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Impacts of Biodiesel Fuel on U.S. Light-Duty Tier 2 Engine and Emission Control Systems – Part 2

ABSTRACT

In the United States, the combination of an increased interest in diesel powered vehicles and government-mandated policies to reduce dependency of foreign oil, has consequently sparked an interest in biodiesel fuel blends to supply fuel for these vehicles.

Information regarding the effects on the performance of advanced emission control systems using biodiesel fuels is quite limited. The goals of this project are to evaluate the implementation of a NOx storage and SCR emission control system and to develop them for optimal performance. The primary area of concern lies in the discussion of the differences between the fuels, which is being done for development purposes as well as evaluating useful life-aged components.

Biodiesel operation resulted in only minimal effects on emission controls. A two percent improvement in fuel economy was noted by using biodiesel, which was attributed to its higher combustion efficiency. In addition, there were no measurable impacts on the engine’s mechanical components, as a result of using biodiesel fuel.

INTRODUCTION

Advancements in diesel engine technology for light-duty diesel powered vehicles have led to their increased popularity in the U.S. Additionally, the phasing in of stringent Tier 2 emissions standards for this vehicle class has led to the necessity of adding Emission Control Systems (ECS) to these vehicles. Fuel price increased have renewed interest in bio-fuels, such as biodiesel, as a means of shifting the demand for petroleum-derived fuels.

Two of the key technologies that have risen to the forefront to meet the Tier 2 NOx emission standards for light-duty diesel vehicles are Selective Catalytic Reduction (SCR) with urea and NOx Adsorber Catalysts (NAC). A vast amount of research and development has been conducted over the past decade, which has been centered on the performance and durability of these technologies when used with conventional fuels [1-8]. However, only a limited amount of research has been conducted with the intent of understanding of the effects of bio-fuels, or more specifically biodiesel, on these ECSs.

This renewable fuel source is derived from vegetable oil, animal fat, or waste cooking oil and consists of the methyl esters of fatty acids. It is customarily used as a blending component for diesel fuel in volumes of up to 20 percent. Potentially 5% or more of petroleum diesel could be displaced by biodiesel over the next decade, as indicated by a resource assessment [9]. A life cycle analysis also indicates that using B20 reduces the life cycle of petroleum consumption by 19% [11].

The primary purpose of this study is to examine the impact on emission performance as the result of using a NAC or SCR system, combined with a Diesel Oxidation Catalyst (DOC) and Diesel Particle Filter (DPF). The fuel utilized in the project had the vehicles operating on Ultra-Low Sulfur Diesel (ULSD) and a blend of ULSD and 20% Biodiesel (B20). The chemistry of the SCR and NAC seem to point to several areas where biodiesel blends may perform differently than pure petroleum-derived fuels.

HARDWARE AND SOFTWARE DESCRIPTION

The following five chapters provide an overview of the hardware and software components used during the project. The test vehicle and test engines are procured through the U.S. Department of Energy, while all catalyst hardware as well as the urea dosing system is supplied by the Manufacturers of Emissions Controls Association (MECA).

ENGINE HARDWARE

The engine used for this project in the test cell as well as in the vehicle is a 4 cylinder 2.15L HSDI Diesel engine. The original engine calibration complies with EURO 4 emission standards. The engine utilizes a conventional high-pressure EGR loop, second generation common-rail fuel injection system (1600 bar maximum injection pressure, solenoid injectors) with all actuators electrically controlled. Table 1 lists the main parameters of the test engine.

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Table 1: Engine Specifications

Engine Power 113 kW at 4000 rpm Peak Torque 360 Nm at 2000 rpm Maximum Engine Speed 4700 rpm Maximum BMEP 21 bar Cylinder Number and Arrangement 4 Cylinder Inline Firing Order 1 – 3 – 4 – 2 Valvetrain 4 Valve DOHC Bore to Stroke Ratio 1.0034 Displacement 2.15L Compression Ratio 18 Fuel Injection System 2nd Generation Common Rail

ENGINE HARDWARE

The test vehicle is a conventional four door sedan vehicle in the 1700 kg class. It meets the EURO 4 emission standards and is equipped with a catalyzed DPF as standard feature.

Table 2 lists the relevant vehicle features.

