Gottschalk, W., Kirstein, G., Magnor, O., Schultalbers, M. et al., "Investigations on a Catalyst Heating Strategy by Variable Valve Train for SI Engines," SAE Int. J. Engines 5(3):1177-1200, 2012, doi:10.4271/2012-01-1142.
The objective of this investigation was to evaluate the effects of a variable intake and exhaust valve timing in terms of opening, closing, opening duration, lift curve and number of active valves per pair on a four cylinder direct-injecting SI engine for the catalyst heating idling phase at the beginning of an NEDC emission test procedure. The first step evaluated the engine behavior at a reference point of operation. Its parameters in valve timing were adjusted to match the valve timing of the base production engine. The second step investigated the effects of an earlier exhaust valve opening while the exhaust valve closing time was kept and the exhaust valve opening duration was extended. The third step was to answer the question for the optimum number of exhaust valves in order to minimize the wall heat losses inside the cylinder head. The optimum 3V exhaust valve timing has been defined as the basis for exhaust valve timing for steps four and five. The fourth step contained the variation of intake valve opening and closing. The group of selected optimum valve timing / ignition timing combinations mainly consists of late intake valve opening and decreased intake valve opening durations. The fifth and final step was to evaluate the optimum number of intake valves in order to find out whether it makes sense to add another source of charge motion. It can be stated that a combination of both a late intake valve opening and intake valve deactivation must be excluded for this evaluated catalyst heating point of operation. From this point of view, two alternative VVT strategies with comparable potential can be seen: either late intake valve opening with decreased intake valve opening duration, or intake valve deactivation with standard intake valve opening duration and a slight valve overlap. Both of these intake valve strategies have at least one thing in common: a valve train with the digital ability to change the cams is necessary for their realization. Their general potential can be numbered in a 3-13 % increase of exhaust gas and catalyst temperature, a 5-30 % decrease of gaseous emissions output, and a 80-95% decrease of FSN. It also has the potential of a 5-25 % decrease in related standard deviation of imep.