This paper describes the operation of a heavy duty six-cylinder engine in a dual fuel, Low Temperature Combustion (LTC) mode with very low engine-out NOx und soot emissions according to the US EPA Tier IV final emission limits in the corresponding C1 test cycle. This operation mode makes use of a short pilot injection of diesel fuel, which is injected directly into the cylinder, to ignite a highly diluted, premixed gasoline air mixture. Multicylinder engine operation could be demonstrated over the entire engine operating map with loads of up to 2 MPa BMEP. Expensive aftertreatment systems for NOx and soot emissions are not required.
This paper also discusses the challenges involved with the implementation of this combustion system on a multicylinder engine. When transferring the dual fuel LTC from a single cylinder research engine to a multicylinder engine, the design of some engine components, e.g. the camshaft and the piston, were changed. The intake manifold is modified with port fuel injectors for ideal gasoline mixture preparation and equal distribution to all cylinders. To avoid cylinder imbalances, it is possible to control the injected masses of gasoline and diesel fuel for the pilot injection on a per-cylinder basis. Achieving high dilution for ignition delay via EGR and boosted intake pressure to avoid high pressure rise rates and knocking presents challenges for the two-stage turbocharger design. Additionally, high EGR rates and EGR cooling for increased loads are addressed. Finally, experiments to determine the significant control parameters for the combustion process are performed on the engine.
In the course of these investigations, dual fuel LTC could be transferred from a single cylinder research engine to a multicylinder engine; previously obtained single-cylinder operating conditions could be achieved even at high loads.