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Oxford study shows Catalytic Volatile Particle Remover effective at reducing particle emissions from spray-guided direct injection gasoline engine

Vpr
Experimental setup and construction of the catalytic VPR system (at top). Credit: ACS, Xu et al. Click to enlarge.

A team at the University of Oxford (UK) reports that use of a catalytic volatile particle remover (VPR) on the emissions from a spray-guided direct injection spark ignition (DISI) gasoline engine leads to a marked reduction in the number of particles, especially smaller size (nucleation mode) particles. The paper by Xu et al. is published in the ACS journal Environmental Science & Technology.

Emissions of fine particles from engines have been shown to have a large impact on the atmospheric environment and human health. Various researchers say port fuel injection (PFI) gasoline and DPF (diesel particulate filter)-equipped diesel vehicles show the lowest particulate emissions, while particle emissions from diesel vehicles without DPFs were the highest. The level of particle emissions from a DISI vehicle is between that of the PFI and diesel vehicles, Xu et al. note.

DISI particulate emissions
Although most research has found that particle number (Pn) emissions from DISI engines were higher than those of DPF diesel or PFI gasoline engines by an order of magnitude, research from CONCAWE showed that the Pn emissions from DISI vehicles were of the same order as those from the DPF diesel vehicles over the NEDC cycle.
...it is of interest to further investigate the particle emissions from the most recent DISI engines. It is well-known that when a DISI engine operates in stratified lean conditions, it will emit a high number concentration of particles. It can be argued that, during stratified combustion, soot was formed as a partially premixed flame propagated through locally rich zones and from pool fires caused by thin films of liquid fuel on the piston surface.
However, during stoichiometric operation when using a piezoelectric injector with multiple injections in a DISI engine (to further increase homogenization and reduce spray penetration), then the Pn emissions may be lowered. By using turbocharging for downsizing, stoichiometric DISI engines show advantages on fuel economy over turbocharged MPI engines and lowered Pn emission compared with lean DISI engines.
—Xu et al.

Generally, particles are generated during acceleration and cold start in a drive cycle. For port fuel injection (PFI) gasoline vehicles, after catalyst light-off, particle emissions diminish to near negligible levels at moderate cruise speeds and during deceleration. Researchers have found that DISI engines tend to emit higher levels of particulate emissions during stratified operation, with one team cited by Xu et al. showing that the the difference in particulate number between the homogeneous and the stratified modes was a factor of approximately 40.

As a result, the particle number emissions of DISI engines will be restricted by the forthcoming EU6 standards; the Oxford team notes that the particulate emission level of DISI engines means that they could face some challenges in meeting the EU6 requirement.

There has already been some investigation of the performance of a catalytic stripper-based volatile particle remover system on the PM emissions from a wall-guided direct injection engine; the Oxford team investigated use of the system with a spray-guided direct injection (SGDI) engine. Engine experiments were carried out to study a catalytic volatile particle remover (VPR), and to find out if it can achieve the transmission efficiencies required by the European PMP (Particle Measurement Program) procedures.

The test engine was a Jaguar naturally aspirated, V8 direct injection spark ignition engine. For the tests, the spark timing was fixed at 30 °CA (crank angle degree) before compression top dead center. A single injection during the intake stroke at 280 °CA before compression top dead center generated a nominally homogeneous mixture.

The catalytic VPR system consisted of a temperature-controlled heated tube with an oxidation catalyst inside. The performance of the catalytic VPR was evaluated by varying its temperature and the exhaust residence time. The effect of the catalytic VPR acting as an oxidation catalyst on particle emissions was also tested. Among the findings from the testing under different operating regimes were:

  • With the pre-TWC [three-way catalyst] engine exhaust, nucleation mode particles constitute a large portion of the total particle number but only constitute a small portion of the total particle mass. The catalytic VPR led to a significant reduction in the particle number, especially the smaller size (nucleation mode) particles.

  • The VPR temperature and exhaust residence time (MFMexhaust) [MFM=Mass Flow Rate] did not show much effect on the catalytic VPR performance once the MFMexhaust was above 0.09 g/s. The catalytic VPR transmission efficiencies for different size particles showed similar trends for the various VPR temperatures and MFMexhaust tested. Generally, the transmission efficiencies of the VPR follow the trends of the scaled PMP counting efficiency.

  • When post-TWC exhaust was introduced to the catalytic VPR, it showed a moderate reduction effect on the number of particles. With a stoichiometric airfuel ratio, the performance of the catalytic VPR was not affected by the use of additional air. Even with rich mixture combustion, there was still not a noticeable effect with increasing additional air on the catalytic VPR performance.

  • For each airfuel ratio, the transmission efficiency of the catalytic VPR did not change much with different types and varying amounts of additional gas. A comparison between adding air or nitrogen as a diluent showed that the reduction in particle number through the catalytic VPR is more likely to be due to physical than chemical processes.

Resources

  • Fan Xu, Longfei Chen and Richard Stone (2011) Effects of a Catalytic Volatile Particle Remover (VPR) on the Particulate Matter Emissions from a Direct Injection Spark Ignition Engine. Environmental Science & Technology Article ASAP DOI: 10.1021/es2008209

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