Adaptation of two small European cars for lower emission standards

The Science of the Total Environment, 93 (1990) 215-222 Elsevier

215

ADAPTATION OF TWO SMALL EUROPEANCARSFOR LOWEREMISSIONSTANDARDS R.C. RIJKEBOER and A. DE VOOGD TNO Road-Vehicles Research I n s t i t u t e , P.O. Box 237, 2600 AE Delft (The Netherlands)

SUMMARY An investigation was made into the low-emission capabilities of existing engine designs of engines with a swept volume of about 1 l i t r e . No changes were made to the engines themselves. The engines were f i t t e d with improved mixture formation and i g n i t i o n . This led to an improvement in the emissions of CO and NOx. To control the emission of HC an oxidation catalyst was needed. This could be a cheap metal-supported minicatalyst. INTRODUCTION In the recent discussions on future emission standards for small European cars the relation between cost and effectiveness played a large part. Vehicles with an engine swept volume below 1.4 l i t r e are often low budget vehicles and costly antipollution technology would seriously affect t h e i r economics. On the other hand they constitute about 60% of the European f l e e t and cause about 50% of the emissions. In the discussions there was an urgent need for more information on the p o s s i b i l i t i e s of available antipollution technologies other than closed-loop threeway catalysts. For this reason the Dutch Ministry of Environmental Protection asked TNO to convert two European cars to low emission vehicles and measure t h e i r emission performance. EXPERIMENTAL PROGRAMME Choice of vehicles Since the economics of antipollution technology become more important when the car gets smaller i t was decided to aim at vehicles of about I l i t r e engine swept volume. One vehicle was chosen with a very modern engine design (vehicle I) and for the other a somewhat older engine design was selected (vehicle 2). The major parameters of these engines are shown in Table 1. The engines were mounted in identical vehicles. Strategy An important condition of the project was that nothing should be changed to the engine i t s e l f . The carburettors would be exchanged for t h r o t t l e body fuel injection (TBI) and the standard i g n i t i o n would be exchanged for a f u l l y elec-

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TABLE 1: Engine and vehicle data Vehicle

1

Swept volume Compression r a t i o Combustion chamber Fuelling

2

999 cm3 9.8 : I bath tub single port carburettor conventional X = 1.00 - 1.05 800 kg

Ignition Air/fuel ratio Equivalent i n e r t i a

1116 cm3 9.2 : 1 wedge dual port carburettor conventional X = 0.95 - 1.05 910 kg

tronic i g n i t i o n with mapped advance. Emission reduction would be attempted through the use of leaner mixtures and possibly EGR, in combination with a suitable timing strategy. The use of an oxidation catalyst to control HC-emission was judged acceptable. A major aim was to achieve the emission reduction without any fuel consumption penalty. This was a p a r t i c u l a r challenge in the case of the modern design 1.0 l i t r e engine (vehicle 1) that had been f u l l y consumption optimised by the manufacturer. The catalysts used had a ceramic substrate and a platinum/palladium loading. As an alternative a so-called minicatalyst was used on vehicle 2. This had a metal substrate and a platinum/rhodium loading; i t s size was 1/3 that of the f u l l - s i z e catalyst. Approach The f i r s t a c t i v i t y was the optimisation of the i n s t a l l a t i o n and setting of the TBI-units. For the connection of the TBI to the i n l e t manifold several a l ternatives were t r i e d . Further optimisation was achieved by varying the injec-

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1: Mixture d i s t r i b u t i o n at 5000 rpm, 80% load (vehicle 2) and 35 km/h (vehicle I ) . The lines represent the lambda deviation of each cylinder r e l a t i v e to the mean lambda.

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Fig. 3: Full load torque and specific fuel consumption, standard and converted (vehicle 1). tion timing. In Figure la the mixture d i s t r i b u t i o n is shown with the i n i t i a l and f i n a l version, compared to that of the carburettor. This was done at f u l l speed and 80% load, since that is the most c r i t i c a l condition. In Figure lb the resulting mixture d i s t r i b u t i o n is shown for the 35 km/h load point. In fact 11 such load points ("key-points") were checked. When the i n s t a l l a t i o n of the TBI had been f i n a l i s e d , the f u e l l i n g and i g n i t i o n timing over the f u l l load curve was determined. This had to take account of the fact that unleaded petrol (RON 96) had to be used instead of leaded petrol (RON 98) without any change in compression r a t i o . This resulted in a s l i g h t retarding from MBT (Fig. 2). But the maximum performance hardly suffered and the fuel consumption improved because of the lower fuel

excess with the TBI (Fig.

