Experimental research on vehicles equipped with a GMP Hybrid for the new Euro 7 emissions standard

With the increase the number of vehicles on European roads, the road transport has become the biggest source of air pollution in the cities of the old continent. According to European Commission estimates in 2018, more than 39% of NOx (nitrogen oxides) emissions and 10% of PM2.5 and PM10 emissions (mechanical particles with diameters of 2.5 and 10 micrometers) were attributed to road transport. It is known that, in order to limit to some extent, the uncontrolled increase in harmful emissions from road vehicles, the European Commission has gradually introduced new pollution standards to be met by manufacturers of new vehicles. In the second half of 2022, the Commission presented a new proposal to reduce pollutant emissions from new vehicles sold in the European Union (EU), defining the new Euro 7 pollution standards. Compared to Euro 6 by 2035, according to European Commission estimates, these new Euro 7 standards, together with a progressive increase in the number of electric vehicles sold, aim to reduce pollutant emissions from private vehicles and vans by around 35% for NOx emissions and around 13% for particulate emissions. Therefore, both the Euro 7 and CO2 emissions standard (CAFE) for vehicles have been designed to improve air quality in cities and reduce greenhouse gas emissions, including by increasing the use of electric vehicles. Starting from this hypothesis, the first part of the paper presents the experimental model developed in the AMESim simulation platform of a mid-size vehicle equipped with a hybrid GMP, tested on a roller bench, which simulates pollutant emissions in this new Euro 7 standard. In the second part of the paper, the simulation results and possible causes influencing hybrid GMP behavior leading to the evolution of pollutant emissions are analyzed. Finally, the paper formulates conclusions on the evolution of pollutant emissions.


Introduction
It is known that, in order to limit the negative environmental impact of the continuous increase in the number of cars in Europe, the European Commission has adopted numerous pollution standards, that we know today as Euro 1, Euro 2,..., Euro 6.
The next major step in terms of emissions regulations for cars is the transition to Euro 7 from July 2025.Although the level of accepted pollutant emissions and test conditions are currently at the proposal stage, this standard represents a significant challenge for the car and commercial vehicle industry.The European Commission's proposal to introduce the Euro 7 standard aims to reduce air pollution in major cities, caused by new cars sold in the European Union, and represents an intermediate step to meet the European Green Deal's ambitious goal of reducing to "zero pollution" or "zero CO2 emissions" in 2035 [1].
As road transport is the main source of air pollution in major cities, the new pollution standards 1303 (2024) 012026 IOP Publishing doi:10.1088/1757-899X/1303/1/012026 2 align pollutant emissions of CO (carbon monoxide) and NOx (nitrogen oxides) for vehicles equipped with SI engine (spark ignition engines) and CI engine (compression ignition engines) to their lowest value at Euro 6.The standards also impose for the first time a limit for NH3 (ammonia) emissions for vehicles equipped with SCR (Selective Catalytic Reduction) and a limit for particles resulting from the braking system of the vehicle, figure 1.In order to study the impact brought by the severity of the Euro 7 standard on a mid-size car, the author proposed a mathematical model made with the help of the HOT interface (Hybrid Optimization Tool), integrated into the Simcenter AMESim simulation platform.This interface allows engineers involved in the development of new vehicle models to make an estimate at an early stage of vehicle design, in order to know the energy potential of a vehicle architecture and a chosen test cycle.

