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Effects of Transient Conditions on Exhaust Emissions from two Non-road Diesel Engines


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Biosystems Engineering (2004) 87 (1), 57–66 doi:10.1016/j.biosystemseng.2003.10.001 PM}Power and Machinery
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Effects of Transient Conditions on Exhaust Emissions from two Non-road Diesel Engines
M. Lindgren; P.-A. Hansson
Department of Agricultural Engineering, Swedish University of Agricultural Engineering, P.O. Box 7032, SE 75007 Uppsala, Sweden; e-mail of corresponding author: magnus.lindgren@lt.slu.se (Received 17 May 2003; accepted in revised form 2 October 2003)

Growing interest in quantifying and reducing the amount of engine emissions of carbon monoxide, hydrocarbons, and nitrogen oxides loading the environment has led to increasingly tighter environmental regulations. However, current non-road emission standards are performed according to a steady-state test cycle, which does not include transient effects and thus underestimates the amount of emissions produced in real use of the engine. This study quanti?es the effects of transients in engine speed and torque on the fuel consumption and emissions from two diesel engines intended for non-road mobile machinery. Fuel consumption and emissions from the engines were measured in an engine dynamometer during various transient load conditions. The results showed that during fast transients, the measured fuel consumption was up to twice as high as the corresponding steady-state load conditions. The effects of transients on emissions of nitrogen oxides were even greater, as were the effects of transient load increase with increasing transient conditions i.e. rate of change. The results showed that the effect of transients on fuel consumption and emissions were also dependent on the type of diesel injection pump and the engine equipment used. Furthermore, the results indicated that the air/ fuel ratio was an important contributor to the emission formation process during transient loads.
# 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

1. Introduction During recent decades, there has been a growing interest in quantifying and reducing the amount of engine emissions loading the environment. In Europe, USA and Japan there is an obligation for manufacturers to certify new engine models to emissions performance standards. Non-road mobile equipment (non-road diesel engines) is currently tested according to an eight-mode steady-state test cycle, ISO 8178 (ISO, 1996). Another standard widely used is ECE Regulation No. 49 (ECE, 2000) which includes 13 steady-state test modes. In a steady-state test cycle, emissions are measured at a sequence of several modes of ?xed engine speed and torque. Average emission values for the whole cycle are obtained by summarising the measured emissions at the individual modes, according to different weighting factors. The procedure of the standards is one reason why most efforts in engine development recently have
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been directed towards improving the characteristics at static load conditions. Transients in engine speed and torque, i.e. changes in engine speed and torque, occur frequently during the normal operation of non-road vehicles. Lindgren et al. (2003) have shown the occurrence of transients for several operations with non-road vehicles, from relatively low transient operations, such as pumping urine fertiliser, to high transient operations, such as loading with a wheel loader. Similar results have been reported by Starr et al. (1999) and Ullman et al. (1999). In order to use test procedures more related to the actual working conditions of agricultural vehicles, the trend is to replace the steady-state test cycles by transient tests, as is already the case for heavy-duty diesel engines in Europe, USA and Japan. A transient test cycle include sections of acceleration as well as steadystate conditions and emissions are measured continuously over the whole transient test cycle. A transient
# 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd

