The thermal efficiency of an Otto-cycle engine is directly related to the compression ratio (CR). CR can be defined as the max volume within the cylinder of the engine at piston bottom dead center (BDC) over the minimum volume within the combustion chamber when the piston reaches top dead center (TDC). The static CR is typically set once an initial engine design is complete with the bore and stroke being fixed due to engine hardware limitations. Dynamic CR can be reduced in some SI engines with the use of variable valve timing taking advantage of late intake valve closing.

The experimental 4-cylinder naturally aspirated spark-ignited (SI) engine tested within this study however took advantage of a four-bar linkage mechanism to vary the static CR. The variable compression ratio (VCR) engine used for this study began with the setup of standard four-stroke Otto-cycle production engine. The base engine selected was a 2.0L GM-LNF SI 4-cylinder engine with a stock CR of 9.2:1. The factory bore and stroke of the LNF engine is 86mm and 86mm respectively. The engine was modified to allow the centerline axis of rotation of the crankshaft to rotate about a fixed point. With the use of the four-bar mechanism, and larger dome volume pistons, a range of 8:1 up to 11.4:1 static CR was achievable.

The experimental engine architecture includes the design of the four-bar mechanism and engine hardware used for increasing the CR. The factory components on the engine included the cylinder head, connecting rods, crankshaft, and cylinder bores. The modified components used for achieving the VCR engine include cylinder block, main bearing linkage arms, four-bar linkage mechanism, larger dome volume pistons, drive gears, power take-off shaft, power-screw, and an external oil supply cart. The assembled VCR was analyzed experimentally, and with the aid of simulation to determine the CR’s achievable with the mechanism built.

The head gasket thickness, combustion chamber, deck clearance, and piston dome volume were experimentally measured in order to determine the volume in the combustion chamber at TDC. The clearance volume measured at TDC was then measured throughout the range of modulated crankshaft centerline axis positions in order to determine the range of volumes achievable at TDC. The swept volume was then simulated using computer aided modeling at corresponding four-bar linkage and crankshaft centerline axis positions. A range of 8:1 up to 11.4:1 is determined as the achievable limits through the measurement of combustion chamber volume, piston dome volume, in combination with a simulation of the swept volume.

The 4-cylinder VCR engine was tested and analyzed at three different CR’s at a fixed load of 600 kPa indicated mean effective pressure net (IMEPnet) at an engine speed of 1000 revolutions per minute (RPM) with a 10-degree crank angle 50 percent mass fraction burned (CA50) sweep. The 87.1 AKI fuel was controlled to a stoichiometric 14.04:1 mixture for each case. The CR’s selected for engine testing were 8.0:1, 9.8:1, and 11.2:1. The analysis of tests completed includes averaged cycle analysis of each of the test conditions including combustion phasing, combustion duration, and cycle variation. The combustion data was also analyzed to determine overall heat release, indicated gross, net, pumping  mean effective pressures, indicated specific fuel consumption, and indicated fuel conversion efficiency for each of the CR’s. The studies show an indicated fuel conversion efficiency of 32.3% for the 8:1 compression ratio. As the CR was increased to 9.8:1 and 11.2:1 the relative increase in efficiency was 8.4% and 11.1% respectively.


Design, Actuation, Experimental Setup and Testing of a 4-Cylinder Gasoline Spark Ignited Variable Compression Ratio Engine

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

ICEF2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.