on board emission and fuel consumption measurement campaign on petrol-driven passenger cars

Pergamon Atmospheric Environment Vol. 31, No. 22, pp. 3753-3761, 1997 © 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain P I I : S1352-2310(97)00212--4 1352-2310/97 $17.00 + 0.00

ON-BOARD EMISSION AND FUEL CONSUMPTION MEASUREMENT CAMPAIGN ON PETROL-DRIVEN

PASSENGER CARS

I. DE V L I E G E R Flemish Institute for Technological Research (VITO), 200 Boeretang, 2400 Mol, Belgium

(First received 8 November 1996 and in final form 17 April 1997. Published August 1997)

Abstract--Realistic emission and fuel consumption rates of petrol-driven cars were determined by on-the- road experiments in 1995. A validated, in-house developed, on-board measuring system was used. Six three-way catalyst (TWC) cars and one carburetted non-catalyst car were measured. The effects of road type, driving behaviour and cold start on CO, HC and NOx emissions and fuel consumption were analysed. In real traffic situations, emissions for TWC cars were found to be at least 70% lower than for the non-catalyst car. For TWC cars, emissions decreased across the board from city to rural and motorway traffic. Without a catalyst, motorway traffic resulted in the highest NOx emissions. Compared to normal driving, aggressive driving gave emissions which were up to four times higher. Except for NOx, calm driving resulted in lower emissions still. Comparable fuel consumption rates were obtained from normal and calm driving. Those from aggressive driving were higher, by as much as 40% in city traffic. Cold starts resulted in significantly higher CO and HC emission values than hot starts. These differences were less pronounced for NO=,. Emissions from TWC cars were higher than generally expected, compared to the European emission limit values (91/441/EEC) and the emission factors used in Flanders and the Netherlands (Klein,1993) for the national emission inventories. Low-emitting cars during the emission test on a chassis dynamometer, as prescribed by the 91/441/EEC directive, did not necessarily give low emissions in real traffic situations. © 1997 Elsevier Science Ltd.

Key word index: Realistic emissions, fuel consumption, on-the-road measurements, driving behaviour, cold start.

l. INTRODUCTION

Exhaust emissions from new passenger cars are regulated within the European Union by directives published in the Official Journal of the European Communities. Limit values are set for carbon monox- ide (CO), total hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter. Type approval tests of this kind are performed on a chassis dynamometer under standardized conditions in laboratories, because a common basis is essential for comparing emission results. If the aim is to obtain realistic emission figures, these tests, though reproducible, are not repre- sentative of more dynamic real traffic conditions (Kruse and Huls, 1973; Ashbaugh and Lawson, 1991; Fujita et al., 1992; Nelson and Groblicki, 1993; Pis- chinger, 1993; Kirchstetter et al., 1996). This was also borne out by the results of the present investigation. Furthermore, in standard laboratory tests little or no regard is paid to driving behaviour, driving conditions, roadworthiness, etc.

On-board testing under real traffic conditions offers a suitable approach for determining realistic emission rates, yet it has to be seen as being complementary to emission laboratory tests (Staab et al., 1989). Two

on-board measurement systems were built and validated by the Flemish Institute for Technological Research (VITO) (De Vlieger et al., 1994; Lenaers, 1994, 1996). Regulated gaseous emissions (CO, HC and NOx) and fuel consumption are analysed second per second in real traffic situations. The system is used to assess vehicle technologies, fuels and traffic management systems for their environmental impact and energy consumption. In the following, the on-board system is described briefly and the results of measurements on seven petrol-driven cars are discussed. As early three-way catalyst (TWC) technologies have al- ready been evaluated under real traffic conditions (Savage, 1989; Staab et al., 1989; Grauer and Baum- bach, 1993), the emphasis, in our research, was put on latest technology TWC cars.

2. EXPERIMENTAL ASPECTS

2.1. On-board measurement system

VITO's on-board system is called "VOEM", VITO's On- the-road Emission and Energy Measurement system. The block diagram of this system is shown in Fig. 1. It consists of the sampling system of exhaust gases, the gas analysers, the

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measurement of the fuel consumption, vehicle speed, lambda value, the power supply and the data acquisition and auto- mated data processing system. The system is based on a new methodology (Lenaers, 1993, 1996) which sets it apart from its predecessors (Potter and Savage, 1982; Staab and Schiir- mann, 1987; Nelson and Groblicki, 1993). Basically, the emission concentration measurements are combined with the total exhaust gas mass flow which is calculated from the fuel consumption and lambda value determination.

