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The paper presents a 1D-3D numerical model to simulate the scavenging and combustion processes in a small-size spark-ignition two-stroke engine. The engine is crankcase scavenged and can be operated with both gasoline and Natural Gas (NG). The analysis is performed with a modified version of the KIVA3V code, coupled to an in-house developed 1D model. A time-step based, two-way coupled procedure is fully described and validated against a reference test. Then, a 1D-3D simulation of the whole two-stroke engine is carried out in different operating conditions, for both gasoline and NG fuelling. Results are compared with experimental data including instantaneous pressure signals in the crankcase, in the cylinder and in the exhaust pipe. The procedure allows to characterize the scavenging process and quantify the fresh mixture short-circuiting, as well as to analyze the development of the NG combustion process for a diluted mixture, typically occurring in a two-stroke engine.

The use of Compressed Natural Gas as an alternative fuel in urban transportation is nearly established and represents an efficient short and medium term solution to face with urban air pollution. However, in order to completely exploit its potential, the engine needs to be specifically designed to operate with this fuel. In the latest years, the authors have investigated the performances of a Heavy Duty Turbocharged CNG fuelled engine both experimentally and by using some analytical tools specifically developed by them which have been used for the engine optimisation. In the present paper the simulation approach has been enlarged by means of a co-operative use of a CFD code and experimental analysis on the actual engine. The numerical simulation of combustion process has, in fact, been used, to interpret series of pressure cycles, aiming to analyse how cyclic fluctuations influence engine behaviour in terms of combustion efficiency and temperature and pollutant distribution.

This paper discusses a partially stratified technology for engines running lean on natural gas. A single cylinder research engine has been modified to enable direct injection of a small quantity of natural gas through the spark plug to the region of the electrodes, independent of the overall lean homogeneous charge. Thus, a Partially Stratified Charge (PSC) is formed within the chamber allowing significant extension of the lean limit of combustion. Although PSC has been shown to reduce NOX emissions and improve combustion efficiency, high hydrocarbon emissions have been observed and this was thought to be due to poor mixing of the injected fuel air charge. The mixed experimental-numerical activity described herein, carried out by the Universities of British Columbia of Vancouver and Roma Tor Vergata, is aimed at improving the micro-direct injection PSC process.

Ultra lean-burn natural gas engines offer the potential for lower emissions and higher efficiency than conventional SI engines. Combustion instabilities near the lean limit can be addressed by partially stratifying the in-cylinder charge. The Partially Stratified Charge (PSC) approach involves micro-direct-injection of pure fuel, or a fuel-air mixture, to create a rich zone in the region of the spark-plug. This has been demonstrated to improve combustion in an ultra-lean bulk mixture. An experimental premixing apparatus was devised to investigate the effect of changing the stoichiometry of the micro-direct-injected charge. In conjunction, a numerical methodology was used as an aid to understanding the complex in-cylinder processes. Although rich premixed micro-injection improved engine performance over the homogeneous case, the fastest heat release rate was found to occur with a pure fuel PSC charge.

The first part of the paper gives an overview of the environmental conditions with which a future two stroke powered vehicle must comply and explains the reasons for which a direct gasoline injection into the combustion chamber offers a potential solution. The paper continues with a description of the fuel/air mixture injection used in the F.A.S.T. concept and gives a detailed overview of the layout of the 125 cc engine to which it is applied. The structure of its electronic engine management system, mandatory for the necessary control precision, is presented. Hereafter is made a short introduction to the visualization and numerical computation tools used for the engine design optimization. The paper concludes with a detailed presentation and discussion of the experimental results obtained with the engine operated, either in steady state and transient conditions on an engine test rig, and mounted in a classic small dimension two-wheel vehicle submitted to road tests.