Effect of High RON Fuels on Engine Thermal Efficiency and Greenhouse Gas Emissions

Historically, greenhouse gas (GHG) emissions standards for vehicles have focused on tailpipe emissions. However, sound environmental policy requires a more holistic well-to-wheels (WTW) assessment that includes both production of the fuel and its use in the vehicle. The present research explores the net change in WTW GHG emissions associated with moving from regular octane (RO) to high octane (HO) gasoline. It considers both potential increases in refinery emissions from producing HO fuel and potential reductions in vehicle emissions through the use of fuel-efficient engines optimized for such fuel. Three refinery configurations of varying complexity and reforming capacity were studied. A set of simulations covering different levels of HO gasoline production were run for each refinery configuration. Two engine designs were considered: one which could take little advantage of higher octane fuel to increase efficiency, and one which could be adjusted further to take advantage of the higher octane. WTW GHG emissions were analyzed within a life cycle analysis framework, where the upstream emissions of raw material and utility inputs to the refinery were added to the direct refinery emissions and product combustion emissions from the vehicle. The ‘well-to-tank’ (WTT) GHG emissions changes with increasing HO production were relatively insignificant compared to the total WTW emissions for gasoline and on average insensitive to the refinery configurations and operations studied. In terms of ‘tank-to-wheel’ impacts, the more octane-responsive engine design supported larger increases in fuel efficiency than the less octane-responsive engine design. The net change in WTW GHG reductions (gCO₂/mile) was therefore strongly influenced by the design of engine.