The sampling system dilutes the exhaust gas at a constant rate to prevent condensation of water. An ejector pump, driven by nitrogen, continuously draws sample gas and de- livers it to the emission monitors. Condensat ion of heavy hydrocarbons (diesel vehicles) is prevented by a high-temperature heating (190°C) system.

The measurement principles of the gas analysers are the same as those normally used in vehicle emission laboratories. Carbon dioxide (COz) and CO contents in the exhaust gases are measured by non-dispersive infraRed (NDIR) equipment. The total HC content is analysed by a Flame Ionization Detector (FID), and the NOx by a Chemilumine- scence Analyser (CLA).

A dedicated piece of apparatus, the P L U 401-108, is used to measure the fuel consumption. It consists of a volumetric sensor and a support system. The sensor is accurate to within 1% in the range of 0.5-60/ ' h - 1. The measured volume ftow together with the fuel density yields the mass flow. Accurate determination of speed and distance travelled is realized with an optical device. The power supply, from a 12 V battery, is divided into 12 V DC and 220 V AC and is delivered by a DC-AC inverter.

The data acquisition processor and data processing system, a laptop PC, handles on-line collection and real-time processing of the measured data. Accurate emission values can be determined for CO, HC, NO~ and CO 2 in g s - 1 and g k m - 1 . The accuracies were estimated by comparative emission measurements on a chassis dynamometer (Lenaers, 1993, 1994). All errors were found to be below 10%, except- ing 20% for CO and 25% for HC in the case of a diesel engine.

The effective surplus weight of a car equipped with VOEM is roughly equal to that of two persons. This on-board measurement system can be fitted into any vehicle.

2.2. Test pro#ramme

2.2.1. Tested ~,ehicles. Seven popular, small or medium- sized petrol-driven passenger cars were tested as received. Six of them were common types of high-tech cars, equipped with a fuel injection system and a closed-loop controlled three- way catalyst (TWC). Additionally, one older car without a catalyst and equipped with a carburettor was measured. Selection of these cars was based on information supplied by the Belgian Institute of Statistics and the car importers. Car details are given in Table 1. Only one vehicle has been measured for each model. The purpose of this study was to determine realistic emission levels and fuel consumption rates and not to rank models as such. Consequently, the results shown are not related to specific makes or types in the following.

2.2.2. Road types. Three types were considered: urban driving, rural driving and motorway driving. The measurements for urban driving were performed in a small town, Turnhout , not far from Antwerp. The total distance of this cross-town journey was 9 km. Emissions and fuel consumption data were determined starting with either a warm or a cold engine. The total distance of the rural and the motorway journeys was 11.5 and 26 km, respectively. These tests were performed starting with a warm engine only.

2,2.3. Driving behaviour. The effects on the exhaust emissions were investigated for three types of driving behaviour: calm, normal and aggressive ( = sporty). Calm driving means anticipating other road users' movements and avoid- ing sudden acceleration. By aggressive driving is meant sudden acceleration and heavy braking. Normal driving implies moderate acceleration and braking. Note that during all tests the national city (50 km h-1), rural (90 km h-1) and motorway (120 km h - i ) speed limits were observed.

Emission and fuel consumption measurement campaign

Table 1. Test pool of petrol-driven passenger cars

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Cylinder capacity Max. power Mileage Year of Model (cc) (kW) TWC (km) manufacture

Ford Escort 1299 43 yes 5607 1993 Toyota Corolla 1332 65 yes 469 1995 Opel Corsa 1195 33 yes 844 1995 Opel Astra 1389 44 yes 11,640 1995 Renault Clio 1171 43 yes 115 1995 Volkswagen Golf 1781 66 yes 3321 1995 Peugeot 205 1360 55 no 77,780 1992

TWC = three-way catalyst.

