Numerical and Experimental Investigation of the Atkinson Cycle on a 2.0 liter 4-cylinder Turbocharged SI-engine

Abstract

A realization of the Atkinson cycle provides a way of increasing fuel efficiency through an over-expansion. One way of achieving an over-expanded cycle is through the implementation of Late Intake Valve Closing (LIVC). The aim of this master thesis is therefore to investigate LIVC as a strategy towards improving fuel efficiency. The research includes a numerical study with simulations performed in GT-power, but also an experimental study where tests were performed on a Volvo 2.0 liter 4-cylinder turbocharged SI-engine. This was done for the purpose of verifying the findings from the simulations. For the numerical study, different intake valve closings between 60 and 100 crank angle degrees after bottom dead center were simulated in GT-power with respect to decreased break specific fuel consumption (BSFC). The effects of varying the valve lift were also investigated. Simulations were also performed with increased geometric compression ratio to compensate for the decreased effective compression ratio caused by LIVC. The simulation results suggest that BSFC could be reduced with 7-8% over a wide range of loads and engine speeds. The baseline engine was modified with a new intake camshaft and new pistons in order to be consistent with the simulated engine. An experimental study of the modified engine was then performed and instead of the 7-8% BSFC reduction suggested by the simulations, the experimental results showed a 2-6% reduction of BSFC. This was found for engine speeds up to 2100 rpm and loads up to 8-9 bar. At 3000 rpm the BSFC was instead increased which was an unexpected outcome that deviates from what the simulations suggested. This is believed to be partly caused by increased pumping losses at this engine speed as a consequence of high back pressure in the exhaust manifold.