engine-based test method for determining the ignition

RESE ARCH  Fuels

PROF. DR.-ING. KARL HUBER is Professor of Thermo­ dynamics and Combustion Engines at the Technische Hochschule Ingolstadt (Germany).

The cetane number, used as a measure of ignitability, is a vitally important attribute of diesel fuels. It is of special significance both in engine design for the specification of minimum compression ratio as well as in engine operation, since it has a significant influence on the noise level and exhaust emissions behaviour of diesel engines. Testing methods for determining the ignition quality of diesel fuels are thus of particular importance for the fault-free operation of diesel engines. At the Technische Hochschule Ingolstadt, the standardised methods were performed and an approach for a new engine-based testing method developed.

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DIPL.-ING. (FH) JOHANN HAUBER is Research Associate at the Institute for Applied Research at the ­Technische Hochschule Ingolstadt (Germany).

ENGINE-BASED TEST METHOD FOR DETERMINING THE IGNITION QUALITY OF DIESEL FUELS

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1 INTRODUCTION 2 L ATEST TECHNICAL STANDARD 3 DESCRIP TION OF THE BASF ME THOD 4 VEGE TABLE OILS 5 BASF TEST ENGINE INVESTIG ATIONS 6 NE W ME THOD 7 SUMMARY AND OUTLO OK

1 INTRODUCTION

The diversification of fuels [1-5] and types of drive is continually progressing in the face of finite natural oil resources. In the field of liquid hydrocarbons in particular, an increase in biogenic and other alternative fuel additives can be observed all over the world and this trend is expected to continue. The importance of fuel testing is increasing at the same rate. In the case of diesel fuels, ignitability is immensely important. It is characterised by the ignition delay, the time between start of injection (SOI) and start of combustion (SOC). The cetane number (CN) as a measure of ignition quality is determined in accordance with EN 590 for all approval tests for diesel fuels. Alongside noise emissions, such as the so-called diesel knock, the cetane number has a considerable influence on the limited emissions of diesel engine combustion. The production of nitrogen oxides and soot [4, 6] is influenced to a significant extent by the ignitability of the fuel. This combustionrelated parameter is traditionally determined using engine-based test methods. Within the context of the BioFIRe (Biogenic Fuel Ignition Research) research project lasting several years, which is supported by the Fachagentur Nachwachsende Rohstoffe (specialist agency for renewable primary products [FNR]), the ignition behaviour of fossil and biogenic fuels was analysed in engine tests. A new method was developed based on the BASF test procedure which will be presented below.

so-called constant volume testers for alternative non-engine-based testing methods. The fuel is injected into a combustion chamber filled with synthetic air preconditioned in terms of pressure and temperature, ❸. However, the ignition conditions can only be compared to those in engine testing to a limited extent due to the static procedure with lack of charge movement to improve carburetion (physical ignition delay). A threshold value trigger resulting from a measurable increase in pressure in the combustion chamber is used as a criterion for determining the SOC. To achieve a better correlation of the cetane number (Derived Cetane Number, DCN) established on the basis of the combustion chamber test with the engine results, the CID 510 from PAC evaluates the time between SOI and 50 % increase in pressure as a key value for the conversion rate of the fuel in addition to the ignition delay. The deviceand process-specific measured variables of the ignition delay determined in a constant volume combustion chamber are converted to a DCN using defined correlation equations. Depending on the fuel composition, a certain DCN determined this way can be related more or less well to the CN measured in a test engine [10]. For most fossil fuels without ignition improver, the cetane index is also suitable for characterising the ignition quality. This can be calculated using the measured values for fuel density, which are easy to determine using laboratory instruments, as well as point(s) on the boiling curve of a two- (ASTM D976-06) or fourparameter (ASTM D4737 respectively DIN EN ISO 4264) calculation formula. If cetane improvers are added to the fuel or if the fuel contains alternative/biogenic components, inapplicable results will be returned [6]. 3 DESCRIPTION OF THE BASF METHOD

The BASF testing method for evaluating the cetane number is based on the test engine described with indirect injection into a turbulence chamber. The parameter of ignition delay required for

