Aftertreatment Systems - Function, Cost, and Care

Manufacturers specify cleaning intervals for each engine platform. Exceeding these intervals increases the risk of costly component damage covered in the sections below.

Manufacturer recommended cleaning intervals:

Cummins - 200,000mi or 4,500 engine hours

Detroit -  300,000 miles

Mack - 250,000mi or 4,500 engine hours

International Maxxforce - 2008-2013 - 150,000 miles

        2014 or newer - 200,000mi or 4,500 engine hours

Paccar - 200,000mi or 4,500 engine hours

Volvo Extended Intervals - Heavy haul - 250,000mi or 4,500 engine hours

       Volvo Regional or long haul - 400,000mi or 10,000 engine hours

Section 1: Introduction

The aftertreatment system is a multi stage exhaust device that converts harmful pollutants into nitrogen, water vapor, and carbon dioxide. It uses a series of catalyst filters, sensors, injectors, and recirculation components, to convert and capture carcinogenic byproducts of the diesel combustion process. 

There are three subsystems within a typical aftertreatment system.

Exhaust gas recirculation (EGR): This system injects a small portion of exhaust gas into the engine’s intake airstream in order to reduce the formation of NOx gasses. This system is typically comprised of an EGR cooler, valves, sensors, and piping. Over time, excess soot in the exhaust stream can clog any of these components. In most circumstances, a regen can help reduce deposits.

Exhaust gas filtration: Diesel combustion byproducts are scrubbed out of the exhaust stream using a series of filters. A standard system is comprised of three or more filters.

• DOC: First, the diesel oxidation catalyst (DOC) passes the exhaust stream over a hot ceramic surface in order to break down and burn off suspended particles in the exhaust gasses. Fuel injection sprayed directly into the exhaust stream aids in the exhaust gas heating process during regen scenarios. The catalyst uses a coating of precious metals in order to build up heat and burn off emissions.

• DPF: Once the exhaust stream passes through the DOC, the diesel particulate filter (DPF) captures any unburnt soot or ash. The DPF has open pores on the inlet side that close off on the outlet side. This forces suspended particulates to pass through the filter material. Differential pressure sensors will look for excessive soot captured in the DPF. Normal vehicle operation and regens will burn out most of the captured material.

• SCR: Lastly, the exhaust stream will pass over the selective catalytic reduction (SCR). Similar to the DOC, the SCR is an open channel catalyst that uses heat and a stream of diesel exhaust fluid (DEF) to convert harmful emissions into nitrogen and water. This is typically the final filtration stage on EPA compliant systems.

DEF system: The diesel exhaust fluid (DEF) system is typically comprised of a DEF tank, level and quality sensor, pump, and a DEF injector that sprays into the SCR. This system reduces harmful NOx emissions by up to 90%.

There is also an assortment of other subsystems that vary by manufacturer, but typically include the following:

• Temperature sensors

• Pressure sensors

• NOx sensors

• Efficiency sensors

• Venturi valves

• Bypass valves

• 
  

Section 2: DPF Service

Diesel particulate filters (DPF) are designed to filter up to 100% of soot pollutants. The porous design of the filter will capture and hold particles up to 1000 times smaller than a human hair. Over time, the DPF will accumulate buildup, this is found in two forms; soot and ash.

• Soot is the combustible byproduct of diesel emissions, typically comprising of unburnt fuel and oil. Soot will burn off during normal operations and regens, permitted there is not an excessive load.

• Ash is the incombustible byproduct of diesel emissions. Ash is typically formed when soot is broken down completely and incombustible metallic byproducts are left behind. Metals such as calcium, zinc, and phosphorus are common fuel and oil additives to protect the motor. These will not burn off during normal operations or regens, and usually will not pass through the filter. 

Oil, coolant, and fuel contamination are also common DPF issues. Scenarios such as a blown turbo, worn piston oil rings, and failed head gaskets can cause fluids to leak into the exhaust stream. Typically, fluid contamination will not allow a truck to passively regen correctly and can cause sensor failures, DOC stripping, and even melted DPFs. Filter replacement is recommended if there is high contamination found. 

Section 3: Cost of Failure

DOCs and SCRs are manufactured using precious metals. Similar to a catalytic converter, the filters typically contain platinum and palladium coatings. Once the precious metals are stripped away from the filter due to improper maintenance or system failure, the filter will cause the truck to derate. The DOC or SCR is no longer recoverable through a regen or cleaning and must be replaced. 

DPFs are manufactured with a highly specialized process that creates a uniform ceramic or silicon carbide cell structure. When the cell structure is clogged, the DPF may often be recovered through a cleaning. The health of the DPF directly impacts the performance of the DOC and SCR. A clean DPF can prevent DOC/SCR precious metal stripping and damage to other components.

