Combustion engine study

The combustion engine study aims to investigate comparative results of using crude palm diesel (blending 10% of CPO in diesel, 10% CPO diesel) on engine combustion of a CI IDI swirl chamber engine. The experiments, conducted on a Ford Ranger WL81 2.499 litre engine, were composed of two parts. First, measure and analyse in-cylinder pressure and fuel injection line pressure by using crude palm diesel and diesel fuel. Second, study combustion phenomena of both fuels in the swirl chamber by means of engine visioscope. Results show details of phenomena of spray, flame propagation. Two-colour method was also employed to evaluate flame temperature and distribution of soot in flame. And finally, to compare results of visualised combustion phenomena with heat release that estimated from in-cylinder pressure information.

Many properties of the 10% CPO diesel fuel can be attributed directly to the thickening effect of the CPO on the diesel fuel. In this study, blending 10% of CPO by volume in diesel can meet the Thai diesel fuel specification. The primary properties of both the baseline diesel and the crude palm diesel blend are shown in Table 23.1. The higher density and higher viscosity of CPO compared with diesel fuel resulted in slight increasing of these properties in the resulting crude palm diesel blends. The blend also has roughly 5% less energy per volume and less cetane value than diesel fuel. The 10% CPO diesel shows the slight reductions in T90 point that may affect the poor long-trip economy. The addition of CPO to diesel fuel will degrade the cetane number of the resulting 10% CPO diesel blend. The flash point of 10% CPO diesel is controlled by high flash point of the CPO. The flash point of 10% CPO diesel is higher than that of diesel fuel.

Подпись: Engine type Pre-chamber Displacement Bore Stroke Compression ratio Injection pump Injector starting pressure Подпись: WL 81 Swirl pre-chamber 2499 cm3 93 mm 92 mm 21.6 Rotary distributor type 11.4-12.1 MPa

The engine under study is a commercial IDI, water cooled four cylinders, in-line, natural aspirated engine. The following chart displays the main dimensions:

Table 23.1 Comparative diesel and 10% CPO diesel properties17

Properties

Unit

Test method

Reference

diesel

10% CPO diesel

Thailand

diesel

specification

Specific gravity

ASTM D1298

0.8266

0.8360

0.810-0.870

@ 15.6/15.6°C Cetane index

ASTM D976

58.9

47 min

Cetane number

ASTM D613

59.3

55.5

47 min

Viscosity @ 40°C

CST

ASTM D445

3.10

3.910

1.8-4.1

Pour point

°C

ASTM D97

-3

-6

10 max

Distillation

IBP

°C

ASTM D86

10% recovered

°C

50% recovered

°C

90% recovered

°C

350.6

346.2

350 max

Lubricity by HFRR

pm

CEC F-06-A-96

522 (+LA =

209

460 max

Total acid number

ASTM D974

398)

0.04

1.02

Gross heating value

J/g

45 968

44 982

44 500 min

The engine was connected to an AVL alpha 40 eddy-current dynamometer. In-cylinder pressure was taken by AVL piezoelectric pressure transducer model GU12P. Fuel line pressure was taken by a KISTLER 607C1 pressure transducer.

Indicating data were captured with Cussons P4503 shaft encoder and Cussons P4500 autoscan. Direct photography was taken with an AVL Engine Visioscope. The system consists of a PixelFly VGA Colour CCD camera (resolution 640 x 480 pixel), an AVL control unit, AVL 364C crank angle encoder, an optical linkage to the camera and the endoscope. The optical access for the endoscope to the swirl chamber of the fourth cylinder was prepared through the cooling system of the cylinder head. The visioscope software controls the triggering of the digital camera within a crank angle tolerance of 0.1°CA. The endoscope has a viewing angle of 30° forward view. To capture the spray images, the light source unit with fibre optic (40 mJ/flash with 20 ps duration at frequency of 10 Hz) was used.

The schematic arrangement of experimental set up is shown in Fig. 23.12.

The experiments were carried out at constant speed, steady state conditions at selected high probability operating points along ECE 15 driving cycle, as shown in Table 23.2.17,18 For the combustion analyses, images of simultaneous complex spray, inflammation and combustion processes in the swirl chamber were taken. Speed, torque, fuel consumption, engine operating pressure and temperature for both fuels were recorded during each test.

