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

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.

Increasing CR from 8 to 18 while maintaining MBT spark timing increased efficiency about 16 percent, but also increased NO_x and unburned fuel (UBF) emissions. Some previous studies have reported decreased NO_x emissions with increased CR, possibly because MBT spark timing was not maintained. Results of this study indicate that constant NO_x emissions can be maintained by retarding spark timing while increasing CR to improve efficiency. Retarding spark timing, however, only marginally reduced UBF emissions.

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.