This is the scope of the present work. The diameters of the two types of particles correspond to the two peaks in Figure 1: a typical size spectral density curve obtained through experimentation. If the temperature of the exhaust sample is decreased by mixing with colder dilution air, the saturation pressure of the volatile particles is decreased simultaneously and this can also lead to condensation. Samuel The optimisation process showed that the pore diameter appears to have the greatest impact on both nucleation and agglomeration mode particles; increasing pore diameter leads to decreased capture efficiencies of both particles. The work used a deep bed filtration model to simulate and validate experimental findings.
Until recently they were assumed to have no effect on soot. Therefore the theory that the actual filtration efficiency is simply closer to zero rather than negative is lent more credence by the observation that collection efficiencies at higher engine speeds and loads include zero when error bars of one standard deviation are added to the data sets. Capture through Brownian diffusion involves particles deviating from their direction of flow due to Brownian diffusion movement leading to their collection when they come in contact with the filter material. Therefore, these results would suggest that Brownian diffusion is the main capture mechanism for both types of particles. Figure 4 Experimentally determined capture efficiency of nucleation mode particles vs. Stephen Samuel is a Senior Lecturer in Thermodynamics and Internal Combustion Engines at Oxford Brookes University. There were large increases in the number of nucleation mode particles with increasing dilution ratio while agglomeration mode particle number remained largely the same.
With the three major model parameters highlighted in numerical simulation methodology section optimised to produce results that most accurately replicate experimental results, the effect of varying particle diameter was studied against filtration efficiency predicted by the model. Samuel the particle spectrometer will translate into an error in the efficiency calculated by the model. Three major model parameters were identified to have a significant impact upon predicted particle capture efficiency: pore diameter, filter wall thickness and porosity. The results indicate that the particle mean diameter is the most significant parameter associated with the incoming exhaust that affects particulate filtration. Three major parameters which appear to have a significant effect on capture efficiency are: 1 pore diameter 2 filter wall thickness 3 filter porosity. In case of nucleation mode particles the results match in 10 out of 14 cases. As these soot particles travel through the flame they continue to be exposed to pyrolysing fuel and experience further growth and agglomeration.
The number of layers can in effect change the characteristics of the filtration process. Figure 3 a shows efficiency variation within a typical nucleation mode particle size range and Figure 3 b shows variation within an agglomeration mode particle size range. Dilution alters the saturation conditions leading to condensation or evaporation back into gaseous form Suresh and Johnson, 2001. Figure 1 Typical particle number spectrum The particle numbers were then converted into mass fractions. .
Actual experimental results show that this is not true; collection efficiencies of agglomeration mode particles are consistently approaching zero or negative. This is accompanied by a simultaneous increase in particles of size greater than 100 nm. Since the concentrations of these particulates are low compared to those from diesel engines, regeneration modes must occur relatively infrequently; therefore the assumption that there is no regeneration used in the modelling process is still justifiable. Modelling results show no impact upon capture efficiency of nucleation mode particles with changing particle concentration around this magnitude. Secondary dilution then takes place. Sample dilution is done to avoid condensation and agglomeration inside the sampling line Dementhon and Martin, 1997.
Samuel number of particles in the stream and therefore needs to be selected carefully. There are two dilution stages: primary and secondary. Of the three, pore diameters had the greatest impact on filtration efficiencies of both particle types. This leads to pyrolysis — the decomposition of the fuel in inadequate levels of oxygen — of fuel leading to the formation of soot precursors. This corresponds well with He et al.
An optimisation function was used to select parameters such that the predicted results are comparable with experimental data. His research contributions are in the field of droplet combustion, combustion and fuel economy of internal combustion engines, combustion generated pollutants, nano-scale particulate matter from gasoline direct injection engines and powertrain modelling. The output is then averaged over the duration of measurement to yield a plot of mean concentration versus particle diameter Xu et al. The other factors showed an impact on nucleation mode particle filtration but negligible effect on agglomeration mode particles. Capture of agglomeration mode particles is probably negligible. After passing through the valve, the sample was pumped into the particle spectrometer. They are installed on exhaust lines of gasoline engines.
In the other six cases, the opposite is true. The experimental filtration efficiencies calculated are compiled into a single figure below. Efficiency ranged between 20% and 60%. The first possibility is discounted as it would imply that some tests will indicate significant positive filtration efficiencies of agglomeration mode particles. The process is described in detail later. This paper was submitted as his dissertation.
Significant reduction in nucleation mode particles, the smaller of the two modes, has been reported by multiple sources Ericsson and Samson, 2009; Chen et al. Vehicle Systems Modelling and Testing, Vol. High filtration efficiencies for smaller particles are attributed to diffusion collection while interception mechanism dominates in collection of larger particles. However, recent work by Whelan et al. A review of packed bed filtration theories identifies Brownian diffusion and direct interception as the primary collection mechanisms for particles in the size range of typical diesel aerosol less than 1,000 nm in diameter Konstandopoulos and Johnson, 1989; Ohara et al. The total efficiency predicted by the deep bed filtration model η is a combination of Brownian diffusion efficiency ηD and direct interception efficiency ηR.
Thus similar graphs were made for each engine operating condition. Error in measured efficiency is determined using the process highlighted in earlier sections while the error in particle diameter measurement is provided directly by the particle spectrometer. In the gasoline engine being tested however particle concentration in the exhaust was never observed to rise above 8. The variation in particle numbers with changing dilution ratio starts to decline at higher dilution ratios Whelan et al. The results also suggest that there is no significant agglomeration of particles within the catalytic converter.