Abstract
ABSTRACT:
One of the most promising ways to improve the global efficiency of SI engines, is the concept of "downsizing". The principle of the downsizing concept consists in reducing the size of the engine to maximize global efficiency (minimize consumption) while keeping the maximum torque constant. To maintain the level of torque, it is essential to supercharge the engine with a large turbocharger, which causes an unacceptable lag during torque transients.
A pneumatic hybridation concept which uses air from a high pressure tank to supercharge a conventional internal combustion engine, makes possible to completely compensate for the aforementioned delay during torque transient phases.
The concept is based on a traditional internal combustion engine on which an additional valve, called the charging valve, connects the combustion chamber of each cylinder to a compressed air tank. The opening and the closing of this valve can be controlled by an additional camshaft or an electric actuator.
During the transient phases where torque needs to be compensated by pneumatic supercharging, the operation of the hybrid engine resembles that of a conventional engine except that, after the closing of the inlet valve, the charging valve opens and an additional air mass is injected into the combustion chamber. This additional mass of compressed air at a high pressure (10-20 bars) produces a higher work of the thermodynamic cycle. To maximize the work of the cycle, the supercharging phase must be carried out as late as possible. The optimal strategy would be to close the charging valve when the cylinder pressure reached the pressure of the tank, and optimizing the opening angle of the charging valve for managing the additional air mass.
During load increase transient phases in hybrid supercharged mode, the intake air mass is higher than the intake air mass of a conventional engine. Consequently, the exhaust flow as well as the energy provided to the turbocharger turbine are higher with the hybrid engine, providing a faster turbocharger spool-up and thus, a decrease in intake manifold pressurization time. In addition, the required couple is obtained instantaneously thanks to the injection of compressed air directly into each cylinder.
The advantages of hybridization are notable:
• The mechanical work provided by a 2 liter engine at full load can be ensured at any time by a smaller engine (around 1.2 liters) functioning in pneumatic supercharged mode.
• For two engines to the same size, one with turbocharger and the other functioning in pneumatic supercharged mode, the desired turbocharger speed is reached more quickly by the hybrid engine.