Search Results

Turbocharging, increasing the compression ratio, and downsizing a spark-ignition engine are well known strategies for improving vehicle fuel economy. However, such strategies increase the likelihood of engine knock due to higher in-cylinder pressures and temperatures. A high octane fuel, such as E85, effectively suppresses knock but is not necessary in most parts of the engine operating map. To better utilize a high octane fuel, dual fuel injection has been suggested where high octane fuel is injected only when the engine is about to knock. However, the effects of downsizing, retarding spark timing, and increasing compression ratio on dual fuel applications are not well understood. To investigate these questions, GT-power simulations along with engine experiments and engine-in-vehicle simulations for a passenger vehicle and a medium-duty truck were conducted.

Octane appetite of modern engines has changed as engine designs have evolved to meet performance, emissions, fuel economy and other demands. The octane appetite of seven modern vehicles was studied in accordance with the octane index equation OI=RON-KS, where K is an operating condition specific constant and S is the fuel sensitivity (RONMON). Engines with a displacement of 2.0L and below and different combinations of boosting, fuel injection, and compression ratios were tested using a decorrelated RONMON matrix of eight fuels. Power and acceleration performance were used to determine the K values for corresponding operating points. Previous studies have shown that vehicles manufactured up to 20 years ago mostly exhibited negative K values and the fuels with higher RON and higher sensitivity tended to perform better.

High octane fuel (e.g., E85) effectively suppresses knock, but the octane ratings of such fuels are much above what is required under normal driving conditions. It is important, therefore, to understand the octane requirement of the engine itself over its full range of operation and apply that knowledge to practical driving cycles to understand fuel octane utilization, especially of a turbocharged engine. By carefully defining knock limits, the octane requirement of a 2-liter turbocharged spark ignition engine was experimentally quantified over a wide range of loads and speeds using PRF blends and gasoline-ethanol blends. Utilizing this knowledge and engine-in-vehicle simulations, the octane requirements of various driving cycles were calculated for a passenger car and a medium duty truck model.

1 Downsizing and turbocharging a spark-ignited engine is becoming an important strategy in the engine industry for improving the efficiency of gasoline engines. Through boosting the air flow, the torque is increased, the engine can thus be downsized, engine friction is reduced in both absolute and relative terms, and engine efficiency is increased. However knock onset with a given octane rating fuel limits both compression ratio and boost levels. This paper explores the operating limits of a turbocharged engine, with various gasoline-ethanol blends, and the interaction between compression ratio, boost levels, and spark retard, to achieve significant increases in maximum engine mean effective pressure and efficiency.