## 1 Introduction  
Hybrid vehicles used in urban environments significantly reduce fuel consumption compared to conventional internal-combustion-engine cars [1, 2] mainly due to three primary factors: an enhancement in engine efficiency, the utilisation of regenerative braking, and the minimisation of idling periods [3, 4]. One would like to think they should also be less polluted in all cases, but some results show that the opposite is true [5]. Studies by Chinese researchers [6] show that CO emissions were higher for hybrid cars in almost all modes. The main reason for this was the longer warm-up time of the engine and catalyst. Unfortunately, in some cases, the CO\({}_{2}\) emissions claimed by plug-in cars are as much as 20% lower than they are [7, 8]. The reason for this is the distance travelled by the electric motor. According to the manufacturers, plug-in cars should cover a much greater distance than they do [9]. There is a paradox: plug-in hybrid electric vehicles (PHEVs) have a larger battery than hybrid electric vehicles (HEVs) [10; 11], but the battery is not used for its intended purpose. Instead, it adds weight to the car and increases fuel consumption [12]. On the other hand, promoting HEVs and PHEVs as environmentally friendly vehicles may result in a notable surge in overall vehicle usage, which could ultimately negate their green credentials due to the increased number of vehicles on the road [13]. The main advantage of a hybrid car is that it can have a low-power but economical and environmentally friendly ICE [14], and there is always an electric assist for traction. The emissions and, of course, the fuel consumption of a hybrid car depend strongly on the size of the car's battery [2], the type of engine, and the type of transmission. The highest fuel consumption and probably the highest emissions are found in vehicles with a serial kind of transmission [15], where the ICE is not directly coupled to the wheels but to a generator. The energy produced by the generator is fed to the electric motor that turns the wheels. A car with a parallel hybrid transmission uses less fuel [16] due to lower energy-conversion losses. And power-split hybrids will have even better fuel economy because it allows the efficiency of both engines to be used optimally in different driving modes. In addition, the flywheel hybrid electric vehicle has a commendable environmental performance [17]. But it is not just about the type of transmission. It is also essential to consider how the car is driven, i.e., daily mileage, idling, engine, and catalyst temperatures, during engine operation, traffic congestion, and other characteristics [18]. The cold-start mode always produces the highest and most toxic emissions compared to warm-up mode [1]. In some cases, these problems can be solved by optimising the control algorithm [16; 19]. Hybrid vehicles, which integrate an ICE with an electric motor, pose unique testing challenges due to their dual powertrains. The WLTC's multi-phase structure is particularly beneficial for analysing hybrids, especially during the transition between electric and gasoline-powered modes. Hybrid systems, applied in vehicles, such as cars, buses, and vans, are promising technologies for improving energy efficiency, offering an alternative energy source to reduce the environmental impact of fossil fuels and lower emission levels [20; 21; 22]. As these systems continue to evolve, they are increasingly offered as an optional feature in various vehicle models, highlighting a critical gap in emissions research and the standardisation of testing protocols, as indicated in [15]. The same study compared a vehicle equipped with a serial-hybrid-drive system and one powered by a gasoline engine, focusing on fuel consumption and CO\({}_{2}\) emissions during the NEDC (New European Driving Cycle) and WLTC Class 2 cruise cycles. The findings revealed a 3.3% increase in CO\({}_{2}\) emissions with the downsized serial-hybrid drive system during the NEDC cycle.