2 LATEST TECHNICAL STANDARD

The engine-based test procedures as there are the CFR-F5 [7] and the BASF method [8] are used to test the ignition quality of diesel fuels. What these two methods have in common is the determination of ignitability at a constant ignition delay of 13 or 20 °CA, ❶. In the case of the CFR-F5 method, the ignition delay is influenced by a change in compression ratio, whereas with the BASF method the quantity of intake air is regulated. This means that with ignitable fuels the quantity of intake air must be reduced with the latter, the effect of which is an unintentional increase in residual gas content and the combustion-air ratio λ is reduced. With both methods, start of combustion must take place at the TDCF (top dead centre firing). The compression ratio or air quantity required is used as a measure of ignition quality for a certain fuel. Cetane number is determined by comparison with n-cetane/1-methylnaphthalene mixtures with the eponymous cetane as the ignitable fuel component. This means that for the ease of ignition of the test fuel, the cetane number can be quoted as a reference parameter for a two-component mixture with equivalent ignition characteristics. An application of the intake air values achieved under standard conditions can be found in ❷ in the form of a daily calibration curve according to [9]. There are currently three testing devices approved as   11I2013   Volume 74

BASF

CFR-F5

Standard

DIN 51773

ASTM D613

Engine speed [rpm]

1000 ± 10

900 ± 9

Bore [mm]

95

82.55 (3.250“)

Stroke [mm]

120

114.3 (4.500“)

Compression ratio [-]

18.2

8 – 36

20

13

8 ± 0.5

13 ± 0.2

Turbulence chamber

Turbulence chamber

20 (constant)

13 (constant)

Intake air quantity

Compression ratio

20 ± 5

66 ± 1

Oil temperature [°C]

70 ± 5

57 ± 8

Type of cooling, cool-

Free convection

Free convection

ant temperature [°C]

(boiling), 100 ± 2

(boiling), 100 ± 2

Start of Injection [°CA before TDCF] Injection quantity [cm³/min] Mixture formation Ignition delay (ID) [°CA] Ignition delay control Intake air ­t emperature [°C]

❶ Operating conditions for the engine-based cetane number testing method [7, 8, 10]

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RESE ARCH  Fuels

4 VEGETABLE OILS

100 CN mixtures Diesel fuel samples

90

BASF calibration curve Cetane number [-]

80 70 60 50 40 30

0

5

10

15 20 25 Air flow measurement [%]

30

35

40

❷ Calibration curve at 20 °CA ignition delay for determining the cetane number according to [9]

evaluating the fuel characteristic according to DIN 51773 (reference value: 20 °CA constant) is measured by two inductive sensors. These are a needle-lift sensor mounted on the injection nozzle for detection of the SOI as well as a combustion pressure transducer which emits a signal about proportional to the rate of pressure change in the turbulence chamber. Both sensors are made available to the so-called ignition delay measuring device as input parameters for determining ignition delay [9]. Due to the method chosen, at SOC almost the complete fuel quantity is injected into the combustion chamber. Combustion begins at top dead centre (TDC) and proceeds almost isochorically, whereby the whole air-fuel-mixture ignites suddenly and leads to high rates of heat release, resulting in a steep increase in pressure which is much more pronounced than as a result of compression. This type of process management is the pre-condition for the sensor technology and evaluation logics which are used to draw conclusions directly about SOC from the pressure pattern without the heat release being explicitly calculated, ❹. If fuels such as FAME (first generation biodiesel) reveal a lower combustion speed, ④ middle, this leads to inaccuracies in determining the SOC for the BASF method, which means that the vagueness in determining CN increases.