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test cycle, ‘non-road transient cycle’, is under consideration for use in forthcoming emission regulations, stage III B in Europe (EU, 2002) and tier 4 in USA (US EPA, 2003). To deal with the coming regulations, knowledge concerning fuel consumption and emissions from diesel engines at different load conditions, especially transient conditions, is of vital importance to manufacturers of engines and engine components such as turbochargers and fuel injection systems. Another reason to search for more knowledge on the effects of transient loads on engine fuel consumption and emission levels is that emission data not including these effects will underestimate the fuel consumption and amount of emissions produced (Hansson et al., 2003; Lindgren et al., 2003). These errors will in?uence the national statistics and may also lead to wrong conclusions in the work of quantifying and decreasing the environmental load caused by different food production strategies, for example by use of life cycle assessments (LCA) methodology (Wrisberg & Udo de Haes, 2002). In previous research, several factors have been identi?ed to explain how fuel consumption and emissions at transient conditions differ from those at steadystate conditions. One of these factors is the turbo-lag. It has been found that turbochargers have time lags of several seconds (Benajes et al., 2000; Bane, 2002). Bane (2002) studied the behaviour of a turbocharger during trapezoidal variations in engine load. The engine load was oscillated with different periods from 20% full power to 100% full power while maintaining a constant engine speed of 1500 min?1. The turbocharger was unable to respond to the variations in engine speed and torque during oscillations faster than a 4-s period. Benajes et al. (2000) found time lags of approximately 5 s before the ?nal boost pressure was reached after an increase in engine torque from idle to full load with ?xed engine speed. Rakopoulos and Giakoumis (1999) have shown that the response time for a mechanical governor, and thus the fuel pump rack position, to reach steady-state conditions after a steep increase in engine speed was considerable. While the engine speed was changed from 1500 to 1300 min?1 in 0?08 s, the governor needed more than 5 s to reach its ?nal steady-state conditions. Moreover, a change in engine speed means that the inertia of the engine will change as well. Under normal driving conditions, the engine will make use of more energy to increase the engine speed than will be used during a decrease in engine speed e.g. braking. Long-term thermodynamic transient responses i.e. non-periodic temperature oscillations, have been studied by Rakopoulos et al. (1998). During transient conditions in engine speed or torque, the turbocharger and

the mechanical governor reached steady-state conditions in about 5 s and thus a rather stable air/fuel ratio. However, according to Rakopoulos et al. (1998) the structural temperature needs several hundred of seconds to reach a thermodynamic equilibrium, while the brake mean ef?cient pressure and in-cylinder temperature reach fairly steady-state conditions in a few seconds. Hansson et al. (2003) found that the fuel ef?ciency for an agricultural tractor decreased with increasingly transient operation. Increasing proportions of transients were studied in four different operations; ?xed speed and torque, acceleration, on-farm driving and front end loading. The fuel ef?ciency decreased by 13% during the front end loading operation compared to the corresponding steady-state conditions, while the decrease in fuel ef?ciency for the ?xed speed and torque operation was less than 0?5%. Moreover, Lindgren et al. (2003) showed that during normal operation with a wheel loader, the emissions of carbon monoxide (CO) increased by more than 200% compared with steady-state conditions. During the same operation the fuel consumption and emissions of nitrogen oxides (NOx) and hydrocarbons (HC) increased by 14, 16, and 60%, respectively. Most of the work on quantifying emissions during transient operation has dealt with either comparison of speci?c emissions and fuel consumption between steadystate cycles and transient cycles (Samulski & Jackson, 1998; Swain et al., 1998), or transient emission prediction models without any aim to speci?cally quantify the transient effects (Ouenou-Gamo et al., 1998; Ramamurthy et al., 1998; Traver et al., 1999). The limited research reported in the literature has concentrated on engine acceleration and engine torque increase (Callahan et al., 1985; Arcoumanis, 1992; Arcoumanis et al., 1994). The objective of this work was to study the effects of different transients in engine speed and torque on fuel consumption and exhaust emissions from two non-road diesel engines. Both positive and negative transients, i.e. increases and decreases in engine speed and torque, were studied on two non-road diesel engines using an engine dynamometer.

