## 1 Introduction  
The typical use of motor vehicles is inherently associated with the occurrence of dynamic conditions [1; 2]. While driving in real road traffic, a vehicle accelerates, decelerates and stops, and even the driver's intention to maintain a constant velocity can usually be realized only with a certain approximation [3]. Variations may occur due to alterations in external factors, e.g., motion resistance, even despite consistent settings of vehicle engine control. The changing conditions of real road traffic result in fluctuations in the time-dependent values that represent the states of individual vehicle systems, including the operating states of combustion engines. Among the properties of motor vehicles, pollutant emissions and fuel consumption are particularly sensitive to dynamic conditions [4]. For this reason, in order to evaluate pollutant emissions and fuel consumption under conditions that correspond to typical traction use of a vehicle, researchers commonly conduct empirical studies in driving tests with a chassis dynamometer to simulate the actual use of the vehicle in real driving scenarios [5; 6]. Despite the numerous advantages of driving tests, including the replicability and repeatability of conditions thanks to the regime of laboratory procedures, discrepancies are observed between the pollutant emission values obtained in such tests and those obtained in real road traffic [6]. This was the reason for introducing new research procedures called Real Driving Emissions (RDE) [7], in which measurements are conducted outside the laboratory with a vehicle driven on an actual road with varying ambient conditions and traffic intensity. The development of research methods was possible mainly due to the emergence of advanced mobile exhaust emission measuring systems, which expanded the possibilities of assessing the performance characteristics of vehicles in real operating conditions [5]. The vast majority of studies on pollutant emissions and fuel consumption of motor vehicle engines in RDE tests are limited to statistical evaluations of empirical testing results. For example, pollutant emission analyses leading to the determination of average road emissions of individual pollutants in the RDE test are occasionally divided into test sections corresponding to specific traffic models: urban, rural and motorway. Such studies enable the assessment of the final effect but do not contribute to increasing knowledge about the processes responsible for the phenomena being studied. For this reason, it is advisable to study pollutant emissions from combustion engines in dynamic conditions in a continuous manner. Further expansion of knowledge about the processes determining pollutant emissions and fuel consumption in the conditions of actual use of engines is made possible by considering these conditions as probabilistic and treating the properties of combustion engines in these conditions as stochastic processes. The reasoning behind this idea is supported by the notable variability in time of the quantities describing the operation of engines, especially the rotational speed and engine load. Consequently, the non-repeatability of empirical research results is observed, even under laboratory driving test conditions. The processes related to pollutant emissions and fuel consumption are particularly sensitive to the changing operating conditions of engines. This paper aimed to apply a stochastic approach to the analysis of pollutant emission and fuel consumption test results of a motor vehicle driven in real traffic conditions. The empirical data regarding vehicle velocity, engine working parameters, emissions of selected pollutants and fuel consumption were obtained by the authors according to the RDE procedures in real road traffic conditions. The tested object was a passenger car with a spark-ignition engine and an automatic transmission, equipped with a mobile onboard exhaust measuring apparatus.