Near Nozzle Field Conditions in Diesel Fuel Injector Testing

Abstract

The measurement of the rate of fuel injection using a constant volume, fluid filled chamber and measuring the pressure change as a function of time due to the injected fluid (the so called "Zeuch" method) is an industry standard due to its simple theoretical underpinnings. Such a measurement device is useful to determine key timing and quantity parameters for injection system improvements to meet the evolving requirements of emissions, power and economy. This study aims to further the understanding of the nature of cavitation which could occur in the near nozzle region under these specific conditions of liquid into liquid injection using high pressure diesel injectors for heavy duty engines. The motivation for this work is to better understand the temporal signature of the pressure signals that arise in a typical injection cycle.A preliminary CFD study was performed, using OpenFOAM, with a transient (Large Eddy Simulation -LES), multiphase solver using the homogenous equilibrium model for the compressibility of the liquid/ vapour. The nozzle body was modelled for simplicity without the nozzle needle using a nozzle hole of 200μm diameter and the body pressurised to values typical for common rail engines. Temperature effects were neglected and the wall condition assumed to be adiabatic. The chamber initial static pressure was varied between 10 and 50 bar to reflect typical testing conditions.Results indicate that vapour formation could occur in areas 10-30mm distant from the nozzle itself. The cavitation was initiated around 100 μs after the jet had started for low ΔP cases and followed the development period required for the formation of vortices associated with the vortex roll up of this jet. These vortices had localised sites, in their core region, below the vapour pressure and were convected downstream of their initial formation location. It was also found that vapour formation could occur at chamber static pressures up to 50 bar (the highest tested) due to cavitation in the shear layer and this vortex effect. The pressure signal received at the chamber would therefore be more difficult to interpret with additional error components. IntroductionFuel injectors are under constant scrutiny as one of the prime factors in governing the overall efficiency and emissions output from heavy duty diesel engines. Manufacturers of fuel systems are therefore pursuing increasingly challenging targets for the accurate quantification of the instantaneous mass flow rate and total fuel quantity delivered. The characteristics of the fuel delivered such as the time rate of change of mass flow for a given pressure differential (i.e. rail pressure) are as important as the total quantity delivered for a given "shot". The rate of change of fuel mass flow is an important consideration to engine developers as it allows more complex combustion strategies that can be tailored for a given engine configuration and application [1,2]. This 'rate shaping' is a useful tool in order to gain the incremental improvements of combustion that...

Fuel Injector Testing Through Development of a New Test Rig

“…Studies are made to measure the rate of fuel injection, nature of cavitation which could occur in the near nozzle region. This is done by analyzing the temporal signature of the pressure signals that arise in a typical injection cycle [9]. The test methods have been designed to evaluate injector deposit effects on performance in real world driving conditions.…”