Standard

In principle, diesel engines can not only be operated with fossil diesel fuels and FAME, they can also be operated with comparatively high-viscose fuels such as vegetable oils [2, 3, 14]. Up to now, however, reliable measurement of these biogenic fuels has not been possible using the engine-based test methods [15], which is mainly due to the poor mixture formation during engine operation with vegetable oils at low fillings (BASF method) or respectively low compression ratio (CFR method). In practice, the injection nozzles of a diesel engine are usually equipped with an electric resistance heater to improve vegetable oil operation, in order to reduce the viscosity of the bio-fuel and improve the spray pattern and thus the mixture formation [14]. To get an impression of the spray quality, a high-speed camera was used to record injection sprays of different fuels at room temperature and with the nozzle holder heated to 63 °C. In ❺ (left), the spray pattern pictures show conventional EN-590 diesel at both temperatures, whereby no real difference can be recognised between the quality of atomisation in these two cases. In contrast, with vegetable oil at room temperature, a solid injection jet could be seen even more than 100 mm away from the nozzle, with only a few droplets deviating from the main jet. When the injection nozzle was heated, on the other hand, there was a significant improvement in the spray pattern, although it was still not as good as the atomisation quality of diesel fuel. Supported by strong charge movement in the turbulence chamber, it was no problem to switch to vegetable oil operation with the engine at operating temperature and an increased air quantity coupled with a heatable nozzle holder. However, it is still not possible to throttle the test engine to the intake air quantity necessary to achieve 20 °CA ignition delay required for determining cetane number according to the BASF method [8, 9], which meant that further improvements to the testing method were necessary. For this, the BASF method described and the respective testing engine were first subjected to detailed analysis. 5 BASF TEST ENGINE INVESTIGATIONS

In order to investigate the engine process, the test engine was equipped with various temperature sensors, an intake manifold

FIT – FUEL IGNITION TESTER

IQT – IGNITION QUALITY TESTER

CID 510 – CETANE ID 510

ASTM D7170

ASTM D6890

ASTM D7668

Dresser / Waukesha

AET

PAC

Combustion air

Synthetic air

Synthetic air

Synthetic air

Filling pressure [bar]

24.0 ± 0.20

21.37 ± 0.07

20.0 ± 0.20

510 ± 50

540 ± 25

567.5 ± 32.5

Ignition delay [ms]

2.9 – 5.0

3.3 – 6.4

2.8 – 6.5

Calibrated CN range [-]

60 – 35

61 – 34

60 – 30

DCN FIT = 171 / ID

DCN IQT = 4.46 + 186.6 / ID

DCN CID = f(ID, CD)

Manufacturer/distributor

Combustion chamber or filling temperature [°C]

Derived cetane number [-] ID – ignition delay [ms] CD – time until 50 % pressure increase [ms]

❸ Operating conditions for the non-engine-based ignitability testing method [10-13]

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pressure sensor as well as a crank angle encoder and a piezoelectric pressure sensor for combustion chamber pressure indication, in addition to the already existing inductive sensors. The crank angle based recording of the pressure pattern and possible additional averaging over several combustion cycles during stationary operation allows the real heat release to be calculated using thermodynamic analysis [6]. With the simultaneous evaluation of the needle lift sensor signal, the exact ignition delay can be calculated

Fossil  diesel fuel

as the interval between SOI and SOC independently of the process management used, ❻. In addition to determining the start of combustion, the calculation of the combustion curve is also used to define the injection quantity with regard to a constant release of energy. According to [9] this is currently to be set to a constant pump flow, whereby neither density nor calorific value of the fuel are taken into account. In addition, it was established that there is great varia-

300

22.5 Cylinder pressure  [bar]

250 200

15

150 100

7.5

50 0

0

-‐50

 

350 Ignition delay

  Needle lift signal [mV] Combustion pressure indicator [mV] (Magnetic inductive sensor) 

400

30

-‐100 FAME

350 300

22.5

250  

Cylinder pressure [bar]  

Ignition delay

200

15

150 100

7.5

50 0

0

-‐50

Needle lift signal [mV] Combustion pressure indicator [mV] (Magnetic inductive sensor)   

400

30

-‐100 55  cetane

350

Ignition delay

300

Cylinder pressure [bar]  

22.5

250 200

15

150 100

7.5

50 0

0

-‐50 330

345

360

375

Needle lift signal [mV] Combustion pressure indicator [mV] (Magnetic inductive sensor)   

400

30

-‐100 390

φ [°CA] Cylinder pressure

  11I2013   Volume 74

Combustion pressure indicator

Needle lift signal

❹ Signals from injection and combustion pressure transducer and cylinder pressure curve with BASF test conditions in accordance to [9]

