Analysis of a Prechamber Ignited HPDI Gas Combustion Concept

High-pressure direct injection (HPDI) of natural gas into the combustion chamber enables a non-premixed combustion regime known from diesel engines. Since knocking combustion cannot occur with this combustion process, an increase in the compression ratio and thus efficiency is possible. Due to the high injection pressures required, this concept is ideally suited to applications where liquefied natural gas (LNG) is available. In marine applications, the bunkering of and operation with LNG is state-of-the-art. Existing HPDI gas combustion concepts typically use a small amount of diesel fuel for ignition, which is injected late in the compression stroke. The diesel fuel ignites due to the high temperature of the cylinder charge. The subsequently injected gas ignites at the diesel flame. The HPDI gas combustion concept presented in this paper is of a monovalent type, meaning that no fuel other than natural gas is used. The high-pressure gas jet is ignited with the aid of flame torches from a gas-scavenged prechamber. The design and layout of the combustion process and the prechamber shape were simulation-based. It’s predesign and validation on a single-cylinder research engine (SCE) were presented in SAE Technical Paper 2019-01-0259. More detailed results of the SCE measurements are presented and discussed in this paper. The results clearly demonstrate the high number of degrees of freedom of the prechamber ignited HPDI gas combustion concept. The publication shows investigations on the influence of injection timing, ignition timing in the prechamber and gas supply pressure. Combustion can be altered by adjusting either injection timing or ignition timing when simultaneously adjusting the other parameter. Different combinations of these parameters have a strong influence on the rate of heat release by altering the amount of high pressure gas that can premix with the cylinder charge. Potential measures for improvement are outlined. An increase in the gas injection pressure helps to increase efficiency for a constant NOx emission level. Finally, a comparison of the prechamber ignited HPDI gas combustion concept and a representative diesel combustion concept reveals that the expected properties have been met. The same combustion performance as with a diesel engine can be achieved using the prechamber ignited HPDI concept.