Computational Fluid Dynamic Simulation of In-Cylinder Pressures to Validate High-Range VCR

Authors Abstract
This article serves as a proof-of-concept and feasibility analysis regarding a variable compression ratio (VCR) engine design utilizing an exhaust valve opening during the compression stroke to vary the compression ratio instead of the traditional method of changing the cylinder or piston geometry patented by Ford, Mercedes-Benz, Nissan, Peugeot, Gomecsys, et al. [1]. In this concept, an additional exhaust valve opening was used to reduce the virtual compression ratio of the engine, without geometric changes. A computational fluid dynamics model in ANSYS Forte was used to simulate a single-cylinder, cold flow, four-stroke, direct injection engine cycle. In this model, the engine was simulated at a compression ratio of 10:1. Then, the model was modified to a compression ratio of 17:1. Then, an additional valve opening at the end of the compression stroke was added to the 17:1 high compression model. The valve opening at the end of the compression stroke was used to bleed off a small amount of pressure. This allows an engine to be built for a geometric 17:1 compression ratio (longer cylinder) while also having the ability to “act” as a 10:1 compression ratio engine due to the valve pressure release. By manipulating the timing of the valve opening, the engine would operate with a compression ratio anywhere between 10:1 and 17:1 depending on the load/speed knock limit.
The additional valve opening profile was developed from the initial valve opening during the exhaust stroke. The timing of the additional valve opening was manipulated over multiple simulation iterations to finally achieve approximately the same maximum cylinder pressure as the first simulation trial (Test 11 CR17b). Opening a valve twice in one cycle ideally uses direct injection and camless valve operation. Available valve actuator technology was compared to the required valve actuation speed for this design and was determined to be feasible. In addition, fuel injection timing was investigated regarding near-top dead center (TDC) injection. Fuel must be directly injected after the exhaust valve closes during the compression stroke to avoid fuel loss through the exhaust manifold. This late fuel injection was determined to be feasible based on previous injection-timing studies [12].
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Davis, S., and Washko, F., "Computational Fluid Dynamic Simulation of In-Cylinder Pressures to Validate High-Range VCR," SAE Int. J. Commer. Veh. 11(4):245-254, 2018,
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Oct 22, 2018
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Journal Article