Abstract
KEYWORDS – GDI, GPF, particulate, filter, black carbon
ABSTRACT
Direct injection is a promising way to reduce fuel consumption of spark ignition gasoline engines. Nevertheless, the spatial fuel / air heterogeneities resulting from shortened process of mixing compared to port fuel injection leads to increased particle formation. The work aimed to characterize the major particles emission modes, analyze the physicochemical properties of these particles and assess the reduction efficiency of dedicated gasoline particulate filter (GPF).
The emissions of a production passenger car equipped with a stoichiometric gasoline direct injection engine and complying with Euro 5b emissions standard were extensively examined on a roller test bench following various driving conditions. Besides, the combination of a three-way catalyst and a gasoline particulate filter was accurately assessed, especially on highly emitting running conditions, using a same engine mounted on an engine test bench. A wide range of complementary advanced analytic devices was implemented for these investigations: on-line particle sizers and counters (DMS500, SMPS, SMPS+E, CPC "PMP"), compact time-of-flight aerosol mass spectrometer for time resolved online measurements of particle composition (c-TOF & AMS), multi-angle absorption photometer for aerosol black carbon mass concentration (MAAP), apparatus that collect particles for laboratory analyzes (mini particle sampler (MPS), dilution micro-tunnel, LPI). Depending on the aerosol characteristics and devices requirements, controlled dilution was operated through a FPS-4000.
This passenger car was found to emit very high concentration of particles, though the mass remains below the standards. Black carbon is however detected in rather high concentration, especially during drive cycles. Nucleation and accumulation modes composing the aerosol analyzed at the tailpipe at constant speed are highlighted to have intensities in the same order of magnitude. Electron microscopy images indicate a graphitized structure for the particles, including inorganic elements such as sulfur, and iron and phosphorous to a lesser extent. The emission of particles are intensively emitted whatever the running conditions, but a particularly sharp increase is detected during the first few hundred seconds of engine warm-up. Based on that statement, the engine bench was programmed to run NEDC cycles simulating a conventional as well as an hybridized vehicle, that means starting the engine eight times a cycle. The raw number of particles is much higher in this hybrid mode but the combination of the three-way catalyst, which appears to efficiently remove nucleation particles, and the GPF leads to particle number below the Euro 6c threshold.