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PROCEEDINGS OF THE INSTITUTE OF VEHICLES 1(115)/2018 Jakub Lasocki 1 GEARSHIFT STRATEGY IN THE WORLDWIDE HARMONIZED LIGHT VEHICLES TEST PROCEDURE 1. Introducti on Operation of road vehicles with combustion engines results in emission of various atmospheric pollutants [1, 2]. In order to reduce this negative effect, emission tests are required at vehicle type approval, according to the law of a particular country. Lightduty vehicles are usually tested under standardised laboratory conditions using a chassis dynamometer [3]. The complete vehicle is placed on rollers that simulate road load while the driver follows a driving cycle – a fixed schedule of vehicle operation defined by vehicle speed and gear selection as a function of time [4, 5]. The detailed test procedure and test conditions are specified in the relevant legislation. It is well known that pollutant emission and fuel consumption of vehicles are sensitive to test procedures and conditions [6–10]. These include a gearshift

strategy, ie the sequence of gears engaged at each time interval of a driving cycle, which is of a great significance for vehicles with manual gearboxes  [11]. There are five main gearshift strategies commonly used in driving cycles [12, 13]: – cycle strategy, in which gear selection criteria are included in the design of the corresponding driving cycle (e.g ARTEMIS driving cycles), – fixed vehicle speed strategy, in which gear shifting takes place at fixed values of vehicle speed (e.g New European Driving Cycle – NEDC), – imposed engine speed strategy, in which gear selection criteria are defined in terms of given engine speed, – record strategy, in which gearshift scheme is derived directly from data recorded during vehicle testing in real road conditions, – free strategy, in which gear selection depends on the driver in the laboratory. The influence of different gearshift strategies on pollutant emission was investigated within ARTEMIS project [12]. The results

revealed that the most favourable in terms of pollutant emission was fixed vehicle speed strategy (as in the NEDC) and the least – imposed engine speed strategy. More specifically, the most sensitive to the gearshift strategy was the emission of carbon dioxide (2–15% variation between strategies), followed by carbon monoxide and hydrocarbons emissions, while nitrogen oxides emission was not influenced. The driving cycle that has been employed in the European Union for type approval of light-duty vehicles was the NEDC [14]. In response to its long-standing criticisms, regarding the discrepancy between type-approval and real-world values of pollutant emission and fuel consumption [15], it has been recently replaced by the World-wide harmonized Light-duty Test Cycle (WLTC), along with the new World-wide harmonized Light-duty Test Procedure (WLTP)† [16]. By assumption gear shifting in the NEDC is performed at set values of vehicle speed, which means that it is identical for all 1

Jakub Lasocki, Ph.D Eng; Institute of Vehicles, Warsaw University of Technology 113 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 the vehicles but cannot be considered as representative of real driving [12]. On the contrary, the gearshift strategy introduced with the WLTP is customised for each particular vehicle-engine combination and resembles real driving. The aim of this paper is to give some details on the method of determination of gear shifting points to be used with the WLTP and to compare it with the method imposed by the NEDC. The presented considerations are illustrated by an example that indicates the most significant differences between these two approaches . 2. Gear selection in accordance with the WLTP The detailed method of gear shifting points selection, developed within the WLTP to be used for vehicle testing on a chassis dynamometer, is contained in the Annex 2 to UN/ECE Global technical regulation No.

15 (GTR 15) [16] It depends upon vehicle characteristics and requires the following input data: maximum rated engine power and corresponding engine speed, engine speed at idle, full load power curve of the engine, the number of forward gears, gear ratios, axle (final gear) ratio, wheels (tyres) size, vehicle test mass and the coefficients of the road load curve. The general idea is to keep the right balance between the power required to overcome vehicle driving resistance and the power provided by the engine, by selecting the highest possible gear from the gears available at a given vehicle speed. Since gear availability is limited by the range of engine speed, t his can be summarized in the following two criteria: Pav i,j ≥ Preq j (1) n min ≤ n i,j ≤ n max (2) and where: Pav i,j [kW] – available engine power for gear i at the time point j of the driving cycle, Preq j [kW] – required power at the time point j of the driving cycle, n i,j [rpm] – actual engine speed for

gear i at the time point j of the driving cycle, n min , n max [rpm] – minimum and maximum engine speed. The first criterion (1) of gear selection is based on the relation between available and required power. The available engine power for each gear i at the time point j of the driving cycle is given by the equation: Pav i,j = Pnorm (n norm i,j )∙Prated ∙ks (3) where: Pnorm – normalised engine power at full load at normalised engine speed n norm i,j , Prated [kW] – rated engine power, ks – correction factor for the difference between engine power at full load determined in stationary conditions (power curve) and engine power available during transient conditions of a driving cycle, Normalised engine speed for each gear i at the time point j of the driving cycle is calculated as: n norm i,j = ri∙v j - n idle n rated - n idle (4) 114 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 where: ri [rpm/(km/h)] –

