Are the GHG emissions different between regular (E87) and premium (E92) gasoline blends?   [Ray M.]

First off you do not need to run a higher octane fuel than the engine manufacturer recommends,  so the higher price per gallon paid would be a waste of money. Regular E87 has a RON (Research Octane Number) of 91 and premium has a RON of 95 to 98. The higher octane gasoline is only required for engines that have a higher compression ratio and is used to prevent self- ignition or ‘knocking’ which leads to lower engine efficiency and possibly damage over time. Using a higher octane gasoline when not required may improve engine efficiency under high loads such as  acceleration,  but this inherently wastes more fuel than driving that minimizes quick stops or starts. Modern engines that require higher octane can run on lower octane gasoline since they are equipped with adaptive spark control and knock sensors. This could result in lower in engine performance and slightly reduced gas mileage.  Anytime your engine is running at a lower efficiency it will consume more fuel and emit more CO2.

Since higher compression engines (11.5:1) can be made lighter and more efficient, studies have been done to explore reducing CO2 emissions by using a gasoline with an octane rating of 98 or higher (Ethanol has a RON of 109 or greater) .  It is estimated that this could reduce  gasoline consumption by 2-5% and greater than 5% by use of a turbocharger. This reduction in gasoline consumption and associated CO2 however is partially  offset by increased emissions from fuel production of the higher octane fuels and the fuel additives used.  Even if the net reduction is 5% we are still left with 95% of the problem that current petroleum based transportation causes – Over 1.7 Billion metrics tons of CO2 emitted annually!  It remains to be seen if we will see higher octane fuels than we currently have being requested by the US auto industry to meet the higher CAFÉ targets required for 2025.