Scientists from the Center for Environmental Research and Technology at the University of California in Riverside performed a major study to identify how adding ethanol to gasoline impacts the exhaust emissions of modern hybrid vehicles. Despite the fact that ethanol is traditionally believed to be a cleaner fuel vs gasoline, its interaction with the motors is not studied in detail yet. A plug-in hybrid electric vehicle (PHEV) was used in the experiment. It was equipped with a motor capable of operating correctly on mixes with high percentage of alcohol.
The researchers tested this vehicle on three types of fuel – E10 containing 10% of ethanol, E30 (30%) and E83 (83%). They analyzed the composition of gaseous emissions, the amount and properties of solid matters, and the biological effect of the exhaust emissions on the cells of human respiratory tracts. One of the most noticeable results was the decrease of nitrogen oxides (NOx) emissions. For E83 fuel, these emissions were almost half as big as for standard E10 fuel, in aggressive motion modes they were even 79% less than fir E10. Ethanol evaporates with heat intake and cools the mix in the cylinder, and lower combustion temperature suppresses the formation of NOx. Similar dynamics was observed with respect of solid matters: the mass of soot and black carbon decreased by dozens of percentages, sometimes – even by almost three quarters. Meanwhile, solid matters are considered to be one of the most hazardous air pollutants.
However, for a series of other parameters the situation turned out to be more complicated. The higher ethanol percentage is in the fuel, the more intense the organic compounds effluents are – from standard hydrocarbons to formaldehyde and acetic aldehyde. Aldehydes are generated as direct products of ethanol degradation while burning, hence, their concentrations grow many-fold when operating on mixes E30 and especially E83. These compounds increase ozone-forming capacity of exhaust emissions, i.e., their ability to participate in photochemical reactions resulting in formation of ground-level ozone. On top of that, contrary to common expectations, the emissions of certain aromatic hydrocarbons grew, most probably because the cooling effect of ethanol slows down the evaporation and oxidation of light aromatic components of gasoline.
The situation with solid matters also turned out to be ambiguous. Yes, indeed, their mass goes down, but their average size decreases even more resulting in the growth of thee share of nanoparticles with the diameter less than 23 nanometers. Such particles have almost no impact on the mass of the emissions, but they can easily penetrate into deeper areas of lungs and hence are considered to be more hazardous. They are especially actively formed during the cold start, when the motor is already operating on the rich burn, and the catalytic neutralizer is not warmed up yet.
The most interesting results are associated with the toxicity of the matters. The scientists tested how solid emissions impacted the cells of human bronchial epithelium. The particles generated during combustion of E10 and E30 fuel had a noticeably weak impact and practically did not break the integrity of cellular membranes. However, the particles generated when operating on E83, demonstrated prominent cytotoxicity and damaged more cells. The paradox was that as for their chemical properties these particles looked less prone to generating active forms of oxygen. Hence, there were other mechanisms of damaging the cells, probably, connected with organic components specific for high-ethanol exhaust, which activate cellular tracts of stress and inflammation.
The scientists emphasize that their study was performed using just one vehicle, and its results are strongly dependent of the motor design, the hybrid system calibration and the fuel properties. That is why they plan to scale-up their efforts testing different PHEV models and different motors, as well as to study in greater detail the chemical composition of the particles and the mechanisms of them affecting the cells.
