Turbocharging a Bi-Fuel Engine for Performance Equivalent to Gasoline

A bi-fuel engine capable of operating either on compressed natural gas (CNG) or gasoline is being developed for the transition to alternative fuel usage. A Saturn 1.9 liter 4-cylinder engine was selected as a base powerplant. A control system that allows closed-loop optimization of both fuel delivery and spark timing was developed. Stock performance and emissions of the engine, as well as performance and emissions with the new controller on gasoline and CNG, have been documented. CNG operation in an engine designed for gasoline results in power loss because of the lower volumetric efficiency with gaseous fuel use, yet such an engine does not take advantage of the higher knock resistance of CNG. It is the goal of this research to use the knock resistance of CNG to recover the associated power loss. The two methods considered for this include turbocharging with a variable boost wastegate and raising the compression ratio while employing variable valve timing. For the first method, which is the subject of this paper, a turbocharger was installed to increase the density of the intake charge and thereby regain the volumetric efficiency lost with CNG. The wastegate was electronically controlled to allow precise control of the engine performance. Equivalent power to gasoline operation with CNG was achieved with the turbocharger. It was necessary to bypass the turbocharger during gasoline operation, on both the inlet and exhaust sides, to avoid knock. Reductions from baseline in hydrocarbon (HC) and carbon dioxide (CO₂) emissions were achieved at power levels equivalent to and slightly higher than the baseline. Brake thermal efficiency values were not significantly different in any case.