The analysis of the PEMS measurements of the exhaust emissions from city buses

using different research procedures

Jerzy Merkisz, Jacek Pielecha, Pawel Fuc, Piotr Lijewski Poznan University of Technology, Institute of Internal Combustion Engines and Transport

60-965 Poznan, ul. Piotrowo 3, Tel. +48 61 665 2207, Fax: +48 61 665 2204 E-mail: {jerzy.merkisz, jacek.pielecha, pawel.fuc, piotr.lijewski}@put.poznan.pl

Abstract—The paper discusses the possibilities of measurement of the exhaust emissions generated by city buses operating in public transit. The currently used methods of exhaust emission measurement under laboratory conditions on engine test beds do not entirely reflect the varied driving conditions under which the vehicles are operated. The paper presents the results of the in-use exhaust emission measurements performed on city buses using a Portable Emissions Measurement System (PEMS). The paper focuses on the road tests methodology as well as the interpretation of the obtained results.

I. INTRODUCTION

The paper discusses the possibilities of measurement of the exhaust emissions generated by city buses operating in public transit. The currently used methods of exhaust emission measurement under laboratory conditions on engine test beds do not entirely reflect the varied driving conditions under which the vehicles are operated. The paper focuses on the road tests methodology as well as the interpretation of the obtained results. The results of this study is the comparison of the on-road exhaust emissions according to 3 measuring methods: (1) NTE methodology, (2) methodology based on the measurement of work during the test and (3) author’s method: methodology comprising the whole driving tests.

II. METHODOLOGY

The objects of the research were modern city buses: a conventional and a parallel hybrid bus used by the Poznan bus operator. The exhaust emission tests were carried out on a distance of several dozen kilometers on a highly congested bus route. The tests aimed at determining of the unit exhaust emissions of the tested objects and at the same time constituted a basis for the evaluation of the research method determined in the NTE (Not-to-Exceed) [1] on-road tests as per the 582/2011 standard [2] (Fig. 1 and Table I).

During the tests, the exhaust emissions were measured with a portable emission analyzer Semtech DS by Sensors (the equipment is EU 582/2011 and USA CFR 1065 compliant) [3]. A portable Semtech DS was used for the emission measurements. The device allowed a measurement of the fuel consumption and emission level. The central unit of the

analyzer received data directly from the OBD system and a GPS. Using the portable system an emission level of CO, HC, NOx, CO2 with the resolution of 1 second was performed as well as the variations in the engine speed and torque – the parameters obtained from the vehicle OBD (CAN SAE J1939) and then used to calculate the engine unit energy. The recording of the geographical location (GPS) enabled a visualization of the trajectory and the calculation of the length of the road portions [4–7].

Fig. 1. The view of the measuring equipment (exhaust emission measurement unit Semtech DS) fitted in the bus during the tests.

TABLE I TECHNICAL DATA OF THE TESTED BUSES

Parameter Conventional bus (diesel engine)

Hybrid bus (diesel engine + electric engine)

Engine type DAF PR 228 S2 Cummins ISB 250

Displacement [cm3] 9200 6700

Power max [kW@rpm] 231@1900 185@2600

Torque max [Nm@rpm] 1275@1100–1710 1075@1200–1700

Emission standard Euro 5 Euro 5

Transmission VOITH DIWA 86 4.5 ALLISON Ep50

Vehicle weight [kg] 16,700 17,800

III. EMISSION TESTS RESULTS AND ANALYSIS

The authors have carried out in-use exhaust emission tests (CO, HC, NOx and CO2 performed under actual traffic conditions) on a selected bus route on a conventional bus and a hybrid bus. The tests were repeated several times but for the analysis presented in this paper two most representative tests were selected. The obtained data served to determine the unit emission of the said exhaust components calculated according to different research tests. The results of this study is the comparison of the on-road exhaust emissions according to 3 measuring methods: (1) NTE methodology, (2) methodology based on the measurement of work during the test and (3) author’s methodology [8]: methodology comprising the whole driving tests.

1. NTE Method The NTE test, as defined in CFR 86.1370-2007, establishes

an area (NTE control area) under the torque curve of an engine where emissions must not exceed a specified emission cap for a given pollutant (Fig. 2). A controversial issue is the applicability of the NTE limits to the real-world driving. In order for NTE standards to apply, the engine needs to remain within the NTE zone (limits include operation at a minimum of 30% of rated power) for at least 30 seconds. a) b)

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Fig. 2. The course of the combustion engine operation with the NTE test measurement limitations marked: a) conventional bus, b) hybrid bus.

Concerns arose that performing this action could prove to be difficult, as each time the driver removes the foot from the accelerator pedal, or shifts gears on vehicles with manual transmission, the engine leaves the NTE zone (Fig. 3 and Fig. 4).

