Abstract
Facing the future, whether working alone or with electric motors, a new type of variable compression ratio (VCR) engine that can achieve a high thermal efficiency under heavy load conditions is necessary. Hence, we propose a dual shaft control variable compression ratio (DSC-VCR) engine based on a gear-driven eccentric sleeve. With the improved position of gears, DSC-VCR allows for double larger gears to share the load, and the engine can operate with a larger eccentric size and a narrower adjustment range compared to other similar mechanisms. This helps to reduce the difficulty of chamber shape design, avoid collisions between valves and piston, and above all, makes the engine operate with a larger overexpansion ratio (OER, the ratio of expansion stroke and compression stroke) under all conditions to improve engine efficiency. Based on a 1.5 T four-cylinder engine, the OER can be increased to over 1.16 with the eccentric size of 6.5 mm. According to the theoretical thermal efficiency calculation while considering turbocharging, the per millimeter increase of eccentric size improves the theoretical efficiency by 0.0025–0.006. The predictive simulation presented that the reduction of residual gas helps to increase the compression ratio (CR) from 9.5 to 10.1 under a full load condition. Larger OERs and CRs help to reduce brake specific fuel consumption (BSFC) by 6%–8%. In the case of a 75% load, about 3.51% of efficiency increase was realized. All of these prove that DSC-VCR is a high-efficiency potential mechanism for the future.
Highlights
Automotive engines are simultaneously developing towards a high self-efficiency and a high electrification
In order to increase the OER and further improve the thermal efficiency of an engine under heavy load conditions, we propose the concept of the dual shaft control (DSC)-Variable compression ratio (VCR) [25], which places the transmission gears on both sides of the crank pin
The dual shaft control variable compression ratio (DSC-VCR) improved about 6.57%, 4.81%, and 6.19% of torque, and it about
Summary
Automotive engines are simultaneously developing towards a high self-efficiency and a high electrification. With the development of energy-saving technologies (Miller cycle, high tumble system, lean combustion, and fuel upgrading), engine thermal efficiency itself is improving. Variable compression ratio (VCR) engines show advantages when combined with the technologies shown above. The mass-produced variable compression turbocharging (VC-T) engine of Nissan realizes nearly 40% of brake efficiency with the combination of VCR, the Miller cycle, and a tumble valve [1]. Engines achieve an efficiency of 45% for both Toyota [3] and Honda [4] cars that operate with the Miller/Atkinson cycle and a high exhaust gas recirculation (EGR) rate. The key to expand the lean burn limit includes a high CR, a high tumble, and an improved ignition system. For many alternative fuel engines, such as bio-diesel [5,6,7]
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