Bench conversion efficiency testing of exhaust after-treatment systems (DOC, DPF, SCR, ASC and TWC) verifies how effectively a catalyst or filter converts NOx, CO, HC and particulate matter under controlled engine dynamometer and synthetic gas bench conditions. This article explains the test scope, sample types, measured parameters, test methods, applicable standards such as ISO 8178 and GB 17691, and typical engineering applications supported by accredited third-party laboratories.
Technical Overview
An exhaust after-treatment system is the set of catalytic and filtration devices installed downstream of an internal combustion engine to reduce pollutant emissions before exhaust gas is released to the atmosphere. Typical architectures combine a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF or catalyzed DPF), a selective catalytic reduction (SCR) catalyst with urea-based reductant injection, and an ammonia slip catalyst (ASC). For gasoline engines, a three-way catalyst (TWC) is the dominant configuration.
Bench conversion efficiency testing measures the ratio of pollutant mass removed by the device to the pollutant mass entering it, evaluated under precisely controlled speed, load, temperature and exhaust flow conditions on an engine dynamometer bench or a synthetic gas reactor bench. Because emission limits under frameworks such as China VI (GB 17691, GB 18352.6) and Euro VI are extremely stringent, even a few percentage points of conversion loss can decide whether an engine or vehicle passes certification. Bench testing therefore serves as the decisive evidence of catalyst functionality, system matching and durability.
Compared with chassis-level or in-field measurements, bench testing offers superior repeatability and diagnostic depth: inlet and outlet concentrations are sampled simultaneously, exhaust temperature and space velocity can be swept systematically, and deactivation mechanisms such as sulfur poisoning, ash accumulation and thermal sintering can be quantified in isolation. Accredited third-party laboratories typically perform these tests under quality systems aligned with ISO/IEC 17025 to support type approval, research and development, production quality control and failure analysis.
Testing Samples
Diesel Oxidation Catalyst (DOC) Units
Bare monoliths, canned DOC assemblies or brick-core samples cut from full-size substrates. The substrate is usually cordierite or metal foil coated with platinum group metals. Testing evaluates CO and HC oxidation activity, NO to NO2 oxidation ratio and light-off behavior.
Diesel Particulate Filters and Catalyzed DPF
Wall-flow filters in cordierite or silicon carbide, with or without precious metal coating. Samples may be virgin, artificially soot-loaded, ash-loaded or field-aged units returned from service, allowing filtration efficiency, pressure drop and regeneration behavior to be characterized at defined soot loadings.
SCR Catalysts and Ammonia Slip Catalysts
Copper-zeolite, iron-zeolite or vanadium-based SCR cores, full-size bricks and complete SCR cans, together with downstream ASC samples. These are tested for NOx conversion efficiency, ammonia storage and release, N2O selectivity and ammonia slip suppression, using diesel exhaust fluid (DEF) conforming to ISO 22241.
Three-Way Catalysts for Gasoline Engines
TWC samples for stoichiometric gasoline and gas engines, evaluated for simultaneous conversion of CO, total hydrocarbons and NOx, oxygen storage capacity and light-off characteristics under stoichiometric and rich-lean perturbation feed gas.
Complete After-Treatment Systems
Fully packaged close-coupled plus under-floor systems, retrofit boxes for non-road machinery, marine and stationary engines, and series-connected DOC-DPF-SCR-ASC modules tested as installed to verify system-level integration, thermal management and dosing strategy.
Testing Items
Depending on the device under test and the applicable regulation, bench conversion efficiency programs typically cover the following measured parameters:
- NOx conversion efficiency of SCR and TWC systems across the operating temperature window
- CO conversion efficiency and total hydrocarbon (THC) conversion efficiency of DOC and TWC samples
- Particulate matter (PM) mass filtration efficiency and particle number (PN) filtration efficiency of DPF samples
- Ammonia (NH3) slip concentration downstream of SCR and ASC units
- Nitrous oxide (N2O) formation and selectivity of SCR catalysts
- NO2/NOx ratio produced by DOC samples, which governs passive DPF regeneration capability
- Light-off temperature T50 and full-conversion temperature T90 during controlled temperature ramps
- Exhaust backpressure and pressure drop curves versus mass flow and soot loading
- Active and passive DPF regeneration performance, regeneration temperature and completeness
- Formaldehyde, methane and non-methane hydrocarbon conversion where required for gas engines
- Post-aging residual activity after hydrothermal, sulfur or ash exposure protocols
Testing Methods
Engine Dynamometer Hot Mapping
The after-treatment system is installed on a fully instrumented engine test bench. The engine is stabilized at selected speed and load points spanning the emissions map, and exhaust gas is sampled upstream and downstream of the device using raw exhaust analyzers: chemiluminescence detection (CLD) for NOx, non-dispersive infrared (NDIR) for CO and CO2, heated flame ionization detection (HFID) for HC, and Fourier transform infrared spectroscopy (FTIR) for NH3, N2O and NO2. Particulate mass is determined gravimetrically using a partial-flow dilution tunnel in accordance with ISO 8178. Instantaneous conversion efficiency at each point is calculated on a mass basis, where efficiency equals the difference between inlet and outlet pollutant mass flow divided by inlet mass flow, multiplied by one hundred percent.