Table 2: Vehicle Specifications

Criteria Unit Value Vehicle Mass kg 1700 Air Drag Coefficient - 0.29 Frontal Surface Area m2 2.20 Transmission Gear Ratio 1st 4.99

2nd 2.82 3rd 1.78 4th 1.25 5th 1.00 6th 0.82

Axle 2.65 Tires Rear 205 / 55 R 16 91 H Front 205 / 55 R 16 91 H

CATALYST SPECIFICATIONS

As part of the ECS all catalysts are provided by MECA. Figure 1 shows the system architecture for the NAC as well as the SCR system. The NAC in the upper portion of the graph is located close-coupled, directly after the turbine exit with minimal distance to the catalyst to allow fastest possible heat-up of this NOx treatment unit. In addition the SCR system dimensions are displayed on the bottom as the SCR catalyst which is the critical unit from heat-up perspective is moved to the under-floor location. All the piping between the DOC exit and the SCR inlet is made of dual wall exhaust pipe for increased heat preservation during operation.

Table 3: Catalyst Specifications

Unit Volume [L]

Cell Density [cpsi]

PGM loading [g/ft3]

DOC 0.8 400 150 DPF (AT)

3.3 300 60 NAC System

NAC 4.1 400 120 DOC 1.23 400 150 SCR

System DPF (SiC)

4.1 300 60

3

SCR Fe-ZSM-5

4.43 300 N/A

CATALYST SPECIFICATIONS

The design process of the ECS software was aimed to be as modular as possible. Each control module can be easily removed or substituted by an alternative routine. Figure 2 shows the high level structure of the controller which is implemented in the rapid prototyping environment.

The input as well as the output module converts the signals to useful conditions for each side of the controller. The core of the ECS algorithm is within the intervention handler module. The different intervention requests coming from the State Release Module and a release system are transformed into the corresponding output signals through a patented multi-variable control structure, which allows the control of temperature and lambda simultaneously.

Turbo

20”

Temperature Sensor

Urea dosing valve

NOx Sensor

Diff. Pressure Sensor

12”

DOC

Flex 13”

115º

135º

115º

2.047”(52mm)

Mixer

SCR DPF

Figure 1: Emission Control System Architecture

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State ReleaseModule

Intervention Handler Module

DeSOx

DeNOx

DPF Regeneration

Rapid Warm-Up

Output Module

Catalyst Protection

End of Control

Input Module

ReleaseSystem

Temperature

Lambda

Merge System

Control System

SCR

State ReleaseModule

Intervention Handler Module

DeSOx

DeNOx

DPF Regeneration

Rapid Warm-Up

Output Module

Catalyst Protection

End of Control

Input Module

ReleaseSystem

Temperature

Lambda

Merge System

Control System

SCR

Figure 2: Emission Control Software Architecture

FUEL SPECIFICATIONS

The ultra low sulfur Diesel fuel (ULSD) also called the base fuel was used to blend the 20% by volume (feedstock is soy methyl ester). Table 4 lists the fuel specifications for each tested fuel type. The effects on the fuel specifications as a result of the increased amount of methyl ester become particularly noticeable in the heating value, the cetane number, the oxygen content and of course have secondary effects on the remaining specifications.

Table 4: Fuel Specifications

ULSD B20 Net Heating Value [ASTM D240]

MJ/kg 42.534 41.522

Cetane Number [ASTM D613]

- 41.4 45.6

Density at 15°C [ASTM D4052]

kg/m3 847.5 852.9

Viscosity at 40°C [ASTM D445]

mm2/sec 2.429 2.685

Carbon [ASTM D5291]

wt% 87.04 85.01

Oxygen [ASTM D5622]

wt% 0.00 2.29

Hydrogen [ASTM D5291]

wt% 12.96 12.70

SCR UREA DOSING SYSTEM DESCRIPTION

The urea dosing system and its control is one of the key components for a successful system conceptualization. The utilized system supplies the urea from a supply unit at 10 bar pressure to a urea dosing valve. A Dosing Electronic Control Unit or DECU controls the system pressure in a closed loop control mode as well as the urea dosing quantity. Figure 3 shows the various subcomponents of the dosing system as well as its installation in the vehicle upstream the catalyst.

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Figure 3: Urea Dosing System

DPF MANAGEMENT

The first phase of the development work was the calibration of the temperature control module for the DPF regeneration mode as this serves both NAC as well as SCR ECS. This task has to precede all other activities as the function of the DPF regeneration allows for the safe and continuous operation of all other system interventions. The DPF regeneration strategy development as well as calibration were started under steady-state conditions in the test cell and were ultimately transferred to the vehicle.

As the project goal was the determination of the influences of biodiesel effects on engine and ECS, a mature emission control calibration, in this case the control of the DPF regeneration, was used to compare the ULSD base fuel with the B20 blend. Figure 4 shows the behavior of the two compared fuel blends at 600°C and 650°C (these tests were conducted in the engine dynamometer test cell). In both cases the DPF was loaded to 5 g/L (5 grams of soot per liter of DPF volume). The soot burn-out rate is calculated based on the feedback signal from the differential pressure sensor. It is evident that at the lower set-point temperature the regeneration rate of the biodiesel blend is faster compared to the base fuel, as has been observed in previous studies [10]. This has been attributed to changes in PM morphology and to the addition of oxygen to the PM surface caused by the inclusion of biodiesel in the fuel. At the higher temperature set-point the differences disappear.

