The Role of Intermediate-Temperature Heat Release in Octane Sensitivity of Fuels with Matching Research Octane Number

Eshan Singh,S Mani Sarathy

doi:10.1021/acs.energyfuels.0c03883

Abstract

Improving the efficiency of internal combustion engines is important for reducing global greenhouse gas emissions; the efficiency of spark ignition (SI) engines is limited by the knock phenomenon. As opposed to naturally aspirated engines, turbocharged engines operate at beyond research octane number (RON) conditions, and fuel octane sensitivity (OS = RON – motor octane number (MON)) becomes important under such conditions. Previous work by this group [ Energy Fuels 2017, 31, 1945−1960, DOI: 10.1021/acs.energyfuels.6b02659] elucidated the chemical kinetic origins of OS; this study is extended to provide a qualitative, as well as quantitative, definition of OS, based on fundamental ignition markers. A varying amount of toluene is blended with various primary reference fuels to match the ignition delay of the targeted research octane number fuels, allowing a range of octane sensitivities for each research octane number. This study establishes a correlation between OS and heat release rates at low, intermediate, and high temperatures. The significance and chemical origins of intermediate-temperature heat release in defining the OS of toluene blended in a mixture of iso-octane and n-heptane is also clarified. For the toluene–iso-octane–n-heptane mixtures considered here, low-temperature reactivity was not found to be a key marker of OS. The results also show areas of improved efficiency in beyond RON operating conditions, where high-sensitivity fuels could be beneficial.

Highlights

In even the most optimistic electrification scenarios for the near future, passenger cars will continue to have internal combustion engines, primarily in hybrid engines that employ both electric and combustion power
The simulations were performed using a chemical kinetic model proposed by King Abdullah University of Science and Technology (KAUST)/LLNL, capable of simulating gasoline surrogates in engine conditions.[45]
Even though knocking is caused by end-gas autoigniting ahead of the flame front, researchers have had some success in relating knocking tendency of fuels in engine with their calculated ignition delay time

Summary

Introduction

In even the most optimistic electrification scenarios for the near future, passenger cars will continue to have internal combustion engines, primarily in hybrid engines that employ both electric and combustion power. Significant benefits can be realized by improving the efficiency of such engines.[1,2] Continued improvement in engine efficiency, along with a shift toward low-carbon-intensity fuels, are important methods for future reduction of carbon dioxide emissions from internal combustion engines. Most passenger cars in the world today are equipped with spark-ignited (SI) engines that operate at peak efficiency under high-load conditions; at low loads, throttling losses reduce efficiency. To achieve a high load, similar to their larger counterparts, downsized engines require a turbocharger, which enables higher-than-atmospheric pressure at the intake. Under such high-load conditions, engine efficiency is limited by the abnormal combustion phenomenon engine knock.[3,4]

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