A Simulative Study for Post Oxidation During Scavenging on Turbo Charged SI Engines

Fulfilling exhaust emissions regulations and meet customer performance needs mainly drive the current engine development. Turbocharging system plays a key role for that. Currently turbocharging should provide highest engine power density at high engine speed by also allowing a very responsive performance at low end. This represents a trade-off in turbocharger development. A large scaled turbine allows having moderate exhaust gas back pressure for peak power region, but leading to loss of torque in low engine speed. In the last years of engine development scavenging helped to get away a bit from this trade-off as it increases the turbine mass flow and also reduces cylinder internal residual gas at low engine speed. The mostly in-use lean strategy runs air fuel ratios of closed to stoichiometric mixture in cylinder and global (pre catalyst) of λ = 1.05 to l = 1.3. This will be out of the narrow air fuel ratio band of λ = 1 to ensure NO_x conversion in the 3-way-catalyst. The use of the rich strategy (in cylinder λ < 1, pre catalyst stoichiometric) increases significantly the brake specific fuel consumption and lowers the exhaust gas temperature, which leads to loss of turbine power. Nevertheless as a result of this procedure, fresh air and large amount of unburned fuel enters the catalyst in short sequences. This can lead to high temperature in the catalyst and thus reduces its live cycle.

The present paper aims to show the potential of oxidation of unburned species pre turbine to empower the turbine and also reduce the amount of emissions needed to convert by the catalyst. Within this paper the use of detailed reactions kinetics mechanism of CO and H₂ will be shown. Furthermore some 3D CFD calculation results will be presented to show the mixture in the exhaust manifold. To show the maximum potential 1D simulation including a strongly reduces oxidation mechanism has been carried out.