A Comparative Study on Influence of EIVC and LIVC on Fuel Economy of A TGDI Engine Part II: Influences of Duration of Intake Event and IVC Timing on In-Cylinder Charge Motion and Mixture Combustion

Paper #:
  • 2017-01-2246

Published:
  • 2017-10-08
Author(s):
Abstract:
Atkinson cycle realized with a late intake valve closing (LIVC) and Miller cycle achieved with an early intake valve closing (EIVC) have been recognized as effective approaches for improving the gasoline engine fuel economy. In both Atkinson and Miller cycles, the engine can be designed with a higher geometric compression ratio for increasing the expansion work and the effective compression ratio is governed by the intake valve close (IVC) timing for the knock control. Duration of the intake event and IVC timing affect not only the pumping loss during the gas exchange, but also have strong influences on the friction torques of the intake cams and the turbulence intensities for the in-cylinder charge motion. The latter governs duration of combustion and EGR tolerance, both of which have impacts on the engine thermal efficiency. In order to better understand how Atkinson cycle and Miller cycle influence the engine fuel consumptions at different engine speeds and loads, an experimental investigation was conducted to compare influences of EIVC and LIVC on the fuel consumption of a 1.5L turbo-charged gasoline direction injection (TGDI) engine. The engine was tested with three different intake cams, covering durations of the intake event IDR = 251 degCA (base engine), 196 degCA (Miller engine), and 274 degCA (Atkinson engine). Compression ratios are 11,5:1 for the Atkinson engine and Miller engine and 9.5:1 for the base engine, achieved with piston modifications. The results of this investigation will be reported in three papers focusing respectively on characteristics of the engine friction, in-cylinder charge motions for different intake events, and combustion and fuel economy without and with EGR for the naturally aspirated mode and boost mode. The present paper is Part II of this study. In this paper, the results of computational fluid dynamic (CFD) analyses on the characteristics of the in-cylinder charge motion during the intake and compression strokes were conducted for 6 bar-bmep / 2000rpm and 12 bar-bmep / 3000rpm, representing naturally aspirated and boost-mode operations respectively. For both load points, although the Atkinson engine shows weaker turbulence than that of the Miller engine in duration of the intake event, its turbulence intensity for the charge motion becomes greater in the compression stroke than that of the Miller engine at the end of the compression, due to decaying less in the compression stroke; the higher the load, the greater the difference. This explains the experimental observations that, for the load points investigated, combustion durations for the Atkinson engine were about 10 degCA faster than those for the Miler engine. This difference in the turbulence intensities at the end of the compression makes the Atkinson engine have a better thermal efficiency at high loads and a better EGR tolerance than those of the Miller engine.
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