Ignition Delay at Various High Pressures. An Experimental Study

Table Contents

INTRODUCTION
1.1 Diesel Combustion
1.2 Ignition Delay
1.2.1 Factors Affecting Ignition Delay:
1.2.2 Chemical Factors
1.3 Diesel Fuel Chemistry
1.3.1 Different properties of Diesel fuel are discussed below
1.5 Test Procedure

RESULTS AND DISCUSSION

CONCLUSION AND FUTURE INVESTIGATIONS

REFERENCES

ABSTRACT An experimental study for the measurement of ignition delay characteristics of burning fuel sprays in Cylindrical Combustion Chamber is carried out on hot air and high pressure. The objective of the study was to investigation the effect of hot air temperature and a well as high pressure on ignition delay of diesel fuel sprays. The effect of blending of n-Pentane with pure diesel was investigated. An experimental set up was design for this purpose with the emphasis on optical method for measurement of ignition delay at various pressures. The results presented here show that ignition delay of diesel fuel spray decreases with increase in the temperature and pressure of hot air. Results also show the effect of methyl group being more dominant at low ignition temperatures and that of alkyl group being more dominant at higher temperature.

Blending of n-pentane with diesel fuel, increase its ignition delay at low ignition temperatures. However, as the concentration of blending fuel was increased beyond 30%, the ignition temperature increase. Ignition temperature for 40% pentane blends is much higher that the pure diesel.

INTRODUCTION

Diesel engines have high thermodynamic efficiency therefore they have always been the first choice for heavy duty vehicles. However, future emission regulation poses a challenge for upcoming diesel engine combustion systems. Future emission regulations are becoming more restrictive, forcing engine designers towards lower exhaust values. With this perspective, knowledge of the injection and the combustion processes is currently being considered as a major research objective. Particularly, the analysis is focused on direct injection Diesel engines, where the fuel-air mixing process plays a dominant role. Only with a good understanding of these phenomena it will be possible to reduce the emission levels without impairing the engine performance and efficiency. [1]

1.1 Diesel Combustion

The combustion process is neither completely non-premixed (diffusion) nor premixed. On the contrary the initial portion of the fuel being injected is vaporized and forms a premixed mixture, in a period known as ignition delay.

Premixed burning takes place with the instantaneous rise in the flame temperature and consequently, the maximum rate of heat release. It is burning phase during which although only 20% of total fuel injected is burnt but nearly 50% of NOx are formed tie to the high temperature during this phase. This premixed burning at the end of ignition delay is followed by homogeneous diffusion burning at different location of combustion chamber.

Abbildung in dieser Leseprobe nicht enthalten

Figure1.1: ROHR Curve for Diesel Engine [2]

This burning phase is followed by heterogeneous diffusion burning during which individual droplets continue to burn and eventually end the combustion process well advance in the exhaust stroke. [2]

1.2 Ignition Delay

When the charge (fuel air mixture) is ignited with the help of a spark plug (in case of SI Engines) or making contact with hot compressed gases in case of CI engines), it doesn’t get burned immediately, rather there is a time last between the two stages known as ignition delay of the fuel.

Ignition delay is subdivided into two parts:

- Physical delay
- Chemical delay

Physical delay is the part in which fuel-au mixing takes place and the resulting mixture is raised in temperature. Chemical delay involves reactions until local inflammation or ignition occurs. Generally, chemical delay is longer than physical delay. However, it depends on temperature. At higher temperatures chemical reactions are quicker and physical delay exceeds chemical delay.

1.2.1 Factors Affecting Ignition Delay:[8]

Abbildung in dieser Leseprobe nicht enthalten

1.2.2 Chemical Factors

Fuel ignition Quality

The most important ignition quality of diesel fuel is its auto ignition property. Auto ignition property of diesel is expressed in terms of Cetane number. The certain number scale is defined by blends of two pure hydrocarbon reference fuels Cetane (n-hexadecane, C6H34), a hydrocarbon with high ignition a1ity, represents the top of the scale with a Cetane number of 100. An isocetane, heptamethylnonane (HMN), which has a very low quality, represents the bottom of the scale with a Cetane number of 15. Thus, Cetane number is given by CN = Percent n-cetane + 0.15 x Percent I-IMN

A calculated Cetane index (CCI) is often used to estimate ignition quality of diesel fuels [9]. It is based on API gravity and the mid- boiling point (temp. 30 percent evaporated). Its use is suitable for most diesel fuels and gives number that corresponds quite closely to Cetane no. A diesel index is also used. It is based on their fact that ignition quality is linked to hydrocarbon composition: n-paraffin has high ignition quality, and aromatic and naphthalene compounds have low ignition quality. The aniline point is the lower temperature at which equal volumes of the fuel and aniline become just miscible, is used, together with the API gravity to give the diesel index:

Abbildung in dieser Leseprobe nicht enthalten

The diesel index depends on the fact that aromatic hydrocarbons mix completely with aniline at comparatively low temperatures whereas paraffin require considerably higher temperatures before they are completely miscible. The diesel index usually gives values slightly above the Cetane number. It provides a reasonable indication of ignition quality in many cases. [11]

1.3 Diesel Fuel Chemistry [15]

Diesel fuel is a very complex mixture of thousands of individual compounds; most with carbon numbers between 10 and 22. Most of these compounds are members of the paraffinic, naphthenic, or aromatic class of hydrocarbons. These three classes of hydrocarbons have different chemical and physical properties. Different relative proportions of the three classes are one of the factors that make one diesel fuel different from another.

