Operating compression ignition engines

When ethanol is used in a standard spark ignition or compression ignition engine designed to bum conventional hydrocarbon-based fuels, ethanol performance varies substantially if no engine modifications are made, and it becomes difficult to rate the value of ethanol as an effective fuel. Consequently, flexible fueled vehicles (FFVs) or variable fueled vehicles (VFVs) have been developed to operate on either gasoline or alcohol-based fuels. These vehicles are equipped with on-board sensors and controls to adjust the spark advance and the fuel injector timing to help correct for the differences in fuel performance. 

Results of a study sponsored by the State of California Energy Commission comparing the exhaust emissions from compression ignition engines designed to operate on methanol fuel are shown in TABLE 12-11. The results are compared to typical exhaust emission levels obtained from conventional diesel engines. 

Certain operating characteristics of the Diesel or compression-ignition engine result from the nature of the combustion process. In particular, the engine operates over a comparatively wide range of over-all fuel/air ratios because fuel and air are not premixed. At a given speed the range is from approximately 0.01 pound per pound (580% excess air) at no load to about 0.055 pound per pound (20% excess air) at full load. Excess air is essential to ensure complete combustion and avoid smoke production. 

Table 22. Main design and operation differences between compression ignition engines and spark ignition engines.

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