With common rail fuel injection, the combustion process can be optimized to achieve low pollutant levels along with lower fuel consumption. Fuel is injected into the combustion chamber through a common rail under high pressure. The electronic control system ensures that the start, amount and duration of injection are independent of engine speed. In 1996, with the 4000 Series engine, MTU became the first major diesel engine manufacturer to offer Common Rail fuel injection as a standard feature.

What Do We Do?

The injection rate determines when and how much fuel is injected into the cylinder. In order to reduce emissions and fuel consumption, the current phase of development of the injection system for MTU engines divides the fuel injection sequence into three separate stages (see Figure 1). The start timing, duration and amplitude of the injection are defined by the user based on the engine performance map. The main injection stage delivers fuel to create the power output of the engine. A pre-injection stage initiates forward combustion to ensure controlled combustion of fuel in the main injection stage. This reduces nitrogen oxide emissions because flash combustion prevents high peak temperatures. The post-injection phase, shortly after the main injection phase, reduces particulate emissions. It improves the mixing of fuel and air in the late stage of combustion to increase temperatures in the combustion chamber and promote soot oxidation. Depending on the operating point of the engine, the main injection stage may be supplemented by including pre-injection and/or post-injection stages as necessary.

Comparisons

Comparison of injector sizes for engines with different cylinder capacities, including injectors for current MTU Series 1600, 2000, 4000 and 8000 engines. (light grey: non-MTU engines)

Injector with integrated fuel tank

Since 2000, MTU has been using an advanced version of the Common Rail system for the 4000 and 8000 Series engines, and since 2004 an advanced version in which the fuel injectors have an integrated fuel tank for the 2000 Series (see Figure 4). This allows the fuel lines between the injectors and the common rail to have a relatively small cross-section. All that happens during an injection sequence is that the pressure in the injector’s own fuel tank drops slightly. This prevents pressure fluctuations in the Common Rail system and thus momentary under- or over-fuel delivery to the injectors.