Experimental and Numerical Investigations on the Transient Performance of a Turbocharged Diesel Engine

In the highly competitive automotive market the share of diesel engine is continuously increasing. They are extensively used in passenger cars as well as in long distance haulage sector vehicles to impart motive force. Regulations for diesel engine emissions as well as public concern for fuel economy have forced the research community to address the combustion and emission issues associated with the use of the diesel engines in general and during the transient operation mode in particular. Turbocharging of diesel engine is the most extensively used technology for the improvement of power density, emissions and enabling downsizing of engines without compromising power. However, turbocharged diesel engine suffer from turbo-lag which is a common phenomenon especially during the rapid transient conditions. Turbo lag causes engine performance deterioration and increased emissions during transient events. The studies on reducing the turbo lag and maintaining the desirable air fuel ratio are considered to be very important for making turbocharged engine compliant with the current environmental regulations.

The thesis covers the important aspects of a turbocharged diesel engine with a specifc focus on the transient response of the diesel engine. In the first facet of this research, computer-based investigations using commercialengine simulation packages are performed to simulate the transient response of the system using different methods. Torque assistance of 0.16 Nm reduces the turbo lag by 3.6 sec for both compressor exit pressure and compressor speed. Optimum value of inertia reduction is found to be -10% which reduces the turbo lag by 2.9 sec for compressor exit pressure and by 0.6 sec for compressor speed. The effect of 2.5 bar air injection is found to reduce the turbo lag by 3.9 sec for both compressor exit pressure and compressor speed. A comparison is made for the assessment of relative improvement in transient response brought by the three methods and based on this comparison, air injection system is chosen that yields maximum benefit in the performance of the engine. The experimental results from the real CI engine is used to validate the simulation model. A good agreement is achieved between the simulation and experimental results.

The effect of air injection on a heavy-duty turbocharged diesel engine under various operating conditions forms the second facet of this study. The turbocharger response parameters are analyzed under the effect of air injection for different transient operating conditions of speed and load transients. For speed transients, considering the energy imparted for air injection, 1 bar is the optimum injection pressure with turbo lag reduction per unit energy as 0.290, 0.392 and 0.555 per joule for compressor exit pressure, turbine inlet pressure and turbine inlet temperature (TIT) respectively. Faster recovery time is noted for 1 sec rapid acceleration than that of 2 sec with the application of air injection. For load transients, optimization of air injection is also performed for injection pressure and orifice diameter. Under constant load step, maximum improvement in turbo lag reduction is observed at 1000 {1400 rpm whereas its effect on maximum attainable value is at 1600 { 1800 rpm. For load magnitude variations, the optimum injection pressure of 3 bar at 15 mm orifice diameter brings more improvement for lighter load than for stronger load. Maximum improvement is noted for 50-70% load step. 3 bar air injection at 15mm orifice diameter is the optimum injection pressure which brings more improvement in terms of turbo lag reduction for faster load application. The effect of this optimized air injection is more beneficial for 1 sec load schedule than for 2 sec.

During the transient operation of diesel engines, exhaust emissions are the imperative issue that needs to be addressed. In the third facet of the research exhaust emissions are analyzed to make sure that the applicationof air injection technique does not need compromise on emissions. The air injection technique is evaluated for exhaust emissions through simulation. 3 bar air injection at 10 mm orifice diameter is the optimum air injection for emissions under speed transient which satisfies Euro 6 standard for CO and HC emissions. This air injection reduces the concentration of CO and HC emissions by 5% and 0.4% respectively. Under load transient the optimum value of air injection is 1.2 bar at 10 mm orifice diameter which reduces the concentration of CO, HC and NOx emissions by 0.8%, 0.01% and 0.4% respectively.

The study reveals that air injection technique while improving the transient response of the turbocharged diesel engine doesn't increase the emissions, highlighting the magnitude of contribution of the technique to the overall performance of the system. The novelty in the research is the compilation of these methods in a cohesive approach of modeling the transient response of turbocharged engine system.