3). As the next step the mixture

218

strength and i g n i t i o n timing were selected f o r each " k e y - p o i n t " . On engine 1 the influence of EGR was also studied. Figure 4 shows the results f o r one key-point. A f t e r optimisation of the engines on the engine test-bed they were i n s t a l l e d in the vehicles and optimised f o r t r a n s i e n t behaviour. This was done with exhaust systems f i t t e d with "dummy" c a t a l y s t s . D r i v e a b i l i t y was r e g u l a r l y checked on the road. Only a f t e r f u l l optimisation the f u l l

size active catalysts were i n s t a l l e d

and the f i n a l emission performance was determined. The vehicles were tested over the ECE-15 urban cycle (both c o l d s t a r t and h o t s t a r t ) and at 90 km/h and 120 km/h in both 4th and 5th gear. In the tables the f i g u r e for the highspeed r e s u l t is the mathematical average of the four highspeed points. 4.00 NOx,g/h

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Fig. 4: NOx versus fuel consumption trade-off for various approaches, on the engine test-bed and in the vehicle at 90 km/h in 4th gear (vehicle 1).

RESULTS L~an mixture settin 9 The use of a TBl-system in combination with leaner mixtures resulted in a considerably lower emission of CO in the urban cycle and of NOx at the highspeed points (Fig. 5). The emission of HC was not or hardly improved with vehicle 1 (all

conditions) and in the coldstart urban cycle of vehicle 2. Addition of the

oxidation catalyst improved CO by another 60%, and brought HC down to acceptable levels (75-80% reduction) in the coldstart urban cycle. The emission of NOx was of course not affected, In the hotstart tests hardly any CO or HC remained, as could be expected. The d r i v e a b i l i t y of the f i n a l version was judged acceptable; this notwithstanding the fact that i t had not been possible to optimise the inl e t manifold for better mixture d i s t r i b u t i o n . Even with the standard manifold the engine accepted 30% a i r excess without any protest. The fuel consumption had

219

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Fig. 5: Emissions, standard and converted, for the coldstart and hotstart urban d r i v i n g cycle (UDC) and a combination of highspeed conditions (HS): vehicle I (top) and 2 (bottom). not deteriorated, neither in the coldstart or hotstart urban cycle, nor in any of the highspeed points. The use of the TBI had in fact resulted in a slight improvement of consumption at the setting of highest efficiency (up to about 3% improvement), but this margin had been "invested" in lowering the NOx-emission. From Figure 4 i t can be seen that a consumption benefit in the order of 5% is possible relative to a stoichiometric mixture ratio as would be used with a closed-loop threeway catalyst. The use of the minicatalyst on vehicle 2 resulted in s l i g h t l y higher emissions of CO and HC, especially in the coldstart test (see Table 2). The emission results of vehicle 1 were reproduced with a carburettor when this was tuned for similar mixture r a t i o ' s , but the d r i v e a b i l i t y was less, due to less optimal mixture formation. Experiments with EGR On vehicle 1 the use of EGR was t r i e d as an alternative. Figure 4 shows the effect of EGR at a stoichiometric mixture and at 20% a i r excess for one keypoint. The use of EGR at a stoichiometric mixture decreased the emission of NOx without any appreciable effect on fuel consumption, whereas leaning does decrease the consumption. At 20% a i r excess the use of EGR has an effect that is

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TABLE 2: Emission results of vehicle 2 with f u l l - s i z e catalyst and minicatalyst in g/km

coldstart urban cycle hotstart urban cycle highspeed conditions

f u l l size catalyst

minicatalyst

CO

CO

HC

NOx

1.75 0.35 0.85 0.05 0 . 0 2 0.65 0.01 0 . 0 2 1.80

HC

NOx

2.90 0 . 6 5 0.85 0.15 0 . 1 0 0.65

equivalent to further leaning. At very low EGR-rates EGR is more effective than further a i r

excess, but higher rates (the 8-10%) are equivalent to the same

amount of extra a i r .

This meant that generally speaking similar results were

obtained with moderately lean mixtures plus EGR as with more lean mixtures. Only at 120 km/h EGR was more effective. At that condition the engine already needs so much a i r that only very reduced leaning of the mixture is possible. In that case a limited amount of EGR is more effective in reducing NOx than the same amount of excess a i r .

