Modeling Low-Pressure Injections in Diesel HCCI Engines

Homogeneous Charge Compression Ignition (HCCI) combustion is being considered as an alternative to conventional engine combustion systems due to its high efficiency and low engine-out emissions. To prepare a homogeneous mixture for diesel HCCI combustion, two types of low pressure (5MPa~20MPa) injectors were considered: a swirl injector and a multi-hole injector. A modified version of the KIVA-3V R2 code, was used to simulate the two types of injections. The Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) hybrid breakup model, which is often used to simulate droplet breakup processes of high-pressure (50MPa~300MPa) diesel injections, was recalibrated and extended for low-pressure, multi-hole injection applications. Two techniques were used to improve the prediction of spray behavior: use of an independent collision mesh with random rotation, and coupling the gas and liquid phases using polar interpolation. The numerical models were validated by comparing simulation results with experiments under different conditions. The simulation results show that the spray structure of the swirl nozzle injection is sensitive to the intake flow field and in-cylinder gas density, while the spray structure of a multi-hole nozzle injection is less influenced by the in-cylinder flow and gas density. The simulation results also show that swirl injectors are more suitable for low ambient pressure (<0.3MPa) conditions because at high pressures (>0.3MPa), the hollow-cone spray collapses into a solid-cone spray. Multi-hole injectors are more suitable for high ambient pressure conditions because at low pressures, the spray penetration is too long, which can cause spray-wall impingement. Corresponding author Introduction Future diesel engine technologies will need to incorporate advanced combustion strategies for achieving low emissions while maintaining good fuel economy and power density. It will be necessary to operate seamlessly over broad load and speed ranges when conditions change between different combustion regimes. HCCI combustion is being considered as an alternative to conventional engine combustion systems. It has the potential to eliminate noxious engine-out emissions while producing higher engine efficiencies. To prepare a homogeneous mixture, fuel should be injected early into the cylinder to allow enough time for fuel/air mixing. However, when direct injection of fuel into the cylinder during the intake or early compression stroke is used for mixture preparation, the use of conventional high pressure common-rail injection systems is limited by the relatively low in-cylinder gas density due to spray impingement on the cylinder walls [1]. Too much wall impingement can not only deteriorate the quality of the charge mixture, increase fuel consumption and unburned hydrocarbon emissions, but also lead to lubrication oil contamination. Therefore, it is of interest to consider low-pressure injection systems as an alternative. In this study, the KIVA-3V Release2 code [2] with improved numerical models was used to explore the charge preparation in a diesel HCCI engine using two types of low-pressure injectors: a swirl injector and a multi-hole injector. Several numerical models in KIVA were updated and the improved models were validated by comparing simulation results to experiments. An inhomogeneity concept is also proposed to evaluate the effects of injector type and SOI timing on diesel HCCI charge preparation. Numerical Approach The CFD code used in the simulations was a version of the KIVA-3V Release2 code with improvements in various physical and chemistry models developed at the Engine Research Center, University of Wisconsin-Madison. The RNG k-ε model [3] was used for the incylinder turbulence simulation. For hollow-cone sprays from swirl injectors, the Linearized Instability Sheet Atomization (LISA) breakup model [4] was used to calculate the primary breakup process of the fuel droplets, and the Taylor Analogy Breakup (TAB) model [5] was used for the secondary breakup calculation. For low-pressure multi-hole sprays, the KH-RT hybrid breakup model, which is used to simulate the droplet breakup process of high-pressure diesel injections, was recalibrated and extended for low-pressure injection applications An advanced unsteady vaporization model [6] was applied to predict the droplet evaporation process. A droplet collision model based on the stochastic particle method [7] was used to calculate the droplet collision and coalescence. The effects associated with spray/wall interactions including droplet splash, film spreading due to impingement forces and motion due to film inertia were considered in the wall-film model [8]. Engine Specifications and Operation Conditions The engine considered was a Caterpillar 3401E single cylinder oil test engine (SCOTE). The specifications of the engine are listed in Table 1. Engine type Caterpillar 3401E Bore× Stroke (mm) 137.2 × 165.1 Engine valve timing (°CA)* EVC=-355, IVC=-143, EVO= 130, IVO= 335