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DPF

bed

tem

p. [d

egC

]

300350400450500550600650700

Engine speed = 2500 rpm, BMEP = 5 bar

B20 ULSD

Lam

bda

[-]

1.401.451.501.551.601.651.701.75

ULSD

B20

Soot

[g]

0

2

4

6

8

10

Time [sec]0 100 200 300 400 500

B20

ULSD

DPF

bed

tem

p. [d

egC

]

300350400450500550600650700

B20 ULSD

Engine speed = 2500 rpm, BMEP = 5 bar

B20

ULSD

B20

Lam

bda

[-]

1.401.451.501.551.601.651.701.75

ULSD

Soot

[g]

0

2

4

6

8

10

Time [sec]0 100 200 300 400 500 600 700 800 900

Figure 4: DPF Regeneration at Low and High Temperature

The temperature increase upstream of the DPF was realized through a combination of air- as well as fuel handling parameter variations. The engine out temperature is raised through intake air throttling in conjunction with lowered EGR rates. An early post injection (close to the main injection event) raises the engine out temperature further while a late cycle post injection provides reactants to the DOC, which generates an exothermic reaction and controls the temperature at the setpoint level.

The temperature control module was transferred to the vehicle and its performance evaluated under various driving conditions. It was found that during transient operation the control parameters had to be recalibrated in order to obtain stable control of DOC outlet temperature.

NOX ADSORBER DEVELOPMENT

LEAN-RICH MODULATION

NAC regeneration development is the cornerstone of meeting the NOx emission standards with the NAC system. A short pulse of rich exhaust gas desorbs the NOx and reduces them subsequently following the well known chemistry of the three-way-catalyst. The challenge is to obtain rich exhaust conditions with a diesel engine that typically operates under lean conditions. The approach taken in this project utilizes a multi-variable controller, which allows the activation of all four actuator components (boost-pressure, EGR level, intake air throttling, and in cylinder post injection) at the same time to adjust lambda to a given setpoint value. The controller adjusts the setting for each parameter using a wide range oxygen sensor as feedback signal. The location of the sensor is upstream of the NAC. A second wide range oxygen sensor located downstream the NAC serves as controller feedback signal determining the completion of the regeneration event.

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The downstream NAC lambda signal shows stoichiometric conditions until the regeneration is complete and then drops below λ = 1 (with an upstream NAC being at λ < 1). This effect is used to not only control the NOx regeneration, but also effectively minimize the hydrocarbon breakthrough during these events.

The effect of the biodiesel in this example is negligible as both lambda traces for ULSD as well as for the biodiesel blend are nearly identical. The investigation attempting to determine the fuel effects during the lean-rich transition was performed under numerous steady-state conditions covering a large area of the engine map with the same results. In all cases the lean duration was chosen sufficiently long to assure a fully saturated NAC. This was considered as the only condition that allows repeatable results as intermediate loading levels are difficult to maintain. The definition of fully saturated NAC was tailpipe NOx emission levels at or in excess of 80% of the engine out NOx level.

Also in this case the effects of biodiesel are evaluated. Noteworthy is that all the actuators commands that result in the rich pulse act virtually identical for the two different fuels. As stated above this investigation was performed in various operating conditions with similar results, that is, there was no impact of the different fuels on the emission controls in regard to the lean-rich modulation calibration. There is however some effects on the emission formation during the rich regeneration process. The ULSD shows a trend of higher unburned HC as well as CO, which indicates greater combustion efficiency with the biofuel blend, as it carries additional oxygen. This larger amount of reactants manifest themselves in higher exhaust temperatures upstream the NAC.

DESULFURIZATION

The basic principle of NAC catalysts is the adsorption of NO2 during the lean operating phases of the engine. The NO2 is adsorbed by alkali oxides forming nitrates such as Ba(NO3)2. The nitrates become unstable and release the NO and NO2 under high temperature (thermal release of NOx) or during rich exhaust conditions. The rich exhaust conditions enable the utilization of the three-way-catalyst mechanism to reduce the released NOx into N2 and CO2. In addition to the described desired functions, NAC exhibit the undesired function of adsorbing SO3, forming BaSO4 which is a considerably more stable compound requiring high temperatures and stoichiometric conditions to be released. This release or the so called desulfurization has to occur frequently to avoid catalyst deactivation. The frequency of this event is dependent on the fuel sulfur level as well as the engine lubricating oil contribution into the exhaust system.

