Thermodynamic Study of Turbocharger Matching and Combustion Optimization for Better Low End Torque and High Speed Power

摘要

Diesel Engines are known for its low fuel consumption coupled with high specific power output. Downsizing the engines with turbocharging and common rail injection technologies are the recent trends in improving the efficiency and performance of diesel engines. It is very challenging to match the torque targets at low speed and power targets at high speed range of a diesel engines due to system hardware limitation. Torque at lower engine speed will improve a greater extent to the drivability of a vehicle. Formation of black smoke is a major problem in lower engine speeds due lack of air availability. The use of variable geometry, two stage turbocharging and four valves per cylinder are some of the solutions which make the task simpler, also involves additional cost and fundamental design changes. At the same time commonly used waste gate turbocharger for boosting the airflow, fails to deliver required air flow at lower engine speeds. We took the challenge of matching a waste gate turbocharger to the engine torque and power targets. Several iterations have been done at engine test bed to finalize the A/R ratio for turbine followed by the compressor trim optimization to get best low speed torque and high speed power by maintaining the performance limits such as surge, choke margin and turbo speed. This study highlights the thermodynamics involved in matching a turbocharger to an internal combustion engine and standardize the process of turbo matching. Common rail diesel injection technology gives the flexibility in controlling the timing, quantity and number of injections. The injection mass ratio and dwell between the injections of a double-pulse injection strategy have great effect on fuel distributions and air-entrainment inside the sprays. Using the split-injection with small quantity and an appropriate dwell between next injections will play a major role in governing the premixed burn. The subsequent injection of double-pilot injection strategy has a turbulent effect on the fuel-air mixing in diesel sprays. This split injection strategy significantly improve the fuel-air mixing by allowing more air entrain into the spray. Thus, for a direct injection diesel engine, utilizing the turbulent effect of split-injection may enhance the combustion in the later stage and re-burning of the particulate matter in earlier combustion stage, thus reducing the black smoke formation. The in-cylinder pressure and ROHR studies have been performed to better understand the in-cylinder combustion during the split-injection