the ratio obtained by dividing engine speed by vehicle speed for each gear i, v j [km/h] – vehicle speed at the time point j of the driving cycle, n idle [rpm] – engine idling speed, n rated [rpm] – rated engine speed at which maximu m power is developed, The ratio ri for each gear i can be obtained from direct measurements of vehicle speed and corresponding engine speed or alternatively calculated taking into account gear ratios, axle (final gear) ratio and wheels (tyres) size. The required power at each time point j of the cycle is given by: Preq j = f0 ∙v j + f1 ∙v 2j + f2 ∙v 3j 3600 + k r ∙aj ∙v j ∙m 3600 (5) where: f0 , f1 , f2 [N, N/(km/h), N/(km/h)2 ] – coefficients of the road load curve, kr – correction factor for the inertial resistance of vehicle drivetrain during acceleration (k = 1.1), aj [m/s] – vehicle acceleration at the time point j of the driving cycle, m [kg] – vehicle test mass. The second criterion (2) of gear selection takes into

account engine speed. Its values are determined in accordance with the principles set out below. If the vehicle speed is lower than 1 km/h, the engine is assumed to work with idling speed with the gear lever placed in neutral position and the clutch engaged. In other cases actual engine speed for gear i at the time point j of the driving cycle is calculated using the equation: n i,j = ri ∙v j (6) The limits of engine speed n min and n max have to be calculated. For the first and the second gears only the minimum values have been established, as these gears are changed up as soon as the engine speed considerably increases. The minimum engine speed for the first gear is the idling speed. When the second gear is engaged and r2 ∙v j ≥ 0.9∙n idle (7) n min = max[1.15∙ n idle , 003∙ (n rated - n idle ) + n idle ] (8) r2 ∙v j < max[1.15 ∙n idle, 003∙ (n rated - n idle ) + n idle ] (9) the minimum engine speed equals Furthermore, if the clutch should be

disengaged. For gears higher than the second the following limits apply: n min = 0.125∙ (n rated - n idle) + n idle (10) n max = 1.2∙ (n rated - n idle ) + n idle (11) 115 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 The sequence of gear changing, obtained based on the equations presented in this chapter, usually requires some corrections in order to avoid too frequent gearshifts and to ensure drivability and practicality of a driving cycle. For example, only the first gear can be used when starting from standstill, gears cannot be skipped during vehicle acceleration but may be skipped during vehicle deceleration. Detailed guidelines on this matter are included in the GTR 15 [16]. 3. Application example of the WLTP approach to gear shifting As an example, gear shifting points dedicated to vehicle speed profile of driving cycle WLTC class 3.2 were determined according to the WLTP method In addition, results were

compared with gear shifting points set at fixed vehicle speed, as in the NEDC. The calculations were performed for passenger car Volkswagen Passat Variant 2.0 TDI (fig. 1) manufactured in 2017 and equipped with a four-cylinder turbocharged compression-ignition engine having a displacement of 1968 cm3 and common rail fuel injection system. More detailed technical specifications of the car, regarding data needed for gearshift points determination, is given in table 1. Fig. 1 Volkswagen Passat Variant (2017) Table 1. Technical specifications of Volkswagen Passat Variant 20 TDI (2017) [17] Parameter Rated engine power Engine speed at rated power Engine speed at idle The number of forward gears Gear ratios: 1st , 2nd , 3rd , 4th , 5th , 6th Axle (final gear) ratio Tyres size Vehicle test mass Road load curve coefficient f0 Road load curve coefficient f1 Road load curve coefficient f2 Unit kW rpm rpm kg N N/(km/h) N/(km/h)2 Value 110 3500 750 6 3.77, 196, 126, 087, 086, 072 2.92 215/60

R16 1534.25 160.14 801.57·10-3 28.34·10-3 116 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 Normalised power curve of the engine at full load is presented in fig. 2 Raw data was derived from [18], engine power normalised to rated power, while engine speed to the span between idling and rated engine speed. Fig. 2 Normalised power curve of the engine at full load Gear sequences were determined in compliance with the provisions of relevant legislation, which are UN/ECE GTR 15 [16] in the case of WLTP and UN/ECE Regulation No 83 [6] in the case of NEDC. It should be noted, however, that these provisions leave a certain margin of discretion. For example, NEDC gearshift pattern was originally designed for vehicles with 5-gear gearbox. In order to take into account the increased variety of gearbox design, with more than five gears , current version of Regulation No 83 allows additional gears can be used, according to

