Optical investigations using optical fibres were carried out in the first available direct injection SI-engine, the Mitsubishi GDI, in the driving mode. The optical access to the combustion chamber was realized by 8 optical sensors evenly distributed in a ring on the ground electrode of the standard spark plug.
All investigations, steady state (constant load and velocity) and unsteady state (engine starts), show, that there is preferred flame propagation to the intake valves, caused by a reverse tumble in-cylinder flow. As the inflammation depends on thermodynamic conditions, flow characteristics and the actual air/fuel-ratio at the spark plug, the optical sensors can be used to describe the quality of stratification. The time between spark and the first flame detection at the sensors is short in case of high velocity and high load (induction stroke injection) and nearly comparable to conventional SI-engines, and in case of low velocity and low load (compression stroke injection) the time duration until flame detection is long.
The behaviour during start of the GDI-engine was tested, too. The measurements using a Fast Response FID show very high emissions of hydrocarbons, both during cold-start and during warm-start. In any case the engine starts in the homogeneous mode to assure a safe driving behaviour and changes depending on oil- and coolant temperature to the stratified mode.