Model-Based Analysis and Control of a Diesel-Ignited Gas Engine

Abstract

The Diesel-ignited gas engine is a promising concept for reducing the emission of climate-damaging greenhouse gases from passenger cars. The reduction of CO2 emissions that becomes possible with this concept is a result of using an alternative fuel with increased efficiency. In the dual-fuel combustion process methane, as the primary fuel, is ignited by a small amount of Diesel. In addition to the dual-fuel combustion process, the Diesel-ignited gas engine can also be operated in pure Diesel combustion mode, like a conventional Diesel engine. While the operating strategies of conventional Diesel or gasoline engines are well known and optimized, the operation of the Diesel-ignited gas engine is still the subject of research. Specifically, the operation at low torque levels is demanding. The contradictory requirements of the two fuels lead to high raw emissions of unburnt methane. In addition, the removal of these raw emissions from the exhaust gas is difficult due to the low exhaust gas temperatures. The Diesel-ignited gas engine is thus not capable of covering low torque requirements using the dual-fuel combustion process, which is why only the transition to the conventional Diesel operation remains possible. The goal of this thesis is to describe the operational limits of the dual-fuel operation and to develop the essential operating strategies. For this purpose, the air path as well as the exhaust aftertreatment system is described by control-oriented models which are then used in numerical optimization methods. Finally, this thesis shows that the dual-fuel operation at low loads is only feasible with a dedicated control strategy that avoids the emission of unburnt methane at the tailpipe. The reaction heat originating from the conversion of carbon monoxide thereby plays an important role in the effective post-oxidation of the methane in the catalyst. Given the limited effectiveness of today’s catalyst, the stoichiometric operation, with its high raw emission of carbon monoxide, is the preferred dual-fuel strategy. For achieving torque levels beyond the operational limit of the dual-fuel combustion process, the transition to Diesel operation is crucial in terms of the practicability of this engine type. However, the transition between the combustion modes is a challenging task as various actuator settings, from both the fuel and air paths, change significantly during the transition. This thesis shows that only the optimal design of the oxygen concentration trajectories in the intake and exhaust manifolds yields a smooth transition with significant reductions of both the torque deviation and the emissions of pollutants.