Presentation of the Model
The proposed model is specific to a car, from the Volkswagen Golf GTE class, equipped with a rechargeable parallel electric hybrid propulsion system (PHEV -Plug-in Hybrid Electric Vehicles).
This vehicle has been approved for the Euro 6 emissions standard and is equipped with a powertrain combining a 1.4l combustion engine and an electric car, coupled to a 6-speed DSG (Direct Shift Gearbox) dual clutch gearbox.This rechargeable hybrid powertrain provides the vehicle with outstanding dynamic, economical and environmentally friendly performance, such as reaching a top speed of 222km/h, acceleration from 0 to 100km/h in 7.6s and an announced fuel consumption of 1.5l/100km or 35g/km CO2 emissions.The internal combustion engine is a turbocharged gasoline engine with direct injection TSI (Turbo Stratified Injection), and the main characteristics of the combustion engine are summarized in The electric machine is of the synchronous type with permanent magnets that can be used as a motor to provide the torque necessary to move the vehicle up to 130km/h in electric mode, but also in generator mode, for brake energy recovery (deceleration).Its mass is 34kg and the torque and power characteristic are shown in figure 2   Torque and power characteristic The electric car is powered by the traction battery that stores the electrical energy needed to propel the car and provides electricity to various electrical components: the 12V battery, heating the passenger compartment, powering the air conditioning compressor, etc.The traction battery is of the lithium-ion type, cooled by water and is composed of 96 cells linked in series, divided into eight modules of 12 cells each.The main characteristics of this battery, as well as the standards or indices of protection are summarized in Table 2 [2].In order to reproduce as closely as possible, the behavior of the studied vehicle, when drawing up the model sketch in AMESim, using the HOT interface, a parallel rechargeable hybrid architecture vehicle was chosen, shown in figure 3 [3], [4], [5], [6]. Rm (fixed ratiosthe name is just to differentiate components in function of configuration);  Rd (fixed ratiosthe name is just to differentiate components.Gearbox in this model); The following two stages consist of:  definition of the strategy for battery management, transmission modes calculated based on previously defined parameters at together with the choice of 100% electric mode;  definition of test cases where to choose test cycle, gears selected during its cycle, altitude at which the test is performed, electrical loads activated as well as other parameters such as initial SOC (State of Charge), final SOC target and zero pollutant emission zone (ZEV)if applicable; Finally, after the calculation has been completed, the final post-processing step provides an overview of the vehicle's economic and dynamic performance, as well as a distribution of lost energy on each component.Figure 5 shows the sketch of the model with the desired architecture obtained from the launched calculation, which can be run in AMESim independently of the HOT interface, modified or optimized.The key component of the model shown in the figure 4, responsible for the economic and ecological performance of the modelled vehicle is the ECMS (Equivalent Minimization Strategy) submodel.This component, similar to an ECU (Electronic Control Units), is dedicated to hybrid vehicles and is not found in the IFP Drive library but is available when developing the model architecture with the HOT interface.The submodel monitors vehicle behavior by optimizing fuel consumption over the selected driving cycle [3], acting on the main components: Optimizing fuel consumption is done by minimizing the cost of energy H, using s as the price of electricity, in relation 1 [3], [4], [5], [6]: where: HLHV is the lower calorific value of the fuel, Pech is the electrochemical consumption of the battery and mf is the fuel consumption.The strategic parameter for optimal control is the equivalence factor s, the theoretical value of which is given by HOT for the same vehicle configuration and driving cycle.To avoid calculation problems, a random s factor cannot be used, although it has a limited effect on fuel consumption.
In the case of a parallel hybrid vehicle, the following parameters are called:  the minimum time between two gear changes, required only if there is a gearbox, but it should be borne in mind that in the case of electric cars with large inertia (over 0,05 •  2 ) problems may arise, because the system is not yet adapted;  clutch position such that acceleration is allowed;  factor Gc during clutch slip, used to spin the clutch when the combustion engine has to be started at a reduced vehicle speed so as to ensure engine idling speed.The clutch position value Cl is calculated by the formula 2 [3], [4], [5], [6]: where: Vtar is the desired vehicle speed and Vveh is the actual vehicle speed;  the factor for controlling the rotational speed Ge during clutch slip and Meng are calculated with the relationship 3 [3], [4], [5], [6]: where: Widle is the rotational speed of the engine at idle and Weng is the actual engine speed.
Finally, the equivalence factor s per cycle is calculated using formula 4 [3]: where: s0 is the initial value of s and the SOCref is the SOC endpoint.
For the model made using the HOT interface, originally intended to estimate only the energy potential of various types of architectures, it was necessary to map the main pollutant emissions of SI engine based on pollution tests carried out at the pollution bench.The next step to the emission mapping was to model the SI engine cooling system to replicate the coolant temperature increase and the 'artisanal' creation of an electrically heated catalyst component, knowing the severity of the CO emission depollution standard and knowing that a vehicle equipped with a hybrid GMP (powertrain) has a low catalyst efficiency, because of frequent deactivations of combustion engine on the depollution cycle.At the time of simulations, this component is not currently available in the IFP Drive library.