The ECS layout with the NAC upstream of the DPF dictates a desulfurization strategy that continuously switches between lean and rich conditions under high temperatures. The switching is necessary to oxidize the undesired H2S species. Figure 5 shows an example of a desulfurization event.

No biodiesel effects were observed during the desulfurization development. As discussed in the previous section the lean-rich modulation was also not affected by the differences in fuel properties. Since the desulfurization was conducted as lean-rich modulation under elevated temperatures, the conclusions from the NAC calibration development remain valid for the entire ECS function. Effects as result from the lower sulfur content were not observed as the base fuel and the biodiesel blends contained very low sulfur levels which make it exceptionally challenging to determine differences on the sulfur poisoning.

NA

C

Tem

pera

ture

[°C

]

500

600

700

800

Lam

bda

0.70.80.91.01.11.21.31.41.51.61.7

Time [s]500 600 700 800 900 1000 1100 1200 1300

SO2

[ppm

] / H

2S [p

pm]

SO2 after NAC H2S after NAC

Figure 5: Extract of a Dynamic Desulfurization Event

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SYSTEM PERFORMANCE WITH NAC

All tests for the FTP75 testing portion were performed at the National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Michigan. It was observed during these tests that the NOx tailpipe emission numbers running on B20 fuel are significantly more stable than on ULSD. The amount of successful regenerations was higher for B20 fuel (B20: 10 regenerations for cold LA4, 8 for hot LA4; ULSD: 6 for cold/hot LA4). This is a result of the engine-out emission calibration, which was performed on a B20 fuel blend and therefore is specifically optimized for this fuel.

The engine-out NOx emissions are higher for the B20 fuel blend compared to the ULSD is similar to what has been observed in other testing [10]. It is important to note that this trend reverses at the tailpipe location. The reason for this is the result of higher exhaust temperature upstream of the NAC for ULSD. The temperature excursions above 450°C result in less favorable NOx adsorption efficiencies with partial thermal desorption during which the NOx emissions increase.

The overall emissions performance met the 50,000 mile standard for all emissions with B20, while 2 out of 3 tests fail to not meet the NOx standard with ULSD.

SCR SYSTEM DEVELOPMENT

AMMONIA STORAGE AND RELEASE

Since the certification cycle for Tier 2 Bin 5 emission is a cold start FTP 75 cycle, where the temperature of the engine, aftertreatment system and ambient are kept at around 25°C it was necessary to study the behavior of the SCR under cold conditions. For this purpose a special testing protocol was developed in the test cell where the engine is started in cold and remained in idle for few seconds and then the engine speed and load are gradually increased to 2000 rpm 5 bar BMEP. By doing this the temperature of the exhaust system gradually increases from 25°C to 400°C allowing the study of its transitional behavior. During this study, a clean SCR (no stored NH3) without urea injection is used.

It is clear from this part of the study that when the SCR is cold and during the cold to warm transition that is in the time period of 0 to 230 seconds of the test cycle the SCR conversion efficiency is poor due to the release portion after reaching the critical temperature, and later in the cycle the efficiency is increased to about 80%, although it is possible to achieve more than 90% efficiency in the second phase of certification test cycle. The poor cold start NOx conversion efficiency during the aggressive, high speed and load driving conditions emits enough NOx mass making it difficult to meet the Tier 2 Bin 5 emissions standards.

COLD START AND RAPID WARM-UP

A Rapid Warm-Up (RWU) calibration was developed to enhance the heat-up of the SCR catalyst during cold start tests. As observed in the above cold start tests, the longer the duration the SCR stays in cold conditions, the more NOx emission it absorbs and will be subject to release during warm transition spoiling the overall conversion efficiency. Figure 6 describes the results of the rapid warm-up strategy and calibration and its effects on increasing the SCR bed temperature. The RWU strategy is phased in 3 distinct parts:

(a) Operating window 1 (0 – 20 seconds after cold start): the goal is to minimize HC emissions during this phase. This is accomplished through a tight Lambda control, increased rail pressure, advanced timing, engagement of both pilot injections, reduced boost pressure, and reduced EGR.

(b) Operating window 2 (20 – 40 seconds after cold start): the target in this phase is to light-off the DOC through the increase of engine-out temperature. This is accomplished through increased rail pressure, retardation of the injection, engagement of both pilot injections, reduced boost pressure, and optimized EGR levels.

(c) Operating window 3 (40 – 60 seconds after cold start): the goal is to heat up the SCR catalyst through an exotherm over the DOC. This is accomplished through the reduction of rail pressure, retardation of injection timing, one pilot injection, one post injection, and increased boost and EGR levels.

