Thermodynamic investigation of different natural gas combustion processes on the basis of a heavy-duty engine

Abstract

The main challenges in engine development are the compliance to the stringent and constantly tightened emission limits and the reduction in the CO₂ release. Natural gas as fuel offers one possible way of CO₂ reduction. Several research projects on gas engine investigations of different engine size are executed at the Institute of Internal Combustion Engines and Thermodynamics at the TU Graz. In one of these projects, a single-cylinder-research-engine was derived from a heavy-duty Diesel engine. This single-cylinder-research-engine was then adapted for the combustion of natural gas and equipped with a spark ignition system for the underlying investigations. Different cylinder head and piston configurations were tested for the evaluation of charge motion, combustion chamber geometry and compression ratio, which was varied from 11.8 to 16.5. The engine displacement of the single-cylinder-research-engine of 2 dm³ represents a typical heavy-duty size. Four different combustion concepts of natural gas were realized and analysed, namely, spark ignition compressed natural gas stoichiometric, spark ignition compressed natural gas lean burn, Diesel pilot ignition compressed natural gas lean burn and Diesel pilot ignition compressed natural gas/H₂-mixture lean burn. For the compressed natural gas/H₂ gas mixture, it is also referred to as hythane. These concepts were compared concerning thermodynamic behaviour and emissions. Therefore, a part load point and a high load point were chosen for evaluation, since these are of special interest for heavy-duty emissions legislation. The focus of the investigation is put on a thermodynamic loss analysis, which allows the subdivision of losses starting at the ideal engine. The losses of a natural gas engine show significant differences to those of a Diesel engine, which are taken as reference. As requirement for the combustion concepts, the fulfilment of the emissions legislation EURO VI under the consideration of realistic conversion rates of an exhaust aftertreatment system had to be guaranteed. Additionally, sufficiently stable combustion was required. The engine efficiency was then optimized under these boundaries. For the investigation of the Diesel pilot ignition concept, five parameters were defined for variation which are air–fuel ratio, exhaust gas recirculation-rate, mass fraction burnt 50% hythane proportion and the hythane composition.