Cold-start urban emissions from LPG and diesel passenger cars: a focus on NOx, NH₃ and particle number

The decarbonisation of road transport requires not only the introduction of alternative fuels, but also a robust evaluation of their real-world pollutant emissions under representative urban conditions. Despite ongoing fleet electrification, diesel passenger cars still represent a significant share of urban traffic in Spain, making them a relevant benchmark for transitional technologies such as Liquefied Petroleum Gas (LPG).

 

This work presents an experimental comparison of nitrogen oxides (NOx), ammonia (NH₃) and particle number (PN) emissions from an LPG spark-ignition passenger car and a conventional diesel vehicle under urban real driving conditions, using the urban segment of a Euro-7-oriented driving cycle. The LPG vehicle (Euro-6, three-way catalyst and gasoline particle filter) was retrofitted with a commercial LPG system, while the diesel vehicle (Euro-6) was equipped with oxidation catalyst, particle filter and selective catalytic reduction. Both vehicles were tested using a Euro 7-compliant portable emission measurement system during real driving cycles covering typical urban operation.

 

Tests were conducted at two ambient temperatures representative of moderate and severe urban conditions (23°C and −7°C), with special attention to cold start operation. All experiments were performed on a chassis dynamometer using urban driving cycles characterised by low speeds, frequent stops and high transient operation, where delayed aftertreatment activation strongly influences emissions.

 

Results show that cold-start events dominate urban NOx, NH₃ and PN emissions for both vehicles, with a marked deterioration at −7°C. Under cold ambient conditions, total urban NOx emissions increase significantly, especially for the diesel vehicle, reflecting reduced efficiency of the NOx control system during warm-up. The LPG vehicle exhibits lower overall NOx emissions over the complete urban cycle, although emissions also increase at low temperature due to delayed catalyst activation.

 

Ammonia emissions are substantially higher for the LPG vehicle, particularly during cold-start and early urban operation. This behaviour is mainly governed by air–fuel ratio calibration, which controls NH₃ formation over the three-way catalyst, rather than cold-start itself. This is supported by measurable NH₃ emissions during hot start operation at +23°C. At −7°C, NH₃ emissions during hot operation decrease but remain relevant during the initial cold phases. For the diesel vehicle, NH₃ emissions are generally low, with occasional peaks during transient operation, likely associated with aftertreatment warm-up.

 

Particle number emissions are strongly influenced by cold start for both vehicles. The LPG vehicle shows elevated PN emissions during cold operation, attributed to incomplete combustion and reduced filtration efficiency at low exhaust temperatures, followed by a sharp reduction once thermal stabilisation is achieved. The diesel vehicle exhibits higher PN emissions under cold ambient conditions, particularly at −7°C, indicating reduced filtration efficiency during warm-up.

 

Overall, LPG operation offers advantages over diesel in terms of urban NOx emissions once aftertreatment is active. However, increased NH₃ and PN emissions during cold start remain key challenges, especially at low ambient temperatures, highlighting the importance of cold-start-oriented emission control strategies and optimised calibration in the context of future Euro 7 regulations.

 

Acknowledgement

This work was supported by the Spanish Ministry of Science, Innovation and Universities through project ImMA_7 (PID2022-136437OB-I00).