M.R. Saidzoda, M.A. Abdullo, R.H. Saidzoda, A.J. Rahmonzoda

Annotation:

The paper examines how the exhaust composition of a petrol hybrid passenger car changes as it climbs a mountain road. On 22 September 2026 a Lexus RX 450h (2015) was tested at six stations between 801 and 2615 m above sea level along the Dushanbe – Istiqlol tunnel road, where ambient pressure fell from 92.3 to 74.6 kPa. CO, HC, NOx and CO2 were measured with an Avtotest gas analyser at idle and at elevated crankshaft speed, six readings per series (72 in total). Data were processed with Student's t-distribution (P = 0.95); the altitude effect was assessed by linear regression and one-way ANOVA. Hydrocarbon concentration rose from 15.2 to 32.3 ppm at idle and from 14.5 to 32.7 ppm at elevated speed, i.e. by about 9.4–9.7 ppm per 1000 m (R2 = 0.95–0.98). Nitrogen oxides fell by 83–89 % and the CO2 volume fraction by 8–14 %. No consistent altitude dependence was found for CO, whose mean values stayed at or below 0.18 %. The relationships obtained can support the adjustment of exhaust inspection procedures for vehicles operated in the mountain regions of Tajikistan.

Keywords: mountain road; altitude; exhaust gases; hydrocarbons; nitrogen oxides; hybrid vehicle; gas analyser; Varzob.