With the rapid warm-up strategy, the combustion parameters are optimized in cold condition to attain highest engine-out temperature in the beginning to heat up the oxidation catalyst which is close-coupled to the turbocharger. Once the DOC reached > 200°C post injection was turned on to develop more exothermic temperature across the DOC to heat up the SCR situated 40 inches downstream of DOC. With this warm-up strategy the SCR bed temperature was heated to > 200°C within 55 seconds compared to 98 seconds with the base calibration. This accelerated warm up calibration was made in such a way that it does not affect the total hydrocarbon and NOx emissions, having least impact on fuel economy.

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Time [s]0 50 100 150 200

DrivingLow Idle

Tem

pera

ture

SC

R b

ed [

°C]

0

100

200

300

400

500

600

55 s 98 s

43 s

Tem

pera

ture

afte

r DO

C [

°C]

0

100

200

300

400

500

600

700Te

mpe

ratu

re a

fter t

urbi

ne [

°C]

-100

0

100

200

300

400

500

600

Tem

pera

ture

bef

ore

turb

ine

[°C

]

0

100

200

300

400

500

600

Baseline Rapid Warm-up

Figure 6: Cold Start and Rapid Warm-up

SYSTEM PERFORMANCE WITH SCR

Similar to the emissions test performed with the NAC system, the SCR system underwent the same test protocol using ULSD as well as the B20 fuel at EPA’s NVFEL. The tests for both fuels were conducted using the FTP75.

The challenges of the cold start and the resulting high NOx emission in the cold portion are the evident result of the long distance between the turbine exit and the SCR catalyst. Once the system reaches operating temperature it operates at high efficiency levels. Except for the elevated engine-out NOx level with B20 which was observed during all emissions tests the tailpipe levels are on a very stable and comparable level with both fuels.

The SCR system degradation is possibly a result of the deactivation of the iron-zeolite substrate through the fuel borne alkali metals. Further investigation of the catalysts including a detailed post mortem analysis is subject of future publications.

ECS DURABILITY TESTING

TEST CYCLE

All ECS parts are aged to an equivalent of 120,000 miles or full useful life. In order to accomplish this, the engine and ECS were exposed to an equivalent useful lifetime of fuel in the engine dynamometer test cell. To keep the aging time at a reasonable level, the aging duration for each ECS was set to be accomplished after 700 hours. The following assumptions regarding the fuel consumption for the system were used:

• Highway cycle fuel economy at 55 mpg • City cycle fuel economy at 33 mpg • Split of ¾ highway and ¼ city cycle

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These assumptions resulted in an average fuel economy of 49.5 mpg. At 120,000 miles this equates to 2,424 gallons or 7,708 kg of fuel. With the available 700 hours of aging time an average fuel consumption of approximately 11 kg/hr was established. Three operating phases were established to reflect real in-use operating modes:

1. NAC operation using the systems efficiency control algorithms to determine the frequency of regeneration events. 2. DPF regeneration (300 for full useful life). 3. Desulfurization (25 for full useful life for NAC system only). Table 5 shows the detailed operating conditions and durations for the chosen durability cycle. In phase 1 the engine operating conditions are changed between two operating points (OP1 and OP2) for 120 minutes. In the second phase the system transitions into the DPF regeneration mode with a DPF inlet temperature setpoint of 650°C. Once the DPF regeneration is completed the system returns into the phase 1 operation. This sequence is repeated until a total runtime of 28 hours is obtained. After the 28 hours the system is forced into the desulfurization mode with a setpoint temperature of 700°C and frequent lean-rich transitions as described in the section DESULFURIZATION. In addition to the operating point discussion the table also contains temperature information for the different emission control components as well as the fuel flow rates for each state.

Table 5: Durability Cycle Operating Conditions

Operating point

Duration Reps Fuel consumed

[rpm], [Nm] [min] - [gallons] OP 1 2000 rpm, 210

Nm 5 0.262

OP 2 2600 rpm, 160 Nm

5 0.287

Phase 1

(OP 1 + OP 2) x 12

300 120 6.234

Phase 2

DPF regeneration 2600 rpm, 110 Nm

20 300 1.062

Total per cycle

Phase 1 + Phase 2

140 300 7.296

Phase 3

Desulfurization 2200 rpm, 75 Nm

20 25 0.668

SYSTEM PERFORMANCE EVALUATION

After completion of the durability testing with each ECS, the components were installed in the project vehicle and underwent the same test sequence as with the parts during the development phase. It could be proven that with the NAC, ECS emissions can be met under end-of-life conditions. The SCR system performance degraded to a point which made it impossible to meet the limits for Tier 2 Bin 5. Figure 7 shows the system performance in the course of the aging process for both ECS.

