An investigation of the performance of a free-piston engine using Miller cycle and renewable fuels

Abstract

A dual-piston type two-stroke spark-ignition free-piston engine prototype has been developed. A comprehensive review on recent published researches and patent documents from academia and industrial organizations on free-piston engine, especially the application of renewable fuels such as hydrogen and ethanol, was conducted. Relevant parameters affecting the operating performance and a number of challenges had been identified as the common denominator for this technology. Modelling and simulations using one-dimensional tools were conducted in parallel with the development activities. Three main simulation models for the crankshaft engines were developed, validated and optimised before converted into the free-piston engine model. This was done by using imposed-piston motion sub-model. The two-stroke free-piston engine model had undergone parametric study for valve timing optimisation. By using similar methods, a two-stroke hydrogen free-piston engine was developed from the prototype and it was validated by experimental data. Emission performance, along with the relationship between equivalent ratio, in-cylinder temperature and NOx emission on the hydrogen FPE was investigated. Series of ethanol-gasoline blend free-piston engine were developed, essential parameters such as compression ratio, air/fuel ratio and valve timing, etc., were amended for each model and both engine performance and emission performance were analysed. Miller cycle has been applied on three types of free-piston engine models: gasoline FPE, hydrogen FPE and ethanol FPE. Two types of Miller cycle (LIVC and EIVC) were both applied and analysed, and different late/early intake valve closing angles (from 5°CA to 35°CA) have been converted to specific time to apply on the free-piston engine. Engine performance and emission performance of Miller cycle free-piston engines have been obtained and individually presented. It turns out that large late/early intake valve closing angles (time) are not suitable in renewable free-piston engines due to the efficiency reduction.