gearshifts calculated according to the WLTP used all gears up to 6 in a rational ... portions of the two test cycles were conducted in the same (top) gear, with the ...
F2014-CET-139
THE WLTP AS A NEW TOOL FOR THE EVALUATION OF CO2 EMISSIONS 1 1
Bielaczyc, Piotr*; 1Woodburn, Joseph; 1Szczotka, Andrzej BOSMAL Automotive Research and Development Institute Ltd, Poland
KEYWORDS – exhaust emissions; carbon dioxide (CO2); WLTP test procedure; WLTC test cycle; NEDC; FTP-75. ABSTRACT – This paper presents the results of a preliminary experimental study examining the World Harmonized Light-duty Vehicles Test Procedure (WLTP) and in particular its test cycle – the World Harmonized Light-duty Vehicles Test Cycle (WLTC). Exhaust emission testing was carried out in an advanced, climate-controlled test facility at BOSMAL Automotive Research and Development Institute. In general, testing was carried out in accordance with the demands of current EU legislation: all the vehicles were tested on a chassis dynamometer and the exhaust gas was diluted and sampled using a constant volume sampler. The results indicated limited differences between CO2 emissions over the NEDC and WLTC, but with some exceptions; differences between the WLTC and the FTP-75 were greater. Further work is required to compare the entire WLTP procedure with current EU and US test procedures. TECHNICAL PAPER – INTRODUCTION Carbon dioxide emissions measurements in the current automotive development context Automotive emissions control has expanded considerably over the years since its inception in California in the 1960s. The range of compounds which must be measured has expanded considerably from the original requirements, and testing of carbon dioxide (CO2) emissions and fuel consumption is now required for all major markets. Carbon dioxide emissions are not subject to set legal limits, but all the major automotive markets have set limits for fleet average (sales weighted) CO2 emissions (sometimes limited in terms of fuel consumption, which is strongly proportional to CO2 emissions). These legal fleet average limits, which are the subject of considerable controversy, have increased in importance over the years to the point where reducing CO2 emissions is arguably just as great (if not greater) challenge as reducing regulated emissions such as hydrocarbons, carbon monoxide, oxides of nitrogen and particulates (1). In short, CO2 emissions data are required for a wide range of reasons – not least for legal ones (calculation of fleet average CO2 emissions); commercial reasons (calculation of fuel consumption figures to be provided to consumers); and technical/engineering reasons (testing and development of emissions reduction technologies, fuel types, etc). Being the most prevalent measured species in the exhaust gas, CO2 is often relied upon to determine test-to-test repeatability, even if CO2 measurements are not the aim of the test in question. Research on CO2 emissions and the relationship between CO2 emissions and fuel consumption has been ongoing for many years (2).
Current moves towards a more harmonized and comparable test procedure are of considerable importance in the area of CO2 emissions testing. The development of a new test procedure and the test cycle that forms the basis of the procedure are of paramount importance in terms of development work on automotive CO2 emissions. Since it is well known that the test conditions and test cycle used can have a major impact on emissions, any discussion of future CO2 emissions is somewhat dependent on future test procedures, as well as changes in vehicle and engine technology, fuel types, etc. The proposed World Harmonized Light-duty Vehicles Test Procedure (WLTP) In response to long-standing criticisms of the New European Driving Cycle (NEDC; the test cycle specified in current EU legislation for measuring CO2 (3)), and more recent work on the disparity between type approval values and in-use emissions (and fuel consumption) – see (4), as well as a desire for regional and global harmonization and standardization, work on a proposed harmonized procedure began as early as 2007/2008 (5), (6). At the time of writing, the WLTP is still a work in progress, but has undergone a variety of changes during its development (7). The fundamental emissions testing strategy unchanged: vehicles are to be tested on a chassis dynamometer