Table 2. Average measured emission factors (VITO) in g km- 1 of three-way catalyst cars under normal and aggressive driving conditions. Comparison

with the emission factors used in the Netherlands (NL)

Aggressive driving Normal driving

Pollutant Road type EF VITO EF VITO EF NL

CO City CS 27.9 -I- 8.6 15.1 + 4.5 7.7 City HS 14.8 _+ 6.8 7.2 _ 5.0 - - Rural 11.8 + 6.9 4.5 + 3.4 0.7 Motorway - - 2.2 + 1.5 0.5

HC City CS 3.7 + 1.2 2.2 _ 1.1 0.74 City HS 0.93 + 0.65 1.1 + 1.0 - - Rural 0.63 + 0.38 0.54 ___ 0.50 0.09 Motorway - - 0.15 + 0.11 0.08

NOx City CS 0.54 + 0.21 0.32 + 0.20 0.29 City HS 0.34 + 0.18 0.25 ___ 0.20 - - Rural 0.21 + 0.13 0.18 + 0.15 0.18 Motorway - - 0.14 _ 0.10 0.47

EF = emission factor; CS = cold start; HS = hot start.

2.2.4. Measurements. CO, HC, NOx and CO 2 emission values and fuel consumption rates were measured for each car. For each combination of car/road type/driving behaviour, two to four tests were performed. All test cars were driven under normal driving conditions, six cars under aggressive driving conditions and four under calm driving conditions. Highly polluting cars were also tested on a chassis dynamometer as prescribed by the 91/441/EEC directive. These tests were performed at the TNO Road Vehicle Re- search Institute (Delft, the Netherlands).

3. RESULTS AND DISCUSSION

3.1. Emissions from petrol-driven cars fitted with a catalyst

3.1.1. Normal driving behaviour. Table 2 shows the average measured CO, HC and NOx emissions for T W C cars under normal and aggressive driving conditions, together with the standard deviation. The average values of all the experimental emission data from the 6 tested T W C cars are given.

The CO, H C and NOx emissions for T W C cars decreased according to road type in the following order: city cold start, city hot start, rural road and motorway. The steadier the traffic flow was, the lower the emissions. The dynamic behaviour of the emission

control system resulted in high emissions under tran- sient conditions. This was especially the case for CO and HC, but less pronounced for NOx. These findings show good agreement with the results of Hansen et al. (1994), who analysed emissions from a catalyst vehicle fleet as a function of speed and speed deviation.

City journeys begun with a cold engine and a cold T W C instead of a hot engine and a warmed-up T W C resulted in much higher CO and HC emissions. For NOx the cold-hot start differences were considerably smaller, as perceived by Laurikko (1994).

To illustrate the scatter of the measured emission data, measured CO, H C and NOx emission rates for the rural journey are shown in Fig. 2 for each car. The emissions per kilometre travelled may differ substan- tially from car to car. Also the scatter of emission values for a specific test vehicle may differ widely. Apart from the fourth T W C car, H C emission values for rural driving were fairly comparable. As far as CO and NOx were concerned, the differences between cars were more pronounced.

3.1.2. Effects of aogressive and calm driving. Table 2 shows the average measured CO, H C and NOx emissions for T W C cars for aggressive driving, together with the standard deviation. The average

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values of all the experimental emission data from the 5 TWC cars tested are given. As the sixth TWC car was the only one tested in motorway traffic for aggressive driving behaviour, emission results are not given for this traffic situation. To illustrate the effect of aggressive and calm driving behaviour, Fig. 3 shows the CO, HC and NOx emission results for the sixth TWC ear.

In common with normal driving, emissions for aggressive and calm driving decreased from a city cold start over rural to motorway traffic. Calm driving with a hot engine and a warmed-up catalyst gave small differences in CO and HC emissions for the different road types.

Aggressive driving resulted in averaged measured CO emission factors up to three times higher than those for normal driving. In the case of HC and NOx, emission factors were up to two times higher. For

motorway driving the differences between normal and aggressive driving were much smaller (5 to 20%). Surprisingly, for some cars normal driving in city traffic with a hot start resulted in a slightly higher HC emission than for aggressive driving.

Emissions for calm driving were always lower than for aggressive dr iving-up to a factor of 10 lower in some cases. As to CO and HC, the emissions were also significantly lower than those for normal driving. NOx emissions were comparable or higher depending on the vehicle tested.

