Title: Comparative Evaluation of Various Strategies for Improving the Characteristics of Performance of a Pilot Ignited Natural Gas/Diesel Engine
Abstract: Automotive engineers have proposed various solutions towards the simultaneous reduction of NO and soot emissions from direct injection compression ignition engines, one of which is the use of "natural gas/diesel dual-fuel engines". Natural gas is considered to be a promising alternative fuel for vehicular applications, particularly in urban transit bus fleets due to its clean combustion. It has high auto-ignition temperature compared to other gaseous fuels facilitating thus its use on future and existing compression ignition engine fleets without serious modifications of their structure. According to extended experimental and theoretical investigations conducted in the past by the present research group, natural gas/diesel operation results to curtailment of NO emissions accompanied by significant reduction of soot emissions while it affects negatively engine efficiency and CO emissions compared to normal diesel operation. In an effort to minimize these negative side effects the present research group has investigated various technical solutions and specifically: the quantity of injected liquid fuel and the introduction of exhaust gas recirculation (EGR) to control NOx. In order to be examined the effect of the aforementioned parameters a theoretical investigation has been conducted using a comprehensive two-zone phenomenological model. From the results of the present investigation, important information is derived revealing the applicability of each one of the aforementioned techniques on an existing pilot ignited diesel/natural gas engine. This has been achieved by making a comparative assessment of the relative impact of each technical solution on engine performance characteristics and emitted pollutants. The information derived from the present work is valuable, especially if one wishes to define the optimum combination of examined strategies to improve the behavior of an existing direct injection compression ignition engine running under natural gas/diesel operating mode.