Analysis of operating speed and efficiency of a 2-stroke linear engine

Abstract

A new approach to generate electricity with a device of high efficiency and low emissions is being performed. A small-bore 2-stroke linear engine has been designed to generate electricity linked with a linear alternator for portable use. It has an advantage that the compression ratio can be varied depending on the operating condition, so it can keep operating on high efficiency region with low exhaust emission. The current engine used in this study uses Dimethyl Ether (DME) premixed mixture as the fuel and combustion is initialized through spark ignition. During the operation, the amount of fuel consumed per stroke is influenced by the equivalence ratio of the mixture and the mixture intake pressure, where these 2 parameters are defined as fueling conditions. By varying the ignition timing, the best efficiency point can be found under a given fueling conditions, and the corresponding piston velocity can also be obtained. For a given fueling condition, thermal efficiency means the amount of work converted from the fuel, and the piston velocity means the amount of work that can be done per unit time. The final engine output is greatly determined by both factors; therefore the objective of this study is to investigate the effect of fueling conditions to engine’s efficiency and operation speed. It was found that increasing the intake pressure was preferable rather than increasing the equivalence ratio with respect to higher efficiency and faster frequency. However, change of strategy can be expected when the delivered fuel amount becomes large due to the increase intake pressure, because the loss of the short circuited fuel through the exhaust port become significantly high and offsets the high efficiency characteristics of the linear engine.