The above results show that emission rates were subject to how dynamically the car was driven. This did not depend solely on road type, driving behaviour was important too. Maximum and average values of acceleration are ideal parameters for defining the dynamics of journeys and driving behaviour. Ranges for average accelerations on city journeys, as defined

Emission and fuel consumption measurement campaign 3757

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in the measurement campaign, were: 0.45-0.65 m s-2 for calm driving, 0.65-0.80ms -2 for normal driving and 0.85-1.10 m s-2 for aggressive driving. Max- imum accelerations in city traffic were 2.2, 2.6 and 3.0 m s- 2, respectively. Aggressive driving resulted in the most dynamic driving of all. For motorway traffic, average accelerations only ranged from 0.08 to 0.20 m s -2 .

In the Netherlands, Van den Beukel et al. (1993) studied the sensitivity to driving behaviour of exhaust emissions. Assuming that all today's motorists have normal driving behaviour, HC and NO~ emissions of the passenger car fleet could be reduced by 22 and 49%, respectively, if motorists reverted to calm driving behaviour. The HC and NO~ levels could be

raised by 14 and 49%, respectively, by a switch to aggressive driving. The NOx gain through calm driving appears to be more pronounced compared to the VITO results. Still, the Belgian and Dutch studies clearly demonstrate that driving behaviour is a key parameter when estimating emissions from the passenger car fleet.

3.1.3. Comparison with emission values in the Neth- erlands. At present, no site-specific emission factors are available for Belgium and Flanders. In Flanders, emission values from the Netherlands (Klein, 1993) are used, as the climatological and geographical conditions in both regions are comparable. The emission values in the Netherlands are the result of many emission measurements conducted on a chassis

3758 I. DE VLIEGER

dynamometer with simulated driving cycles, i.e. no real traffic situations have been analysed.

Table 2 shows the differences between VITO's average measured CO, HC and NOx emission factors and those of the Netherlands under normal driving conditions for all road types. For city traffic the Nether- lands assume 50% of the trip length to be cold start, whereas VITO included only 20%. For rural and motorway traffic no cold starts were included in VITO's factors, but 8 and 5%, respectively, in those of the Netherlands. Although V1TO's emission factors include less cold start they are generally higher than those of the Netherlands.

For normal driving, VITO's average measured CO and HC emission factors (six cars) were 2 to 6 times higher than those of the Netherlands. NOx values were comparable in urban and rural traffic, and even 3 times lower in motorway traffic compared to those of the Netherlands. As far as individual cars were concerned, the differences were not always that pronounced. More information on these topics can be found in the literature (De Vlieger and Lenaers, 1995 and 1996).

For aggressive driving the measured CO and HC emissions factors were at least four times higher. For calm driving the measured CO and HC factors were comparable to the emission values currently used for estimating road transport emissions in the Nether- lands and Flanders.

3.1.4. Comparison with 91/441/EEC limit values. These values are used to secure the emission approval for new passenger cars (Table 3). Besides the limit values, the test procedure is also laid down by law. Cars are preconditioned and driven for a standard driving cycle on a chassis dynamometer. This cycle comprises a stretch of urban driving and non-urban driving, and is assumed to represent a typical Euro- pean journey (Official Journal of the European Com- munities, 1983; McArragher et al., 1994). New cars pass the emission type approval tests if the limit values for exhaust emissions (including durability testing) and the limits for evaporative emissions (included as from 1993) are met.

For reasons of comparison with the limit values for exhaust emissions, the sum of VITO's measured HC and NOx emissions was considered. Furthermore, the weighted average emissions over the three road types were considered. For Belgium, the respective propor-

tions of driving distance in city, rural and motorway traffic were assumed to be 27, 49 and 24% (Eggleston et al., 1993). In city traffic, it was held that 50% of the distance driven is with a cold engine and a cold TWC. For normal driving this resulted in a weighted average emission factor of 12.0 g k m - ~ for CO and 2.2 gkm- for HC + NO~. This is 4 and 2 times higher, respectively, than the limit values. For aggressive driving, the limit values were exceeded by a factor of 7 and 3, respectively. For calm driving the limit values were met.

