Impact of Soot in Engine Oil on Wear
As diesel engine designers and assemblers know too well, more stringent emissions regulations have led to hardware and software mapping changes that can yield unpleasant side effects, such as increased carbon particulate output that can migrate to the crankcase. There is the potential for more internal wear over time, due to soot-loaded engine lube oil. Lubrizol has developed additive technology for API CI-4 Plus and API CJ-4 engine lube oils that help mitigate soot-related wear, to the relief of heavy vehicle assemblers, as well as their customers—the engine owners/fleet operators.
Soot loading in diesel engine oil can present wear problems. Diesel engines consume a carbon-rich fossil fuel that liberates soot as a byproduct of combustion. Soot (see figure below) consists of micrometer-scale particles of elemental carbon. The existence of soot signifies incomplete combustion, which suggests an incorrect air/fuel ratio, improper combustion temperature, insufficient residence time in the combustion zone, and/or non-availability of sufficient oxidants (another way to say that the air/fuel ratio is too fuel-rich). Diesel engines usually run lean with excess air for combustion. However, under acceleration (transient) conditions, much fuel is injected and there is a temporary air starvation situation in the combustion chamber as the turbocharger labors to spool up and deliver sufficient pressurized air to the intake manifold, thus yielding unburned soot.
In most modern well-maintained diesel engines, soot will be oxidized within the combustion chamber or later trapped and oxidized downstream in the emissions system. The U.S. EPA has classified carbon soot, which can find its way deeply into lungs, as a carcinogen. In the face of ever-tighter emissions regulations and subsequent emissions controls technologies visible black smoke (see older heavy diesel truck below), has largely become a nuisance of the past. However, soot load is rising within the engine, including the presence in diesel engine oil.
So, how is it that soot enters the crankcase to mix with the engine oil? As the piston goes down for every power stroke, soot can accumulate on the cylinder liners of each bore and can be scraped down by the oil control piston rings. Soot can be further delivered to the crankcase via blowby of combustion gases past the piston rings (especially worn rings). In addition, the thin motor oil film retained on the bores can partially break down under combustion heat, leaving more soot. Such soot accumulations in the engine oil have been observed in the 2% to 10% range. In concentrations starting around 3 to 5% in the engine oil, soot can become problematic for engine owners/operators.
The growing problem of soot-laden diesel engine oil is linked to ever-stricter exhaust emissions standards. In diesel engines, engineers trade off soot production versus NOx production. In general when one goes up, the other goes down, and vice versa. In the interest of reducing NOx,, which increases with combustion chamber temperatures, engines now have retarded injection timing (that is, fuel injected later in the combustion cycle) which lowers peak combustion temperature, thus boosting soot production but diminishing NOx. To reduce NOx further at the source, diesels today also have a generous dose of cooled EGR (exhaust gas recirculation), which dilutes and cools the combustion charge.
Aftertreatment of the exhaust to trap excess soot via diesel particulate filters (DPF) is easier and cheaper than reducing NOx downstream of the engine (as via selective catalytic reduction—SCR). However, the regulations are now so strict in markets such as the U.S. and Europe that most engine producers have to include both a DPF and SCR treatment, as well as cooled EGR at some level to meet the regulations. So, that was the tuning—less NOx and more soot at the source, which unfortunately put more soot into the engine oil.
Soot is harmful in a number of ways when at excess levels in engine oil. First is the abrasion challenge, especially if the carbon is agglomerated into clumps, which can cause valvetrain, ring and liner wear. Soot loading also causes a viscosity increase, which inhibits oil flow, especially in cold weather starts where the soot-laden engine oil can increase the time it takes for the oil to reach critical engine parts, such as the valvetrain.
Various tests in the API CJ-4 engine oil specifications address the harm that soot potentially could cause, such as:
- MRV TP-1 (Mini-Rotary Viscometer low-temp viscosity/pumpability test)
- Mack T-11 for soot-related viscosity increase.
- Mack T-12 for soot-related wear
- Cummins ISB for valve train wear.
- Cummins ISM for soot-related wear and deposits.
Some OEM specs go beyond the API CJ-4 specs, to provide a better margin of resistance to the deleterious effects of soot loading. Cummins and Volvo (including Mack) add more stringent requirements to their lube oil specs. OEMs may require special testing (such as Volvo VDS-4) to prove field performance of the diesel engine oil.
Lubrizol additives can help mitigate soot-related wear while allowing the OEMs to meet stringent emissions regulations. Lubrizol advanced dispersant systems keep carbon soot particles separated in suspension, thus limiting abrasive wear. Lubrizol detergents and anti-wear chemistry control wear and deposits in both in the modern and heritage engines.
In North America, Europe, Japan, and soon Brazil, heavy highway diesel engine particulate matter emissions regulations force the use of diesel particulate filters (DPFs) to trap and oxidize carbon soot in the exhaust. Installation of DPFs allowed diesel builders to retune engines (more specifically, to adjust fuel injection events) to achieve cooler combustion which reduces the formation of harmful nitrous oxides (NOx) but at the expense of boosting output of unburned...