Testing exhaust aftertreatment systems

Testing exhaust systems’ mechanical resistance to vibration

Exhaust systems, due to being connected to an internal combustion engine, are subjected to constant vibrations. As a consequence, these systems suffer cyclic stresses that lead to fatigue cracks. The catalytic converter assembly, in which the ceramic substrate is housed in a metal casing, is particularly susceptible to damage resulting from vibrations. These systems must be validated in terms of their fatigue strength. Durability tests of a specific catalytic converter unit or particulate filter precede vibration measurements. These measurements are performed using tri-axial accelerometers placed in locations considered to be the most exposed to vibrations. Analysis of the results obtained determines the rotational speed of the engine at which vibrations reach critical resonant frequencies. The speed values thus obtained are the basis for determining the test profile to which the exhaust system will be subjected. The duration of the test depends on the required number of displacement cycles which the element under test should be able to withstand.

Testing and development of Euro 6 catalytic converters and particulate filters (DOC, NSC, DPF)

Examinations of exhaust gas aftertreatment systems fitted to internal combustion engines allow for validation, evaluation and development of the assemblies under test. Measurements of exhaust gas composition are carried out using measuring equipment of the highest quality. It is possible to measure concentrations of gaseous exhaust gas components such as THC (total hydrocarbons), CH₄ (methane), NO_x, NO, NO₂ (nitrogen oxides), CO (carbon monoxide), CO₂ (carbon dioxide), O₂ (oxygen), NH₃ (ammonia) and N₂O (nitrous oxide). The quantity of solid particles is determined by measuring the mass of particles (PM), the number of particles (PN), the particle size distribution, and the opacity and emission opacity of the exhaust gas. The measurements of the efficiency of the exhaust aftertreatment system may be carried out in accordance with standardised approval procedures, as well as procedures determined individually for the purposes of the test. Catalytic reactors and filters can be subjected to durability and ageing tests, the purpose of which is to determine the viability or development direction of the test system.

Tests and development of Euro 6 SCR systems

Selective catalytic reduction systems, which are more and more often used in exhaust aftertreament systems, require advanced testing and validation methods. Optimisation of the AdBlue dosing strategy depending on the exhaust temperature and the level of SCR reactor saturation requires detailed testing of the nitric oxide reduction efficiency and any possible ammonia emissions (so called ammonia slip).

Other types of tests include evaluation of the precipitation of solid deposits in the exhaust system and visual inspections using a videoscope. The heterogeneity of the decomposition of AdBlue on the surface of the reactor can be determined by means of automated sampling probe measurements.

Ageing and degradation testing of catalytic reactors TWC (petrol, CNG, LPG)

Ageing and degradation of three-way catalytic converters (TWC) is conducted on the engine dyno during stationary engine tests. This type of procedure is characterised by time and cost savings in comparison to on-road ageing/degradation.

Converter ageing can be achieved according to various procedures, for example, reproduction of highway driving cycles or operating the engine under heavy load. The aim of the tests is to determine or verify reactor durability and changes in its efficiency at different ageing stages. During the ageing procedure, measurements of catalytic reactor efficiency are conducted periodically by means of emission tests. Such tests can be conducted on an engine on the engine dyno equipped with an emissions analysis system, or with the TWC mounted on a vehicle in the exhaust emission laboratory. Emissions test can be conducted according to standardised procedures, for example, type approval procedures, or according to individual procedures created for a particular test plan.

A TWC can also be subjected to degradation testing. Contrary to ageing, a TWC degradation cycle is characterised by artificial changes in the parameters of the exhaust gas flowing into the converter. Exhaust emitted by the engine is enriched with air, which intensifies the oxidation process in the converter. As a result, the converter is exposed to very high temperatures reaching 1200°C, and the degradation time is relatively short.

A simplified method to specify the level of a TWC’s degradation is available through measurement of its oxygen storage capacity. More detailed efficiency tests can be conducted according to the same procedures as in ageing programmes.

Three-way catalytic converters are used in spark ignition engines. Tests using engines powered by: RON95, RON98, E85 fuels or gaseous CNG and LPG fuels can be conducted. The tests are conducted using engines provided by customers or those owned by BOSMAL.

