Study: 30% of Particulates Formed During Engine Braking

As much as 30% of the particulates in an engine's exhaust are formed when the powerplant is decelerating and not being fueled, according to a seven-member Finnish-Greek research team.

The group, which studied both diesel and gasoline engines, reports its findings in the American Chemical Society's journal Environmental Science & Technology.

The greatest volume of particulate emission occurs during strong acceleration, and emission tests already take that into account. But the researchers say controlling particulates formed during engine braking when a vehicle is moving downhill or approaching a stop while still in gear has been ignored.

With fuel injected engines, fuel consumption during engine braking is zero. But air continues to be drawn into the engine, compressed to high temperature and exhausted into the atmosphere.

The team's analysis indicates that particulates continue to be formed under those conditions by the presence of engine oil. The particles are below 7 nanometers in diameter for heavy-duty diesels and roughly 10-20 nm for passenger car gasoline engines.

The researchers say the particles contain zinc, phosphorus and calcium. They point out that the "extremely high and instantaneous increases" in particle emissions they found during engine braking represent a health risk in urban areas where pedestrians are likely to aggregate: at crossroads, traffic lights and bus stops.