manufacturer recommendations, in the last phase of the NEDC where vehicle speed reaches 100 km/h and more. For the purpose of present calculations, it was assumed that the sixth gear is engaged at this speed. Another aspect worth mentioning is gear reduction in the last phase of vehicle deceleration to a stop. According to the GTR 15, in this situation the clutch may be either disengaged or the gear lever placed in neutral and the clutch left engaged. However, the last phase, in which this occurs , is not strictly defined ‡ On the other hand, Regulation No 83 clearly specifies when the currently selected gear should remain engaged with the clutch disengaged. The problem is that it is not easy to adopt these settings to vehicle speed profiles other than in the NEDC. To simplify calculations it was assumed that gears in the WLTC are always reduced until the car stops. Gearshift strategies for the WLTC class 3.2 driving cycle, determined with the use of two different methods, are

presented in fig. 3 It can be seen that the general character of both gearshift sequences is similar. However, despite the overall similarity, gearshifts in the NEDC-derived sequence are more frequent and sometimes the driving time for a given gear can be very short, even less than 5 s (fig. 4) It can be explained by the fact that the course of vehicle speed in the WLTC resembles the actual usage of the vehicle, what is the opposite of the steady speed phases of the NEDC. When driving in accordance with the dynamic speed course of the WLTC, the vehicle sometimes crosses fixed speed values, related to gear shifting in the NEDC, several times, forcing frequent, impractical gear changes. 117 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 Fig. 3 Gearshift strategies for vehicle speed (v) in the WLTC class 32 determined with the use of the WLTP method (g WLTP ) and the NEDC method (g NEDC) Fig. 4 Sample section of the WLTC with

marked gear changes according to the WLTP and NEDC gearshift strategies 118 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 In the WLTP gearshift strategy the sixth gear is used much more frequently. The time share of its use is over 25%, while in the NEDC gearshift strategy it is only about 10% (fig. 5) At the same time the shares of the WLTC conducted using fourth and fifth gears are relatively small in the case of the WLTP gearshift strategy. Possible explanations include close values of 4th and 5th gear ratios and the emphasis on maximizing the use of the highest possible gear, which is inherent to the WLTP. On the contrary, the gearshifts determined according to the NEDC method are more uniform in terms of the time share of gear use. However, this can be only an idealized picture of the actual use of the car. Fig. 5 Comparison of the time share (ts) of gear use in the WLTC class 32 according to different gearshift

strategies The NEDC gearshift strategy was optimized for the vehicle speed profile of this particular driving cycle. In contrast, vehicle-dependent gear selection as proposed in the WLTP is supposed to be more flexible and universal. Therefore it is worth considering the application of the WLTP gearshift strategy for the NEDC in order to compare it with the standard solution (fig. 6) Again, there is a large similarity between two gearshift strategies with the exception of the use of three top gears. On the basis of calculations in accordance with the WLTP method, the vehicle would spend almost 19% of the driving cycle with the sixth gear engaged, which is more than twice as much as in the case of standard NEDC gearshift strategy – around 8% (fig. 7) In contrast to the previous case, where gear changes were applied to the WLTC, a reasonable gearshift frequency was obtained using both methods. This proves that the WLTP method is in fact more flexible and universal than the one derived

from the NEDC. 119 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 Fig. 6 Gearshift strategies for vehicle speed (v) in the NEDC determined with the use of the WLTP method (g WLTP ) and the NEDC method (g NEDC) Fig. 7 Comparison of the time share (ts) of gear use in the NEDC according to different gearshift strategies 120 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 4. Summary Testing procedures for vehicle emission and fuel consumption need to be adjusted in line with the technical progress in automotive engineering. One of such adaptations is the introduction of the WLTP in Europe for light vehicle testing on a chassis dynamometer. This study focused on one aspect of testing procedures, namely gearshift strategy, which is markedly different for the WLTP and previously used NEDC procedures. Gear shifting in the WLTP is customised to vehicle characteristics,

which brings the chassis dynamometer testing closer to the real-road conditions of vehicle use. Determination of gearshift sequence according to the WLTP is quite complex and timeconsuming, because after performing calculations according to the scheme it requires final control and possible modification of the sequence. The emphasis is on maximizing the use of the highest possible gear, while respecting the power output capabilities of the engine. This approach may sometimes lead to a selection of higher gear than the one which would normally be chosen by a driver in real-world driving conditions. On the contrary, gear shifting in the NEDC takes place at fixed speed values and is identical for all the vehicles . In spite of the fact that it can be relatively easily implemented, especially to steady-speed cycles such as the NEDC, it is not representative of real driving. Therefore for driving cycles such as the WLTC, which was developed as the combination of vehicle speed samples

