Correcting mass measurement of diesel particulate filters at non-ambient temperatures

Diesel particulate filters (DPFs) are becoming a widespread method for reducing the particulate matter (PM) emissions from both on-highway and off-highway automotive diesel engines. Mass measurements of DPFs are commonly used to determine rapidly both the amount of PM trapped by the filter and the amount regenerated (removed) by regeneration systems. To avoid issues with adsorption of atmospheric water the filters are often weighed at elevated temperatures. It is shown in this work that at elevated temperatures the filters weigh less than at lower temperatures as a direct result of the buoyant hot air within the filter substrate. This study shows that consideration of the buoyancy forces allows for correction of the mass measurement for the errors relating to the non-ambient temperature of the filter, allowing mass measurements at elevated temperatures while avoiding adsorption of atmospheric water on to the filter substrate and, therefore, improving the accuracy of mass-measurement-based studies of filtration and regeneration performance of DPFs. It is demonstrated that a filter with approximately 85 per cent overall porosity weighed at 150 °C in ambient temperatures will have an error of about 0.3 g/l (typically about 10 per cent of the trapped PM mass) in the mass measurement when not correcting for the temperature. By way of an example, this is shown to have potentially an important effect on the calculated trapped PM

Analysis and optimization of gel-cast ceramic foam diesel particulate filter performance

Gel-cast ceramic foams potentially offer a more robust configurable alternative filtration medium to monolithic wall flow filters (WFFs) for the reduction in particulate matter (PM) emissions from diesel internal combustion engines. The fundamental back pressure and filtration efficiency characteristics of gel-cast ceramic foam diesel particulate filters (DPFs) have been investigated. Methodology is developed for the first time that allows the calculation of the effect of local PM loading on the pressure drop characteristics from experimental data without problems caused by the non-uniform PM loading in the filter that can be applied to all depth bed filtration media. The back pressure and filtration efficiency relationships were used to develop graphical design spaces to aid development of application-specific DPFs. Effects of PM distribution on the pressure drop of the filter are presented. Filters with a non-even distribution of PM were found to have lower pressure drops than filters with an evenly distributed PM for the same average specific PM loadings. The predictions showed that gel-cast ceramic foams can achieve comparable back pressure, filtration volume, and PM holding capacity with WFFs with a lower filtration efficiency of about 80 per cent. The model demonstrated that greater than 90 per cent filtration efficiency can be achieved with filter volumes of about 0.6 times the volume of a WFF with a lower PM holding capacity

Measuring pore diameter distribution of gelcast ceramic foams from two-dimensional cross sections

Increasing applications for gelcast ceramic foams is making the effective, accurate and cost effective measurement of pore diameter and distribution of significant value to a wide range of research fields. Current methods either do not directly measure pore diameter or they require high equipment and time costs. Measuring pore diameter directly from sample cross sections is both rapid and cost effective but, due to the random nature of the pore location during sectioning of the sample, it under predicts the pore diameter. The proposed method identified that the mean measured pore diameter was 79% (2 s.f.) of the actual pore diameter. Numerical methods for correcting the pore distribution as well as the average pore diameter are presented

A finite-volume-based two-dimensional wall-flow diesel particulate filter regeneration model

Many existing diesel particulate filter (DPF) models do not sufficiently describe the actual physiochemical processes that occur during the regeneration process. This is due to the various assumptions made in the models. To overcome this shortcoming, a detailed twodimensional DPF regeneration model with a multistep chemical reaction scheme is presented. The model solves the variable density, multicomponent conservation equations by the pressure implicit with splitting of operators (PISO) scheme for inlet and outlet channels as well as the porous soot layer and filter wall. It includes a non-thermal equilibrium (NTE) model for the energy equation for porous media. In addition, for the first time, experiments on the DPF were conducted to determine the interstitial heat transfer coefficient inside the DPF porous wall. The results compare well with an in-house one-dimensional model and subsequently this was used in the new two-dimensional model. By using this detailed two-dimensional model, some interesting observations of the DPF regeneration process were revealed. These included flow reversals and asymmetry in the filter channels

Turbo-discharging turbocharged internal combustion engines

Turbo-discharging is a novel approach that can better utilize the energy recoverable by a turbine (or series of turbines) mounted in the exhaust flow of internal combustion engines. The recovery of blowdown pulse energy in isolation of displacement pulse energy allows the discharging (depressurization) of the exhaust system to reduce engine pumping work and improve engine fuel economy. This is a novel approach to air system optimization that has previously been studied for naturally aspirated engines. However, to be successful, turbo-discharging should be applicable to turbocharged engines, as downsizing is a promising direction for future powertrain systems. This study uses one-dimensional gas dynamics modelling to explore the effect of turbo-discharging on a turbocharged gasoline engine, particularly focusing on the interaction with the turbocharging system. The results show that the peak engine torque is increased at low to mid speeds with high speed torque slightly reduced due to restrictions in engine breathing with low lift exhaust valves. The engine peak torque as a function of speed with a larger turbocharger and turbo-discharging was comparable to that of the smaller turbocharger without turbo-discharging. Fuel economy improvements were evident over most part-load regions of the engine map, with peak values varying from 2 to 7% depending on the baseline engine air system strategy. Hot trapped residual mass was consistently reduced across a large fraction of the engine map, with the exception of high power conditions, where the valve pressure drop effect dominated. This is expected to enable spark advance and further fuel economy benefit. The results from this study are promising and show that the use of some of the available exhaust gas energy for turbo-discharging in preference to turbocharging can have a positive effect on both part-load and full-load engine performance. There remains significant potential for further optimization with application of variable valve actuation and turbocharger control systems (for example, variable geometry turbines)

