The effect of compression ratio on exhaust emissions from a PCCI diesel engine

Single cylinder engines are widely used for engine research because they allow isolation of the detail of behaviour within an individual cylinder rather than an average over multiple cylinders while eliminating the inter-cylinder interactions. A description of the development of a single cylinder test facility is presented, being based on a production 4-cylinder DI diesel engine and designed to allow study of the emissions characteristics over a very wide range of operating conditions. The objective was to establish how engine out NOx emissions can be reduced to the estimated levels required by the next emissions target ‘Euro 6’ and thus be able to apply the findings to the original 4-cylinder engine and minimise the requirement for currently immature NOx after treatment. To meet these needs, both the intake and exhaust systems were developed so that inlet air conditions and exhaust back-pressure could be individually varied over wide ranges independently of the other engine operating conditions. Several key operating points were defined across a load speed map to represent a typical drive cycle requirement. Baseline emissions data at these key points had been taken for the 4-cylinder engine and were reproduced on the single cylinder engine. It has been proposed that further reduction in compression ratio beyond current levels would be beneficial to engine out emissions and specific power, and could be facilitated by developments in cold start technology. The results of a study using this single cylinder facility to evaluate the effect of reducing compression ratio from 18.4 to 16.0 are presented. This was achieved by modifying the piston bowl while maintaining the production engine squish clearance. Injection timing investigations were performed at a number of the key operating points and the corresponding pressure-time curves analysed to help explain the measured results. It was found that, although there was a small CO and HC penalty, either reducing the compression ratio or retarding the injection timing greatly reduced NOx and soot emissions when both premixed and diffusion-combustion phases were present. This effect was less significant when the combustion was solely premixed.