in a laboratory under controlled conditions, using a constant volume sampler (CVS) and emissions testing bags. The test procedures specified by the various regional authorities use different test cycles – and this is also the case for the WLTP, which introduces a new test cycle – the World Harmonized Light-duty Vehicles Test Cycle (WLTC). Many of the changes in the WLTP development process have concerned the driving cycle (6), (8) (the current version at the time of writing is 5.3 – the version used for all testing described in this paper) and the procedure to be used for calculating the gearshifts to be used with the cycle, which are customised for each particular vehicle/engine combination (6), (8). Figures 1 and 2 and Table 1 present and compare the NEDC and WLTC test cycles in terms of speed traces and key characteristics and metrics. 140
160 UDC
100 Vehicle speed [km/h]
Low
140 Vehicle speed [km/h]
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40 20
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0 0
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Figure 1. The New European Driving Cycle (NEDC)
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Figure 2. The proposed Worldwide Harmonized Lightduty Test Cycle (WLTC), version 5.3, for class 3 vehicles (power to mass ratio ≥34 kW/tonne)
Parameter ratio likely to affect CO2 emissions (at least somewhat)? Yes Yes No (irrelevant)
Parameter
Unit
NEDC value
WLTC value
Parameter ratio (WLTC/NEDC) [-]
Total distance Duration Number of phases Number of pull away events Number of pull away events per km Number of gear changes (for a manual transmission) Idle time (before initially pulling away) Total idle time Idle time (proportion) Maximum speed Average speed (all phases, including idling) Maximum positive acceleration
km s -
10.982 1180 2
23.266 1800 4
2.12 1.53 2
-
13
8
0.62
km-1
1.18
0.34
0.29
-
22
Varies, but always >>22
>>1
Possibly
s
10
11
1.1
No
s % km/h
280 23.73 120
234 13 131.3
0.84 0.55 1.09
km/h
33.51
46.50
1.39
Yes
m/s2
1.04
1.67
1.61
Yes
9.13
20.57
2.25
Yes
2
Maximum value of v·a
Yes
Yes Possibly
3
m /s , W/kg
Time at which speed ≥ s 16 17 1.06 No 15 km/h is first reached Time at which gear Typically higher than second is s 133 ~0.15-0.20 Possibly ~20-25 first used Proportion of cycle for % 5.76 10.11 1.76 Yes which v≥100 km/h Time elapsed at first occurrence of v≥100 s 1067 1559 1.46 Possibly, in some cases km/h Remaining cycle distance after a km ~4 ~15 ~4 Probably characteristic warmup 3 time of ~10 seconds Table 1. A comparison of key parameters and metrics for the WLTC and NEDC, with a brief consideration of their relevance for cycle CO2 emissions results in the context of the test vehicles employed in this study
TEST PROGRAM A laboratory test programme was undertaken to investigate the proposed WLTP and its implications. The experimental work described in this paper did not include all the proposed stipulations of the WLTP regarding road load determination and setting, but focused on the test cycle and gearshift strategies – which are markedly different for the WLTP and current EU procedures. Test facility All testing described in this paper was conducted in the exhaust emission laboratory at BOSMAL Automotive Research and Development Institute Ltd (Bielsko-Biala, Poland) (Figures 3 & 4). This advanced, climate controlled vehicle emissions testing laboratory has been described in detail elsewhere (9), (10) and has been employed in a wide range of recent emissions testing studies focusing on the quantification of CO2 emissions from various
vehicle and engine types under standard and non-standard test conditions (e.g. (11), (12), (13), (14), (15), (16), (17)).
Figure 3. Schematic BOSMAL’s climate controlled exhaust emissions testing laboratory, with all emissions testing systems shown. See (9) and (10) for further details
Figure 4. Interior view of BOSMAL’s emissions laboratory, showing a vehicle mounted on the chassis dynamometer, the connection to the emissions sampling system (foreground), and the windspeed fan (background), all within the climatic chamber. See (9) for further details
Test vehicles A range of modern European test vehicles were selected for testing. Key characteristics of the test vehicles are presented in Table 2. Since all test vehicles had a power/weight ratio of ≥ 34 kW/tonne, class 3 of the WLTC was applicable.
