Investigation of Scavenging, Combustion and Knock in a Two-Stroke SI Engine Operated with Gasoline and CNG

Abstract

The paper reports a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with either Gasoline and Natural Gas (CNG).It is widely recognized that for two-stroke, crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, point wise lambda field, etc.) plays a fundamental role on both engine performance and tailpipe emissions. To properly characterize the engine behavior in terms of scavenging patterns and combustion, a detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed starting from preliminary 1D computed boundary conditions provided by a in-house developed 1D model of the whole engine.In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of the instantaneous in-cylinder pressure history for steady-state operations of the engine are at first performed and shown in the paper. Subsequently, the activity is focused on the investigation of knock occurrence. In order to limit the computational cost of the simulations, calculations are at first carried out within the 1D modeling framework, where customized quasi-dimensional combustion and knock models are used. In particular, the 1D model is used to compute a numerical knock index which can be useful to address the tuning of the spark advance, given a prescribed and controlled percentage of knock released heat. At the end of the simulation process, the 1D knock index is qualitatively compared to results obtained from full 3D knocking analyses for different incylinder compositions and spark timings. The intrinsic knock-resistance of the CNG fuel is finally numerically exploited, through variations of both compression ratio and spark advance.

Development of a 1 kW Micro-Polygeneration System Fueled by Natural Gas for Single-Family Users

“…The determination of the potential achievable by an optimal CNG engine configuration has been performed through a 1D engine simulation software, including predictive combustion, turbulence and knock models developed at the DII of the University of Naples Federico II [29][30][31] and a commercial CFD tool for the full-3D multi-cycle analysis of the combustion process and knock-occurrence. This last activity was performed by the University of Modena [32,33].…”

“…A 3D thermo-fluid dynamic simulation of the engine was performed in order to properly characterize the development of the combustion process under CNG operation at the fixed running speed of 5000 rpm by assuming a compression ratio (CR) equal to 14 and a spark advance (SA) of 30.5 CAD BFTDC respectively. These represent the combination of knock-limited values of those two engine parameters at 5000 rpm identified in [32,33], where the reason for selecting the specific engine operation at 5000 rpm is also discussed. Engine performance has been evaluated by means of the calculation of brake power, overall efficiency, gross IMEP and related gross BSFC (Figure 15).…”

The Key Role of the Vector Optimization Algorithm and Robust Design Approach for the Design of Polygeneration Systems

“…Moreover, to perform detailed energetic and economic analyses, the electric loads of the user (P e (t)), the CHP electric power (P eCHP (t)), and therefore the rate of P eCHP (t) self-consumed by the user (P esel f (t)) have been characterized according to the considered TOU tariff, allowing for the definition of vectors P e,Fi , P eCHP,Fi , and P esel f ,Fi . The individual contributions in Equations (6) and 7are calculated through the following equations:…”

“…Due to improvements in energy efficiency and to an increasing use of low-carbon energy and renewable energy sources, energy-related CO 2 emissions stalled in 2015, as reported in [4]. However, with the development of innovative technologies in the future [5] in addition to traditional engines [6][7][8][9][10][11], increasing exploitation of energy sources and increasing share of renewable energies in final energy consumption will still be fundamental if we are to cope with the challenges due to the world energy balance and recognized by the Paris Agreement on climate change. In particular, the World Energy Outlook of 2016 (WEO-2016) highlighted how a key player in further emissions reductions is recognized to be the wider use of renewable energy sources in the power sector.…”