The Effects of Orifice Shape on Diesel Combustion 2004-01-2920
Combustion and emission formation in a diesel engine are governed by fuel spray formation and mixing processes. Spray formation depends mainly on the fuel injection system and its use, i.e. injection strategies. Since future emission legislation will focus on sharply reducing emissions such as NOX and particulate, spray formation and consequently the fuel injection system take on increasing importance. Over the years, the adoption of electronically controlled fuel injection systems and higher fuel injection pressure have led to a decrease in fuel consumption, and compliance with emission legislation has been achieved. To achieve higher injection pressures, much energy needs to be provided from the engine, i.e. parasitic losses increase. Developments in manufacturing techniques now make different shapes of the orifices on the fuel injector nozzle possible. This includes very small orifice diameters, high inlet corner rounding and, more recently, conically shaped orifices beside several rows of orifices on the nozzle. A differently shaped orifice could lead to lower emissions at the same injection pressure level, i.e. emission reduction without an increase in the parasitic losses. This would then enable a better trade-off between fuel consumption and emissions.
This paper investigates the effects on emissions and fuel consumption of five nozzles with different orifice shapes by using a heavy-duty single-cylinder engine under the same injection pressure conditions. To further try to distinguish the effects of the orifice shapes, the minimum orifice diameter was kept constant. The results show that orifice shapes resulting in lower fuel flow rates can yield lower fuel consumption and lower emissions than orifice shapes that have larger fuel flow rates. A negative conisity (divergent) orifice shape yields lower emissions and has lower fuel consumption than the reference nozzle with a cylindrical orifice shape. It is also shown that in terms of fuel consumption, a 50% HE grinded orifice is a more beneficial selection than a positive conisity orifice shape.