Experimental Analysis on the Effects of Passive Prechambers on a Small 2-Stroke Low-Pressure Direct Injection (LPDI) Engine

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Abstract
Two-stroke (2S) engines still play a key role in the global internal combustion engine (ICE) market when high power density, low production costs, and limited size and weight are required. However, they suffer from low efficiency and high levels of pollutant emissions, both linked to the short circuit of fuel and lubricating oil. Low- and high-pressure direct injection systems have proved to be effective in the reduction of fuel short circuiting, thus decreasing unburnt hydrocarbons and improving engine efficiency. However, the narrow time window available for fuel to be injected and homogenized with air, limited to few crank-angles, leads to insufficiently homogenized fuel-air mixtures and, as a consequence, to incomplete combustions. The use of prechambers can be a well-suited solution to avoid these issues. In fact, the faster combustion which results from the ignition process by hot turbulent jets, allows us to delay the ignition advance, thus achieving a more homogenous mixture inside the combustion chamber; furthermore, the improved mixing process which occurs inside the prechamber extends the lean flammability limit of the engine. The present paper shows the preliminary experimental results of the use of passive prechambers on a crank-scavenged Low-Pressure Direct Injection (LPDI) 2S 50 cm3 engine for motorcycle use. Analyses were carried out at full load conditions in order to understand the effects of adopting a prechamber on the engine performance, in terms of Indicating Mean Effective Pressure (IMEP), heat release, and cycle-to-cycle variation. Diameter of orifices was varied so as to achieve best performance on the investigated engine. Results prove that a faster and more effective combustion process can be achieved in the LPDI engine. Furthermore, the analysis highlights the importance of choosing the right flow-passage area of orifices, which is related to the prechamber volume, to avoid excessive in-cylinder pressure peaks or a too early pressure decrease during the expansion phase

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