Vehicle no. 1 2 3 4 5 6 7 Vehicle type PC PC PC PC PC PC LDV Engine type SI SI SI SI DISI CI CI Test fuel Petrol Petrol Petrol CNG Petrol Diesel Diesel Approx. displacement 1.6 1.2 1.4 1.4 1.8 1.3 3.0 [dm3] Emissions standard Euro 5 Euro 6 Euro 5 Euro 5 Euro 5 Euro 5 Euro 5 EU test inertia [kg] 1360 910 1130 1130 1360 1020 2270 Reg No. 83 ‘alternative’ 7.1 5.7 6.4 6.4 7.1 6.1 12.9 chassis dyno model 0 0 0 0 0 0 0 [N] 0.0481 0.0385 0.0433 0.0433 0.0481 0.0412 0.08762 [N/(km/h)] [N/(km/h)2] Notes: PC – passenger car; LDV – light-duty vehicle; SI – spark ignition; DISI – direct injection spark ignition; CI compression ignition; CNG – compressed natural gas; Reg. No. 83 – (18) Table 2. Key characteristics of the seven test vehicles used in this study
Test procedures followed While this paper includes a discussion of the new test procedure (WLTP), the experimental work reported here focused on the new cycle (WLTC), rather than the entire procedure. As previously mentioned, while the current EU procedure and the proposed WLTP procedure differ in certain ways, there are multiple areas of overlap. Therefore, it was possible to test the vehicles in a way which satisfied the requirements of both test procedures in terms of climatic conditions: an ambient temperature of 25°C and 45%-50% relative humidity was used for all testing. The preconditioning procedures used were the ones specified in the respective test procedures. In addition, good engineering practice was used to ensure that the vehicles were thermally stabilised, thereby facilitating repeatable results. As previously stated, this study did not focus on the road load determination and setting procedures of the WLTP; for all testing, the current EU test inertias and alternative chassis dynamometer models were employed. Standard, commercially available European fuels (petrol, CNG, Diesel) were used for test vehicles 1-7. Since the same fuel was used in each emissions test, the test fuel was not a variable in this test programme, so no fuel parameters are presented here. For each test procedure, three emissions tests were performed on each vehicle. The standard deviation of the emissions results obtained were briefly analysed, and the mean values were taken. All results presented in this paper refer to mean values, rather than the raw results of the emissions tests themselves. RESULTS & DISCUSSION Overall observations Notwithstanding the dynamicity of the WLTC, no major drivability issues were encountered for the new test cycle. The customised gearshift approach generally worked well, but on a few occasions the specified gear was perhaps higher than the gear which would normally be chosen by a driver (given the vehicle speed), leading to periods of high load and somewhat sluggish throttle response. However, these problems were reasonably limited and did not prevent the driver from following the speed trace within the specified tolerances. Therefore, in terms of the viability of the test procedure, the specified methodology of using customised gearshift schedules was found to be unproblematic. Execution of the FTP-75 was similarly unproblematic, even though that cycle is also more dynamic and demanding than the NEDC.
Test fleet results: WLTC The CO2 emissions results obtained from all seven test vehicles are shown in Figure 5. While there was considerable variation in the absolute CO2 emissions results between the test vehicles, some key trends (high emissions for the cold start Low phase; lower emissions for Mid and High; emissions for Ex High close to those for Low) were apparent, as visible in Figure 5. 300 Vehicle 1
Vehicle 2
Vehicle 3
Vehicle 4
Vehicle 5
Vehicle 6
Vehicle 7
CO2 emissions [g/km]
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WLTC
Figure 5. CO2 emissions results shown for each phase of the WLTC, as well as the entire test cycle
Test fleet correlations: NEDC/WLTC Results obtained over the WLTC were compared to emissions results obtained over the NEDC, using otherwise identical test conditions. In general, results were numerically similar for the two test cycles (Figures 6 & 7), a general tendency previously reported (19). Two vehicles (vehicle 1 & 2) emitted slightly more CO2 over the NEDC; one vehicle’s (vehicle 6) CO2 emissions were statistically indistinguishable; the remaining four vehicles (vehicle 3, 4, 5 & 7) emitted significantly less CO2 over the WLTC than over the NEDC. The mean deviation (WLTC-NEDC) for all seven test vehicles was 2.74%; the mean deviation for the vehicles with SI engines was 1.6%; the mean deviation for the vehicles with CI engines was 5.6%. Notwithstanding these differences, the relationship between NEDC and WLTC CO2 emissions for the vehicles tested in this study followed a relationship close to perfectly linear, with