NTE emissions results are computed for each thirty second period of continuous NTE operation. NTE brake-specific emissions are computed by summing the total mass of emissions in grams and dividing by the total work (kWh) for each of these thirty second periods of continuous NTE operation.

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Fig. 3. Exhaust emission and the course of the bus speeds with the work marked as per the NTE requirements – conventional bus.

The calculated values of the unit emission in the urban drive according to the NTE method were as follows (Fig. 5):

– for a bus fitted with a conventional powertrain CO2 =

= 367 g/kWh, CO = 1.07 g/kWh, NOx = 2.37 g/kWh and HC = 0.13 g/kWh.

– for a bus fitted with a hybrid powertrain CO2 =

= 490 g/kWh, CO = 0.45 g/kWh, NOx = 3.53 g/kWh and HC = 0.07 g/kWh.

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HC [mg/s]

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Fig. 4. Exhaust emission and the course of the bus speeds with the work marked as per the NTE requirements – hybrid bus.

a)

3.67

1.07

2.37

0.130

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CO2

[kg/

kWh]

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/kW

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CO2

[kg/

kWh]

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, NO

x, H

C [g

/kW

h]

Fig. 5. On-road unit exhaust emission (actual traffic conditions) as per the NTE method: a) conventional bus, b) hybrid bus.

During the testing of the conventional bus the time share of the operation meeting the NTE conditions was 22%, but only 3 measurement windows could have been distinguished lasting a bit over 30 s. For the hybrid vehicle the time share was 29% and 10 measurement windows could have been distinguished (the characteristics of the obtained results – Table II).

TABLE II

THE CHARACTERISTICS OF THE OBTAINED RESULTS COMPLIANT WITH THE NTE REQUIREMENTS

Exhaust emissions[g/kWh]

Conventional bus (diesel engine)

Hybrid bus (diesel engine + electric engine)

Avg. NTE

Max NTE

Std. Dev.

Avg. NTE

Max NTE

Std. Dev.

CO2 367.45 371.88 3.724 490.25 498.46 5.024

CO 1.066 2.025 0.701 0.450 0.612 0.095

NOx 2.376 2.677 0.333 3.532 3.622 0.139

HC 0.134 0.155 0.016 0.025 0.032 0.002

2. UE 582/2011 Method The emissions shall be integrated using a moving averaging

window method, based on the reference work. The principle of the calculation is as follows: The mass emissions are not calculated for the complete data set, but for sub-sets of the complete data set, the length of these sub-sets being determined so as to match the engine work measured over the reference laboratory transient cycle. The moving average calculations are conducted with a time increment Δt equal to the data sampling period. These sub-sets used to average the emissions data are referred to as ‘averaging windows’. The duration of the averaging window is determined by: W(Δt) ≥ Wref, where W(Δt) is the engine work measured between the start and time t (in kWh), Wref is the engine work for the WHTC (in kWh).

In order to realize that task the authors needed to estimate the value of work in the WHTC test for the conventional and hybrid buses. In order to determine work in the WHTC test we need to know the course of engine speed and the torque in this cycle. In order to determine the actual engine speeds and loads a denormalization was carried out of the engine operating parameters. Its first stage is the determination of the characteristics of the full power for each of the engines. Based on that, the actual parameters (engine speed and torque) of the engines were calculated, according to the methodology given in regulation 582/2011 (Fig. 6).

The estimated value of work of a conventional engine in the WHTC test was 33.4 kWh, and the engine used in the hybrid vehicle – 28.4 kWh.

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Fig. 6. WSHC cycle, engine speed and torque (after denormalization): a) conventional bus, b) hybrid bus. a)

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[kg/

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Fig. 7. On-road unit exhaust emission (actual traffic conditions) as per the method determined in EU regulation 582/2011: a) conventional bus, b) hybrid bus.

Knowing the reference values of work for the tested vehicles the unit exhaust emission was summed up only in the measurement windows for which the total engine work exceeded the previously given values. For the conventional vehicle the number of all proper measurement windows in the whole test was 872 (the time of duration of the measurement window was a range from 2099 s to 2143 s) and for the hybrid vehicle – 391 (the time of duration of the measurement window was a range from 1956 s do 2334 s). The results presented in Figure 7 are averaged values with the standard deviations marked for each exhaust components.

3. Author’s Method The values of the unit exhaust emissions in the next method

were determined using the data collected during the whole bus test drive (separately for the conventional and hybrid vehicles). These results reflect the actual, in-use operating conditions and at the same time constitute a basis for comparing the ecological nuisance of the city transit. On this basis we can classify various bus routes and the possibilities of replacing of the conventional buses with hybrid ones (Fig. 8). a)

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Fig. 8. The results of the unit emission tests calculated with the use of data recorded while the vehicle was in operation on the bus route: a) conventional bus, b) hybrid bus.