Comparison of in-cylinder pressure, fuel line pressure, fuel injection rate, heat release rate, net heat release and mass fraction burned is shown in Fig. 23.13.18 The measurement of in-cylinder pressure and fuel injection line pressure has

Table 23.2 Engine test points (selected high probability operating points along ECE 15 driving cycle)

Test point number

Speed

(rev/min)

Torque

(Nm)

Statistical frequency (%)

1

Idle speed

39.49

2

1000

30

2.05

3

2000

30

7.69

4

2000

50

n. a.

5

2250

20

1.02

6

2750

20

12.31

image178

indicated that 10% CPO diesel has approximately 1° of early injection timing compared with diesel. The 10% CPO diesel also has longer ignition delay and higher amount of fuel injected mass (mf) due to its lower energy density. The maximum in-cylinder pressure of 10% CPO diesel is similar to diesel. Net heat release and mass fraction burned of 10% CPO diesel are also lower than diesel.

Comparison of maximum in-cylinder pressure (Pmax), SOI, ignition delay and fuel injected mass (mf) as engine operates with diesel and 10% CPO diesel are summarised in Table 23.3.

image179

image180

image181

23.13 Comparison of in-cylinder pressure, fuel line pressure, fuel injection rate, heat release rate, net heat release and mass fraction burned as engine operates with diesel and 10% CPO diesel at 2000 rev/min, 30 Nm.18

Table 23.3 Comparison of maximum in-cylinder pressure (Pmax), SOI, ignition delay and fuel injected mass (mf) as engine operates with diesel and 10% CPO diesel18

Pmax SOI Ignition delay mf

(bar) (°CA) (psec) (mg/cycle)

Test point

Diesel

10%

CPO

diesel

Diesel

10%

CPO

diesel

Diesel

10%

CPO

diesel

Diesel

10%

CPO

diesel

Idle

53.26

53.31

-4.0

-4.0

2.08

2.2

6.22

7.04

1000 rpm, 30 Nm

58.45

59.45

-10.5

-11.5

2.08

2.17

9.63

10.77

2000 rpm, 30 Nm

61.48

61.84

-11.0

-11.5

1.54

1.50

9.99

10.88

2000 rpm, 50 Nm

61.72

61.74

-10.0

-10.0

0.46

0.46

12.64

13.97

2250 rpm, 20 Nm

64.98

64.97

-10.5

-11.0

0.78

1.04

8.72

9.81

2750 rpm, 20 Nm

63.90

64.66

-9.0

-9.0

0.21

0.21

9.56

10.46

The images of spray formation at selected operating points of reference diesel and 10% CPO diesel are shown in Fig. 23.14 (a) and (b), respectively.1718 The figures show that 10% CPO diesel has approximately 1°-2° of early injection timing compared with diesel. The early injection timing is probably due to the higher isentropic bulk modulus and higher viscosity of CPO compared with diesel, resulting in a slight increase in these properties in the resulting blends.19 The comparison of the observed spray formation between reference diesel and 10% CPO diesel are summarised in Table 23.4. It was found that, using OEM injection pump and standard injector in a pre-chamber, with 10% CPO diesel the observed sprays were wider than that of reference diesel. The difference in spray angle tends to reduce with increasing speed. The observed spray penetration with 10% CPO diesel is also longer than reference diesel in low to medium engine speed range. The higher the engine load, the longer the spray penetration was observed.

Summarising the results of these sections, as shown in Fig. 23.15, it can be noted that the visible combustion course in a swirl chamber occurs without any starting aids.17,18 The visible inflammation appears above the fuel jet. From there the flame engulfs the whole swirl chamber very quickly. This process needs some delay times. The comparison of the observed luminous spray combustion between reference diesel and 10% CPO diesel is shown in Table 23.5. It was found that 10% CPO diesel has shown a longer ignition delay period than diesel. The combustion for both fuels tends to start faster with increasing speed. After this ignition delay, the burning area rotates under the influence of the swirl. This motion can be observed for nearly the entire burn duration after complex luminous inflammation has occurred. In the low speed and load range, 10% CPO diesel

image183

23.14 (a) and (b) Images of liquid fuel spray in the pre-chamber for reference diesel and 10% CPO diesel respectively. The crank angles at which the images were acquired are written on the left of the images.

