1990-10-01

An Investigation of Injection Rate Controlled Heat Release of Low Cetane Fuels in a Direct Injected Diesel Engine 902061

Assisted ignition and subsequent combustion of various fuels differing in volatility and viscosity in a Diesel engine is herein described. This study was conducted to investigate the feasibility of igniting low cetane fuels at the immediate vicinity of the nozzle orifice in an attempt to produce injection rate controlled heat release. Four fuels were studied: a high viscosity, low volatility Diesel blend, a low viscosity, high volatility Diesel blend, strait gasoline, and No.2 Diesel fuel which was used as a baseline for comparison purposes. A droplet ignition delay model was used to provide insight into the various physical processes that occur when heat release is controlled by rate of injection. Split injection timing predicted by the model resulted in the successful occurrence of rate controlled heat release for all of the fuels tested. Further, results of this study demonstrate that high pressure gradients normally associated with low cetane fuels can be significantly reduced or entirely eliminated with this ignition technique. The fuels examined in this study bracketed a broad spectrum of fuel distillate thereby demonstrating the usefulness of this technique for other fuels as well as those tested.
SINCE THE BIRTH of the mobility industry around the turn of the century, the transportation and petroleum industries have worked together towards a common goal of achieving maximum efficiency in the I.C. engine. The availability of high quality fuel for engine operation made this optimization process relatively easy. The primary objective was to obtain the maximum amount of power output for a given quantity of fuel. Various engine designs utilizing high quality fuels were used to achieve that goal.
One of the candidates for the future combustion of alternate fuels is the direct injection stratified charge (DISC) engine. The DISC engine is actually a hybrid between spark ignition and compression ignition, incorporating the best features of both concepts, such as high thermal efficiency and fuel metering flexibility. The DISC engine also yields itself to such concepts as spark assist and glow plug assist to combust low cetane alternate fuels. With the proper combination of fuel injection parameters and combustion chamber configurations, the DISC engine is under consideration for becoming a useful tool to the combustion engineer of the future.
In addition to engine type, various fuelling arrangements may be used to introduce the fuel into the combustion chamber. One promising method for improving the combustion of low cetane fuels is the use of pilot injection. In the past, researchers have used various forms of pilot injection to decrease the ignition delay of low cetane fuels. These methods range from the introduction of intermediates into the chamber to decrease the ignition delay, to the use of pilot spray combustion to ignite the main injection of fuel. As will be seen in this paper, pilot injection is a definite candidate in the search for a reliable technique to repeatedly produce injection rate controlled combustion in a Diesel engine.

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