Externally Divided Exhaust Period in a Turbocompound Heavy Duty C.I. Engine

Abstract

Turbocompounding has been a development on heavy diesel engines to improve the utilisation of exhaust heat energy. This system, however, introduces additional pumping losses and exhaust back pressure. Previous research has shown that the introduction of a divided exhaust period (DEP) to the turbocompound engine using staggered timing on exhaust valves feeding individual manifolds lowers the exhaust back pressure, improving engine breathing and behaviour. Exhaust flow is divided into two distinct flow paths beginning at the exhaust valves – through the turbocharger (blowdown), and bypassing it (scavenging). However, research showed that using one exhaust valve for each flow path can significantly lower the effective flow areas for exhaust gases at higher engine speeds. This leads to highly choked flow and consequently reduced engine output. A solution proposed was the division of the flow paths external to the exhaust port, henceforth named external DEP (ExDEP). In this solution, both exhaust valves open identical to those of the original engine, but the gas flow is divided downstream with the use of independent valves. By the use of ExDEP on a turbocompounded diesel engine the above research resulted in a brake specific fuel consumption benefit of up to 4% over a regular turbocharged engine. This thesis project, conducted at the Advanced Technology and Research division of Volvo Group Trucks Technology in association with the Combustion department of Chalmers University, aims to implement and evaluate the effects of ExDEP on an existing GT-SUITE simulation model of a two-stage turbocompounded Volvo HDE13 engine. Various flow components (valve types) and architectures are modelled. The effect of timing and phasing of the ExDEP valves on engine performance is investigated. Resizing of the existing turbine is investigated due to the altered gas flow. Rough CAD models are prepared to determine the feasibility of physically incorporating such systems on existing engines. A further adaptation to the existing engine model is the incorporation of CAD data of a novel engine concept currently being developed at Volvo GTT, in order to study the feasibility of ExDEP with this engine. With the implementation of ExDEP, a primary advantage foreseen is that the exhaust flow into the turbines can be constantly controlled. This eliminates the need for variable geometry turbochargers, bringing large reductions in system cost and complexity. It can enable other innovative technologies such as Miller cycle operation, air hybrid operation when an air tank is coupled to the exhaust manifold.