collected in road tests, gearshifts can be too frequent or the time spent in a given gear can be very short, even less than 5 s. There is no doubt that the change of vehicle testing procedure will have an extensive impact on the automotive industry in Europe. Until recently, new vehicles used to be designed and optimized for NEDC-based type approval. Some optimizations might be no longer effective after the introduction of the WLTP. It also applies to the gearshift characteristics, which can be modified by the vehicles manufacturers , affecting the realworld performance of vehicles.  Vehicles equipped with automatic gearboxes are tested using standard factory settings, while vehicles with semi-automatic gearboxes can be tested like vehicles with manual gearboxes or by employing gear shift strategy in accordance with the manufacturers instructions (depending on the test procedure). † T he WLTP officially applies to new types of cars, i.e models that are introduced on the market

for the first time, since September 2017. It will apply to all new car registrations from September 2018 ‡ In practice, the moment of clutch disengagement depends on the characteristics of a vehicle drivetrain and therefore is determined individually for a particular vehicle. References: [1] Jakubiak-Lasocka J., Lasocki J, Siekmeier R, Chłopek Z: Impact of traffic-related air pollution on health, Advances in Experimental Medicine and Biology, 2015, 834, pp. 21–29, [2] Jakubiak-Lasocka J., Lasocki J, Badyda A J: The influence of particulate matter on respiratory morbidity and mortality in children and infants, Advances in Experimental Medicine and Biology, 2015, 849, pp. 39–48, [3] Chłopek Z., Biedrzycki J, Lasocki J, Wójcik P, Samson-Bręk I: Modelling of motor vehicle operation for the evaluation of pollutant emission and fuel consumption, Combustion Engines, 2017, 171(4), pp. 156–163, [4] Boulter P. G, Barlow T J, Latham S, McCrae I S: Emission factors 2009: Report 1 –

a review of methods for determining hot exhaust emission factors for 121 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05 [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] road vehicles, Published Project Report PPR353, UK Transport Research Laboratory, 2009, Chłopek Z., Biedrzycki J, Lasocki J, Wójcik P: Emisja zanieczyszczeń z silnika samochodu w testach jezdnych symulujących rzeczywiste użytkowanie trakcyjne, Zeszyty Naukowe Instytutu Pojazdów, 2013, 92(1), pp. 67–76, Chłopek Z., Biedrzycki J, Lasocki J, Wójcik P: Assessment of the impact of internal combustion engine dynamic states on its useful properties, Eksploatacja i Niezawodnosc – Maintenance and Reliability, 2015, 17(1), pp. 35–41, Chłopek Z., Lasocki J: Correlation investigations into pollutant emission and the operational states of compression-ignition engines in dynamic tests , Combustion Engines, 2017, 169(2), pp.

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2022-01-05 Abstract The article discusses the method of determining gearshift sequence according to the WLTP (Worldwide harmonized Light vehicles Test Procedure) used for vehicles testing on a chassis dynamometer. The necessary input data is listed and a detailed calculation scheme is presented. As an example of the application of the discussed method, a gearshift sequence for a passenger car with a compression ignition engine was determined. The calculations were made for the vehicle speed course as in the WLTC (Worldwide harmonized Light vehicles Test Cycle) and, for comparison, as in the NEDC (New European Driving Cycle). The most important differences in the results obtained using both methods were indicated. Keywords: WLTP, WLTC, NEDC, gear shifting, gearshift strategy STRATEGIA ZMIANY BIEGÓW WEDŁUG ŚWIATOWEJ UJEDNOLICONEJ PROCEDURY BADAŃ POJAZDÓW LEKKICH Streszczenie W artykule omówiono metodę wyznaczania strategii zmiany biegów według światowej ujednoliconej

procedury badań pojazdów lekkich WLTP (Worldwide harmonized Light vehicles Test Procedure), stosowaną do badań pojazdów na hamowni podwoziowej. Wyszczególniono niezbędne dane wejściowe i przedstawiono szczegółowy schemat obliczeń. Jako przykład zastosowania metody wyznaczono sekwencję zmiany biegów dla samochodu osobowego z silnikiem o zapłonie samoczynnym. Obliczenia wykonano dla przebiegu prędkości pojazdu w cyklu jezdnym WLTC (Worldwide harmonized Light vehicles Test Cycle) oraz, dla porównania, w cyklu jezdnym NEDC (New European Driving Cycle). Wskazano najważniejsze różnice w wynikach uzyskanych przy zas tosowaniu obydwu metod. Słowa kluczowe: WLTP, WLTC, NEDC, zmiana biegów, strategia zmiany biegów 123 Pobrano z http://repo.pwedupl / Downloaded from Repository of Warsaw University of Technology 2022-01-05