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NM

HC

[g/m

ile]

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

CO [g/mile]0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

PM [m

g/m

ile]

0

2

4

6

8

10

12

NOx [g/mile]0.00 0.05 0.10 0.15 0.20 0.25

Tier2 Bin5 50k Tier2 Bin5 120k

NAC DPF : 0hr Composite FTP 75 NAC DPF : Full Useful Life Composite FTP 75 SCR DPF : 0hr Composite FTP 75 SCR DPF : Full Useful Life Composite FTP 75

Figure 7: Emission Results with Useful-Life Aged Components

The system degradation of the SCR system is only partially due to the loss of cold start performance. Overall the SCR system degraded over the entire operating range to a degree that did not allow meeting the emission standard.

ECS COMPARISON

Both Emission Control Systems (ECS) were developed on one engine and vehicle platform, which allows this project to directly compare the emissions and fuel economy impacts for each system. Figure 8 shows the comparison of four different configurations in the course of the program. The left bar represents the baseline vehicle as it was delivered from the dealership. The NOx as well as the fuel economy are at their highest level since the vehicle was certified to EU 4 emissions standards and was operated over the U.S. FTP75 which is a significantly more aggressive driving schedule. The subsequent reduction in NOx represented as the Tier 2 Bin 5 engine-out bar results in a fuel economy penalty of 10% to 12% over the FTP75 and a few percent points more over the HWFET. The measurement for the Tier 2 Bin 5 engine-out level was conducted without any ECS relevant interventions. The implementation of the NAC or respectively the SCR system results in a total NOx reduction of over 90% for both, FTP75 and HWFET. An additional fuel economy impact of 2% to 3% has to be accounted for when applying these ECS. The additional fuel economy penalty is a result of the increased backpressure, the NAC regeneration events, and the rapid warm-up particularly with the SCR system.

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Fuel economy [mpg]

35.28

31.76

30.72 30.74

34.84

29.9630.06

30.73

27

28

29

30

31

32

33

34

35

36

Base EU4 T2B5 engine out

Tailpipe NAC

TailpipeSCR

Fuel

eco

nom

y [m

pg]

B20 Fuel economy [mpg]

ULSD Fuel economy [mpg]

10%

12% 13%

14%

13%

14%

FTP75NOx emissions [g/mile]

0.74

0.34

0.03 0.04

0.31

0.05 0.04

0.72

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Base EU4 T2B5 engine out

Tailpipe NAC

Tailpipe SCR

NO

x [g

/mile

]

B20 NOx [g/mile]ULSD NOx [g/mile]

57%

92%

96% 94%

94%

54%

NOx emissions [g/mile]

0.49

0.35

0.07

0.020.010.03

0.43

0

0.1

0.2

0.3

0.4

0.5

0.6

Base EU4 T2B5 engineout

Tailpipe NAC

Tailpipe SCR

NO

x [g

/mile

]B20 NOx [g/mile]ULSD NOx [g/mile]

30%

84%

95% 97%

95%

Fuel economy [mpg]

59.07

50.2548.97

50.73 50.58

47.7349.49

58.18

27

32

37

42

47

52

57

62

Base EU4 T2B5 engineout

Tailpipe NAC

Tailpipe SCR

Fuel

eco

nom

y [m

pg]

B20 Fuel economy [mpg]

ULSD Fuel economy [mpg]

15% 17%18%

14%13%

HWFET

Figure 8: Impact of Emission Reduction on Fuel Economy

COMBUSTION ANALYSIS

The fuel economy numbers discussed in the previous section lead to more detailed investigations to determine the root cause of improved fuel economy operating with Biodiesel fuel blends. This effect is counter intuitive as B20 has a 2.5% lower heating value when compared to ULSD. The lower heating value typically results in a decrease in power output; therefore, increasing specific fuel consumption. This observation was reported in an earlier publication [13].

Figure 9 shows the combustion comparison of the two different fuels at the same operating point on the engine map (1500 rpm, 15 mm3/cycle fuel injection quantity command BMEP/IMEP varied as function of combustion efficiency, see Table 6:). The heat release rate as well as the total heat release was consistently higher for the Biodiesel fuel. As a byproduct of the faster combustion, the pressure rise rate is also higher. The analysis was done in a wide range of the engine map resulting in the same conclusion.

