Turbocharging of high performance compressed natural gas SI engine for light duty vehicle
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Natural gas as an automotive fuel has many benefits in comparison with traditional fossil fuels. Favorable anti-knock properties of methane allow us to utilize higher boost levels and the engine power than that of gasoline engines. High level of intake boosting male possible to achieve loads, comparable to the state of the art diesel engines without soot and PM emission. Stoichiometric operation within the full range of the complete engine map enables the use a relatively simple exhaust gas aftertreatment, based on a three way catalyst. The paper describes a chosen 1-D thermodynamic modelling studies, calibrated and validated by experimental data. The investigations were performed on a spark ignition, direct injection, four cylinder engine with 1.6 l displacement. The engine was optimized for mono fuel operation with compressed natural gas. Due to complexity of gaseous fuel infrastructure in vehicles, compared to the traditional fuels, it is desirable to keep the turbocharging system as simple as possible. Traditional variable geometry turbine systems were tested. Practical design constraints as peak cylinder pressure, turbine inlet temperature, compressor outlet temperature and others were met. Various strategies on how to achieve high load at low engine speed were investigated. The authors propose a single stage turbocharger to cover the demand for a high torque at low engine speed and high power at full speed, with boost levels comparable to a dual stage turbocharging. It was concluded that the single stage turbocharging enables the engine to operate with maximum BMEP of 3 MPa between 1500 and 2750 rpm. Maximum engine speed had to be limited to a similar value that is usually applied in a diesel engine due to limited control range of turbocharging.
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