Compression Ratio Effects on Performance, Efficiency, Emissions and Combustion in a Carbureted and PFI Small Engine

Abstract

This paper compares the performance, efficiency, emissions and combustion parameters of a prototype two cylinder 430 cm 3 engine which has been tested in a variety of normally aspirated (NA) modes with compression ratio (CR) variations. Experiments were completed using 98-RON pump gasoline with modes defined by alterations to the induction system, which included carburetion and port fuel injection (PFI). The results from this paper provide some insight into the CR effects for small NA spark ignition (SI) engines. This information provides future direction for the development of smaller engines as engine downsizing grows in popularity due to rising oil prices and recent carbon dioxide (CO 2) emission regulations. Results are displayed in the engine speed, manifold absolute pressure (MAP) and CR domains, with engine speeds exceeding 10000 rev/min and CRs ranging from 9 to 13. Combustion analysis is also included, allowing mass fraction burn (MFB) comparison. Experimental results showed minimum brake specific fuel consumption (BSFC) or maximum brake thermal efficiency (η TH) values in the order of 220 g/kWh or 37% could be achieved. A maximum brake mean effective pressure (BMEP) of 13 bar was also recorded at 8000 rev/min.

Effect of compression ratio on the emission, performance and combustion characteristics of a gasoline engine fueled with iso-butanol/gasoline blends

“…However higher CRs make gasoline engines subject to engine knocking if lower octane number is used, also known as detonation [17]. Attard et al [18] researched the effect of CR on the engine performance, emissions and combustion parameters. The tests were carried out on a double-cylinder, fourstroke, 430 cc, naturally aspirated, gasoline engine.…”

Combustion System Development and Analysis of a Downsized Highly Turbocharged PFI Small Engine

“…Across all modes, it was evident that the test engine (~70 mm bore) could operate with considerably higher CR for a given MAP when compared to larger bore (80-100 mm), lower speed engines. This was demonstrated with normally aspirated results showing potential for engine operation at a compression ratio exceeding 13 [9].…”

“…The faster initial burn is caused by the higher mixture density, which improves the flame initiation and hence kernel growth. Furthermore, as the engine speed increases, the 0-10% MFB data shows identical trends when compared to the normally aspirated data [7,9,36] with increased CA durations over reduced time intervals. It is interesting to note that the increased retained residuals in the non-knock compensated regions above 6,000 rev/min, due to the higher backpressure previously described, slows the initial burn duration at high speeds as seen in the middle diagram of Figure 7.…”

“…Figure 1 highlights a design image together with a sectional view of the engine design. To put the research into context, experimental engine results at the maximum specific output setting and associated CR of 10 are displayed in Figure 2, with further detail on the test engine previously published [7][8][9][10][11][12][13]. To reliably achieve 25 BMEP, the asymmetric (odd) fire configuration required the uneven flow conditions in the intake and exhaust systems to be accommodated in the engine design [12,13].…”

“…To reliably achieve 25 BMEP, the asymmetric (odd) fire configuration required the uneven flow conditions in the intake and exhaust systems to be accommodated in the engine design [12,13]. In addition to the turbocharged results displayed in Figure 2, the test engine achieved a maximum of 37% brake thermal efficiency in normally aspirated configuration [7,9]. However, this was only possible after CR optimization to compensate for the higher levels of dissociation, friction and heat losses associated with the smaller cylinder capacity.…”