Pres

sure

[bar

]

0

10

20

30

40

50

60

70

80

Inje

ctio

n Si

gnal

Hea

t Rel

ease

[J]

0

100

200

300

400

500

°CA-30 0 30 60 90 120 150 180

B20 ULSD

Pres

sure

Rat

e [b

ar/d

eg]

-2

-1

0

1

2

3

4

°CA-5 0 5 10 15 20

Hea

t Rel

ease

Rat

e [J

/deg

]

-20

0

20

40

60

°CA5 10 15 20

Figure 9: Combustion Analysis 1500 rpm, 15 mm3/cycle

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Table 6: lists additional details that corroborate the findings discussed above. At the same commanded fuel quantity the IMEP is approximately 2.5% higher which is an indicator of higher combustion efficiency. The energy supplied is approximately 1.5% lower however does not follow the 2.5% heating value reduction of the fuel specifications. The offset is a result of the increased actual fuel quantity (due to higher fuel density of Biodiesel fuel blend) injected into the cylinder which compensates for the 1% of the total energy difference.

Another indicator of higher combustion efficiency is the gaseous emissions listed. NOx is significantly higher operating on B20, while HC and CO are elevated running on ULSD. Also the smoke number, as a combustion efficiency indicator, corroborates the superior combustion efficiency of the Biodiesel fuel blend.

Table 6: Combustion Analysis - B20 versus ULSD

555.13546.35Energy supplied [J/cyc]

1.080.55Smoke [FSN]

269.16165.02HC [ppm]

852.91648.23CO [ppm]

76.96102.59NOx [ppm]

88.6392.04Mass burn Efficiency [%]

13.05113.119Injection mass [mg/cyc]

5.615.75IMEP [bar]ULSDB20

555.13546.35Energy supplied [J/cyc]

1.080.55Smoke [FSN]

269.16165.02HC [ppm]

852.91648.23CO [ppm]

76.96102.59NOx [ppm]

88.6392.04Mass burn Efficiency [%]

13.05113.119Injection mass [mg/cyc]

5.615.75IMEP [bar]ULSDB20

IMPACT OF BIODIESEL OPERATION ON ENGINE MECHANICS

After completion of the durability test for the NAC as well as the SCR system the engine underwent an accelerated aging schedule representative of twice the useful life or approximately 240,000 miles. At the conclusion of the project it was disassembled and each component was carefully analyzed. All moving parts such as bearings, pistons, piston rings were inspected and measured. None of the components of the engine, including the injectors showed any sign of excessive wear or other signs of deterioration as result of the extended Biodiesel operation. The flow characteristics of the injectors remained comparable to the levels before the start of the durability study. Figure 10 shows several engine components after the disassembly.

The cylinder bore in the upper left corer shows the honing pattern which indicates that there was no loss in oil control for this engine. The main bearing shown below the cylinder bore shows no sign of deterioration or wear. This component is typically most prone to wear due to oil dilution and the resulting loss in lubricity of the engine oil. The main journal, as well as one camshaft, is shown on the right side of the picture. No visible signs of wear can be detected.

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Figure 10: Selected Engine Components

CONCLUSION

The following list represents the main steps for the complete development program, which resulted in the integrated engine – emission control system configuration:

• Measurement of the existing production hardware to provide a baseline • Recalibration for the reduced engine-out level • Implementation and calibration of the NAC system • Implementation and calibration of the SCR system • Aging of the NAC system • Aging of the SCR system The combination of engine hardware, engine control strategy and calibration resulted in an engine-out NOx increase under Biodiesel operation, which consequently provided small improvements in fuel economy. The performance of the NAC system using B20 fuel was at a higher rate compared to ULSD, which resulted in a more optimal temperature especially when the FTP75 cycle was conducted. The results of the testing showed only a negligible impact on NAC or DPF regeneration events. In addition, the performance of the SCR system did not change when using the two fuels.

The NAC system is able to provide sustainable conversion efficiencies throughout the useful life, which was proven by the durability tests on both emission control systems. The SCR system experienced a significant degradation in system performance, which was attributed to the deactivation of the iron-zeolite through thermal shock that resulted from HC adsorption.

The hardware was exposed to an accelerated aging protocol of twice the useful life. No Biodiesel related wear or engine mechanical deterioration was found as a result of the accelerated aging process.

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ACKNOWLEDGMENTS

The authors would like to express their gratitude to all of the participating team members that supported the project and actively contributed to the technical discussions.

This work was jointly supported by the United States Department of Energy, Office of Vehicle Technologies and by the National Biodiesel Board. Special thanks to Kevin Stork and Dennis Smith of the U.S. DOE, as well as Steve Howell of the NBB.

Also, thanks to Joe Kubsh and Rasto Brezny of MECA and its team members for the in-kind contributions of all exhaust aftertreatment hardware and to Charles Schenk of the U.S. EPA for providing the opportunity to conduct the vehicle tests.

REFERENCES

1. Tomazic D., Tatur M., Thornton M. “Development of a Diesel Passenger Car Meeting Tier 2 Emissions Levels,” SAE Paper 2004-01-0581, March 2004, Detroit, Michigan.