an R2 correlation coefficient of 0.9890 (Figure 7). From Figure 6, it is apparent that two vehicles stand out as having higher deviations: vehicles 5 & 7. Regarding vehicle 5, one possible explanation is that the vehicle featured a six-speed gearbox, meaning that the gearshifts calculated according to the WLTP used all gears up to 6 in a rational (rather than arbitrary) manner, better reflecting the power output capabilities of the engine. Indeed, in accordance with the calculated gearshift schedule, vehicle 5 spent almost 38% of the WLTC in sixth gear. For vehicles 1-5, which all featured five-speed gearboxes, the high speed portions of the two test cycles were conducted in the same (top) gear, with the additional speed required for the WLTC provided by increased engine speed. Vehicle 7 also featured a 6-speed gearbox, spending almost 33% of the WLTC in sixth gear. However, the observed CO2 deviation was much greater than for vehicle 5. One possible explanation (in addition to the aforementioned impact of the gears) is that the warmup time of the 3 litre CI engine featured in vehicle 7 is quite long – such that the engine is warming up for most of the
duration of the NEDC, but a sizable portion of the WLTC is conducted with the engine and its fluids at their normal operating temperatures (c.f. Table 1). Further investigations on this vehicle (and others featuring CI engines of displacement ≥2.0 litres) would perhaps give an indication of how typical such behaviour is. 300
0%
R² = 0.9890
WLTC CO2 [g/km]
Deviation (WLTC-NEDC)
2%
-2% -4% -6% -8%
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-12%
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Vehicle Vehicle Vehicle Vehicle Vehicle Vehicle Vehicle 1 2 3 4 5 6 7
Figure 6. Deviations in the CO2 emissions results obtained over the entire NEDC and WLTC
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NEDC CO2 [g/km]
Figure 7. A scatterplot of CO2 emissions results obtained over the entire NEDC and the WLTC
Test fleet correlations: NEDC/FTP-75/WLTC Two vehicles (vehicles 4 and 6) were also tested over the US FTP-75 test cycle, in accordance with the specified emissions test procedure for that test. Relative results for the three test cycles used on these two vehicles are summarised in Table 4, compared to CO2 emissions obtained over the NEDC. Test cycle relative CO2 results [%] Vehicle 4 (CNG) Vehicle 6 (Diesel) NEDC 100.00 100.00 WLTC 97.57* 100.06 FTP-75 112.12* 102.51* * Statistically significant difference at the 95% confidence level Table 4. CO2 emissions results for the NEDC, WLTC and FTP-75 cycles for vehicles 4 and 6
As previously stated, CO2 results for these two vehicles were very close over the NEDC and WLTC cycles, but somewhat greater values were found for the FTP-75. Both vehicle 4 and vehicle 6 were of European manufacture, so it is to be expected that their calibration is optimised for the current EU test procedure. For this reason, it is unsurprising that results over the FTP-75 were higher. However vehicle 4’s substantial difference between the NEDC and FTP-75 results were somewhat unexpected. In the case of vehicle 4, the spread of the three emissions results was quite considerable – around 15%. Further investigation, with detailed consideration of cycle characteristics, etc, would be required in order to comment further upon this observation. Emissions trends – an examination of the behaviour of two test vehicles Two of the test vehicles (2 and 6) were tested using the modal emissions technique for continuous (modal) analysis of regulated pollutants and CO2 in the diluted exhaust gas. Figures 8 & 9 show modal CO2 emissions results and observed correlations for vehicle 2. The fundamentally different speed traces and the duration of the test cycles cause the CO2 emissions traces for the two cycles to appear very different. However, both cycles show the characteristic short-lived surges in CO2 emissions coinciding with periods of load acceptance and acceleration. Notwithstanding the fact that the speed trace for the NEDC features only
linear accelerations, whereas the WLTC features no linear accelerations, the dynamicity of the CO2 emissions traces were found to be roughly comparable for the two cycles. The CO2 scatterplot (Figure 9) revealed that the maximum CO2 emissions observed at any given vehicle speed were observed over the WLTC. However, the linear accelerations and periods of constant speed encountered during the NEDC lead to a much smaller range of engine speed and load conditions, which is reflected in the CO2 emissions scatterplot. While the maximum emissions at any given vehicle speed were higher for the WLTC, a large proportion of the WLTC’s datapoints lay below the data plotted for the NEDC. Interestingly, the ‘excess’ speed of the WLTC (those greater than encountered during the NEDC, i.e. 120 km/h+) caused CO2 emissions that were scarcely and higher than the emissions occurring at speeds around 120 km/h during the NEDC. The impact of the differences in the early portions of the two test cycles on warmup behaviour and the resulting CO2 emissions is a topic worthy of thorough investigation. Comparing the two cycles (Figures 1 & 2), it is apparent that the WLTC features elevated speeds much earlier in the cycle – the first ‘ramp’ of the NEDC reaches a maximum (steady) speed of 15 km/h, whereas the first ‘hill’ of the WLTC reaches a maximum (unsteady) speed of 44.45 km/h. 10