2. Materials and methods 2.1. Engines Two turbocharged diesel engines, a six-cylinder Volvo TD 63 KDE and a four-cylinder Valtra 420 DWRE, were studied. The engines including injection systems, turbochargers, and engine cooling were typical for nonroad applications. The engines were originally mounted

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in a Volvo L70 wheel loader and a Valtra 6650 HiTech agricultural tractor respectively. Some basic data describing the two engines are presented in Table 1. The TD 63 KDE engine was ?tted with an in-line injection pump with a plunger and barrel assemblies with a lower helix only, thus causing the pumping to begin at the same stroke travelling independently of fuel quantity injected (Bosch, 1996). In contrast to the in-line injection pump, the delivery of fuel to the cylinders in a distributor pump always ends at the same stroke travelling independent of fuel quantity injected (Bosch, 1996). The distributor pump in the 420 DWRE engine was equipped with a centrifugally controlled timing device for correction of start of delivery. Moreover, the distributor pump was also equipped with a boost control device, a boost pressure actuated diaphragm mechanism that adjusts the maximum fuel delivery from the pumping element relative to the boost pressure. The in-line pump was without corresponding characteristics. However, the in-line pump changed the injection characteristics during low loads and speeds.

All tests were conducted with a low sulphur (maximum 10 ppm sulphur) diesel fuel with low aromatic content, classi?ed as Swedish environmental class 1 diesel fuel. Emissions of carbon dioxide (CO2), CO, HC and NOx were measured, as well as fuel consumption. Carbon dioxide and CO were analysed with two nondispersive infrared (NDIR) analysers, Maihak MULTOR and Maihak UNOR 610, respectively. Hydrocarbons were analysed with a heated ?ame ionisation detector (HFID) mod 109A. A chemiluminescent instrument, ECO Physics CLD 700 Elht, was used for measuring emissions of NOx (the sum of NO and NO2). In addition, the measurement instruments had linearity better than 1% for all emissions and a drift less than 1% for CO and 1?5% for HC. The ECO Physics CLD 700 Elht instrument had no registered drift. 2.3. Steady-state test cycle Fuel consumption and emissions of CO2, CO, HC and NOx were measured according to a 20-mode steadystate cycle. The 20-mode steady-state cycle used was based on the 13-mode European ECE Regulation No. 49 test cycle (ECE, 2000) and extended with nine additional modes in order to increase the resolution, see Fig. 1. The test cycle ECE Regulation No. 49 includes 11 unique engine load conditions, the remaining two modes
Torque, % of maximum torque

2.2. Engine test facility The engines were tested at the Swedish Machinery ( , Sweden. The engine Testing Institute located in Umea dynamometer used in the transient tests was a fast response Schenck eddy-current dynamometer with 400 kW maximum power. Engine output speed and torque were controlled through an electronic fuel pump rack control in combination with the dynamometer brake power. The engine dynamometer and control system are described in more detail in Wetterberg et al. (2002). During transient measurements a partial ?ow of exhaust gas emissions from the engine was collected in a heated steel container with a volume of 60 l, which caused an average retention time of 90 s. Samples for analysis were taken continuously from the container until steady-state conditions were obtained.

100 80 60 40 20 0 0 20 40 60 80 Engine speed, % of rated speed 100

Fig. 1. Twenty-mode steady-state test cycle based on ECE Regulation No. 49 ?D? and extended with nine additional modes ( ? )

Table 1 Speci?cations of the engines studied Engine type Displacement Cylinders Max. power Max. torque Idle speed Rated speed Injection pump Model year Volvo TD 63 KDE 5?48 l 6 91 kW at 2150 min?1 624 N m at 1100 min?1 650 min?1 2315 min?1 In-line 1999 Valtra 420 DWRE 4?4 l 4 81 kW at 2200 min?1 460 N m at 1400 min?1 850 min?1 2400 min?1 Distributor 2000

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Torque, N m

are repetitions of the low idle mode. The fuel consumption and emissions were measured in accordance with the ISO 8178 regulation (ISO, 1996).

