Loading and Regeneration Analysis of a Diesel Particulate Filter with a Radio Frequency-Based Sensor

Accurate knowledge of diesel particulate filter (DPF) loading is critical for robust and efficient operation of the combined engine-exhaust aftertreatment system. Furthermore, upcoming on-board diagnostics regulations require on-board technologies to evaluate the status of the DPF. This work describes the application of radio frequency (RF) – based sensing techniques to accurately measure DPF soot levels and the spatial distribution of the accumulated material. A 1.9L GM turbo diesel engine and a DPF with an RF-sensor were studied. Direct comparisons between the RF measurement and conventional pressure-based methods were made. Further analysis of the particulate matter loading rates was obtained with a mass-based soot emission measurement instrument (TEOM). Comparison with pressure drop measurements show the RF technique is unaffected by exhaust flow variations and exhibits a high degree of sensitivity to DPF soot loading and good dynamic response. Additional computational and experimental work further illustrates the spatial resolution of the RF measurements. Based on the experimental results, the RF technique shows significant promise for improving DPF control enabling optimization of the combined engine-aftertreatment system for improved fuel economy and extended DPF service life. INTRODUCTION Motivated by increasingly stringent emissions regulations, diesel particulate filters have seen widespread use as the only technically and economically feasible means for meeting current and future particulate matter (PM) emissions limits. In the United States, all 2007 and newer on-road diesel engine are equipped with particulate filters. Despite work on DPFs since the early 1980's [1], and their first serial introduction as original equipment in automotive applications in 2000 [2], current systems suffer from a number of inefficiencies. Although extremely effective at trapping soot, typical commercial filters can achieve trapping efficiencies in excess of 99%, this level of performance is not without considerable cost. Both the DPF itself and the accumulated soot impose additional backpressure on the engine, which translates into a fuel consumption penalty. This fuel consumption penalty generally scales with back pressure and the amount of soot accumulated on the filter. Recent computational studies report fuel consumption penalties in the range of 1.5% to 2% due to increased backpressure with a DPF, depending on the level of PM load [3]. Additional experimental investigations report potential fuel savings of 0.4% to 2.0% by reducing DPF 2 backpressure [4]. Further, the soot must be periodically removed (oxidized) from the DPF during filter regeneration, which generally incurs an additional fuel penalty in most actively regenerated systems. The fuel penalty attributed to DPF regeneration has further been reported between 2% and 5% [3]. Accurate knowledge of soot levels in the DPF at any given time is critical for proper control of the regeneration process to both minimize the fuel consumption impact and to avoid damaging the DPF and other aftertreatment system components. If the DPF is allowed to accumulate too much soot, the large amount of heat released upon regeneration can not effectively be dissipated, resulting in filter damage such as by the formation of cracks, or regions which may be locally melted. On the other hand, regenerating the DPF too often incurs unnecessary fuel penalties with associated CO2 emissions. Further, advanced control strategies may also be required to adjust the conditions under which the filter is regenerated, such as exhaust temperatures and flow rates, based on the amount of soot in the DPF [5]. Despite the important role accurate knowledge of the DPF loading state plays in optimizing the DPF regeneration and control processes, the same basic measurement systems used to estimate DPF soot levels in the early 1980's are still widely used today. DPF SOOT LOAD MEASUREMENTS The amount of PM accumulated in the DPF is a function of many different factors. Figure 1 presents a simplified illustration of the key parameters influencing the mass of soot accumulated in the DPF, DPF M , specifically the engine-out soot emissions rate, in m& , the soot oxidation rate, ox m& , and the soot escaping from the DPF, out m& . For typical commercial DPFs with trapping efficiencies near 99%, the amount of soot escaping from the filter is generally negligible on a mass basis under most conditions. However, recently increasing scrutiny has been placed on PM number emissions as well. m in m out m ox M DPF Figure 1. Mass balance of soot trapped in DPF. Aside from the specific emissions and oxidation rates listed in Figure 1, the DPF's operating history, i.e. the variation of these rates in time, are equally as important. A mass balance for the soot accumulated in the DPF can, therefore, be formulated as follows: dt t m dt t m dt t m t M out ox in DPF ) ( ) ( ) ( ) ( ∫ ∫ ∫ − − = & & & Equation (1) While the above formulation appears straightforward, the individual terms are influenced by a large number of factors, which are often not well-known. To illustrate the point, engine-out PM emissions are a function of the specific engine operating conditions, combustion characteristics, vehicle drive cycle, fuel type, lubricant type, and