Injector Nozzle Hole Parameters and their Influence on Real DI Diesel Performance

University dissertation from Stockholm : KTH

Abstract: A modern diesel engine is capable of running efficiently with low exhaust gas emissions over a wide operating range. This is thanks to techniques such as turbocharging, EGR, charge air cooling and an advanced fuel injection process. The fuel injection process is important for the combustion and emission formation in the diesel engine. The fuel injector has to atomize and vaporize the fuel as it is injected. During the combustion the emission formation has to be kept to a minimum. Very strong pressure gradients are present in a modern diesel injection nozzle, this causes cavitation to occur in the nozzle holes. The influence of cavitation on flow parameters such as the various discharge coefficients is discussed. The occurrence of cavitation helps the spray break up and it can keep the nozzle holes free from deposits. Excessive amounts of cavitation can lead to hole erosion and thus impact the long term operation of the nozzle in a negative way. Hole erosion as well as other mechanisms can cause hole to hole variations in fuel spray impulse, mass flow, penetration etc. This is a very important issue in any low emission diesel engine, especially during transients, as less than optimal conditions have to be handled. The influence of hole to hole variation on fuel consumption and emissions is not very well known and this thesis contributes to the field. As a part of this work a fuel spray momentum measurement device was developed and tested. Any automotive engine needs to be able to perform quick transitions between different loads and speeds, so called transients. In a turbocharged diesel engine with EGR issues related to the turbocharger and the EGR-circuit arise. A diesel engine has to run with a certain air excess in order to achieve complete combustion with low emissions of soot. When turbocharging is used the turbocharger turbine uses some of the exhaust enthalpy to drive the turbo compressor, in this way the engine is provided with boost pressure. In order for the engine and turbocharger to function at the higher load and thus higher mass flow rate the turbocharger has to increase its rotational speed and the surface temperatures have to settle at a new thermodynamic state. Both of these processes take time and during this time the combustion process may have to proceed under less than optimum circumstances due to the low boost pressure.