Effect of Compression Ratio on Exhaust Emissions and Performance of a Methanol-Fueled Single-Cylinder Engine

Abstract

<div class="htmlview paragraph">One of the reasons methanol is considered an attractive alternative fuel for automobiles is its high octane quality, which may allow the use of high compression ratio (CR) engines. To evaluate compromises between engine efficiency and exhaust emissions, a methanol-fueled single-cylinder engine was run at CR's from 8 to 18. At each CR, engine speed and airflow were constant at 1200 rpm and about half throttle, respectively; equivalence ratio (ø) was varied from 0.7 to 1.1; and spark timing was varied from best power (MBT) to 10° retarded. Knock was observed only at CR = 18 with MBT spark timing.</div> <div class="htmlview paragraph">Increasing CR from 8 to 18 while maintaining MBT spark timing increased efficiency about 16 percent, but also increased NO<sub>x</sub> and unburned fuel (UBF) emissions. Some previous studies have reported decreased NO<sub>x</sub> emissions with increased CR, possibly because MBT spark timing was not maintained. Results of this study indicate that constant NO<sub>x</sub> emissions can be maintained by retarding spark timing while increasing CR to improve efficiency. Retarding spark timing, however, only marginally reduced UBF emissions.</div> <div class="htmlview paragraph">Vehicle tests are necessary to define the optimum CR for methanol fueling because exhaust emission trends and knocking tendency may be different than those observed with this single-cylinder engine.</div>