Repeatable steady-state measurement of particulate number emissions in engine experiments

Particulate number count is an important consideration for engine developers due to changes in emissions legislation. Changes are driven by an increasing body of evidence that particulate number, particularly smaller particles, have a deleterious effect on human health. This article presents the results of an investigation into the key factors influencing particulate number emissions measurement repeatability during dynamometer-based testing of a gasoline direct-injection engine. At the outset of this work, a review of literature summarises some of the current discussion concerning particulate formation, evolution and measurement to identify the key factors that influence these three things. Having established what these factors are a number of engine experiments are undertaken to determine how sensitive particulate number measurements are to change in these factors and therefore how great an influence they are on measurement repeatability in engine experiments of a similar type. The investigation highlights a number of important results, showing that particular regard ought to be given to the pre-conditioning of engine internal surfaces which begins when the engine is started. In addition, the effect of coolant temperature (including the dynamics of the control system) is observed and highlighted as another key source of variation as is intake air temperature.