500 400 300 200 100 0 0 1 2 Time, s 3 4 5

2.4. Synthetic transient test cycles The synthetic test cycles were developed in order to study the effect of transients in engine speed and engine torque independently of each other. Moreover, effects of positive and negative transients were studied independent of the other. An example of a synthetic test cycle over a positive transient in torque is shown in Fig. 2. The engine speed was kept constant over the whole cycle. Section (a) in Fig. 2 shows the positive transient in torque, while section (c) shows an opposite change in torque that was used to form a complete cycle i.e. the last points of engine speed and torque in the test cycle were equivalent to the ?rst points. Each section of change in engine speed or torque was followed by a section of steady-state conditions, sections (b) and (d), where the engine speed and torque were held constant for 2 s. The rate of change over the studied transient segment, section (a) in Fig. 2, was varied between 10, 15, 20, 25 and 40% s?1, see Fig. 3, while the rate of change over the other transient change, section (c) in Fig. 2, was kept constant at ?2% s?1. The rate of change was derived as normalised change of engine speed or torque per second as shown by ni ? ni?1 dn ? =?ti ? ti?1 ? ? 100 ? 1? nrated ? n0 dt ? ti ? ti ? 1 =?ti ? ti?1 ? ? 100 tmax ? 2?

Fig. 3. Different rate of change during positive transients in engine torque at ?xed engine speed: } ? ? }, 10% s?1; } ? }, 15% s?1; } }, 20% s?1; ? ? ?, 25% s?1;}}, 40% s?1

speed and low idle speed respectively; t was torque in N m with subscript max representing maximum torque; and t was time in s with subscripts as above. During negative transients, the rate of change over section (a) in Fig. 2 was kept constant at 2% s?1 and the rate of change over section (c) was varied between ?10, ?15, ?20, ?25 and ?40% s?1. Four different categories of transient operations were studied: (1) increase in engine torque from 30% of maximum available torque to 70% of maximum available torque; (2) decrease in engine torque from 70% of maximum available torque to 30% of maximum available torque; (3) increase in engine speed from 30% of rated speed to 70% of rated speed; and (4) decrease in engine speed from 70% of rated speed to 30% of rated speed. Each category of the transient load change was repeated with three different levels of the constant variable, i.e. ?xed engine speed during transients in torque and ?xed torque during transients in engine speed. The torque was equivalent to 20, 50 and 70% of the normalised torque of the engines and the engine speed was equivalent to 25, 50 and 80% of the normalised speed of the engines. Barsic (1984) have shown that the variability of testto-test measurements of heavy-duty diesel engine emissions within a laboratory are considerable although measurements had been taken to varying ambient conditions. Therefore, an additional synthetic test cycles with a rate of change of 2% s?1 were performed as a start of each measurement occasion, six in total per engine, and the results used to compensate for zero point drift and systematic errors. A rate of change of 2% s?1 was considered to generate the same amount of emissions as steady-state conditions.

where: dn and dt were rate of change in engine speed and torque in % s?1; n was engine speed in min?1 with subscripts i, i?1, rated and 0 representing engine speed at current time, engine speed at previous time-step, rated
a b 400 c d 1500 1000 200 500 0 0 Speed, min-1

Torque, N m

0

10 Time , s

20

Fig. 2. Synthetic test cycle for a positive transient in engine torque and ?xed engine speed: (a) positive torque transient zone; (b) and (d) steady-state zones; (c) negative torque transient zone; }}, torque; } }, speed

