Transient response of turbocharged diesel engine for transient operation using air injection assist system

Great improvements have been made in engine performance and emission control due to recent intensive research on engine systems. Further improvements are still required and turbocharging is aiding further improvements in emissions, power and downsizing. Turbocharged vehicles, however, exhibit a weak point of poor drivability under transient running conditions.

This poor drivability has a detrimental affect on the engines emission output during the transient phase. Various methods are being researched to improve transient performance of turbocharged engine systems. This study analyses the effectiveness of injecting air into the inlet manifold and the compressor at improving the transient performance of a turbocharged diesel engine when performing rapid load changes. The paper expands upon previous work carried out which demonstrates the potential of the system.

Computer simulations have been used to analyse the transient engine performance including the drive train. The transient engine models have been developed to analyse the transient engine performance with various injection strategies. The air injection system was modelled by injecting air into the plenum on the intake manifold and using an additional torque on the compressor wheel. The injection pressure was set at a pressure of 3 bar, which is in commonly used range. The valve and additional compressor torque were programmed to activate at the same instant that the transient regime begins.

To assess the effectiveness of the air injection strategies at improving the transient response of the engine the load step simulation was undertaken. During the simulations the air was injected either into the inlet manifold, compressor or both manifold and compressor also known as two point injection. A parametric study has been undertaken analysing vehicle speed, outlet pressure of the compressor, turbine inlet temperature and driver demand.

It has been observed that the air injection systems improve the transient response of the turbocharger and the overall performance of the engine. The vehicle speed response has been shown to be greatly improved, with the time required for the engine to reach 30kph when air injection is used, reduced by 71% with an additional minor gain for two point injection. These Improvements to the transient performance lead to a significant reduction in engine emissions.