Results in the extra-urban d r i v i n 9 cycle At the time of the original conversion experiments there was no highspeed driving cycle. So the emissions at 90 km/h and 120 km/h were determined in 4th and 5th gear. Since then an extra-urban driving cycle (EUDC) was agreed in the EEC. When this agreement had been reached the experimental vehicles were tested over t h i s cycle. But no baseline figures for the EUDC are available since the original vehicles were no longer available. The version now on the market in the Netherlands is a so-called $6 version; t h i s is a version meeting 88/76/EEC minus 15% as required for the Dutch tax incentive scheme. Another version marketed is f i t t e d with a 1.5 l i t r e

engine and closed-loop threeway catalyst. Both these

versions were tested over the coldstart UDC and the EUDC. In Table 3 they are comparedto vehicle I with EGR and catalyst and vehicle 2 with lean mixture and f u l l - s i z e catalyst. The figures are given for the colds t a r t UDC and the combination of coldstart UDC plus EUDC. As can be seen the converted vehicles compare well on CO and HC, even in relation to the vehicle TABLE 3: Emission results from the coldstart urban cycle and the combination of coldstart urban cycle plus extra-urban cycle ( f u l l cycle) in g/test urban cycle

Standard $6 Vehicle 1 Vehicle 2 Standard 3way-catalyst

f u l l cycle

CO

HC

NOx

CO

HC

NOx

30.3 9.0 9.8 17.7

6.60 2.90 2.10 2.20

3.95 2.90 2.60 0.45

66.1 12.40 18.3 20.8 4.00 10.9 10.5 2.40 9.7 19.0 2.45 2.15

221

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urban c y c l e Fig. 6: E m i s s i o n s o f t h e two c o n v e r t e d v e h i c l e s i n t h e c o l d s t a r t (UDC) and the combination of this and the extra-urban driving cycle (EUDC), compared to a large number of production vehicles of different technologies.

with threeway catalyst; but on NOx they cannot approach the threeway catalyst, even though they are a clear improvement over the $6 version. The higher CO and HC of vehicle 1 in the EUDC r e l a t i v e to vehicle 2 is caused by the fact that vehicle 1, with the smaller engine, runs into f u l l - l o a d enrichment in the highspeed part of the EUDC. Overall emission situation For the overall emission situation these emissions in the coldstart urban driving cycle (UDC) and in the combined test (UDC + EUDC) are considered. This last combination is taken since the consensus in the EEC now is that this test w i l l have to be the basis for the new l e g i s l a t i o n . In Figure 6 the emissions of the two converted vehicles are shown r e l a t i v e to those of a large number of $6 vehicles (meeting 88/76/EEC minus 15%), vehicles with open-loop threeway catalysts and vehicles with closed-loop threeway catalysts. I t can be seen that the

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emissions

of

CO are s i m i l a r

to

those

of

the

closed-loop threeway-catalysts;

t h e emissions o f HC+NOx are comparable t o those of t h e best in the group w i t h open-loop threeway c a t a l y s t s ,

which a p p l i e s a l s o t o the emissions of NOx alone•

CONCLUSIONS From the programme the f o l l o w i n g conclusions could be drawn: • Lean o p e r a t i o n proved p o s s i b l e on the two engine designs s t u d i e d , provided t h e m i x t u r e p r e p a r a t i o n and c o n t r o l was s u f f i c i e n t l y

good.

• With lean o p e r a t i o n a NOx-emission o f less than I g/km and a CO-emission o f around

5 g/km in the c o l d s t a r t

urban c y c l e proved p o s s i b l e . The HC-emission

remained around 2 g/km. • With a f u l l - s i z e

o x i d a t i o n c a t a l y s t the HC-emission dropped t o below 0.5 g/km

and t h e CO-emission t o below 2 g/km in the c o l d s t a r t nicatalyst

urban c y c l e • With a mi-

these f i g u r e s were: b e t t e r than 0.75 g/km f o r HC and 3 g/km f o r CO.

• The use of EGR in combination w i t h a moderately lean m i x t u r e r e s u l t e d in s i milar

emissions in

the coldstart

urban c y c l e ;

under highspeed o p e r a t i o n

it

produced somewhat lower NOx-emission. • In the combination c o l d s t a r t of

CO was s i m i l a r

NOx and HC+NOx were s i m i l a r talyst

urban c y c l e plus e x t r a - u r b a n c y c l e the emission

t o t h a t w i t h a c l o s e d - l o o p threeway c a t a l y s t

and those o f

to the best r e s u l t s w i t h open-loop threeway ca-

as measured on a l a r g e number o f p r o d u c t i o n v e h i c l e s .

• A l l these r e s u l t s were obtained w i t h the o r i g i n a l

fuel consumption•