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Each synthetic test cycle was repeated for approximately 15 min in order to obtain average fuel consumption and emission data for the whole cycle. Furthermore, fuel consumption and emissions data were recorded at 1 Hz and engine speed and torque data were recorded at 5 Hz. 2.5. Data analyses The data analyses were based on the following steps: (1) registration of engine speed, torque, fuel consumption, and emissions from the 20-mode steady-state test cycle; (2) development of four matrices of fuel consumption and emissions of CO, HC and NOx respectively, by two-dimensional interpolation of the data from part (1) for all possible steady-state combinations of engine speed and torque data for each engine, within the operational range of the engine, and the engine speed and torque data rounded towards the nearest integer; (3) registration of engine speed, torque, fuel consumption and emissions from the synthetic test cycles; (4) calibration of the zero point of the matrices from part (2) to the current conditions at each measurement occasion by use of fuel consumption and emission data from an additional synthetic test cycle, one for each measurement occasion; (5) calculation of the assumed fuel consumption and emissions if the transient effects were neglected, using the recorded engine speed and torque data from part (3) as inputs to the calibrated matrices from part (4); and (6) calculation of the effects of transients in engine speed and torque on the fuel consumption and emissions x in g g?1 g at steady-state according to    tcycle EM ? EC x? ?1 ? 3? EC ttransient where: EM was measured transient fuel consumption and emissions from part (3) in g cycle?1; EC was calculated steady-state fuel consumption and emissions from part (5) in g cycle?1; tcycle was the total cycle time in s; and ttransient was the duration of the transient zone i.e. section (a) in Fig. 2 in s. The difference between measured fuel consumption and emission amounts and calculated fuel consumption and emission amounts were assumed to derive to the transient zone solely, hence the time quotient in Eqn (3). At a rate of change of 0% s?1 i.e. steady-state conditions, measured fuel consumption and emissions were de?ned to be equal to calculated fuel consumption

and emissions thus causing the ratios to be equal to unity. Obvious errors in measured fuel consumption or emission values were omitted from the results. Moreover, synthetic test cycles containing incorrect measured engine torque data due to large vibrations in the engine– dynamometer system were also deleted from the comparison.

3. Results Measured fuel consumption values compared with calculated for various transients are shown in Fig. 4. Engine TD 63 KDE showed in most cases a minor increase in fuel consumption during transients in both engine speed and torque compared with corresponding steady-state conditions. However, during positive transients in engine speed fuel consumption decreased for the 100 N m constant engine torque scenarios. Engine 420 DWRE showed a more pronounced increase in fuel consumption during negative transients in both engine speed and torque. The fuel consumption during positive transients in engine torque showed a minor increase, while the fuel consumption decreased during positive transients in engine speed. The results showed a modest increase in emissions of CO with increased positive transients in both engine speed and torque for both engines, see Fig. 5. Engine 420 DWRE showed an increase in emissions of CO, while emissions decreased for engine TD 63 KDE during negative transients in engine speed and torque. Furthermore, the results indicated that emissions of CO had a local minimum at approximately ?20% rate of change in engine torque for TD 63 KDE. Emissions of NOx increased for all transients studied except for positive transients in engine speed for engine 420 DWRE and the 100 N m constant engine torque scenario for engine TD 63 KDE, see Fig. 6. The results showed a marked increase in emissions of HC during transients in engine speed and torque for the transients and engines studied, see Fig. 7. The coef?cients of variation for the last 300 s of measurement were on average 0?6% for NOx, 0?5% for CO and 1?8% for HC.

4. Discussion The results showed that transients in engine speed and torque in most cases increased the fuel consumption and emission amounts. During fast transients, fuel consumption and emission amounts often increased by more than 100% compared to the corresponding steady-state

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2.0 Fuel consumption, g g-1 at steady state Fuel consumption, g g-1 at steady state 1.5
M. LINDGREN; P.-A. HANSSON

2. 0 1. 5

-40

-20 0.5 0.0

20

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-40

-20 0. 5 0. 0

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(a)

Rate of change in engine torque, % s-1 2.0

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Fuel consumption, g g-1 at steady state

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40

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-20 0. 5 0. 0

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(c)

Rate of change in engine torque, % s-1

(d)

Rate of change in engine speed, % s-1

Fig. 4. Measured transient fuel consumption relative to calculated steady-state fuel consumption for different rates of change in engine speed and torque: (a) and (b) engine TD 63 KDE; (c) and (d) engine 420 DWRE; , 2000 min?1; , 1600 min?1; , 1100 min?1; , 400 N m; , 350 N m; , 250 N m; , 100 N m

