Diesel exhaust aftertreatment systems (DOC + DPF + SCR + ASC with DEF/AdBlue dosing) · emission limits (EPA Tier 4 Final, Euro VI), NOx conversion, DEF quality to ISO 22241-1, dosing pressure and rate, catalyst volume, exhaust back pressure
A diesel exhaust aftertreatment system treats engine exhaust after combustion: a diesel oxidation catalyst (DOC) oxidises HC and CO, a diesel particulate filter (DPF) traps soot, and a selective catalytic reduction (SCR) catalyst, fed with a urea solution (DEF or AdBlue) by a pump, dosing control unit and injector, converts NOx to nitrogen and water, with an ammonia slip catalyst (ASC) last. Ask three things first: which emission standard and engine power class the system must certify to, what catalyst volumes and back pressure it imposes on your engine, and what DEF dosing hardware, pressure and heating come with it.
Complete diesel aftertreatment systems and their parts for trucks, construction, agricultural and generator engines: SCR catalyst and its canned housing, DOC, ASC, DEF/AdBlue injection systems (supply pump module, dosing control unit DCU, injector or dosing module, heated lines and filter). Stand-alone diesel particulate filters are covered on the particulate filter page and DEF tanks with level sensors on the DEF tank page; gasoline three-way catalysts and exhaust gas analyzers are outside this page.
01
EPA's summary of nonroad compression-ignition engine standards lists the final Tier 4 limits for 2014 and later engines from 130 kW to 560 kW as 0.19 g/kW-hr NMHC, 0.40 g/kW-hr NOx, 0.02 g/kW-hr PM and 3.5 g/kW-hr CO; above 560 kW the 2015+ NOx limit is 3.5 g/kW-hr (0.67 g/kW-hr for generator sets) and PM 0.04 g/kW-hr (0.03 g/kW-hr for generator sets). Tier 4 engines are tested for transient and steady-state emissions under 40 CFR Part 1039 Subpart F; transient standards do not apply to constant-speed engines or engines above 560 kW. [1]
For on-road heavy-duty engines, ICCT states that Euro VI limits NOx to 0.4 g/kWh and PM to 0.01 g/kWh with a particle number limit of 8.0 × 10¹¹, and that US 2010 limits NOx to 0.27 g/kWh (0.2 g/bhp-hr). Both are met with an aftertreatment system of DOC, DPF, SCR and ASC in series: exhaust passes the DOC and DPF, then a urea doser, the SCR catalyst and the ASC, with at least two temperature sensors between the units and a NOx sensor between the ASC and the tailpipe. [2]
ICCT notes the DOC also oxidises nitrogen oxide to nitrogen dioxide, improving SCR conversion, and requires fuel of 500 ppm sulfur or lower; wall-flow DPFs achieve PM reduction above 95%. SCR catalysts are vanadium-based (effective at 300–450 °C) or zeolite-based (iron, 350–600 °C; copper, 150–450 °C); zeolites are used when the SCR is paired with a DPF because of the regeneration temperatures. [2]
Editorial calculation: ICCT gives the Euro V NOx limit as 2 g/kWh and Euro VI as 0.4 g/kWh, so an engine emitting 2 g/kWh upstream of the SCR would need (2 − 0.4) ÷ 2 = 80% NOx conversion, before any compliance margin; the conversion your system needs depends on your engine-out NOx and the test cycle. [2]
Give the supplier the engine model, rated power and displacement, the emission standard and test cycle (EPA Tier 4 Final, Euro VI, or other), engine-out NOx and PM, exhaust flow and temperature over the duty cycle, fuel sulfur level, allowed back pressure, installation space and mounting, DEF tank location and line lengths, supply voltage, ambient temperature range, and quantity.
02
Bosch describes its Denoxtronic system as a modular AdBlue dosing system made of a dosing control unit, which calculates the required quantity, a supply module, which brings AdBlue to pressure, and a dosing module, which atomises and injects it into the exhaust ahead of the SCR catalyst. Published generations run at 5.0 bar (Denoxtronic 2.1 and 6-5) or 9.0 bar (2.2 and 6-HD), on 12 V or 24 V, with maximum dosage of 5.4 kg/h (6-5), 12 kg/h (2.2) and 15 kg/h (6-HD). The supply module contains a membrane pump, a filter and a 4/2-way valve; heating is electric or through the cooling-water circuit. [3]
Bosch states AdBlue is up to 32.5% urea and up to 67.5% demineralised water and freezes at −11.5 °C, so its filters use compressible compensation elements. A CF Industries DEF specification stated to meet ISO 22241-1 lists urea concentration of 31.8–33.2%, refractive index at 20 °C of 1.3814–1.3843, biuret 0.3% max, aldehydes 5 ppm max, insoluble matter 20 ppm max, and metals capped at 0.2–0.5 ppm. [3] [4]
ICCT's cost model sizes catalysts by the ratio of catalyst volume to engine displacement: 0.75 for the DOC, 2.0 for the DPF and 2.5 for a Euro VI zeolite SCR, giving 9.0 L, 24 L and 30 L of catalyst for a 12 L engine; the ASC is sized at 0.2 × displacement and the urea tank at 12 × displacement (144 L for 12 L). Its urea system includes the tank, level sensor, pump, injector, stainless steel tubing, a 200 W, 12 V DC heating system, temperature and NOx sensors, a mixer and a dosing control unit. [2]
DieselNet notes that every engine has a maximum allowable exhaust back pressure set by its manufacturer; generator-set engines from 15 kW to over 1000 kW have limits of 6.7–10.2 kPa, while the Swiss VERT programme recommends at most 40 kPa below 50 kW, 20 kPa at 50–500 kW and 10 kPa at 500 kW and above for retrofit filters. Higher back pressure raises fuel consumption, PM, CO and exhaust temperature. [5]
Editorial calculation: with ICCT's ratios, a 9 L engine needs 0.75 × 9 ≈ 6.8 L DOC, 2.0 × 9 = 18 L DPF, 2.5 × 9 = 22.5 L SCR and 0.2 × 9 = 1.8 L ASC, about 49 L of catalyst in total. [2]
03
The companies in the manufacturer list below show this product on their own websites. Confirm the legal seller, factory and scope for your order.
