If you have ever been behind a tractor-trailer leaving a long white trail of smoke, you’ve experienced the ugly side of modern diesel emissions systems. That white, choking smoke is a sign of a failing DPF filter. According to a 2023 study by the Society of Automotive Engineers, that haze is mainly Sulfur Trioxide. By itself not harmful, but when in contact with water forms sulfuric acid. Probably not the trail you want to be leaving for others to breathe.
Regeneration Cycle Overview
When diesel fuel is burned various pollutants are released into the air. These include
- Particulate Matter- Particles in a solid state, as in carbon soot and dust.
- O3- Ozone
- CO- Carbon Monoxide
- Sulfur
- NOx– Oxides of Nitrogen
Major Components
- DPF- Diesel Particulate Filter
- DOC- Diesel Oxidation Catalyst
- SCR- Selective Catalyst Reduction
- EGR- Exhaust Gas Recirculation
- Turbocharger
- Post Combustion Fuel Injection
Diesel Particulate Filter
The DPF in conjunction with the DOC is responsible for catching particulate matter. The honeycomb design coated with precious metals enables the filtration and oxidation of PM.
Selective Catalytic Reduction
The SCR is responsible for Nitrogen reduction.
When DEF fluid is injected into the SCR, it reduces to ammonia, which itself reacts with
the NOx compounds, reducing them back into
oxygen and nitrogen.
Exhaust Gas Recirculation
The objective of the EGR system is to recirculate “dead” exhaust gas into
the intake. The idea is to reduce NOx emissions by lowering combustion
heat. This occurs at the expense of engine power and fuel economy. It
also routes contaminates through the intake and cylinders.
Variable Geometry Turbo
and Post Fuel Injection
The turbocharger has an extra role in
modern emission systems. No longer
just accountable for producing manifold
pressure and HP but now must add heat into the DPF during low throttle and idle conditions. By utilizing vanes inside the housing it can behave as if it were a smaller unit, producing more
pressure/heat at idle. Combine a form of post fuel injection and the DPF can stay up to temperature under prolonged low throttle operation.
Why Proper DPF Maintenance
Matters
Aside from the obvious air pollution emitted from diesel engines, not paying attention to warning lights or maintenance intervals can
be very expensive. The local Ford shop wants $200 an hour to work on your regular F250 or F350. Simple things like not allowing your
truck to excessively idle and using the correct coolant have major impacts on the longevity of the DPF, EGR, and SCR.
Common Issues
Low Power / Excessive Smoke:
These symptoms are usually the result
of a clogged or damaged DPF. This is where most mechanics get in trouble. 9 times out of 10 the DPF is clogged due to a fault in one of the systems upstream. Beware of a mechanic that tells you to replace sensor after sensor to no avail.
Clogged or Damaged
Aftertreatment Injectors
Most heavy-duty on-highway equipment are inline 6-cylinder engines with a separate after treatment injector called a 7th injector. On most light-duty v8 diesel engines such as your Powerstroke and Duramax, the after treatment fuel injection is handled by the rearmost two cylinders, 7 and 8. This is where the major problem arises. To heat the DPF up 7 and 8 must over-fuel. Fuel then passes by the exhaust valve and into the exhaust stream where it can be oxidized for heat. Some of this fuel will slip past the piston rings and into the engine
oil. Have you ever heard the term “Making oil”? This is where it comes from.
Seized or Sticking Variable
Geometry Turbochargers
At idle conditions, the engine relies on the turbocharger to help create heat. It does this by closing the vanes and increasing efficiency. The actuator that opens and closes these vanes is subjected to tremendous heat. They must be cooled by engine coolant. Even with adequate maintenance all actuators eventually wear out and
start to stick. I have seen more Holset turbos replaced due to faulty actuators than actual turbine failure.
Exhaust and Charge Air Leaks
For the DPF to reach regeneration temperatures, enough exhaust must make it there in the first place. An extremely common and overlooked issue is a simple exhaust or charge air cooler leak. Charge air cooler leaks don’t usually have quite the effect as exhaust though a large enough leak will severely de-rate engine performance.
Summary
Every year OEMs like Cummins, Ford, and Chrysler, add more and more electronics to these already complicated systems. That doesn’t mean it should be feared. Electronics are just a means of monitoring a mechanical system. 90% of all the issues I see can be traced down to the base engine. Even a damaged DPF filter is still usually not the culprit. The filter was damaged by something. Whether it was a clogged injector broken valve, or stuck turbo, it’s never caused by magic.
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