Air Quality Implications of Transport Biofuel Consumption

With sophisticated engines and aftertreatment technology, the effect of a fuel’s chemical and physical characteristics on tailpipe emissions is greatly reduced. For vehicles that comply with the latest emissions standards, most tailpipe air pollutant emissions reach very low levels regardless of the fuel used. For older vehicles, however, fuel type can significantly influence air pollutant emissions. Therefore, fuel selection is particularly relevant in many emerging economies and developing countries with large urban agglomerations and older vehicle fleets.

The IEA’s ‘Advanced Motor Fuels’ technology collaboration programme (TCP) compared the emissions of various fuels by category of air pollutant. For cars with less sophisticated engines, air pollutant emissions are higher from diesel engines than from spark-ignition (gasoline) engines (Table 1). With cars that meet the equivalent of a Euro 3 emissions standard, e.g. manufactured around the year 2000 in Europe, biomethane has the most potential to deliver low air pollutant emissions.

E85 blends can deliver lower PM emissions than fossil gasoline in older cars, but the reduction is relatively minor. For more widely consumed low-ethanol blends, e.g. E10, the effects on air pollutant emissions are less pronounced than for higher blends, although in older engines without catalysts, E10 can reduce CO and VOC emissions to a small extent. In catalyst-equipped cars the impact of ethanol blending on emissions is minor.