Divided Exhaust Period - A Gas Exchange System for Turbocharged SI Engines

The necessity to limit the boost pressure in turbocharged gasoline engines results in higher exhaust pressure than inlet pressure at engine speeds when the wastegate is opened. This imbalance has a negative influence on the exhaust scavenging of the engine and results in high levels of residual gas and consequently the engine is more prone to knock.

This paper presents a study of a gas-exchange system for turbocharged SI engines. The concept aims at improving the performance and emissions of a turbocharged SI engine by dividing the exhaust flow from the two exhaust valves into two different exhaust manifolds, one connected to the turbocharger and one connected to a close coupled catalyst. By separating the valve opening period of the two valves and keeping the duration of both valve opening events shorter than 180 crank angle degrees, the disturbance of the exhaust blowdown pressure pulse during valve overlap in a four cylinder engine can be completely eliminated.

The study was carried out both experimentally on a four cylinder turbocharged SI engine and with extensive 1-D simulation of the system. A positive pressure difference over the engine could be realized over the entire speed range by using the concept system. Simulations show up to 60 % reduction of residual gas content compared to a standard turbocharged engine. The study also showed that the time to catalyst light off could be reduced with over 35% compared to a standard turbocharged engine with a conventional exhaust system.