Low-metal Engine Oil Reduces Ash Loading

To meet the strict Euro V limits, exhaust gas aftertreatment systems such as diesel particulate filters must also be used in commercial vehicles and mobile machinery. The problem: The particulate filters often clog and have to be cleaned manually, resulting in expensive downtime. Chevron has developed a new engine oil that does away with 60 % of the usual metallic components, which can clog the filter.

To ensure that diesel-powered commercial vehicles and mobile machinery comply with increasingly stringent emissions regulations, the use of exhaust gas aftertreatment systems such as Exhaust Gas Recirculation (EGR), Selective Catalytic Reduction (SCR) and the Diesel Particulate Filter (DPF), Figure 1. In 1999, Stage I European standards for engines used in Non-Road Mobile Machinery (NRMM) were implemented, and over the years the requirements for NO_x and particulates have become more stringent, which has helped to make the engines significantly lower in emissions. Whilst Stage III and Stage IV led to limited use of DPFs, Stage V is driving widespread adoption of DPFs, which can be very effective at reducing particulate matter emissions but require periodic cleaning to remove incombustible materials that accumulate over time.

Figure 1

Changes made necessary by amendments to EU standards (DOC: Diesel Oxidation Catalyst) (© Chevron)

Exhaust Gas Aftertreatment in Heavy-duty Vehicles

Unlike passenger cars, however, operating and maintenance costs are dominant in commercial vehicles and mobile machinery. Due to their very long operating times, additional costs associated with exhaust gas aftertreatment must also be considered. Particulate filters clogged by ash play a key role here: in a survey commissioned by Texaco Lubricants 2020 in the UK, 67 % of respondents reported problems with the exhaust gas cleaning systems used - many of them specifically with the DPFs. In fact, nearly half of the survey respondents indicated that they needed to force a manual regeneration of the DPF to restore its operation in service - an unplanned maintenance activity that keeps the truck off the road and out of service, and that also requires burning of fuel. DPF blockages that required cleaning or replacement were the most commonly reported issues.

The main reason for these problems is ash that results from metallic lubricant additives and remains in the DPF. The DPF collects up to 98 % of particulate matter emissions in the form of soot and ash, the majority of which is burnt off during typical DPF regeneration cycles, Figure 2. Unlike soot particles, however, the ash element is incombustible material derived primarily from metallic lubricant additives, that over time clog the DPF. Thus, manual external cleaning is required, resulting in expensive downtime. In addition, if too much soot and ash build up, the large amount of heat produced upon regeneration can result in DPF damage. DPF clogging increases engine back pressure, and regeneration cycles, resulting in higher fuel consumption. This is because once the particulate filter reaches a sufficiently high operating temperature, it burns soot particles - a process known as passive regeneration. If temperatures do not become high enough to allow continuous regeneration, active or manual regeneration takes place, in which the exhaust gas temperature is raised by injecting fuel into the exhaust to burn soot particles and unclog the DPF. This comes at a cost, not only the upfront cost associated with these devices and the numerous sensors and dosing units that enable them, but also ongoing operational and maintenance costs.

Figure 2

Exhaust flow from the engine through the exhaust aftertreatment system (DEF: Diesel Exhaust Fluid) (© Chevron)

Additionally, clogged particulate filters can have a significant impact on productivity. In some cases, the engine will enter a reduced power mode until active regeneration is complete, resulting in unscheduled downtime. If too much soot and ash build up, the large amount of heat generated during regeneration can cause filter damage - resulting in a cost-intensive DPF replacement and downtime. DPF regeneration can only remove the soot. The ash remains and ultimately determines the filter life. Machine owners are forced to remove affected equipment from service and clean the DPF - usually externally - resulting in further costly maintenance and downtime.

60 % Less Metallic Components

Chevron has developed a new engine oil that contains around 60 % fewer metallic components than conventional high-performance oils. Texaco Delo 600 ADF with patented Omnimax technology reduces ash formation in DPF, protecting both the engine and the exhaust aftertreatment system. Suitable for both on- and off-highway application, the lubricant is formulated with only 0.4 % sulphated ash - far lower than the API and ACEA limit of 1 % sulphated ash, as found in most current heavy-duty engine oils. Even so-called low-ash engine oils can cause high ash loading of the DPF over time. This also happens with the new engine oil, but much more slowly, as it contains 60 % less ash-forming additives. In addition, the ash resulting from the lubricant has a higher density in the DPF. Therefore, a significantly higher active filter volume remains over the lifetime of the DPF.

