Three-way catalyst performance using LPG with two different sulphur levels

Report no. 7/23: An experimental programme was carried out to determine the long-term impact of the level of sulphur present in LPG on the performance of three-way catalytic converters (TWCs) in terms of their elimination (conversion) of gaseous pollutants. Two TWCs were subjected to an engine dyno ageing procedure, with fuel of high (29 ppm m/m) or low (8 ppm m/m) sulphur content used for ageing. The performance of the TWCs was assessed before, during and after this ageing via emissions testing. Emissions testing was conducted on a passenger car (the type for which the two TWCs were designed), running on LPG fuel and tested on a chassis dynamometer over three different driving cycles. Emissions testing was performed in accordance with the European legislative method, supplemented by additional measurements and procedures, including continuous emissions measurements of undiluted gaseous pollutants.

With a single exception, the test vehicle met the applicable Euro 6 emissions limits when tested using its type approval procedure (NEDC), for both TWCs, at all ageing stages. The exception to this came in the form of a single NMHC emissions result, which in one test was found to be above the applicable legislative limit following 250 ageing cycles; the two other repetitions of this test under the same conditions showed NMHC emissions below with the Euro 6 limits, and the mean of these 3 tests was also below the Euro 6 limits. While emissions results obtained using the WLTP test procedure were not legally applicable to the test vehicle, results obtained using that procedure were also below the Euro 6 limits, in all cases, with no exceptions. The relatively low emissions results occurring even after extended ageing indicate high TWC conversion efficiency and durability under the specific ageing conditions tested.

The results showed that the ageing cycle caused a tendency for slightly lower TWC conversion efficiency (and thus slightly higher emissions results). It was found that differences in conversion of regulated pollutants were greatest at low operating temperatures (i.e. during and following cold start) and that differences were very limited under thermally stabilised conditions.

As regards the impact of fuel sulphur level on TWC conversion efficiency, overall there appeared to be no significant difference between the two test objects. In certain cases differences in performance were apparent, but sometimes the High-sulphur TWC outperformed its Low-sulphur counterpart. Even focusing on the results obtained from the cold start phases, no consistent, significant difference between the two test objects could be observed. It was concluded that the fuel’s sulphur content had not exerted a clear, significant impact on TWC performance.

A possible explanation for this is that the high speed, high load driving simulated in the engine dyno ageing cycle (but which can also be met under real driving conditions, e.g. during highway driving) led to continuous desulphatation of the TWCs tested, hence leading to a potential non-harmful effect of sulphur content on the TWC conversion efficiency. A complementary study was performed to evaluate the relevance of this explanation (details in the appendix). It consisted in chemical analyses of the aged TWCs, using advanced laboratory methods. Unfortunately, this study remained inconclusive, as it were unable to directly link the sulphur level measured in the TWCs to their conversion efficiency loss. Consequently, lacking a clear and systematic explanation about the effect of sulphur on the TWC conversion efficiency, this study cannot be 100% conclusive regarding the harmful/non-harmful effect of sulphur content in LPG on the TWC conversion efficiency, even if the engine/vehicle tests tend to show that a higher sulphur content is not harmful.