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RESE ARCH  Fuels

22 °C

63 °C

22 °C

50 mm

50 mm

50 mm

Vegetable oil (rapeseed oil)

50 mm

Common diesel fuel

63 °C

❺ Comparison of spray patterns for conventional diesel fuel (left) and vegetable oil (right) each at 22 and 63 °C

30

1600

25

1300

20

1000

15

700

Ignition delay

10

400

5

100

0 340

350 Cylinder pressure

360

370

φ [°CA]

Temperature

380 Needle lift signal

390 Heat release

-200 400

Gas temperature [K]

possible with the BASF testing engine and the misfiring rate strongly increases. For fuels with lower ignition quality, further methodical and engine-based measures would be required to guarantee reliable engine operation (reduction in engine speed, increase in compression ratio, pre-heating of the intake air, supercharging etc.). Since the cetane number range < 30 CN is less significant for standard diesel engines, however, the method was not pursued further in this direction. In contrast, a cetane number specification of fuels with particularly short ignition delays (> 80 CN) is not limited by engine operation and still possible even with increased throttling (that is to say intake manifold pressure 400 mbar), whereby the measuring inaccuracy in CN increases slightly due to the flatter curve. As a supplement, the curves for determining the DCN from the ignition delay were entered by means of simple conversion of the nominal ignition delay time of two constant volume testers in °CA. It can clearly be seen that the static ignition delay specification takes place via the constant volume tester for ignition delays which in the case of the FIT correspond well to those in engine operation at 400 mbar intake manifold pressure. The IQT

Needle lift signal [-] Heat release [J/°CA*10]

Cylinder pressure [bar]

tion in the quantity of injector leak oil and thus the actual injection quantity is depending on the radial clearance of the nozzle needle and viscosity of the fuel. If, on the other hand, the injection quantity is set to a constant energy released per working cycle, comparability is improved in relation to thermal load in the turbulence chamber and combustion air ratio with different fuel types. Independently of the dynamics of the pressure curve (dp/dφ) at start of combustion and of the fuel-dependant conversion rate, the real ignition delay can thus be calculated. The development of the ignition delay depending on the fuel composition is shown in ❼, whereby two-component mixtures of n-cetane and 1methylnaphthalene were used as fuels. For this, the test engine was operated with different fuel mixtures at three different intake manifold pressures and the respective ignition delay was determined. The SOI was set in such a way that the SOC was within a window of ±4 °CA around TDCF. The significant influence of the cylinder filling, which results in an increase in ignition delay when intake manifold pressure is reduced, can easily be seen. Equally, the extremely progressive development of the ignition delay with reduction of the cetane content is considerable, which is why engine operation with fuels below approximate 30 CN is hardly

❻ Results of the pressure curve analysis and ignition delay calculated from this

58

36

6 Intake manifold pressure 400 mbar Intake manifold pressure 600 mbar

5

Ignition delay FIT Ignition delay [°CA]

24

4

Ignition delay IQT

18

3

12

2

6

1

0 20

30

40

50

60

70

80

Ignition delay (FIT, IQT) [ms]

Intake manifold pressure 500 mbar

30

0 100

90

❼ Dependence of the ignition delay on intake ­ anifold pressure and cetane number as well as m conversion equation for DCN specification in FIT and IQT

Cetane number (mixing ratio n-cetane/1-methylnaphtaline) [-]

works with comparably poorer ignition conditions and thus even higher ignition delays, which makes higher resolution easier. In contrast to these two methods, ③ , the cylinder pressure during ignition delay in the BASF testing engine is only between 10 and 16 bar with an intake manifold pressure of 400 mbar, ❽ . This means that the intensive charge movement in the turbulence chamber and the higher gas temperature must be responsible for the comparatively unexpected short ignition delay. A new method for specifying the cetane number can be derived from the results gained in the analysis.