2. Tomazic D., Tatur M., Thornton M. “APBF-DEC NOx Adsorber/DPF Project: Light-Duty Passenger Car Platform,” DEER Paper, August 2003, Newport, Rhode Island.

3. Geckler S., Tomazic D., et al. “Development of a Desulfurization Strategy for a NOx Adsorber Catalyst System,” SAE Paper 2001-01-0510, March 2001, Detroit, Michigan.

4. McDonald, J. “Progress in the Development of Tier 2 Light-Duty Diesel Vehicles,” SAE Paper 2004-01-1791, March 2004, Detroit, Michigan.

5. Webb, C., et al. “Achieving Tier 2 Bin 5 Emission Levels With a Medium-Duty Diesel Pick-Up and a NOx Adsorber, Diesel Particulate Filter Emissions System-Exhaust Gas Temperature Management” SAE Paper 2004-01-0584.

6. Whitacre, S., et al. “Systems Approach to Meeting EPA 2010 Heavy-Duty Emission Standards Using a NOx Adsorber Catalyst and Diesel Particle Filter on a 15L Engine” SAE Paper 2004-01-0587.

7. Tatur M., Tyrer H., Tomazic D., Thornton M., McDonald, J. “Tier 2 Intermediate Useful Life (50,000 miles) and 4000 Mile Supplemental Federal Test Procedure (SFTP) Exhaust Emission Results for a NOx Adsorber and Diesel Particle Filter Equipped Light-Duty Diesel Vehicle,” SAE Paper 2005-01-1755.

8. Thornton M., Tatur M., Tyrer H., Tomazic D. “Full Useful Life (120,000 miles) Exhaust Emission Performance of a NOx Adsorber and Diesel Particle Filter Equipped Passenger Car and Medium-Duty Engine in Conjunction with Ultra-Low Sulfur Fuel,” DEER 2005.

9. Tyson, K.S., Bozell, J., Wallace, R., Petersen, E., Moens, L. “Biomass Oil Analysis: Research Needs and Recommendations”, Technical Report, National Renewable Energy Laboratory/TP-510-34796, June 2004.

10. R.L. McCormick, A. Williams, J. Ireland, M. Brimhall, and R.R. Hayes ‘Effects of Biodiesel Blends on Vehicle Emissions’ Milestone Report NREL/MP-540-40554 October 2006.

11. John Sheehan, Vince Camobreco, James Duffield, Michael Graboski, Housein Shapouri ‘An Overview of Biodiesel and Petroleum Diesel Life Cycles’, NREL/TP-580-24772, May 1998.

12. Williams, et al., “Effect of Biodiesel Blends on Diesel Particulate Filter Performance” SAE 2006-01-3280 13. Tatur et al. ‘Biodiesel Effects on U.S. Light-Duty Tier 2 Engine and Emission Control Systems’, SAE 2008-01-0080

CONTACT

Marek Tatur – Department Manager, Diesel Engine Development, FEV Inc. Mailto:tatur@fev-et.com

Dr. Matthew Thornton – Senior Engineer, National Renewable Energy Laboratory. Mailto:mattew_thornton@nrel.gov

DEFINITIONS, ACRONYMS, ABBREVIATIONS

BTDC: Before Top Dead Center B20: Biodiesel with 20% renewable fuels content B5: Biodiesel with 5% renewable fuels content CA: Crank Angle CO: Carbon Monoxide CO2: Carbon Dioxide CDPF: Catalyzed Diesel Particle Filter DOE: U.S. Department of Energy DPF: Diesel Particle Filter ECM: Electronic Control Module ECS: Emission Control System

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ECU: Engine Control Unit EGR: Exhaust Gas Recirculation EPA: Environmental Protection Agency FSN: Filter Smoke Number FTP: Federal Test Procedure HC: Hydrocarbon HD: Heavy-Duty HFET: Highway Fuel Economy Test (HWFET) HSDI: High-Speed Direct Injection LA4: Urban Dynamometer Driving Schedule (UDDS) NAC: NOx Adsorber Catalyst NMHC: Non-Methane Hydrocarbon NO: Nitric Oxide NO2: Nitrogen Dioxide NOx: Oxides of Nitrogen O2 Oxygen OEM: Original Equipment Manufacturer PM: Particulate Matter PCR: (Boost) Pressure Control Regulator RPM: Revolutions per Minute (engine speed) SCR: Selective Catalytic Reduction SET: Supplemental Emissions Test SFI: Secondary Fuel Injector THC: Total Hydrocarbon TV: Throttle Valve UDDS: Urban Dynamometer Driving Schedule ULSD: Ultra-Low Sulfur Diesel (The base fuel for this test) VNT: Variable Nozzle Turbine