel economy and higher torque output.



 

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