6 NEW METHOD

In the new method, the heat release is calculated from the measured pressure curve in order to determine start of combustion [6]. Together with the measured start of injection, this allows the ignition delay to be stated. Yet the simple specification of the ignition delay is not sufficient to evaluate the ignition quality of a fuel, particularly since the state variables prevailing during ignition delay influence the start of reaction, ⑦. With the method used previously, the fuel to be tested was bracketed between reference

26 1650

24 22

16

SOI

Pressure [bar]

18

Ignition delay BASF

1150

14 n-cetane mixture: CN 50 Speed = 1000 rpm SOI = 344 °CA pintake = 400 mbar

12 10 8 340

350

360

370

380

Ignition delay FIT

φ [°CA]

Temperature [K]

1400

20

900

650 390

Ignition delay IQT 0

1

p BASF T BASF

  11I2013   Volume 74

2 p FIT T FIT

3 t [ms]

4 p IQT T IQT

5

6

❽ Cylinder pressure curve Needle lift signal

at 400 mbar intake manifold pressure (50 CN n-cetane/1methylnaphthaline mixture) and indicated ignition delay for the FIT and IQT

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RESE ARCH  Fuels

40

40 cetane 45 cetane

45

60 cetane

50

70 cetane 55

90 cetane

60

Cetane number [-]

Diesel fuel 1 0.2

Critical reaction level [-]

50 cetane

70 80 90

0

5

10

15

20

Ignition delay [°CA]

25

❾ Correlation between critical reaction level and cetane numbers for selected diesel fuels

fuels. Since the new method does without comparative measurements, a calculation approach was chosen in the context of the research project, where the cetane number is reached via the intermediate parameter of a so-called critical reaction level as a characteristic ignitability parameter. The intermediate parameter of the critical reaction level is determined by evaluation of the ignition integral of an Arrhenius function with the integration limits start of injection and start of combustion (SOI, SOC) [6]. With the Arrhenius approach, considerations are based on the fact that a certain hydrocarbon-air mixture will self-ignite according to the process management used through the prevailing gas temperature T and pressure p when the ignition integral has reached a defined value.

EQ. 1

SOC

1 Critical reaction level = ∫ __ ​  τ ​ · dφ SOI

with

EQ. 2

  ​  ​ ___ R ·T τ = A · p-n · e ~ EA

Knock models for SI engines [16] where the condition parameters in the end gas put into the evaluation are also based on this correlation. Unlike with knock models where the self-ignition limit can be calculated during engine process simulation on the basis of known knock-resistance of the fuel, both the integration limits and the process parameters for the ignition interval are available in ∼ case of diesel fuel testing. The parameters A, n and EA / R were adjusted as constants for the testing engine in such a way that all fuels with one and the same set of parameters are distinguished with a fixed value for the critical reaction level independently of process management. Finally, the integral value is characteristic for the ignition quality of a fuel, allowing clear conclusions to be drawn about the

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cetane number of the respective fuel, ❾. Thus different environmental conditions and test conditions which are not set exactly, such as intake manifold pressure or start of injection, are automatically compensated. Ultimately, two different ignitable fuel mixtures with a known cetane number are sufficient for calibrating a testing engine. Through calculation of the critical reaction level, the conversion equation for CN determination is thus defined. If limits still to be defined are exceeded, this is an indication of faulty engine function and the engine must be serviced accordingly. This can be caused by a worn or carbonised injection nozzle or incorrect adjustment of the nozzle opening pressure. 7 SUMMARY AND OUTLOOK

A testing method was presented which does not require further reference fuels for determining cetane number apart from two calibration fuels of different ignitability to guarantee fault-free engine function. The engine process management is much closer to real operating conditions than the existing method and is characterised as follows: :: retention of the reliable basic engine with quite stable combustion behaviour :: compensation of different environmental conditions using the Arrhenius approach :: determination of the start of combustion from the heat release :: calculation and control of injection quantity based on heat release :: constant engine operating point for all fuels :: exact measurement of intake manifold pressure :: adaptation of viscosity through nozzle heating :: reference precision of an engine-based test method :: other features comparable with BASF (DIN 51773). Compared to the previous cetane number test method thus result in an improvement in the accuracy of the thermodynamic determination of the ignition delay using online pressure curve analysis, to increase the comparability of the basic process conditions through standard filling values and residual gas contents. Also an

