Experiment!: Planning, Implementing and Interpreting

10.3 Experiment 2: Lift-Off Length in a Diesel Engine

In a diesel engine, the flame is always found some distance from the fuel injection nozzle. This distance is called the lift-off length and, as explained in Example 5.1, it is useful for understanding trends in soot formation in such engines. The lift-off length is generally assumed to stabilize shortly after auto-ignition and remain stationary until the end of the fuel injection period.

Most investigations of how the lift-off length is affected by various parameters are conducted in spray chambers with optical access. These are typically large chambers where important conditions, like pressure and temperature, can be precisely controlled over wide ranges. This is obviously a great advantage from a data collection point of view, but a potential drawback is that spray chambers are not very similar to the combustion chambers of real diesel engines. Due to the limited volume in an engine's cylinder, the combustion inevitably increases both the temperature and pressure, but they remain relatively unaffected in a large spray chamber. Furthermore, to simplify measurements, spray chamber experiments are usually made with single-hole fuel injectors, while injectors in engines often have six to eight nozzle holes. Since spray chamber experiments ultimately are made to understand the processes in real engines, it is important to understand how these differences affect the lift-off length.

In contrast to spray chambers, optical engines ...