This CTS Knowledge Base article describes the key factors controlling the quantity and type of ash that accumulates in DPFs. The build-up of ash deposits in the filter is affected by a number of parameters, which must be considered as a whole, in order to minimize the impact of ash on DPF and engine performance.
System Definition and Input Parameters
An analysis of the factors influencing ash build-up in the DPF must first consider the entire system, which includes the combined engine and aftertreatment system. The emissions generated by the engine directly affect the DPF, and conversely, the processes occurring in the DPF also affect engine performance. Equally important to defining the system is the identification of the relevant input parameters. Figure 1 presents a diagram of the combined engine and DPF system, as well as the key inputs and processes.
Figure 1: Key factors controlling the nature of the ash deposits and the quantity of ash which builds up in the Diesel Particulate Filter (DPF).
The primary sources of ash in the DPF are generally metallic-based detergent and anti-wear additives in the engine oil, although trace metals in the fuel, engine wear and corrosion, and other environmental sources can contribute, but generally to a much lesser extent. The contributions of each of these input parameters was discussed in detail in the article describing ash sources and composition.
Another important factor governing the magnitude of the effect of ash on exhaust back pressure and engine performance is the design of the DPF itself. Specifically the porosity of the filter material, its mean pore size and pore characteristics [1], as well as the filter material and cell geometry can exert a large influence on the sensitivity of the filter to ash accumulation. Asymmetric DPF cell designs, where the inlet channels are larger than the outlet channels, present one means for effectively increasing the filter’s ash storage capacity with the same total DPF volume [2].
Key Processes Influencing Ash
Due to the amount of time ash spends in the DPF between filter cleanings, the ash properties and the manner in which the ash deposits are formed in the filter are affected by a number of processes. Among these processes, the conditions of engine operation, exhaust temperature and flow, and DPF regeneration play key roles which vary significantly for different vehicles, equipment, and applications. Figure 2 shows ash deposits which have formed plugs completely blocking many of the channels in a cordierite DPF. Not all channels show the same degree of ash plugging, as indicated by the ash forming a layer along the channel walls in some of the cells.
Figure 2: Ash plugging in Diesel Particulate (DPF) channels.
Engine Operating Conditions directly influences the rate of ash emissions going into the DPF. Hand-in-hand with the total lubricant sulfated ash level, engine oil consumption is the second most important process (if not the most important) when it comes to the build-up of ash in the particulate filter. Engine operation, whether at idle, low load, or high load, also directly affects the exhaust conditions, including the rate of particulate matter (soot) emissions and exhaust gas composition entering the DPF.
Exhaust Conditions, primarily temperature and flow rate, are largely controlled by the engine operating conditions. Nonetheless, these two parameters play an important role in determining the ultimate characteristics and distribution of the ash in the DPF, including the extent of ash sintering, the ash packing density, and mobility within the filter.
Regeneration Strategy is also linked to the exhaust conditions. Active regeneration typically exposes the ash to higher temperatures relative to passive regeneration, which has been shown to affect the resulting ash particle size. The soot loading level at the time of regeneration, as well as the oxidation pathway, whether O2 or NO2, also plays a role in the formation, mobility, and distribution of the ash deposits [3].
Why is this important for DPF ash cleaning?
Understanding the roles and complex interactions that the various inputs to the engine have on the resulting ash formation in the DPF, as well as the effect that operating conditions and regeneration strategies have on the ash properties, is useful to tailor a cleaning approach best suited to a particular type of ash. Recognizing that not all ash is the same, depending on the DPF application, is crucial to developing an efficient and cost effective filter maintenance program.
References
- Dimou, I., Sappok, A., Wong, V., Fujii, S. et al., “Influence of Material Properties and Pore Design Parameters on Non-Catalyzed Diesel Particulate Filter Performance with Ash Accumulation,” SAE Technical Paper 2012-01-1728, 2012, doi:10.4271/2012-01-1728.
- Aravelli, K. and Heibel, A., “Improved Lifetime Pressure Drop Management for Robust Cordierite (RC) Filters with Asymmetric Cell Technology (ACT),” SAE Technical Paper 2007-01-0920, 2007, doi:10.4271/2007-01-0920.
- Sappok, A., Govani, I., Kamp, C., Wang, Y. et al., “In-Situ Optical Analysis of Ash Formation and Transport in Diesel Particulate Filters During Active and Passive DPF Regeneration Processes,” SAE Int. J. Fuels Lubr. 6(2):2013, doi:10.4271/2013-01-0519.