Table 23.4 Maximum spray penetration (mm) and spray angle (degree)

Test point

Maximum penetration (mm)

Maximum spray angle (degree)

Diesel

10% CPO diesel

Diesel

10% CPO diesel

Idle

23.0

27.8

25.5

24.1

1000 rpm, 30 Nm

27.9

25.6

24.1

26.4

2000 rpm, 30 Nm

29.8

27.1

36.8

41.4

2000 rpm, 50 Nm

28.3

28.7

36.3

39.3

2250 rpm, 20 Nm

25.6

28.4

36.4

39.4

2750 rpm, 20 Nm

28.5

33.7

36.4

40.8

image184

image185

23.15 Images of luminous spray combustion in the pre-chamber for reference diesel and 10% CPO diesel showing the start of luminous flame, the position for maximum area of over 2400 K and end of luminous flame. The crank angles at which the images were acquired are written under the images.

combustion duration tends to have a slightly shorter period than diesel. This may be due to the benefit of oxygen content in the fuel.

Using the ‘Thermovision’ software from AVL List GmbH,20 the temperature of radiating soot particles was calculated from the three spectral intensities in the flame images using the two-colour method. In the temperature images, shown in Fig. 23.16, purple — blue — green — yellow — red — white in the original colour image denote the temperatures ranging from 1800 to 3000 K.

Table 23.5 Comparison of the first appearance of luminous flame, end of luminous flame and luminous flame duration between reference diesel and crude palm diesel in an IDI engine

First appearance End of luminous Luminous flame

of luminous flame (°CA) duration in

flame (°CA) pre-chamber (°CA)

Test point

Diesel

10% CPO diesel

Diesel

10% CPO diesel

Diesel

10% CPO diesel

Idle

3.5

5.0

28.5

25.5

25.0

20.5

1000 rpm, 30 Nm

0.5

2.0

32.5

31.0

32.0

29.0

2000 rpm, 30 Nm

-0.5

-0.5

30.5

28.5

31.0

29.0

2000 rpm, 50 Nm

0.5

-0.5

27.5.

31.0

27.0

31.5

2250 rpm, 20 Nm

-0.5

-0.5

25.5

27.0

26.0

27.5

2750 rpm, 20 Nm

1.0

-1.0

27.5

26.5

26.5

27.5

image186

(a) Diesel.

23.16 Flame temperature images of spray combustion in the pre­chamber for reference diesel and 10% CPO diesel. The crank angles at which the images were acquired are written at the top of the images.

(Continued )

image187

(b) 10% CPO diesel. 23.16 Continued.

The difference in combustion is much more obvious when looking at the flame. The in-cylinder combustion temperature of 10% CPO diesel combustion is lower than diesel combustion. From Fig. 23.17, the flame areas of temperature above 2400 K for diesel and 10% CPO diesel at 2000 rev/min, 30 Nm are compared. It was found that diesel fuel showed greater amount of flame areas of temperature above 2400 K.

In the soot distribution images, the same colour scale denotes soot densities ranging from thin to dense soot. The appearance of luminous combustion flame comes from the radiation of soot particles occurred in the fuel mixture oxidation zone. Prediction of soot density distribution at selected operating points of diesel and 10% CPO diesel are shown in Fig. 23.18. It is noted that soot density in 10% CPO diesel combustion flame tends to be slightly lower than that in diesel.

Comparative studies of engine fuelled with reference diesel and 10% CPO diesel were investigated. Visualised images show the effects of CPO in 10% CPO diesel blend. The injection timing of 10% CPO diesel is approximately 1° earlier compared with the injection timing of reference diesel. Observed 10% CPO diesel fuel sprays have shown either longer spray tip penetration length or wider spray angle than the reference diesel.

image188

23.17 Flame area with temperature above 2400 K for 10% CPO diesel and diesel at 2000 rev/min, 30 Nm.

image189

(a) Diesel.

23.18 Soot concentration distribution images of spray combustion in the pre-chamber for reference diesel and 10% CPO diesel. The crank angles at which the images were acquired are written at the top of the images.

(Continued )

image190

(b) 10% CPO diesel.

23.18 Continued.

Images of spray combustion indicate that the period of 10% CPO diesel combustion phenomena occurred more retardedly with respect to TDC than diesel. As its consequence, together with the lower heat of combustion, the predicted combustion flame temperature and soot density distribution, using the two-colour method, are lower than the reference diesel. The combustion for both fuels tends to start faster with increasing speed. The observed combustion duration of 10% CPO diesel is slightly shorter than that of diesel.