Abstract
The availability of compressed air in combination with downsizing and turbocharging is a promising approach to improve the fuel economy and the driveability of internal combustion engines. The compressed air is used to boost and start the engine. It is generated during deceleration phases by running the engine as a piston compressor. In this paper, a camshaft-driven valve is considered for the control of the air exchange between the tank and the combustion chamber. Such a valve system is cost-effective and robust. Each pneumatic engine mode is realized by a separate cam. The air mass transfer in each mode is analyzed. Special attention is paid to the tank pressure dependence. The air demand in the boost mode is found to increase with the tank pressure. However, the dependence on the tank pressure is small in the most relevant operating region. The air demand of the pneumatic start shows a piecewise continuous dependence on the tank pressure. Finally, a tank sizing method is proposed which uses a quasi-static simulation. It is applied to a compact class vehicle, for which a tank volume of less than 10 L is sufficient. A further reduction of the tank volume is limited by the specifications imposed on the pneumatic start.
Highlights
The use of compressed air to reduce the fuel consumption and improve the driveability of internal combustion engines has been extensively investigated for many years [1,2,3,4,5]
The compressed air required to enable the modes described previously is generated during the vehicle deceleration, when the engine is operated as a piston compressor
The tank volume is limited by the lower limit of the tank pressure
Summary
The use of compressed air to reduce the fuel consumption and improve the driveability of internal combustion engines has been extensively investigated for many years [1,2,3,4,5]. In the pneumatic start mode, compressed air is injected during the expansion stroke to start the engine. Cam design methodologies have been proposed for the boost mode [6] and the pneumatic start mode [13]. They cope with the various constraints, such as a lower limit for the tank pressure to prevent the flow of air–fuel mixture to the tank. In the pneumatic motor mode, the torque is controlled by the valve timing [15]. This requirement cannot be fulfilled with a fixed cam profile. The sensitivities of the results to the initial tank pressure, the tank size and the gear-shifting strategy are evaluated
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