Exhaust Emissions and Combustion Stability in a Bi-Fuel Spark Ignition Engine

A Saturn 1.9 liter engine has been converted for operation on either compressed natural gas or gasoline. A bi-fuel controller (BFC) that uses closed-loop control methods for both fuel delivery and spark advance has been developed. The performance and emissions during operation on each fuel have been investigated with the BFC, as well as the performance and emissions with the stock original equipment manufacturer (OEM) controller using gasoline. In-cylinder pressure was measured at a rate of 1024 points per revolution with piezoelectric pressure transducers flush-mounted in the cylinder head. The in-cylinder pressure was used in real time for ignition timing control purposes, and was stored by a data acquisition system for the investigation of engine stability and differences in the combustion properties of the fuels. Engine-out emissions of hydrocarbons (HC), oxides of nitrogen (NO_x), carbon dioxide (CO₂), and carbon monoxide (CO) were also recorded along with other relevant engine performance data. Ignition delays, combustion durations and statistical cycle-to-cycle variations were calculated from the in-cylinder pressure traces and compared between the two fuels and different control methods. Ignition delay, defined as the time between the spark and when the in-cylinder pressure deviates significantly from a motoring curve, was shorter for CNG than for gasoline. Combustion duration, defined as the time between when 10% and 90% of the mass fraction is burned, was longer for CNG than for gasoline. Engine stability, as measured by the coefficient of variance of the gross indicated mean effective pressure [COV(IMEPg)], was better with the BFC than the stock controller. NO_x emissions were found to decrease as engine stability improved. Emissions of HC, CO, and CO₂ were less dependent on engine stability than they were on other parameters such as fuel type and equivalence ratio. It is the goal of this research to produce an optimized bi-fuel engine and add to the body of knowledge on the differences in combustion between CNG and gasoline.