More effective use of fuel octane in a turbocharged gasoline engine : combustion, knock, vehicle impacts

Abstract

Turbocharging, increasing the compression ratio, and downsizing a spark-ignition engine are well known strategies for improving vehicle fuel economy. However, such strategies result in higher in-cylinder pressures and temperatures which increase the likelihood of engine knock above that of naturally-aspirated engines. A high octane fuel, such as E85, effectively suppresses knock but the octane ratings of such fuels are much above what is required under normal driving conditions. To address this issue, there have been attempts to use octane more effectively by means of Octane on Demand (OOD): higher octane fuel is used only when needed. Engine experiments were performed to understand the combustion characteristics and knock limits of a commercially available turbocharged spark ignition engine. By utilizing data from engine experiments and engine-in-vehicle simulations, this study quantifies the octane requirement of a 2-liter turbocharged engine over its operating range as well as for various driving cycles. The average octane ratings of fuel needed in real-world driving were in the 60-80 RON range (maximum RON required around 90-100.) Engine configurations (boost/downsizing level, compression ratio), spark retard strategies, and vehicle configurations (vehicle type and loading conditions) were important parameters deciding these octane requirements. To analyze the effects of downsizing, retarding spark timing, increasing compression ratio, and vehicle type on dual fuel applications, GT-power simulation was conducted along with engine experiments and engine-in-vehicle simulations for a passenger vehicle and a medium-duty truck. Parametric studies were conducted to analyze the effects of listed variables on the vehicle fuel consumption, ethanol usage, and average engine efficiency. Downsizing a naturally-aspirated engine by 50% resulted in about a 30% increase in fuel economy. Ethanol consumption varied from 5 to 40% (by volume) of the total fuel used, depending on the details. Moderate amounts of spark retard reduced ethanol consumption by half while not deteriorating fuel economy significantly. Increasing compression ratio above 11.5 had a marginal return in fuel economy while demanding a significantly larger amount of ethanol. Finally, two dual fuel systems (twotank and on-board fuel separation) were modeled to compare benefits and disadvantages. Additionally, a new cycle-by-cycle pressure analysis method is presented, which help better explain the cycle-by-cycle variations of the spark ignition engine combustion process.