Transient Cycle Testing
Whole-cycle conversion efficiency is evaluated over regulatory transient and steady-state cycles such as the World Harmonized Transient Cycle (WHTC) and the World Harmonized Stationary Cycle (WHSC), including cold-start and hot-start runs where the standard requires. Cycle-weighted NOx, CO, HC, PM and PN results are compared with and without the after-treatment system, or across its inlet and outlet planes, to yield integrated cycle efficiency. Modal analyzers record second-by-second concentrations so that efficiency can be correlated with exhaust temperature, urea dosing rate and flow transients, following measurement procedures consistent with ISO 8178 and ISO 16183.
Synthetic Gas Bench Testing
For catalyst screening and mechanistic studies, core samples are mounted in a temperature- and flow-controlled reactor fed with a synthetic gas mixture reproducing real exhaust chemistry, including NO, NO2, NH3, O2, H2O, CO2, CO and hydrocarbons. Temperature ramps at defined space velocities generate light-off curves, while ammonia-to-NOx ratio sweeps reveal optimal dosing windows and ammonia storage capacity. This method isolates the catalyst formulation from engine variability and is widely used for quality control of production batches.
DPF Loading and Regeneration Evaluation
Filters are first loaded with a defined soot mass by operating the engine at soot-producing points, with loading tracked by continuous weighing or pressure drop correlation. Filtration efficiency is determined from upstream and downstream PM and PN measurements at several soot loadings. Active regeneration is then triggered through exhaust temperature management or late injection strategies, and parameters such as regeneration onset temperature, peak temperature gradient, regeneration completeness and residual ash mass are recorded to assess thermal robustness.
Durability and Aging Assessment
To demonstrate performance over the regulatory useful life, samples undergo accelerated hydrothermal aging, sulfur exposure, oil-ash accumulation and lubricant-derived poisoning protocols, after which conversion efficiency is re-measured using identical mapping procedures. The efficiency loss relative to the fresh state quantifies deactivation and supports durability declarations required by GB 17691 and equivalent certification frameworks.
Applications
Bench conversion efficiency data underpin nearly every stage of the after-treatment product life cycle:
- Type approval and homologation evidence for heavy-duty engines, light-duty vehicles, non-road mobile machinery and stationary engines
- Catalyst formulation screening and substrate or washcoat development during research and development
- System matching studies optimizing DOC, DPF, SCR sizing, positioning, insulation and urea dosing calibration
- Production quality control and batch release testing of catalytic converters and filters
- In-service conformity verification and periodic technical inspection support for fielded fleets
- Evaluation of retrofit emission control systems for legacy diesel equipment and marine installations
- Failure diagnostics, including thermal sintering, sulfur poisoning, ash plugging and urea deposit investigation
FAQ
What exactly does conversion efficiency mean in after-treatment testing?
Conversion efficiency is the percentage of a target pollutant removed while exhaust passes through the device. It is calculated from simultaneously measured inlet and outlet concentrations converted to mass flow rates, so the result is insensitive to concentration changes caused by combustion variation or exhaust dilution.
Which standards govern the testing?
Gaseous and particulate measurement follows ISO 8178 and, for transient heavy-duty testing, ISO 16183, while test cycles and limit-value procedures derive from regulations such as GB 17691 and GB 18352.6. Urea solution quality is verified against ISO 22241, and reference fuels are typically specified to EN 590 or the equivalent national standard.
Can a small catalyst core be tested instead of a full system?
Yes. Synthetic gas bench and core-reactor testing on drilled or cut samples is a recognized method for comparing formulations, provided the space velocity, gas composition and temperature history are matched to the real application. Full-system conclusions, however, are always confirmed on the complete assembly installed on an engine bench.
How are ammonia slip and N2O addressed during SCR testing?
NH3 and N2O are measured continuously by FTIR at the SCR outlet and after the ASC. Dosing maps are swept to identify the ammonia-to-NOx ratio that maximizes NOx conversion while keeping NH3 slip below the regulatory or customer threshold and minimizing N2O selectivity.
What sample preparation and test duration are typical?
Samples should arrive undamaged, with known history, mileage or aging hours, and with mating flanges or gaskets where possible. A steady-state efficiency map on a single device typically takes one to three bench days, while full cycle testing, DPF loading with regeneration, and aging programs can extend from one week to several weeks depending on scope.
What deliverables does a test report include?
Reports normally present inlet and outlet modal data, point-by-point and cycle-weighted conversion efficiencies, light-off curves, pressure drop characteristics, uncertainty statements and pass or fail conclusions against the agreed criteria, all traceable to calibrated instrumentation within an ISO/IEC 17025-accredited environment.