Abstract
Several previous studies have proven that pneumatic hybridization of an internal combustion engine is a technically viable and cost-efficient alternative to electric hybridization. Because the heat transfer process remains a first order factor while the engine operates in a motored or pneumatic mode without combustion, an accurate instantaneous heat transfer model is required in order to predict the in-cylinder pressure. This study shows that the original Woschni model is not suitable for describing the shape of the measured instantaneous heat flux versus crank angle while the engine operates in a motored pneumatic mode, because of the extinction of tumble motion near Top Dead Center (TDC). A modified form of the Woschni model is therefore proposed here that better describes the instantaneous heat flux during compression and expansion strokes, applicable to pneumatic and motored with fuel cut-off modes. First, the new constant parameter model is identified from experimental measurements performed in various motored mode conditions. Next, the pneumatic supercharged mode without combustion (ignition not performed) is investigated with the new model using the same identification values as for the motored mode. This study shows that the modified model remains fully applicable to the supercharged mode, despite of a certainly modified in-cylinder aerodynamic structure created by the opening of the charging valve under a high pressure ratio.
Highlights
In order to reduce pollutant and greenhouse gas emissions, Internal Combustion Engines (ICE) have to constantly improve their mean efficiency.The first problem of ICE is that peak efficiency is obtained near full load
This paper focuses on a convective heat transfer model while operating in a motored with fuel cut-off or pneumatic modes for two reasons
To take into account the change of in-cylinder air motion during compression and expansion, that causes the asymmetrical shape shifted from Top Dead Center (TDC). of the instantaneous heat flux curve, a modification of the Woschni model is proposed
Summary
In order to reduce pollutant and greenhouse gas emissions, Internal Combustion Engines (ICE) have to constantly improve their mean efficiency.The first problem of ICE is that peak efficiency is obtained near full load. Maximum power and torque leads to increasing engine displacement, to decreasing mean efficiency at part load which corresponds to real life driving conditions. This drawback can be tackled by downsizing the engine while keeping a high maximum power with the use of a turbocharger. This in turn creates the problem of “turbo-lag’’ during a strong acceleration transient phase. Given the high pressure ratio between the air tank and the cylinder, the opening of the charging valve probably modifies the internal aerodynamic structure created during the intake process.
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