Spark ignited engines with direct fuel injection into the combustion chamber can be started by injecting fuel into the combustion chamber of the stopped engine and igniting it afterwards with the spark plug. The explosion of the air fuel mixture initially rotates the crankshaft. The engine is started without motoring the crankshaft with a starter device. The described start procedure is called “Direct Start”.
For a successful Direct Start a maximum oxygen concentration in the “Direct Start Cylinders” (the cylinders utilized initially for the Direct Start) is required, because the oxygen concentration in the start cylinders determines the maximum amount of energy, which can be exploited to move the crankshaft over the following “TDC” (Top Dead Center). After the engine stop the Direct Start Cylinder valves are closed. Therefore the oxygen concentration has to be maximized during the preceding engine shut down.
For the following Direct Start also an appropriate prediction of the oxygen concentration in the Direct Start Cylinders is important, with the aim to be able to meter a suitable amount of fuel for the Direct Start.
In order to measure the oxygen concentration in the Direct Start Cylinder a new measuring technique is developed. Based on an already known hydrocarbon gas sampling valve, a new sampling valve is designed and actuated cycle resolved. The sampling is triggered on the last cycle before engine stop.
With the new method the oxygen concentration in the Direct Start Cylinders is measured, while different throttle strategies are applied during engine shut down.
With a fully closed throttle during engine shut down the oxygen concentration in the Direct Start Cylinders is about 15 %. If the throttle is opened during the engine shut down the oxygen concentration exceeds 20 %.
The oxygen concentration is found to be proportional with the average cylinder pressure in TDC and the average manifold pressure in the preceding “BDC” (Bottom Dead Center). If this relation is established once for a specific engine set up, the oxygen concentration can be predicted by evaluating the cylinder or manifold pressure during engine shut down and therefore can be easily calibrated in the “ECU” (Engine Control Unit).