The detrimental effects of ash on diesel particulate filter (DPF) and now gasoline particulate filter (GPF) performance is of significant concern because the resulting permanent backpressure penalty compromises both fuel economy and soot storage capacity, leading to more frequent regeneration intervals with further fuel economy impact.
The ash is thought to be derived primarily from oil additives but also from fuel, engine wear, and other sources. As these filters accumulate more miles on customer vehicles, the long-term effects are becoming of more concern to vehicle and filter manufacturers.
However, the time taken to accumulate a representative amount of ash—over, say, 100,000 km (62,100 mi)—is significant. Cambustion’s DPG, an automated particle filter testing system, can be used to load DPFs/GPFs with ash over a matter of hours, the company claims.
Initial tests at Cambustion involved mixing oil additive “packs” to the fuel tank of a DPG and burning the resulting fuel/additive mix in the DPG’s burner. Note that the production of ash can be obtained at the same time as soot yield and regeneration events.
While the DPG itself is not new, this is a new application of the test system for studying the effects of ash build-up in DPFs and GPFs. The DPG can handle DPFs up to heavy duty, so it’s suitable for on- and off-highway commercial applications in addition to automotive.
Backpressure results from multiple loads (from clean)
Tests using a 4-L SiC (silicon carbide) DPF have been conducted where a yield of 1.5 g/hour of ash was achieved along with 10 g/hour of soot. The results for three loads—interspersed with regenerations performed on the DPG—on a clean (without soot or ash) DPF are shown in Figure 1.
Load 1 shows the classic pore-filling curve within the first gram of loading. The backpressure then continues linearly until the part is regenerated—then Load 2 begins.
The second load begins at a higher backpressure because a layer of ash has been deposited during Load 1 as a “cake” layer. Note also that the pore-filling and pore bridging phase is absent from Load 2 because ash is preventing as much soot entry into the pores. Also, the gradient of the linear pressure increase thereafter is slightly less than for Load 1. This is because the reduced soot within the pores (displaced by the ash) is unable to dominate the Dp (pressure drop) during the cake layer build-up.
The third (and all subsequent) loads begin at a higher and higher initial backpressure indicating permanent ash backpressure penalty.
The deposition position and quantity have yet to be assessed for the DPG, but the backpressure characteristics and filtration efficiency changes (not shown here) appear to be similar to engine tests performed by a variety of customers.
Further tests are planned using alternative loading temperatures, flow rates, doping rates, and soot rates.
Assessment of ash
Some measurements have been taken with a DMS500 particle sizing instrument to identify the ash and soot particles that are being produced by the additive-in-fuel technique, the results of which are shown in Figure 2.
A clear, small ash size mode was identified centered at about 15 nm (compared with the DPG soot mode at approximately 100 nm). The amount of ash being generated is basically linked to the fuel flow into the burner and the additive dosing rate. The maximum ash yield is therefore occurring during regeneration where the burner is operating in a hot (but non-sooting) mode with high fuel input.
It is interesting to note that the soot load mode and the warm-up mode actually consume very similar fuel rates but where soot is being produced, it is clear that the ash particles are being bound-up onto the surface of the soot particles.
Note that this technique contaminates the entire fuel system with the additive, which can be problematic if the DPG is required for other conventional work. Both the fuel system and the surfaces upstream of the test part need to be cleaned of ash deposit before accurate soot-only tests can resume.
Information about these test results will be available at the company’s SAE 2016 World Congress exhibit (Booth 906), and Cambustion engineers will be able to explain in detail how the instrument works and its applications.
Bruce Campbell and Kingsley Reavell, directors at Cambustion Ltd., wrote this article for Automotive Engineering.