Modelling gas flow pressure gradients in Gelcast ceramic foam diesel particulate filters

New mathematical models are proposed that predict fluid flow pressure gradients in gelcast ceramic foam diesel exhaust particulate filters by considering the foam structure conceptually as serially connected orifices. The resulting multiple orifice mathematical (MOM) model is based on the sum of a viscous term derived from an extended Ergun model and the kinetic energy loss derived from the Bernoulli and conservation of mass equations. The MOM model was calibrated using experimental data obtained from measuring the air flowrate and pressure drop across a physical large-scale three-dimensional model of a cellular foam structure produced using rapid manufacturing techniques. The calibrated model was then validated using fluid flow data obtained from gelcast ceramic foam filters of various cell sizes and was found to require no empirical recalibration for each gelcast ceramic foam sample. The MOM model for clean filters was extended to predict pressure gradients of filters loaded with particulate matter (PM). The prediction of pressure gradients through gelcast ceramic filters using the MOM model for clean and PM-loaded cases was shown to be in reasonable agreement with experimental data. The models were finally applied to design a filter for a turbocharged, charge-cooled, 2.0 l, fourstroke, common rail, direct injection passenger car diesel engine

On the measurement and modelling of high pressure flows in poppet valves under steady-state and transient conditions

Flow coefficients of intake valves and port combinations were determined experimentally for a compressed nitrogen engine under steady-state and dynamic flow conditions for inlet pressures up to 3.2 MPa. Variable valve timing was combined with an indexed parked piston cylinder unit for testing valve flows at different cylinder volumes whilst maintaining realistic in-cylinder transient pressure profiles by simply using a fixed area outlet orifice. A one-dimensional modelling approach describing three-dimensional valve flow characteristics has been developed by the use of variable flow coefficients that take into account the propagation of flow jets and their boundaries as a function of downstream/upstream pressure ratios. The results obtained for the dynamic flow cases were compared with steadystate results for the cylinder to inlet port pressure ratios ranges from 0.18 to 0.83. The deviation of flow coefficients for both cases is discussed using pulsatile flow theory. The key findings include: 1. For a given valve lift, the steady-state flow coefficients fall by up to 21 percent with increasing cylinder/manifold pressure ratios within the measured range given above; 2. Transient flow coefficients deviated from those measured for the steady-state flow as the valve lift increases beyond a critical value of approximately 0.5 mm. The deviation can be due to the insufficient time of the development of steady state boundary layers, which can be quantified by the instantaneous Womersley number defined by using the transient hydraulic diameter. We show that it is possible to predict deviations of the transient valve flow from the steady-state measurements alone

Thermal management in porous ceramic particulate filters: Opportunities and consequences of plasma technology solutions for particulate filter regeneration [Powerpoint]

We remain dependent on combustion sources for many of our essential energy systems, continuously improving technologies to minimise the negative impacts of the energy use on our environment. Local air quality in urban areas is of particular concern and has led to increasingly stringent legislation being applied to the energy sector. Reduction of particulate emissions from combustion sources is being effectively tackled through pre-combustion approaches (e.g. fuel quality), through combustion optimisation and by implementing exhaust gas aftertreatment systems such as monolithic ceramic particulate filters. Although a wide range of types of particulate filters exist, they all require cleaning to avoid excessive pressure drops across the substrate as the amount of trapped particulates increase. Typically this is done through oxidation of the trapped particles, requiring filters that are capable of withstanding high temperatures, high temperature gradients and both reducing and oxidising environments. Significant opportunities exist within the industry to improve vehicle efficiency and reduce cost by developing new and improved regeneration systems. Medium (>1000 K) and high temperature (>10,000 K) plasma technologies for particulate filter regeneration are introduced with a particular focus on their interaction with porous ceramic substrates. When used effectively, no observable damage is present. However, there exists engineering conflicts between minimising energy consumption, achieving fast regeneration and maintaining substrate durability. This conflict is presented showing the limits of current technology. It identifies two clear opportunities. Firstly, the recent application of pulsed plasmas to cordierite substrates demonstrates the opportunities for developing lower thermal requirement (and potentially lower cost substrates). Secondly, how advances in local substrate thermal characteristics can enable significant improvements in efficacy of oxidative plasma solutions

Turbo-Discharging: Predicted Improvements in Engine Fuel Economy and Performance

The importance of new technologies to improve the performance and fuel economy of internal combustion engines is now widely recognized and is essential to achieve CO2 emissions targets and energy security. Increased hybridisation, combustion improvements, friction reduction and ancillary developments are all playing an important part in achieving these goals. Turbocharging technology is established in the diesel engine field and will become more prominent as gasoline engine downsizing is more widely introduced to achieve significant fuel economy improvements. The work presented here introduces, for the first time, a new technology that applies conventional turbomachinery hardware to depressurize the exhaust system of almost any internal combustion engine by novel routing of the exhaust gases. The exhaust stroke of the piston is exposed to this low pressure leading to reduced or even reversed pumping losses, offering >5% increased engine torque and up to 5% reduced fuel consumption. This method has the distinct advantage of providing performance and fuel economy improvements without significant changes to the structure of the engine, the combustion system or lubrication system. The Turbo-Discharging concept is introduced and analyzed. A combination of filling/emptying models and 1-D gas dynamic simulations were used to quantify the energy flows and identify optimum valve timings and turbomachine characteristics. 1-D gas dynamic simulation was then used to predict primary fuel economy benefits from Turbo-Discharging. Secondary benefits, such as extended knock limits are then discussed