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Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
Good intake and exhaust system design makes use of the dynamic effects of gas acceleration and deceleration. Most engine designs incorporate open periods well over 180° of crank angle: intake valves open 5 to 30° before top center and close 45 to 75° after bottom center; exhaust valves open 40 to 70° before bottom center and close 15 to 35° after top center. (The longer valve open times correspond to high performance engines.) Valves are thus open when piston motion is in the opposite direction from the desired gas flow. At high engine speeds, the correct flow direction is maintained by pressure differences and by gas inertial effects. At low engine speeds, an extended valve open period is detrimental to performance. Some engines incorporate variable valve timing to obtain optimum performance over a range of speeds.
The reciprocating piston petrol engine
Published in M.J. Nunney, Light and Heavy Vehicle Technology, 2007
Although the basic valve timing implied by the four-stroke principle was early modified for better engine performance, as long as the valve timing is fixed with respect to crankshaft rotation it must still remain something of a compromise. More specifically, it would be an advantage if the valve overlap period could be varied, so that its duration could be shortened for engine starting and during low-speed running and lengthened for high-speed running. By varying the valve timing in this manner it should therefore be possible not only to optimize engine performance at all running speeds, but also to improve fuel economy and reduce harmful exhaust emissions. In this context it should be noted that any overlap dilution of the air and fuel mixture with exhaust gas constitutes a simple form of exhaust gas recirculation (EGR) (Section 11.2).
Automotive Trends in Europe
Published in Leslie R. Rudnick, Synthetics, Mineral Oils, and Bio-Based Lubricants, 2020
Another major innovation in engine design in recent years has been variable valve timing. This can be achieved using either geared camshafts or electronically controlled camless valves. Variable valve timing allows higher specific power for the same fuel efficiency. However, this is at the expense of variations in NOx emissions and higher combustion temperatures, meaning that oil oxidation and thermal stability improvements may be required. Higher antiwear performance may also be required for geared camshafts.
Experimental analysis of the volumetric and thermal efficiency performance of a novel direct piezo-acting CVVT mechanism
Published in International Journal of Green Energy, 2023
A. Sürmen, M.I Karamangil, A Avcı, B. Dirim, F. Işıklı, M. Tekin, N. Türköz
Before the piezo application, preliminary experiments were carried out to determine the hot (with combustion) and cold volumetric efficiency values yielded by the engine with the original valve timing. In these experiments, it was observed that the vibrations of the experimental setup were considerably high and might cause damage to the piezo which didn’t have any knowledge background. Therefore, it was decided to continue cold operation at the expense of not obtaining quantitative applicable timing values for the real operation (with combustion) of the test engine. However, the main goal is to provide continuous variability in valve timing, while the engine is running and to find timing values that will yield a higher volumetric efficiency than the original timing values at different engine revolutions.
Operational feasibility of a spark ignition engine which is subjected to VTEC management strategy
Published in Australian Journal of Mechanical Engineering, 2020
Lucky Anetor, Edward E. Osakue
Research and practical engine operations have shown that various variable valve lift and timing (VVT) technologies can improve idle speed stability, low speed stability, emissions, power and fuel economy of the engine. In view of this, a lot of automobile manufacturers have started incorporating this technology into their products. Some of the automobile companies that have commercialised and incorporated the variable valve lift and timing technology in various forms are Toyota (i-VTEC), BMW (Vanos), Ford (VCT), Delphi (VCP), Nissan (CVVT), Hyundai (VVT) and Kia (CVVT). The mechanics of these variable valve lift and timing systems is based on camshaft phase modulation which adjusts the valve timing by changing the relative phase of the cam shaft and crankshaft. It is worth mentioning that this method of implementing the variable valve lift and timing can only adjust the valve timing over a certain/restricted range of angles. However, when compared with the traditional valve-cam systems, it does improve engine emissions and performance significantly.