OEM replacement part estimates (not including labor):

• Cummins ISX15

  • DPF - $3,000-$5,000

  • DOC - $4,000-$8,000

  • SCR - $7,000-$12,000

• Freightliner / Detroit DD15 OneBox

  • Onebox - $13,000-$17,000 (including 2 DPF filters)

• Volvo D13

  • DPF - $2,500-$5,000

  • DOC - $3,000-$6,000

  • SCR - $5,000-$10,000

Filter loading can also cause premature sensor wear. 

• Sensor pricing can vary. Part costs can be from $100-$1,000 depending on make, model, and what sensor is needed.

Cost of truck downtime varies by classification and load. The table included is a rough estimate of lost revenue per classification of truck per day. Average truck downtime for DPF replacement depends on diagnosis time, part lead time, and shop availability. Expected downtime of 1-3 days is average, but varies by case.

Lost revenue per day due to truck down scenarios:

Truck Type / Operation	Estimated Lost Revenue Per Truck Per Day	Estimated Lost Net Profit Per Day
Dry Van OTR	$800–$1,500	$150–$400
Reefer	$1,200–$2,000	$250–$600
Flatbed	$1,200–$2,500	$300–$800
Tanker	$1,500–$3,000	$400–$1,000
LTL Linehaul	$2,000–$4,000	Highly variable
Car Hauler	$2,000–$5,000	$500–$1,500
Heavy Haul / Oversize	$3,000–$10,000+	$1,000–$4,000+
Oilfield Trucking	$2,500–$8,000	Highly variable
Dump / Construction	$700–$2,000	$300–$800
Expedited Freight	$1,500–$4,000	Highly variable

A clogged aftertreatment device increases exhaust system backpressure. Referencing FSX Equipment community sourced baselines, the average increase from “green tag” pressure readings (at or near OEM flow rate) vs “red tag” pressure readings (restricted airflow due to cell loading, replacement recommended) is listed below:

• Cummins, International, & Paccar - 41.2%

• Detroit - 25.6% 

• Volvo & Mack - 32.7%

The pressure increase will be reflected via the differential pressure sensor, reading as “high soot load”, “high ash load”, “high pressure”, etc. and can cause forced regens or derates. Before fault codes are produced, however, restriction within the exhaust system will cause excessive strain on critical components such as turbos, cylinder heads, rotating assemblies, and any affiliated sensors or sub systems. Cost of replacement varies, but $10,000+ services are common for any of the listed systems, including aftertreatment system replacement.

Section 4: DPF Cleaning

In order to maintain a healthy aftertreatment system, regular DPF cleanings are recommended. A typical service takes 24-48 hours and will include pre and post inspections to measure system flow and sensor function, as well as check for clogs or signal upstream failures.

Inspection process:

- To be performed by the repair technician:

• Check measured soot/ash load as recorded on truck, regen as needed

• Determine if a cleaning or replacement is recommended according to mileage, hours, or case scenarios such as a high soot/ash reading, failed 7th injector, catalytic efficiency codes, etc.

If a forced regen is needed, it is good practice to perform an off-vehicle cleaning as preventative maintenance post forced regen.

Tip: The DPF and DOC should always be cleaned as a pair.

- Inspection to be performed by the DPF cleaner:

• Perform visual inspection; look for cracked or compromised filter material using light, check for oil/fuel/coolant contamination, and inspect face for plugging

• “Pin depth gauge test”; needle-like rods are dropped into the inlet side of the DPF to check cell structure and uniform soot/ash loading. Uneven loading typically indicates further faults in the system, such as an overcycling 7th injector

• Backpressure test DPF, check DOC for faceplugging

Once the DPF and DOC are determined as cleanable, the cleaning process will be performed.

DPF and DOC cleaning:

• DPF and DOC devices are blown out using an enclosed blast cabinet such as a Donaldson Pulse Cleaner, FSX Trapblaster, or Evacublast Blast Cabinet. If the DPF is oil/fuel/coolant contaminated, solution based cleanings can be performed. If the DOC is contaminated, replacement is necessary.

• Back pressure test and pin depth gauge test, determine if further cleaning is necessary

• Thermal regen using a specialty oven. Industry standard ovens use a multi step temperature curve producing up to 1,400°f over 4-12 hours

• Blow out device and inspect again, referencing bullets 1 and 2

• Ultrasonic or flush clean DPF as necessary. The DPF is soaked in a specialty solution to help break down soot and ash deposits before flushing using water 

Ultrasonic or flush cleanings can be performed in place of blowouts & thermal regens for DPFs and, while typically costing more, offer the most thorough cleaning.