General Motors' 3.6L DOHC 4V V6 engine has been upgraded to provide substantial improvements in performance, fuel economy, and emissions for the 2008 model year Cadillac CTS and STS. The fundamental change was a switch from traditional manifold-port fuel injection (MPFI) to spark ignition direct injection (SIDI). Additional modifications include enhanced cylinder head and intake manifold air flow capacities, optimized camshaft profiles, and increased compression ratio.
The SIDI fuel system presented the greatest opportunities for system development and optimization in order to maximize improvements in performance, fuel economy, and emissions. In particular, the injector flow rate, orifice geometry, and spray pattern were selected to provide the optimum balance of high power and torque, low fuel consumption, stable combustion, low smoke emissions, and robust tolerance to injector plugging. In concert with the injector selection, the intake port, combustion chamber, and piston bowl were designed for strong cold-start capabilities with low engine out HC emissions and quick warm-up of the catalytic converters.
The system development and optimization was facilitated by extensive use of computational fluid dynamics (CFD) for analysis of fuel sprays and their interaction with the piston bowl to achieve the desired mixture formation. The cold-start performance was further optimized through use of extensive cold-start buggy testing and analysis of designed experiment data. This paper details the analytical and empirical work done to optimize the SIDI fuel injector and the combustion system.
