A new study finds that as much as 20–30% of the number of vehicle engine exhaust particles larger than 3 nm may be formed during engine braking conditions—i.e., during decelerations and downhill driving while the engine is not fueled. However, the authors note, these particles have not been taken into account in emission regulations and in the assessment of associated health risks.
The study by researchers in Finland and Greece, published in the ACS journal Environmental Science & Technology, suggests that both the characteristics of these particles and the mechanism by which they form seem to differ significantly from those of soot and nucleation particles. The study also indicates that the particles were non-volatile, formed before the catalyst, and originating from engine oil. Results thus indicate that the emissions of engine braking particles can be reduced using exhaust particle filtration systems.
… changing driving conditions also affect particle emissions. In emissions legislation, this has been taken into account by including transient driving cycles in emission standards. As a result, the number of exhaust studies focusing on emissions at transient conditions has increased. However, specific data for detailed driving conditions have remained limited because the regulated quantity is the particulate mass or number integrated over the test cycle.
In general, it has been observed that during transient cycles, the particle size distributions have two modes, and the highest particle emissions occur under heavy acceleration. Transient driving conditions can affect both the nucleation mode particles and the soot particles. Increased emissions of nucleation particles, affected by fuel sulfur content and exhaust dilution and thus proposed to be formed by sulfuric compounds, has been observed at deceleration as well. As at steady state tests, the particle emissions during transient cycles has been observed to be affected by vehicle technologies like exhaust after-treatment and fuel type.
This work focuses on the observation that engines emit a high number of exhaust particles at engine braking conditions—that is, during decelerations and downhill driving. In practice, “engine braking” refers to the driver’s lifting of the gas (accelerator) pedal and keeping the gear on, and it is typically used to enhance fuel economy or simply to limit or decrease a vehicle’s speed during downhill driving or deceleration. With direct injection engines, the fuel consumption during engine braking is zero.—Rönkkö et al.
The clear benefit of engine braking is its reduction of fuel consumption; for the engines with direct fuel injection technology, fuel consumption during engine braking is zero. The use of engine braking also decreases brake wear.
The inertia of the vehicle maintains the rotation of a crankshaft during engine braking, and thus, the movement of pistons—without fuel combustion. Intake air still goes to the cylinder, where it is compressed to elevated pressures, its temperature raises, and it is exhausted to the atmosphere.
In the study, the team from Tampere University of Technology, Metropolia University of Applied Sciences, and the Finnish Meteorological Institute (all in Finland) and Aristotle University Thessaloniki in Greece, report on three experiments: one with a heavy-duty diesel truck at on-road conditions, and two conducted with gasoline direct injection (GDI) passenger cars on the chassis dynamometer.
Among the findings were:
On-road exhaust particle studies with the heavy-duty diesel truck showed that—with only few exceptions—the highest concentration peaks (frequently exceeding the maximum measuring range of the particle counter after passive dilution) were observed during downhill driving. The peaks were repetitive, appearing throughout the downhill sections of the driving sequence.
The results suggested that the high particle concentrations during downhill driving (engine braking) were dominated by particles sized below 7 nm.
Engine braking particles were also observed in the experiments with modern GDI vehicles when following the European statutory driving cycle. Repetitively high concentrations of particles were observed during engine braking in decelerations, when the sampling was conducted directly downstream of the exhaust manifold.
The mean diameter of these particles varied from less than 10 nm to slightly larger than 20 nm, depending on the deceleration pattern. When sampled downstream of the complete exhaust line including the three-way catalyst, the concentration of particles was lower, and the mean particle size larger, probably due to the strongly size-dependent diffusional particle losses, the researchers suggested.
Analysis indicated that engine braking particles, emitted during transient driving cycles, were associated with engine oil.
The particles’ small size and non-volatility, coupled with the observation that these particles contain lube-oil-derived metals zinc, phosphorus, and calcium, are suggestive of health risks at least similar to those of exhaust particles observed before.
Because of their origin, in principle, no correlation between the emissions of these engine braking particles and CO2 is expected. It is therefore possible that emission factors, typically determined by measuring particles and carbon dioxide concentrations simultaneously in an urban environment and using these quantities to calculate particles per unit of fuel combusted, are at least partly failed. This is, however, dependent for example on the location of urban air measurement station in relation to traffic routes. Also, our chassis dynamometer studies indicate that the high particle concentrations that are in reality caused by deceleration and engine braking are in some studies erroneously associated with idling.
Our study indicates that these [nano]particles are emitted in short time bursts,causing extremely high and instantaneous increases of particle concentrations. In addition to downhill driving, these particles may be emitted while navigating locales with a frequent presence of people, such as when decelerating at crossroads, traffic lights with pedestrian crossings, or bus stops. Particularly in urban environments, this can lead to significant human exposure of these particle emissions.
…our study indicates that the particles were non-volatile, formed before the catalyst, and originating from engine oil. Thus, the results indicate that the emissions of engine braking particles can be reduced using exhaust particle filtration systems. Additionally, another potential but less direct way to reduce the emissions of these particles may be the modification of engine oil characteristics, such as metal content or viscosity. From an emissions control perspective, the problem remains for those older vehicle types that are not equipped with particle filters and possibly for new vehicle technologies for which particle filters have not been mandated, such as gasoline direct injection (GDI) vehicles.—Rönkkö et al.
Topi Rönkkö, Liisa Pirjola, Leonidas Ntziachristos, Juha Heikkilä, Panu Karjalainen, Risto Hillamo, and Jorma Keskinen (2014) “Vehicle Engines Produce Exhaust Nanoparticles Even When Not Fueled,” Environmental Science & Technology doi: 10.1021/es405687m