Turbocharged HCCI Engine, Improving Efficiency and Operating Range

Abstract: Increasing fuel prices and environmental concern drives the research and development for the internal combustion engine. Homogeneous Charge Compression Ignition (HCCI) is a promising combustion concept with high fuel efficiency and low emissions. The auto-ignition in HCCI combustion has no flame propagation and therefore the combustible mixture is consumed rapidly, even at low temperature. This results in high peak pressure rate and therefore high combustion noise, that limits the possible operating range. The focus of this thesis is to research how the fuel efficiency can be the improved and how the operating range can be extended for a turbocharged light duty multi-cylinder HCCI engine. The research evaluates how different engine settings influence the fuel efficiency, combustion noise and exhaust emissions. It is shown which operating limitations a HCCI engine needs to fulfill regarding; combustion noise, combustion stability, peak cylinder pressure, nitrogen oxides and soot emissions. At increased boost pressure the achieved fuel efficiency has to be balanced against these operating limitations and the real operating range can be very limited, especially at high load. Due to the low exhaust enthalpy from a negative valve overlap HCCI engine the turbocharger function is important. It is shown how efficiency and operating range can be improved by finding a better suited smaller turbocharger. At high boost pressure the pumping losses can be very high in this engine configuration. In engine simulation of the turbocharged HCCI, the correct exhaust valve closing position is fundamental since it controls the mass flow through the engine. It is shown that by controlling the exhaust valve closing position in the engine simulation to have in-cylinder temperature of ~1000 K near firing top dead center, gives simulation results that follows experimental engine results well. The short duration exhaust valve event was identified as one source to the pumping losses. The engine simulation results showed that a change of exhaust valve timing could reduce the pumping losses. This was verified with experimental results where the new longer duration exhaust camshaft improved the fuel efficiency and capable operating range. The knowledge on how to operate this turbocharged HCCI engine to improve the fuel efficiency and fulfill the operating limitations, resulted in an operating range from 1000 to 3000 rpm and a load of more than 5 bar BMEP, in that speed range. It is shown that this turbocharged HCCI engine has up to 35 % improvement in fuel efficiency compared to a modern SI engine.