The above differences arise from the definition of the European test cycle and the preconditioning of the vehicles. This test cycle is less dynamic than journeys made in realistic traffic situations. For example, maximum accelerations in rural traffic varied between 2 and 3 m s- 2 depending on the driver, this was only 0.83 m s 2 fog the European non-urban test. In the measurement campaign, cold start tests were carried out at a temperature of 0-15°C, whereas this is 20 to 30~C in the European test.

3.1.5. Monitoring of highly polluting cars. The first and fourth TWC cars were found to be highly polluting during the on-the-road tests (Fig. 2}. These cars were put through an emission test on a chassis dynamometer in accordance with the 91/441/EEC directive. The results of these comparable emission tests are given in Table 3. One can conclude that new cars that gave low emissions during the emission approval. are not necessarily low emission cars in real traffic situations.

3.2. Emissions lrom a petrol-driven car without a catalyst

3.2.1. Normal driving behaviour. Table 4 shows that non-catalyst cars gave the highest CO and HC emissions in city traffic and the lowest on the motorway. High CO and HC emissions are due to incomplete combustion. The reverse was obtained for NOx. High NOx emissions are due to a high flame temperature in the engine, which arises during fast driving.

3.2.2. E~bct of drir'in9 behaviour. Table 4 shows that CO and HC emissions in city and rural traffic were greatly influenced (up to a factor of 4) by driving behaviour. NO~ emissions, on the other hand, were rather comparable, in the main, for all types of driving behaviour.

Table 3. Exhaust emissions during the chassis dynamometer test (91/441/EEC) for big polluters ingkm -~

Chassis dynamometer test 91/441/EEC directive"

Pollutant Car 1 Car 4 Limit values Production conformity

CO 3.14 1.45 2.72 3.16 HC + NO:, 0.72 0.53 0..97 1.13

This directive was in force from 01.01.93 until 31.12.96: limit value for particulate matter = 0.14gkm -1

Table 4. Average measured emission factors non-catalyst car in g km- 1

Driving behaviour

Pollutant Road type Calm Normal Sporty

CO City CS 41.0 47.4 77.1 City HS 27.5 40.8 72.0 Rural 8.5 14.7 36.2 Motorway 5.2 11.3 11.9

HC City CS 9.3 9.4 35.4 City HS 4.1 6.3 28.2 Rural 2.8 3.2 10.4 Motorway 1.1 1.8 2.2

NOx City CS 1.9 2.0 3.3 City HS 1.7 1.6 2.1 Rural 2.9 2.6 2.5 Motorway 3.5 3.5 3.6

CS = cold start, HS = hot start.

3.2.3. Comparison between petrol-driven cars "with" and "without" a catalyst. One can see from Tables 2 and 4 that in real traffic situations CO, HC and NO~ emissions of TWC cars were much lower than those of the carburetted non-catalyst car tested. The CO and HC emissions of TWC cars were about 75 to 80% lower than the emissions of the non-catalyst car; the NO~ emission was 90% lower. Emission reductions for cold start city journeys were smaller: 70 to 75% for CO and HC, and 80% for NO=,. In optimal conditions - e.g. no TWC failure, a hot engine and a warmed up TWC-- the emission reductions are expected to be 90% for CO, HC and NOx (Haskew

m m s p e e d calm DB i J speed normal DB ......... speed sporty DB

.~" fuel calm DB --C]--fuel normal DB A fuel sporty DB

and Liberty, 1991; Sjrdin, 1994). Except for hot start NO~ emissions, optimal emission reductions were not reached under real traffic conditions. It should be noted that only one non-catalyst car was measured. Its results may not be generalized for all non-catalyst cars, so the above percentages could be different if more cars had been involved in the measurement campaign.

3.3. Fuel consumption

All the cars were fuelled with unleaded petrol. Fuel consumption rates for both catalyst and non-catalyst cars exceeded 101 per 100 kilometre (10 / 100 km- 1) in city traffic. For rural and motorway traffic these rates were some 30-40% lower. The measured fuel consumption rates were 10 to 20% higher compared to values obtained from motorists surveys (Testaan- koop, 1993, 1994, 1995; Leyrer, 1995). These differences are explained by the surplus weight due to the VOEM system.