During the development of the new oil formulation, the biggest challenge was to reduce ash without affecting the oil performance. This was realized by retuning the additive system and introducing new, less ash-containing components. This required an entirely new formulation approach and extensive re-balancing of all components to achieve comprehensive protection of the engine and exhaust aftertreatment system, Figure 3.

Figure 3

Comparison between conventional engine oil with 1 % ash (left) and new engine oil (right) (© Chevron)

Test Procedure for Evaluating the New Engine Oil

Wear tests, which are part of the ACEA and API standards, on machines in use confirm the new engine oil's strong wear protection. Tests with operators showed that by reducing 60 % of the ash from the oil, 60 % less ash is found in the particulate filter, Figure 4. In the course of the project, however, other customer benefits became clear: fuel costs are often the most important operating cost of a fleet, but it is often unknown that the condition of the exhaust aftertreatment systems has a significant impact on fuel consumption. In order to quantify the impact of ash quantity on fuel consumption, and to understand how it affects exhaust backpressure and DPF regeneration, the engine oil was put to the test in a systematic engine test. In controlled tests on the engine test bench, the DPF load under identical conditions was simulated and the new engine oil was compared with a conventional oil with 1 % ash. At identical loads and operating times, the system with Texaco Delo 600 ADF had a significantly lower ash content, Figure 5. The study also evaluated how the amount of ash affected the exhaust backpressure that the DPF exerts on the engine. Since ash combines with soot in the exhaust, there can be a significant increase in exhaust backpressure. For the same operating time, the DPF on an engine with the engine oil resulted in almost no increase in exhaust backpressure - not only because of the lower ash content, but also because of the higher density of the ash, which allows for a larger active filter volume, FIGURE 6. Further tests showed that operation with conventional 1 % ash oil resulted in three times more frequent DPF regeneration than with Texaco Delo 600 ADF.

Figure 4

Amounts of ash blown out of the particulate filter after similar operating time (© Chevron)

Figure 5

Visualization of the accumulation of ash at the end of the DPF ducts by means of computed tomography - the significantly lower ash accumulation (right) shows a significantly higher, active filter volume (© Chevron)

Figure 6

Exhaust backpressure comparison between conventional engine oil with 1 % ash (left) and new engine oil (right) (© Chevron)

The increase in fuel consumption when the DPF is fully loaded is significant. In the WHTC test, the engine burned almost 6 % more fuel when the DPF reached its total useful life (on 40 g/l ash load) compared to a new and clean system. In comparison, the DPF of an engine operated with the new engine oil over the same period showed only a 1 % increase in fuel consumption. Averaged over the DPF usage interval, this means a fuel saving of up to 2.3 %. This saving can be achieved in addition to the reduction in fuel consumption due to reduced exhaust backpressure.

Extended Oil Life and Increased Component Protection

Increasing oil change intervals has remained a trend in the automotive and work machine industries. OEMs have therefore included oxidation control as a key criterion in their specifications. Volvo, for example, has introduced the T-13 engine test as part of the API CK-4 standard and their own VDS-4.5 specification. This is a very demanding test where the oil is subjected to an aggressive duty cycle with high operating temperatures and a reduced oil volume. This test will also be part of future ACEA specifications. Oils that meet T-13 test requirements must survive 360 h of operation without significant oxidation or oil thickening. Texaco Delo 600 ADF completed the test with double length of 720 h before the limits were exceeded. Additionally, the engine oil meets or exceeds ACEA E6, E9, and OEM specifications, and has demonstrated oxidation stability in industry, OEM and field tests, providing the opportunity to extend engine oil drain intervals. It has also been formulated to deliver valve-train wear protection and piston deposit control performance. The greatest advantages can be achieved in heavy-duty diesel engines with DPFs. Older equipment retrofitted with a DPF would still see the benefits of DPF life extension as well as increased fuel efficiency.

Summary and Outlook

This engine oil helps reduce the rate of DPF clogging, resulting in extended DPF service life and less frequent DPF regeneration cycles. Tests with a number of heavy-duty diesel engines have shown that cleaning intervals could be extended by a factor of up to 2.5, and fuel consumption reduced by up to 3 % over the filter life. Considering that fuel costs typically account for about 25 % of the total cost of ownership of a commercial vehicle fleet, the fuel savings can provide a reduction in overall costs and increase the fleet's competitiveness.

The major engine builders around the globe are looking hard at how they will meet the pollutant and greenhouse gas emission standards that will continue to phase in over the next seven to ten years, while also continuing to meet customer demands. Very low ash oils will be pivotal to meeting the requirements of future products, while offering engine builders the flexibility to optimize their systems in terms of the space that emission control components occupy in the equipment as well as their up-front cost.