Buyers source this system either as a complete canned module (DOC, DPF, SCR and ASC with mixer and sensors) or as parts: SCR catalyst and its stainless steel housing, DOC, DEF supply pump module, dosing control unit, injector, heated lines and filters. When requesting quotations, ask each supplier for the emission certification the system was tested with, catalyst volumes and substrate type, back pressure at rated flow, dosing pressure and maximum dosing rate, heating method, and DEF quality requirements. For a stand-alone diesel particulate filter see the particulate filter page; for DEF tanks with level sensors see the DEF tank page.
04
This is a functional reference built from the sources cited; it is not one maker's design.
| Part | Role | Fix in the order |
|---|---|---|
| Diesel oxidation catalyst (DOC) | Oxidises HC and CO; makes NO₂ for the SCR. [2] | Volume, fuel sulfur limit. |
| Diesel particulate filter (DPF) | Wall-flow filter, PM reduction above 95%. [2] | Volume, regeneration. |
| DEF supply module (pump) | Membrane pump, filter, 4/2-way valve; 5.0–9.0 bar. [3] | Pressure, heating. |
| Dosing control unit (DCU) | Calculates the dosing quantity. [3] | Interface to engine ECU. |
| Dosing module (injector) | Atomises DEF into the exhaust; up to 15 kg/h. [3] | Rate, cooling. |
| Lines, heating and filter | Stainless tubing; 200 W, 12 V DC heating in a cost model. [2] [3] | Length, heater power. |
| Mixer | Mixes DEF with exhaust before the SCR. [2] [3] | Design, length. |
| SCR catalyst and housing | Vanadium or zeolite catalyst in a stainless steel can. [2] | Chemistry, volume. |
| Ammonia slip catalyst (ASC) | Removes excess ammonia after the SCR. [2] | Volume. |
| Sensors | NOx, temperature, differential pressure. [2] [3] | Number, positions. |
05
An aftertreatment system quotation needs values for your engine. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Emission standard | Tier 4 Final 0.40 g/kW-hr NOx (130–560 kW); Euro VI 0.4 g/kWh. [1] [2] | Certified standard, cycle. |
| Operating temperature (°C) | SCR windows 150–450 (Cu), 350–600 (Fe), 300–450 (V). [2] | Catalyst window; ambient range. |
| Exhaust flow rate (kg/h) | Not stated in reviewed sources. | Rated and minimum flow. |
| NOx conversion efficiency (%) | Not stated as a figure; see calculation above. [2] | Conversion on your cycle. |
| Dosing accuracy (%) | Not stated; maximum dosage 5.4–15 kg/h. [3] | Accuracy and range. |
| Supply voltage (V DC) | 12 V or 24 V. [3] | Voltage, transients. |
| Power consumption (W) | 200 W urea heating in a cost model. [2] | Heater and pump power. |
| IP rating | Not stated in reviewed sources. | IP code per part. |
| Housing material | Stainless steel can. [2] | Grade, thickness. |
| Weight (kg) | Not stated in reviewed sources. | Mass per module. |
| Dimensions (L×W×H) (mm) | Not stated in reviewed sources. | Drawing, mounting points. |
| Catalyst volume (L) | DOC 0.75, DPF 2.0, SCR 2.5 × displacement. [2] | Volume per catalyst. |
| Back pressure (kPa) | Engine limits 6.7–10.2 kPa (gensets). [5] | Pressure drop at rated flow. |
| DEF specification | 31.8–33.2% urea, ISO 22241-1. [4] | Quality the system requires. |
06
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Engine: [model; kW; displacement; engine-out NOx/PM] Standard: [Tier 4 Final; Euro VI; other; cycle] Exhaust: [flow; temperature range; allowed back pressure] Scope: [complete module; SCR; DOC; dosing system parts] Dosing: [pressure; max rate; heating; voltage] Installation: [space; mounting; line lengths] Quantity: [units]
These companies list exhaust aftertreatment system on their own websites.
Its website has a product page for exhaust aftertreatment system.
Product scope checked 2026-10-12
Its website has a product page for exhaust aftertreatment system.
Product scope checked 2026-10-12
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
EPA's standards summary lists 0.40 g/kW-hr NOx, 0.02 g/kW-hr PM, 0.19 g/kW-hr NMHC and 3.5 g/kW-hr CO for 2014 and later engines. [1]
Bosch describes it as up to 32.5% urea in demineralised water that freezes at −11.5 °C; a DEF specification meeting ISO 22241-1 allows 31.8–33.2% urea. [3] [4]
ICCT's cost model sizes them by engine displacement: DOC 0.75×, DPF 2.0× and Euro VI SCR 2.5×, or 9.0 L, 24 L and 30 L for a 12 L engine. [2]
Sources checked 2026-10-11. Sources are an EPA standards summary, an ICCT white paper, a Bosch Denoxtronic service brochure, a DEF maker's specification and a DieselNet technical paper; values for your system must come from the supplier's certification and test data.
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