Performance of a High Speed Engine with Dual Fuel Capability

Concern over dwindling oil supplies has led to the adoption of alternate fuels to power fleet vehicles. However, during the interim period when alternate fuel supply stations are few and far between, dual fuel engines prove a necessity. In the light duty arena, these engines are typically gasoline engines modified to accommodate compressed natural gas (CNG) as an alternate fuel, but they are seldom optimized with both fuels in mind. A Saturn 1.9 liter 4 cylinder dual overhead cam engine was selected as a base for developing an optimized gasoline/CNG powerplant. Baseline data on power and steady state emissions (CO₂, CO, NO_x, HC) were found using the standard Saturn controller. In addition to monitoring standard sensor measurements, real-time pressure traces were taken for up to 256 cycles using a modified head with embedded PCB piezoelectric pressure transducers. A controller was developed that could mimic original Saturn controller performance yet permit excursions on any control variable in order to map engine response. Effect of spark timing and equivalence ratio on emissions and performance were then assessed using gasoline as a fuel. Classical variation of CO, HC, and NO_x with air/fuel ratio variation was observed. Natural gas operation was achieved using four simultaneously firing solenoid operated injectors with a 100 psi feed pressure, situated upstream of the throttle plate. For stoichiometric operation with the speed, torque and location of peak pressure held at the same points as those for gasoline, hydrocarbon emissions were found to be significantly lower and NO_x emissions slightly lower than on gasoline operation. Data on effects of varying timing and air/fuel ratio during natural gas operation were also obtained. This study is working toward the goal of recovering power loss usually associated with CNG operation, while avoiding knock during gasoline operation.