Due to the short mixture formation times in the direct injection of modern gasoline engines, there is an increase in the emission of undesirable particle emissions. It is well known that particulate mass is not very high compared to diesel engines. However, the harmful small particles are a problem, which has led to the legislator limiting the number of particle emissions.
As a result of previous studies with a portable emission measurement system (PEMS) of raw cleaned exhaust gas, the particle number (PN) of the sampling point after the catalyst was higher than before the catalyst at the same process parameters and engine operating points. Based on this reproducible phenomenon, several theories were proposed. The theories set up dealt on the one hand with the question of process control with regards to the formation of particles, but also fundamentally if conventional exhaust gas aftertreatment systems (three-way catalytic converters) are suitable for influencing the number of particles. To verify these theories and to investigate the influence of the catalyst on the PN and the particle size distribution (PD), several series of measurements were taken. In general, the investigations and measurements cover process and component related topics.
Specifically, the authors combined several measurements of injection parameters, such as variation of rail injection pressure and start of injection (SoI) at steady state operating points. Further investigations were conducted with the equivalence ratio and its effects on PN and PD before and after the catalyst, respectively. Such investigations make sense, since normally the so-called forced excitation of the catalytic converter only relates to the conversion of the gaseous pollutants. For the time dependent PN measurement PN(t), the authors found a correlation between the AFR controller and PN. This newly found relationship enables the forced excitation of the catalyst to be optimized for the conversion of particle components. To date, the parameters AFR-setpoint, period duration and period length have been calibrated exclusively according to the criterion of the oxygen storage capacity and thus according to the conversion of the gaseous pollutants.