What’s the difference between soot and ash in diesel engines?

Sectional DPF Filter

Costly DPF issues can easily be avoided

Since the introduction of Diesel Particulate Filters (DPFs) in 2007, soot and ash in the exhaust stream of diesel engines has been a problem that won’t seem to go away.

A Diesel Particulate Filter, as the name suggests, is a critical exhaust aftertreatment device that captures and stores soot and other particulate matter in the exhaust system of diesel engines to reduce harmful emissions. It essentially acts as a soot trap, filtering out the tiny soot particles produced by diesel engines before they enter the atmosphere.

Where does the soot and ash come from?

Microscopic view of soot and ash in a DPF

Soot and ash under a microscope - this is what blocks your DPF if you don't take action

Soot is a byproduct of the combustion of diesel fuel. The very nature of diesel can create a barrier to its complete combustion. Typically, diesel fuel and air do not blend ideally, and consequently, fuel-dense pockets are created within the combustion chamber during the fuel injection process. This leads to incomplete burning of all the hydrocarbon molecules in the fuel, resulting in soot (unburned carbon). Unless measures are introduced to increase the combustibility of the fuel, harmful soot will always be the result.

Ash in diesel engine oil refers to the non-combustible residue remaining after the oil is burned in the combustion chamber and treated with sulfuric acid. It is composed of different compounds, such as calcium, magnesium, zinc and barium, which are typically added to diesel engine oil for various purposes. This unburned ash is expelled into the exhaust stream during normal engine operation, along with soot and other emissions.

Why is it important? 

Soot and ash are sometimes referred to as the same thing, when in fact, they are quite different. It’s helpful to understand the difference in order to appreciate the adverse impact of both soot and ash on the DPF. Ash in the exhaust stream is the primary binder of soot particles, clumping the soot together within the DPF.

Badly blocked DPF

The solution to most DPF problems is to eliminate the root cause

What problems arise with soot and ash in the DPF?

As the ash and soot combine within the DPF, the resultant mass starts to block the filter core. When this occurs, a number of negative consequences follow. These can include reduced fuel efficiency, diminished effectiveness of the DPF, engine shut-down, equipment downtime, pricey maintenance costs, and ultimately, complete component failure.

Other problems…

Soot is not just a harmful emission that affects the environment, it can also cause catastrophic damage to the engine. This is evidenced in two critical areas of engine operation.

  1. damaged engine bearings

    Engine-destroying, gritty, black engine oil can be prevented

    Soot particles created during the combustion process build up on such components as the piston compression rings, the ring grooves, exhaust valve heads, cylinder wall cross-hatch micro grooves (cylinder glaze), piston crowns, ring lands and skirts. This action causes such problems as stuck compression rings, glazed cylinders, broken compression rings, reduced compression, engine blowby, reduced power, and excessive oil consumption.

  2. Soot particles get past the piston rings and end up in the engine oil. Once there, they immediately mix into the oil – turning it black, thick, and gritty. This becomes a dangerous slurry because of its abrasive nature on all lubricated engine components, the considerable change to oil viscosity, and the reduction in the oil's ability to circulate and lubricate properly. Over time, this slurry transitions into engine-destroying sludge.

What is the solution?

There are a number of ways to ensure you can significantly reduce, or even eliminate, the threats of major expenses and inconvenience associated with ash and soot in your diesel engine.

  1. Ensure you use low-ash diesel engine oil in your diesel engine. Use at least API CK-4 spec (1% sulfated ash) engine oil. Ultra-low sulfated ash (≈ 0.5%) engine oil is also available from several manufacturers. Minimizing ash production and accumulation in this way will help prevent premature clogging and degradation of the DPF.
  2. Use FTC Decarbonizer in the diesel regularly. FTC Decarbonizer is a true fuel-borne catalyst, and ensures every fuel molecule is combusted. This causes much less harmful soot to be created during every combustion cycle. It also oxidizes (burns off) hard carbon buildup in the DPF as the engine operates, as well as on the piston rings and ring grooves, cylinder walls (deglazes the cylinders), and other combustion chamber components. It will also de-coke the turbocharger. Engine power and efficiency will increase, and fuel costs will decrease. DPF life will be greatly increased, as well as overall engine life.
  3. Rid your engine of chronic sludge buildup in the lubricating system by flushing with Flushing Oil Concentrate. Highly concentrated detergents (no abrasive or corrosive solvents) will thoroughly dissolve, suspend and flush all sludge and other damaging contaminants from the lubrication system. This overcomes blocked oil-control rings (reduces oil consumption and further soot production), stops oil turning black and abrasive immediately after an oil change, cleans oil lines and valves, restores oil viscosity and lubrication properties, and extends the operational life of the engine.