Combustion System Development and Analysis of a Carbureted and PFI Normally Aspirated Small Engine

Abstract

This paper focuses on the combustion system development and combustion analysis results for a normally aspirated 0.43 liter small engine. The inline two cylinder engine used in experiments has been tested in a variety of normally aspirated modes, using 98-RON pump gasoline. Test modes were defined by alterations to the induction system, which included carburetion and port fuel injection fuel delivery systems. The results from this paper provide some insight into the combustion effects for small cylinder normally aspirated spark ignition engines. This information provides future direction for the development of smaller engines as oil prices fluctuate and CO 2 emissions begin to be regulated. Small engine combustion is explored with a number of parametric studies, including a range of manifold absolute pressures up to wide open throttle, engine speeds exceeding 10,000 rev/min and compression ratios ranging from 9 to 13. Combustion system optimization through compression ratio development enabled the engine to achieve 37% brake thermal efficiency and 13 bar brake mean effective pressure. Hence, the test engine performance and efficiency results demonstrate that smaller bore engines can match or exceed typical larger bore engines found in passenger vehicles. However, this was only possible after compression ratio optimization to compensate for the higher levels of dissociation, friction and heat losses associated with the small cylinder size.

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

“…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.…”