important advantage is the increased engine-based relevance compared to constant volume tests without charge movement. Furthermore, no reference fuels are required for cetane number determination. For the statistical confirmation of the testing method developed in terms of repeatability and comparability, it is imperative that measurements are carried out at different set-up locations, with different testing engines and by different operators. For this purpose, the construction of several prototypes is planned, as well as participation in round robin tests. In addition, the testing method is to be presented to the standards committee for engine-based testing of liquid fuels (FAM AA-643) for a decision. REFERENCES [1] N. N.: Directive 2009/28/EC of the European Parliament and of the Council. 2009-04-23 [2] Harndorf, H.; Schümann, U.; Wichmann, V.; Fink, C.: Motorprozessverhalten und Abgasemissionen alternativer Kraftstoffe im Vergleich mit Dieselkraftstoff. In: MTZ 69 (2008), No. 7/8, pp. 640-646 [3] Harndorf, H.; Wichmann, V.; Schümann, U.; Richter, B.: Requirement ­specification for the use of biofuels in modern high performance engines. ­F NR Conference Neue Biokraftstoffe, Berlin, 2010 [4] Janssen, A.; Jakob, M.; Schnorbus, T.; Kolbeck, A.: Chancen und Herausforderung der Ethanolbeimischung zum Dieselkraftstoff. In: MTZ 72 (2011), No. 7/8, pp. 572-577 [5] Picard, K.: Zielkonflikte im Biokraftstoffmarkt – Konsequenzen für den ­Verbraucher. In: MTZ 69 (2008), No. 4, pp. 294-299 [6] Heywood, J. B.: Internal Combustion Engine Fundamentals. Mc Graw Hill, 1988, ISBN 0071004998 [7] N. N.: Standard Test Method for Cetane Number of Diesel Fuel Oil. ASTM D613, 2010 [8] N. N.: Prüfung flüssiger Kraftstoffe – Bestimmung der Zündwilligkeit ­(Cetanzahl) von Dieselkraftstoffen mit dem BASF-Prüfmotor. DIN 51773, 2010 [9] Lauer, W.: Cetanzahlmessung am Prüfdiesel BASF. Manual, Edition C, 1966 [10] Terschek, R.; Gorek, W.; Feuerhelm, T.: Die Bestimmung der Cetanzahl – Standpunktpapier des Arbeitskreises 643 im Fachausschuss für Mineralöl­ normung (FAM). In: Erdöl Erdgas Kohle (2008), No. 10 [11] N. N.: Standard Test Method for Determination of Derived Cetane Number (DCN) of Diesel Fuel Oils – Fixed Range Injection Period, Constant Volume ­C ombustion Chamber Method. ASTM D7170, 2012 [12] N. N.: Standard Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuel Oils by Combustion in a Constant Volume Chamber. ASTM D6890, 2013 [13] N. N.: Standard Test Method for Determination of Derived Cetane Number (DCN) of Diesel Fuel Oils – Ignition Delay and Combustion Delay Using a ­C onstant Volume Combustion Chamber Method. ASTM D7668, 2012 [14] Wichmann, V.: Konzepte und Betriebsstrategien für die Nutzung von ­R apsölen in Verbrennungsmotoren für den Einsatz in Landmaschinen. Rostock, Technical University, Dissertation, 2008 [15] Attenberger, A.; Remmele, E.: Entwicklung einer Prüfmethode zur ­B estimmung der Cetanzahl von Rapsölkraftstoff. TFZ 6, Straubing, 2003, ISSN 16141008 [16] Franzke, D. E.: Beitrag zur Ermittlung eines Klopfkriteriums der ottomotorischen Verbrennung und zur Vorausberechnung der Klopfgrenze. München, Technical University, Dissertation, 1981

THANKS The results presented were produced by several research projects. The Authors would like to thank the involved companies Rofa Laboratory & Process Analyzers, Bayernoil GmbH, Kistler Instrumente AG and the Gesellschaft für Motoren- und Fahrzeugtechnik as well as the Fachagentur Nachwachsende Rohstoffe (FNR) for their support and funding.

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