IV. CONCLUSIONS

Having performed the analysis the authors obtained varied values for individual exhaust components depending on the applied method of calculation (Fig. 9):

1) for conventional bus: – CO2: 364 g/kWh (at variability of 353–372 g/kWh), – CO: 1.62 g/kWh (at variability of 1.07–1.98 g/kWh), – NOx: 2.44 g/kWh (at variability of . 2.37–2.54 g/kWh), – HC: 0.16 g/kWh (at variability of 0.13–0.18 g/kWh), 2) for hybrid bus – CO2: 508 g/kWh (at variability of 490–519 g/kWh), – CO: 1.18 g/kWh (at variability of 0.45–1.55 g/kWh), – NOx: 4.07 g/kWh (at variability of 3.53–4.37 g/kWh), – HC: 0.05 g/kWh (at variability of 0.04–0.07 g/kWh).

a)

3.67 3.723.53

1.07

1.98 1.822.37 2.54 2.42

0.13 0.18 0.170

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CO2 CO NOx HC

CO2

[kg/

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, NO

x, H

C [g

/kW

h]

1 - NTE method2 - UE 582/2011 method3 - Authors method

b)

4.95.19 5.17

0.45

1.55 1.53

3.53

4.37 4.33

0.07 0.05 0.040

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CO2 CO NOx HC

CO2

[kg/

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; CO

, NO

x, H

C [g

/kW

h]

1 - NTE method2 - UE 582/2011 method3 - Authors method

Fig. 9. The values of unit emissions obtained during the on-road tests and with different calculation methods: a) conventional bus, b) hybrid bus: 1 – NTE methodology, 2 – methodology based on the measurement of work during the test, 3 – author’s method: methodology comprising the whole driving tests.

The highest values of the unit emission were obtained according to the NTE calculation method, and the lowest according to the method using all the data recorded during the whole driving test.

Comparing the obtained test results of the unit emissions for the investigated buses we need to confirm great similarity between the obtained results. It is, however difficult to point to the method that fits the comparative criterion most when measuring the exhaust emissions with the portable emissions analyzers (PEMS). The method determined by NTE is very good for heavy-duty vehicles operating at high loads in the expressway cycle. The methodology introduced by the EU is based on values that need to be known prior to developing the tests (e.g. reference work in the WHSC test).

ACKNOWLEDGMENT

This work has been financed from the resources for science for the years 2009-2012 as a research project No. NR 10 0004 06.

REFERENCES [1] Regulations Commission: “Regulation (EU) No 582/2011 of 25 May

2011 implementing and amending Regulation (EC) No 595/2009 of the European Parliament and of the Council with respect to emissions from heavy duty vehicles (Euro VI) and amending Annexes I and III to Directive 2007/46/EC of the European Parliament and of the Council,” 2011.

[2] United States Environmental Protection Agency, “Control of Emissions of Air Pollution from 2004 and Later Model Year Heavy-Duty Highway Engines and Vehicles; Revision of Light-duty Truck Definition,” 40 CFR Parts 85 and 86, 1999.

[3] T. Bougher, I.A. Khalek, S. Trevitz and M. Akard, “Verification of a Gaseous Portable Emissions Measurement System with a Laboratory System Using the Code of Federal Regulations Part 1065,” SAE Technical Paper Series 2010-01-1069, 2010.

[4] H. Jehlik, Challenge X 2008 – Hybrid Powered Vehicle On-Road Emissions Findings and Optimization Techniques: A 4 Year Summary, Sensors 5th Annual SUN (SEMTECH User Network) Conference, September 2008.

[5] P. Bonnel, M. Weiss and A. Provenza, In-use Emissions Requirements in the New and Future European Motor Vehicle Emissions Regulations: State of Play, In: 8th Annual SUN Conference, Ann Arbor 2011.

[6] K.C. Johnson, T.D. Durbin, D.R. Cocker, J.W. Miller, R.J. Agama and N. Moynahan, “On-Road Evaluation of a PEMS for Measuring Gaseous In-Use Emissions from a Heavy-Duty Diesel Vehicle,” SAE Technical Paper Series 2008-01-1300, 2008.

[7] L. Rubino, P. Bonnel, R. Hummel, A. Krasenbrink, U. Manfredi, G. De Santi, M. Perotti and G. Bomba, “PEMS Light Duty Vehicles Application: Experiences in downtown Milan,” SAE Technical Paper Series 2007-24-0113, 2007.

[8] J. Merkisz and J. Pielecha, “The On-Road Exhaust Emissions Characteristics of SUV Vehicles Fitted with Diesel Engines,” Combustion Engines, No. 2, 2011.