Figure 4 shows that fuel consumption rates decreased from city (cold start) to motorway traffic for all types of driving behaviour. Considering driving behaviour separately, a marked rise in fuel consumption (20 to 40%) for aggressive driving is shown, with the exception of motorway traffic (only 7% higher). Calm driving results in a small decrease (5%) in fuel consumption compared to normal driving.

Although the amount of fuel consumed in city traffic increased as driving behaviour went from calm through normal to aggressive, the average trip speed, including stops, remained almost the same. For rural

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3760 1. DE VLIEGER

traffic, however, there was a significant gain in average speed.

3.4. Cold phase emissions

Cold phase stands for the period needed to warm up the catalyst and/or the engine on cold start journeys. During this phase, exhaust emissions can be very high. In non-catalyst cars the high emissions are due to the fuel/air enrichment for a cold start. Combustion can only be initiated when the petrol has evaporated. The key to the cold start process is the ratio between the air and the petrol in the gaseous phase. The overall fuel/air supply ratio is of secondary impor- tance, because much of the liquid petrol will not evaporate in a cold engine. On TWC cars, enrichment and catalyst warm-up are responsible for the high emissions during the cold phase. The efficiency of emission reduction is low during the warm-up time of the catalyst.

In the case of the carburetted non-catalyst car in city traffic, the cold phase lasted for 130 to 180 s. The corresponding emission values for normal driving behaviour were 217 g CO kin- ~, 53 g HC k m - t and 1.2 NO~ g k in - t. These emissions were 4 to 6 times higher than the hot start emissions. For NO~, the cold phase emission rates were comparable or even lower than those for a hot start (hot engine).

On TWC cars the cold phase lasted from 130 to 280 seconds depending on the car tested. The corresponding driven distance was 1 to 2 kin. The average measured emission value (six cars) during the cold phase for normal driving behaviour was 62 g of CO k m - t, 10 g of HC k m - 1 and 1.4 g of NO~ k m - ~. CO and HC emissions were 4 to 40 times higher than emissions with a hot start (a hot engine + a warmed- up TWC). Differences between the cold phase and hot start NO~ emissions were not always that pronounced. Sometimes they were comparable, sometimes the cold phase NOx emission was a factor 8 higher.

Also during the cold phase, absolute CO and HC emission levels for the carburetted car were much higher than on a TWC car. Cold phase NO~ emission rates were comparable for both technologies. Never- theless, one should realize that cold phase CO, HC and NO~ emission rates from T W C cars remain a major source of pollution.

4. CONCLUSION AND SUMMARY

In 1995 an on-board measurement campaign on 6 three-way catalyst (TWC) cars and 1 non-catalyst carburetted petrol-driven car was conducted, using an in-house developed device. Emissions and fuel consumption were determined while physically driving in urban, rural and motorway traffic under calm, normal and aggressive driving conditions.

CO, HC and NO~ emissions of TWC cars were found to be much lower at least 70% - than those of

the carburetted non-catalyst car. One should note however that cold phase CO, HC and NOx emission rates from TWC cars remain a major source of pollution.

Emissions obtained from aggressive driving in urban and rural traffic were up to four times higher than those obtained from normal driving. Fuel consumption increased by 30 to 40%. In case of cahn driving. the emissions were always lower than those for aggressive driving. On TWC cars, emissions could be lower by a factor of 10. Except for NOx, the emissions were also significantly lower than for normal driving. Calm driving merely resulted in a limited decrease in fuel consumption compared to normal driving.

Apart from NO,, VITO's measured emission factors for TWC cars under normal driving conditions were higher than the emission factors reported in the Netherlands. The measured CO and HC emissions were 2 to 6 times higher. When compared to the limit values of the 91/441/EEC directive, the CO emission under normal driving conditions was four times higher and the sum of HC + NOx was twice the limit value. For calm driving the limit values were met, for aggressive driving the limit values were grossly exceeded.

Comparat ive emission tests were performed on a chassis dynamometer in accordance with the 91/441/EEC directive. Results showed that new cars which produced low emissions during the emission approval, are not necessarily tow emission cars in real traffic conditions.

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