2.5 CO, g g-1 at steady state CO, g g-1 at steady state 2.0 1.5

2.5 2.0 1.5

-40

-20

0.5 0.0

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-40

-20

0.5 0.0

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Rate of change in engine torque, % s-1 2.5

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(c)

Rate of change in engine torque, % s-1

(d)

Rate of change in engine speed, % s-1

Fig. 5. Measured transient emissions of CO relative to calculated steady-state emissions of CO for different rates of change in engine speed and torque: (a) and (b) engine TD 63 KDE; (c) and (d) engine 420 DWRE; , 2000 min?1; , 1600 min?1; , ?1 , 400 N m; , 350 N m; , 250 N m; , 100 N m 1100 min ;

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3.0 NOX, g g-1 at steady state NOX, g g-1 at steady state 2.5 2.0 1.5 -20 20 40

3.0 2.5 2.0 1.5 -20 20 40

-40

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Rate of change in engine speed % s-1

3.0 NOX, g g-1 at steady state NOX, g g-1 at steady state 2.5 2.0 1.5 -20 20 40

3.0 2.5 2.0 1.5 -20 20 40

-4 0

0.5 0.0

-4 0

0.5 0.0

(c)

Rate of change in engine torque, % s-1

(d)

Rate of change in engine speed, % s-1

Fig. 6. Measured transient emissions of NOx relative to calculated steady-state emissions of NOx for different rates of change in engine speed and torque: (a) and (b) engine TD 63 KDE; (c) and (d) engine 420 DWRE; , 2000 min?1; , 1600 min?1; ?1 , 1100 min ; , 400 N m; , 350 N m; , 250 N m; , 100 N m

HC, g g-1 at steady state

HC, g g-1 at steady state

9 8 7 6 5 4 3 2 -4 0 -20 0 20 s-1 40

9 8 7 6 5 4 3 2 -40 -20 0 20 s-1 40

(a)

Rate of change in engine torque, %

(b)

Rate of change in engine speed, %

HC, g g-1 at steady state

HC, g g-1 at steady state

9 8 7 6 5 4 3 2 -40 -20 0 20 s-1 40

9 8 7 6 5 4 3 2 -4 0 -20 0 20 40

(c)

Rate of change in engine torque, %

(d)

Rate of change in engine speed, % s-1

Fig. 7. Measured transient emissions of HC relative to calculated steady-state emissions of HC for different rates of change in engine speed and torque: (a) and (b) engine TD 63 KDE; (c) and (d) engine 420 DWRE; , 2000 min?1; , 1600 min?1; , ?1 , 400 N m; , 350 Nm; , 250 N m; , 100 N m 1100 min ;

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conditions. The results showed that even negative transients, i.e. decreasing engine speed and torque, strongly in?uenced both fuel consumption and emissions. Both fuel consumption and emissions, except for emissions of CO from engine TD 63 KDE, increased during negative transients. This was in contrast to other research works that assumed that negative transients do not in?uence emissions at all (Arcoumanis, 1992). The fuel consumption and formation of emissions from internal combustion engines are intimately coupled with the conditions within the cylinder during combustion e.g. air/fuel ratio, mixing and temperature (Heywood, 1988). Heywood (1988) also stated: ‘The pollutant formation processes are strongly dependent on the fuel distribution and how that distribution changes with time’. During transients in engine speed and torque, the initial in-cylinder conditions resemble those of the preceding engine load conditions. However, depending on time delays the parameters react differently to a change in load conditions e.g. the time delay for injected amount of fuel is less than the time delay for the boost pressure. This has also been con?rmed by Benajes et al. (2000), Bane (2002), Rakopoulos and Giakoumis (1999) and Rakopoulos et al. (1998). The boost control device reduced the amount of fuel delivered during accelerations from low engine loads with engine 420 DWRE and thus increased the fuel ef?ciency during positive transients in engine speed, as indicated by the results. Engine TD 63 KDE was not equipped with a corresponding device and the amount of fuel injected increased accordingly. However, the inline pump changed the injection characteristics during low loads, thus resulting in decreased fuel consumption for the 100 N m transient in engine speed for engine TD 63 KDE. An additional cause of the increased fuel consumption was that the relatively low boost pressure applied less work on the piston, which was compensated for by extra fuel. Moreover, more heat from the combustion was needed to evaporate the injected fuel and increase the structural temperature (due to the increased amount of fuel injected and the lower structural temperature). During positive transients in engine speed and torque, the increased emissions of CO indicated an incomplete combustion. The increased emissions of CO were probably an effect of the low air/fuel ratio, low mixing rate and locally fuel-rich regions. These results indicated that the air/fuel ratio was important for the fuel consumption and emission formation process during transients. This has also been shown under steady-state conditions (Taylor, 1985; Heywood, 1988). In diesel engines, torque is decreased by reducing the fuel supply but this study shows that decreased fuel ef?ciency has the same effect. Benajes et al. (2000) and

