Turbo-Cool Turbocharging System for Spark Ignition Engines

Abstract

The problem of matching a turbocharging system to a piston engine is investigated. The variable-geometry turbine (VGT) turbocharger used for spark ignition (SI) engine application is studied in some detail. A critical factor that has been overlooked in considering VGT turbochargers for SI engine application in the past is the backpressure to the piston engine and, consequently, the resulting residual gas and its effect on the temperature of the in-cylinder mixture at the beginning of combustion. That is, VGT turbocharger operation affects engine backpressure, which affects the in-cylinder mixture temperature, which affects the tendency for engine knock, which limits engine performance. The higher backpressure also raises the turbine entry temperature. A new turbocharging system named Turbo-Cool is proposed in this paper. The Turbo-Cool turbocharging system utilizes more complete exhaust gas expansion in the turbine to achieve a higher engine effective expansion ratio (EER) in a straightforward manner. The novelty of the concept is that the excess power from higher exhaust gas expansion through the turbine (rather than being considered as an object for power recovery, as in the classical turbocompound concept) is used for conditioning intake charge air for the purpose of engine loss reduction. The validation of the proposed concept is carried out using a commercial engine simulation code. The simulation results show that the Turbo-Cool turbocharging system and the VGT turbocharger are indispensable to each other in producing engine performance of superior brake mean effective pressure (b.m.e.p.) and excellent torque back-up, as well as improved engine efficiency. It is argued that the Turbo-Cool turbocharging system is a basic design that might bring about the full potential benefits of turbocharging for SI engines.