10 WLTC
CO2 emissions [g/s]
NEDC
CO2 emissions [g/s]
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Vehicle speed [km/h]
Time [s]
Figure 8. CO2 emissions traces over the NEDC and WLTC for vehicle 2
Figure 9. Scatterplots of vehicle speed against CO2 emissions over the NEDC and WLTC for vehicle 2
Figures 10 & 11 show modal CO2 emissions results and observed correlations for vehicle 6. 10 NEDC
10
WLTC
CO2 emissions [g/s]
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CO2 emissions [g/s]
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NEDC
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Figure 10. CO2 emissions traces over the NEDC and WLTC for vehicle 6
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Figure 11. Scatterplots of vehicle speed against CO2 emissions over the NEDC and WLTC for vehicle 6
150
CONCLUSIONS The results of the emissions tests described in this paper have confirmed that the WLTP and in particular its cycle (the WLTC) are viable and valuable tools for the determination of CO2 emissions. Test-to-test repeatability appears to be comparable to the NEDC. The most basic differences between the test procedures are the test cycle and the gear usage stipulations, but results obtained over the two cycles using the same test vehicles tested under identical conditions revealed reasonably limited differences, with a slight tendency for the WLTC to produce lower CO2 emissions results, though not in all cases. Based on the relatively small number of test vehicles employed in this preliminary study (seven), it was not possible to draw any specific conclusions regarding engine or fuel type (SI/CI/DISI, petrol, Diesel, CNG). The fact that the two vehicles with the greatest deviation in the results both featured six-speed gearboxes was judged to be possible significance. A brief two-vehicle CO2 emissions comparison with the FTP-75 test cycle revealed the WLTC to produce lower results. However, the cycle only forms part of the test procedure – the proposals for altered road load settings and inertia weights (stepless, rather than in classes (20)) will also have a certain impact on CO2 results. Further research is required in this area in order to fully assess the likely impact of the coming changes in the EU legislative test procedure. Notwithstanding the emissions comparisons with the NEDC (and indeed any other test cycle), the WTLC and the wider WLTP procedure appear to place the test vehicle under more realistic and demanding conditions; i.e. conditions encountered by the vehicle during usage on public roads. Given the importance of CO2 emissions reduction efforts and the great need for reliable data in that area (1), work in the area addressed in this paper is an ongoing research priority. ACKNOWLEDGEMENTS The authors would like to thank Alessandro Marotta and Heinz Steven of the WLTP working group for their assistance and provision of information. REFERENCES [1] Bielaczyc, P. “Drivers of CO2 and pollutant emissions reduction from modern IC engines” PTNSS-2013-SC-208. V International Congress on Combustion Engines org. by PTNSS, 24th – 26th June 2013, Industry Session on 26th June 2013 “Powertrain Development for Low-to-Zero Emissions and Efficient Energy Usage”. Industry Session Proceedings on 26th June 2013 on CDROM, ISBN 978-83-931383-5-7. Combustion Engines 4/2013 (155),PL ISSN 0138-0346 [2] Gis, W., Bielaczyc, P. “Emission of CO2 and Fuel Consumption for Automotive Vehicles” SAE Technical Paper 1999-01-1074, 1999, doi:10.4271/1999-01-1074 [3] UNECE, “Addendum 100: Regulation No. 101 – Revision 3” 2013, http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/updates/R101r3e.pdf [4] Mock, P., German, J., Bandivadekar, A., Riemersma, I., “Discrepancies between typeapproval and “real-world” fuel-consumption and CO2 values - Assessment for 2001-2011 European passenger cars” [I], ICCT, 2012, http://www.theicct.org/sites/default/files/publications/ICCT_EU_fuelconsumption2_working paper_2012.pdf [5] UNECE Working Party on Pollution and Energy. “WLTP Initial sessions” [I]. UNECE,
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