Rakopoulos and Giakoumis (1999) have shown that the response time in fuel metering on a change in fuel pump rack position is less than the corresponding response time for the turbocharger. The effects were probably even larger for the 420 DWRE engine due to the more advanced fuel pump e.g. boost control device and timing device. The high air supply and thus mixing rate in combination with the high structural and in-cylinder temperature were likely to decrease the fuel ef?ciency. To counteract the decreased fuel ef?ciency, more fuel was injected in order to maintain the desired engine speed and torque. Decreased emissions of CO during negative transients in engine speed and torque for engine TD 63 KDE indicated a more complete combustion compared to steady-state emissions and thus increased air/fuel ratio. The transients were characterised by initially high boost pressure and fuel consumption and high structural temperature. The air/fuel ratio was probably lower for engine 420 DWRE compared to TD 63 KDE as indicated by the more pronounced increase in fuel consumption during negative transients in engine speed and torque for engine 420 DWRE. However, during fast negative transients in engine torque the emissions of CO seemed to increase and thus indicated an air/fuel ratio closer to the steady-state conditions. High emissions of NOx suggested high peak combustion temperature and vice versa. Furthermore, emissions of NOx showed the same pattern as the fuel consumption; increased amounts coincided with increased fuel consumption. Emissions of HC showed an increase during all transient cycles compared to the corresponding steadystate amounts. This indicated that the air–fuel mix was, locally, either too rich or too lean to combust. Furthermore, overpenetration was likely to occur during positive transients in both engine speed and torque as the air/fuel ratio decreased substantially compared to the corresponding steady-state conditions. Overpenetration leads to increased emissions of both CO and HC (Heywood, 1988). Bulk quenching of the ?ame was another important characteristic that contributed to increased emissions of HC.

5. Conclusions The results clearly showed that transients in engine speed and torque have a major effect on fuel consumption and emissions. Fuel consumption under negative transients increased by up to 100% depending on engine and rate of change. During fast positive transients in both engine speed and torque, emissions of carbon monoxide (CO) increased by a factor of two. Emissions

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of nitrogen oxides (NOx) increased by up to 200% during negative transients in engine speed and torque. A boost control device that adjusts the maximum fuel delivery from the pumping element relative to the boost pressure seems to increase the fuel ef?ciency during positive transients in engine speed. During fast positive transients in engine speed, the fuel consumption decreased by 30% and emissions of NOx decreased by 50%. The methodology for estimating the effects of transients in engine speed and torque on the fuel consumption and emissions seemed useful and produced reliable results. However, the two engines studied gave different results to some extent as a consequence of the differences in construction and equipment. Furthermore, the results indicated that the air/fuel ratio during transients in engine speed and torque plays an important part for the fuel consumption and emission formation process. In addition, the in-cylinder combustion under transient conditions seems to be characterised by a highly irregular mixing formation process.

Acknowledgements This work was supported and funded by the Swedish National Energy Administration and the Swedish National Road Administration.

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