Ageing tests of catalytic DOC+DPF and SCR systems

Ageing of catalytic exhaust gas aftertreatment systems for compression ignition engines is conducted on the engine dyno for stationary engine tests. Such tests are characterised by time and cost savings in comparion to on-road road ageing tests. Diesel oxidation catalysts (DOCs), Diesel particulate filters (DPFs) and selective catalytic reduction systems (SCR) are submitted to ageing tests.

Ageing of oxidation catalysts (DOC) is conducted over work cycles with elevated exhaust temperature. The temperature and mass flow of the exhaust can be regulated, depending on the test requirements.

Ageing of particulate filters (DPFs) uses an accelerated ageing procedure. Exhaust with an elevated number of solid particles enters the filter, which shortens the time required for accumulation of particles in the filter. Next, an active regeneration of the filter is carried out, which, to intensify the ageing effect, can be conducted at an elevated temperature.

Ageing of selective catalytic reduction systems (SCR) is carried out over cycles with varying mass flow parameters, exhaust temperatures and quantities of AdBlue dosing. The temperature and the changes in the degree of SCR ammonia saturation lead to faster ageing.

Ageing procedures are characterised by cyclic measurements of reactor and filter efficiency during emission tests. Such tests can be conducted on an engine on the engine dyno equipped with an emissions analysis system, or with the TWC mounted on a vehicle in the exhaust emission laboratory. Emissions test can be conducted according to standardised procedures, for example, type approval procedures, or according to individual procedures created for a particular test plan. 

Ammonia dosage optimisation for applications with SCR

Dosage of AdBlue in SCR systems is especially difficult, due to the possibility of precipitation of ammonia deposits within the exhaust system. Excessive ammonia emission from downstream of the reactor, so called ammonia slip, is also problematic. Intensive research and development work is currently being carried out in order to limit the occurrence of these unwanted phenomena. One of the methods used is optimisation of the algorithm controlling AdBlue injection or modification of the dosage system. Other innovative solutions like, for example, heating and evaporating AdBlue before it enters the exhaust stream before SCR reactor are also being researched.

Measurements of the ammonia distribution on the inlet surface of SCR reactors

Achieving high homogeneity of AdBlue distribution dispensed to the exhaust system is a major design challenge. Simulation of the distribution of ammonia on the surface of the selective catalytic reduction (SCR) reactor often proves inadequate and the best solution for optimisation of the design is to perform measurements of the heterogeneity of the distribution. The measurements are made by means of an automatic sampling probe that allows mapping of any grid of points on the downstream face of the SCR reactor. Measurements of the concentrations of ammonia and nitrogen oxides are performed, on the basis of which the heterogeneity of their distribution can be assessed. It is also possible to measure the remaining gaseous components of the exhaust gas. Results obtained in this way clearly identify locations that are too rich and too poor in ammonia, thus clearly defining the necessary corrections.

Measurement of exhaust emissions during type approval tests, e.g. WHTC, WHSC, NTE, NRSC, NRTC

Emission measurements for homologation tests are carried out on the engine dyno in accordance with the latest international standards for heavy duty engines. Uniform WH (World Harmonized) tests are carried out for on-road applications, both dynamic (WHTC) and stationary (WHSC), whose results are respected in most global markets. There are also tests for off-road and marine applications like NRSC, NRTC and NTE, as required by the EPA.

Testing particle mass (PM), number (PN) and particle size distribution

Measurements of the mass of particulates (PM) emitted on the chassis dyno are carried out using the gravimetric method and a partial-dilution CVS tunnel. The particulate accumulated on the filters is weighed using a microbalance, which allows measurements of particles emitted by engines complying with the Euro 6 standard. The latest exhaust emission limits also specify the maximum number of particles (PN) that is allowed in individual approval tests. Particle number can be measured both in undiluted and diluted exhaust gas. For a more detailed analysis of the composition of the particles emitted by the engine, it is possible to measure the particle size distribution using a spectrometer.