lubricant consumption rate, among others. Even assuming accurate knowledge of the engine-out PM emissions rate provides only the first term in Equation 1. Once deposited on the DPF, the soot is eventually oxidized either through a passive process, such is the case with catalytic systems, through active DPF regeneration, or some combination of active and passive processes. Exhaust gas temperatures, flow rates, and composition (specifically NOx/PM ratio), DPF soot loading levels, catalyst formulations, and the quality, i.e. completeness, of the regeneration are all factors influencing the amount of soot oxidized over a given time interval. Lastly, while the amount of soot escaping the DPF is generally small, <1%, this value can become much larger when the integrity of the particulate filter is compromised through any one of a number of failure 3 modes. Detecting PM leakage from the DPF above a specific threshold value has become increasingly important from an OBD perspective [6]. Additional complexity arises from the fact that the loading state of the DPF is continually changing. Not only soot, but also inorganic ash accumulates in the DPF over time. This ash consists primarily of incombustible lubricant additives and engine wear and corrosion particles, which unlike soot, are not consumed during the regeneration process. Over time, ash build-up in the DPF leads to increased exhaust flow restriction, a reduction in soot storage capacity, and negatively impacts vehicle fuel economy. As a result, DPFs are periodically removed for ash cleaning, which is mandated to be no more frequent than every 150,000 miles [7]. DPF ash levels after 150,000 miles of on-road use may comprise more than 80% of the total accumulated material (ash and soot) in the DPF. The significant amount of the DPF volume occupied by the ash affects both the soot distribution in the filter as well as local soot loading levels. Previous studies with ash loaded filters subjected to periodic regeneration have shown an increase in local soot loads toward the front of the filter by more than 30%, in some cases [8]. Given the large and complex number of factors influencing the amount of soot accumulated in the DPF, and the increasingly stringent regulatory framework within current systems must operate, it is quite surprising that pressure drop measurements form the backbone of most DPF soot load measurement systems. In addition to the factors outlined above, pressure drop across the DPF is itself also a function of exhaust conditions, namely flow and temperature, DPF type and configuration, as well as the distribution and amount of both soot and ash in the filter. Confounding the issue even further is that fact that many of the most common DPF materials currently in use exhibit a non-linear initial increase in pressure drop with soot load, due to the soot first accumulating in the filter pores (depth filtration) prior to forming a layer on the filter surface (cake filtration). Depending on the filter's operating history, the pressure drop response may exhibit a significant hysteresis as a result of the relative amounts of soot accumulated in the filter pores and cake layer. Several studies have attempted to quantify the variability and error in pressure-based DPF soot load measurements, reported in the range of + 50% of the measurement [9, 10]. Additional concern has recently focused on the ability of pressure-based measurement systems to detect DPF failures in response to upcoming OBD regulations. The results of several recent studies targeting the issue cast serious doubt regarding the ability of pressure-based measurement systems to detect small filter failures which may result in tailpipe-out PM levels exceeding the mandated limits [11, 12]. Additional work has also focused on investigating alternative technologies to measure soot levels downstream of the DPF for purposes of OBD compliance [13]. In order to reduce the variability inherent to pressure measurements, nearly all commercial engine and DPF control systems employ some type of predictive models to better estimate the loading state of the filter. A detailed treatment of these models is well beyond the scope of this study, and there has been extensive work in this field over the past twenty years. In general, these models may estimate many of the key parameters outlined in Equation 1, based on data tables and maps stored in the ECU, or predictive methods utilizing data from various sensors on the engine or vehicle. Additional corrections are generally also applied to account for the effects of exhaust flow and temperature variations on the pressure drop signal. Further, these models are generally calibrated for a specific engine, fuel, and aftertreatment system, rendering the overall measurement method relatively inflexible for use in applications outside the scope of the calibration. The variability of combined pressureand model-based DPF soot load measurement systems has been reported from 1 g/L to 3 g/L in a recent study with a passenger car over a range of urban and highway drive cycles [14], or 16% to 50% of the maximum allowable soot load assuming a limit of 6 g/L for a cordierite DPF. Despite some gains in overall measurement accuracy, current pressureand model-based measurement systems provide only an indirect estimate of soot levels in the particulate filter. The objective of the present work, therefore,