Results and discussion
The ultimate goal of the model used was to carry out a theoretical case study using the newly created eCat component (electric catalyst), simulating on the WLTC cycle the ecological and economic behavior of the vehicle in relation to Euro 7 standard, for a 3wCat (or TWC -three-way catalyst [7]) and four electrically heated catalysts with resistances of different powers: eCat 0,5kW, eCat 1kW, eCat 2kW and eCat 3kW.
Looking at the overall results of engine THC (total hydrocarbons) emissions shown in figure 5, an approximately linear increase is observed for the simulations launched with the 4 electrical catalysts, relative to reference, where a 3wCat was used.The increase of the THC emission is about 1,0-1,7% for every 1kW electrical power losses.This is explained by more frequent and sometimes longer activation of the internal combustion engine to compensate for energy consumed from the battery, needed to heat up the electrical resistance of the catalyst.The overall exhaust THC emissions highlight the expected behavior of increasing the efficiency for the 4 eCat's in relation to 3wCat due to the increase in internal temperature in the 4 electric catalysts.Although we note that the Euro 7 norm is respected in all cases reporting to 3wCat (reference), eCat 0,5kW is 37,3% more efficient, eCat 1kW is 54,8% more efficient, eCat 2kW is 57,2% more efficient, and eCat 3kW is with 60,1% more efficient.Figure 6 allows analysis of overall CO emissions, where a normal upward trend is observed for engine emissions compared to the WLTC 3wCat reference simulation.Motor CO global emissions increased by around 0,7 -2,2 %, for each kW of electricity power losses additionally add when heating the eCat.In terms of exhaust emissions, we note that, in the reference situation 3wCat), the proposed Euro 7 standard is not met, the efficiency of this catalyst being relatively low ~84,28%.At the moment, with the Euro 6 standard in force, this is not a problem, as the CO threshold is set at 1[g/km].In the other 4 simulations (WLTC eCat 0,5kW, WLTC eCat 1kW, WLTC eCat 2kW and WLTC eCat 3kW) the proposed Euro 7 standard is respected with sufficient safety margin.In these 4 simulations, catalyst efficiency at CO aftertreatment is approximately 3,66 -5,09% higher than in the reference situation, reaching a maximum of 89,38% for the WLTC eCat 3kW simulation.

Figure 6. Evolution of engine and exhaust CO emissions
In figure 7 it is presented the overall engine and exhaust NOx emissions.Global NOx emissions follow the same linear upward trend depending on the electrical power consumed by eCat.Comparing the results of the simulations of the 4 eCat catalysts to the WLTC 3wCat reference simulation, an increase of 1,14% for eCat 0,5kW, 2,19% for eCat 1kW, 3,70% for eCat 2kW and 4,13% for eCat 3kW is observed in the case of engine NOx emissions.After analyzing the differences between these increases in engine NOx emissions, an average increase of around 1% was observed for each 1kW of electricity power losses additionally add when heating the eCat.NOx exhaust emissions are below the Euro 7 threshold for all 5 cases in which the simulations were launched.There is a significant decrease in exhaust emissions in the 4 eCat configurations with a maximum efficacity for the eCat 3kW.The overall NOx efficiency calculated for the 3wCat benchmark simulation is 57,59%, while for the other 4 configurations efficiencies of 73,48% were achieved for the eCat 0,5kW, 78,29% for the eCat 1kW, 80,18% for the eCat 2kW and 81,64% for the eCat 3kW.It is known that to determine the fuel consumption of a vehicle it is sufficient to know the CO2 emissions released by that vehicle into the atmosphere.Following the theoretical case study made using the 4 simulations, a linear increase in fuel consumption of approximately 1,4% was observed for every 1kW additional electrical power losses for heating the electrical catalyst, as a result of more frequent activation for the spark ignition engine.

Conclusions
The proposed Euro 7 standard eliminates differences in pollutant emissions between vehicles equipped with SI engine or CI engine and introduces limit thresholds for NH3 and particulate matter from the braking system.
In terms of THC emissions, the behavior observed for the simulated vehicle in the 4 configurations is expected for engine and exhaust emissions.Following the analysis of the results obtained after the launched simulations, CO emissions exceed the limit threshold for the proposed Euro 7 standard in one case (3wCat), but when is using an electrically heated catalyst on a PHEV, the standard is respected.
Following the theoretical case study made on the studied vehicle, for the 4 catalysts eCat 0,5kW, eCat 1kW, eCat 2kW and eCat 3kW an efficiency increases of up to 89,38% was observed, but this remains relatively small compared to the efficiency of aftertreatment systems that equips vehicles with 100% thermal engines, where for carbon monoxide the efficiency of 3wCat is 90...99%.
For NOx emissions as for THC emissions, the behavior observed in the 4 configurations is expected for engine emissions and exhaust emissions.The Euro 7 limit threshold for NOx emissions is at a level that allows us, according to the results obtained from the simulations, a sufficient margin of safety in case of light drifts.
According to the study, the increase in fuel consumption is directly proportional to the increase in power losses that simulate the heating resistance of the electric catalyst.
As last one conclusion, based on the case study carried out, it was concluded that the exit of the engine from the thermal operating regime has direct negative consequences on pollutants emissions and indirectly on the fuel consumption.

Figure 4 .
Figure 4. Vehicle model with parallel rechargeable hybrid architecture made with HOT

Figure 5 .
Figure 5. Evolution of engine and exhaust THC emissions

Figure 7 .
Figure 7. Evolution of engine and exhaust NOx emissions

Figure 8 .
Figure 8. Evolution of CO2 emissions and fuel consumption

Table 2 .
Main characteristics of the traction battery[2]