Understanding the impact of ethanol blending in gasoline as part of the decarbonization journey

Blending of ethanol into gasoline remains an important option for partial decarbonization of the light duty fleet. The recent introduction of E10 (10% by volume ethanol in gasoline) as the standard grade (replacing E5) in the UK has happened very smoothly, and E20 represents a possible next step in the European decarbonization journey. Fuels with much higher levels of ethanol (E85 or E100) are common in some markets such as Brazil. Ethanol also has very different low temperature oxidation chemistry compared to the other hydrocarbons in gasoline. This means that highly non-linear behaviour can be observed in octane properties when ethanol content is increased, with a non-linear increase in Research Octane Number (RON) and an increase in so-called octane sensitivity (RON-MON). In the first part of this paper the role of E20 as part of an overall decarbonization roadmap is summarized. In particular, the presence of 20% ethanol allows sustainable, low-octane hydrocarbon streams produced from renewable sources (such as bionaphtha) to be blended into the gasoline while maintaining an acceptable overall octane quality. It is often useful for research purposes to be able to define gasoline surrogate compositions that can replicate the chemical and physical properties of more complex mixtures. Equations have been developed, based on data generated in an experimental CFR octane engine, to predict the RON, MON, and Dry Vapour Pressure Equivalent (DVPE) of gasoline surrogates containing ethanol (0-25 vol%), isopentane, n-heptane, isooctane and toluene. In the second part of this paper, these equations are used to illustrate the role that ethanol can play as a high-octane blending component to facilitate the use of hydrocarbon streams from renewable sources. Chemical kinetic mechanisms for E20 surrogate blends have been validated against Rapid Compression Machine (RCM) and Shock Tube (ST) data. In the final aspect of this paper, Ignition Delay times, simulated using the validated chemical kinetic mechanisms, are employed to provide an additional perspective into the non-linear octane blending behaviour of ethanol. These show that ethanol may be even more valuable in modern engines where the temperature and pressure conditions can differ from those of the standard octane tests.