Studies on Reducing Cycle by Cycle Variations and Improving Performance of a Small Carbureted Gasoline Engine

Experiments were conducted on a single cylinder 160cc, four stroke gasoline SI engine. Preliminary experiments were conducted on the base engine to characterize the nature of CBC (cycle by cycle) variations and the influencing parameters. The results have indicated that as the ignition advances, Peak pressure increases and its COV (Coefficient of variation) reduces. IMEP increases up to MBT (Minimum advance for Best Torque) timing and its COV reduces. HC emission and BSFC are minimum at MBT timing. The best AFR (main jet) and spark timing are selected based on low CBC variations and good performance. The engine behavior with this best timing and AFR were taken as the base line data for comparison.

The combustion geometry improvement method like dual spark plug and swirl chamber (SC) with multi torch ignition is considered to be more effective for combustion rate enhancement. The flame front area and burned volume were simulated using a MATLAB code to determine the suitable position of the spark plugs. In the case of SC spark plug design, the swirl chamber volume and orifice area were selected using simulation results like change in energy and residual mass in SC. Based on simulation results the existing design was modified and experiments were conducted with their optimum AFR and ignition timing. Both the designs have reduced CBC variations as compared to the base engine. The swirl chamber system is better of the two as it produces higher combustion rates and lower CBC variations.

However it needs richer mixtures and thus leads to higher HC and CO emissions and lower thermal efficiency as compared to dual spark plug system. The throttling losses in the swirl chamber holes also lead to reduced power and thermal efficiency. This system is good at part load.