1.3.1 Different properties of Diesel fuel are discussed below :

(a) Boiling Points: For compounds in the same class, boiling point increases with carbon number. For compounds of the same carbon number, the order of increasing boiling point by class is isoparaflin, n-paraffin, naphtherie, and aromatic. The boiling difference (100-150°F or 60°-80°C) between isoparaffins and aromatics of the same carbon number is larger than the boiling point difference (about 35°F or 20°C) between compounds of the same class that differ by one carbon number. Thus, the compounds that boil at about 500°F, the middle of the diesel fuel boiling range, might be C12 arornatics, C13 naphthenes, C14 n-paraffin, and C15 isoparaffins.

(b) Freezing Point Freezing points (melting points) also increase with molecular weight, but they are strongly influenced by molecular shape. Molecules that fit more easily into a crystal structure have higher freezing points than other molecules. This explains the high melting points of n-parafflns and unsubstituted aromatics. Compared to the melting points of isoparaffins and naphthenes of the same carbon number.

(c) Density: For compounds of the same class, density increases with carbon number. For compounds with the same carbon number, the order of increasing density is paraffin, naphthene, and aromatic.

(d) Heating Value: For compounds with the same carbon number, the order of increasing heating value by class is aromatic, naphthene, and paraffin on a weight basis. However, the order is reversed for a comparison on a volume basis, with aromatic highest and paraffin lowest.

This same trend holds with fuels Lighter (less dense) fuels, like gasoline, have higher heating values on a weight basis; whereas the heavier (more dense) fuels, like diesel, have higher heating values on a volume basis.

(e) Cetane Numbe r: Cetane number also varies systematically with hydrocarbon structure. Normal paraffins have high cetane numbers that increase with molecular weight. Isoparaffin have a wide range of cetane numbers, from about 10 to 80. Molecules with many short side chains have low cetane numbers; whereas those with one side chain of four or more carbons have high cetane numbers.

Naphthenes generally have cetane numbers from 40 to 70. Higher molecular weight molecules with one long side chain have high cetane numbers; lower molecular weight molecules with short side chains have low octane numbers.

Aromatics have cetane numbers ranging from zero to 60. A molecule with a single aromatic ring with a long side chain will be in the upper part of this range; a molecule with a single ring with several short side chains will be in the lower part. Molecules with two or three aromatic rings fused together have cetane numbers below 20.

(f) Viscosity: Viscosity is primarily related to molecular weight and flot so much to hydrocarbon class. For a given carbon number, naphthenes generall) have slightly higher viscosities than parafTins or aromatics.

Normal paraffins have excellent cetane numbers, but very poor cold flow properties and low volumetric heating values. Aromatics have very good cold flow properties and volumetric heating values, but very low octane numbers. Isoparaffins and naphthenes are intermediate, with values of these properties between those to normal paraffins and aromatics.

1.4 Combustion Chamber:

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.2: Combustion Chamber

Table 2.1 Specifications of Combustion Chamber:

Abbildung in dieser Leseprobe nicht enthalten

Combustion chamber for our experimental set-up is a stainless steel cylindrical tank having a volume of 1 liter. A pintle type nozzle is fitted on the head of the combustion chamber. We used in our study is closed type chamber so that providing various high pressures inside the combustion chamber. A spiral hollow tube is fitted tangentially at the top of chamber for the supply of hot air. The spiral tube is connected to the secondary coil of a step down transformer (output current=1 ampere).due to the heating effect of current the spiral tube is heated and the entering the chamber, passing through the tube is also heated. Two collinear tubes are embedded diametrically opposite in chamber. On one end of the tube a light source is fitted. On the end of the second tube a photo sensor is placed. To increase the temperature of air inside the chamber a l000watt heating coil is placed below the chamber. A glass fiber blanket is placed on heating coil through which only heat can pass through but the fuel injected cannot penetrate. Provisions are also made in the chamber for fitting the multi hole nozzle for the further studies.

1.5 Test Procedure :

- First of all, provide air into the combustion chamber at various high pressures by using Air Compressor and then an environment of air (maximum temperature700K) is created hot inside the combustion chamber. When there is no spray or burning activity inside the combustion chamber, light source gives a constant light intensity to the sensor so the output of the oscilloscope is constant.
- When the fuel is injected into the chamber with the help of the arrangement, a spray is formed, thus obstructing the optical path.
- Due the injection of spray the refractive index of the medium inside the chamber (hot air) changes, due which the intensity of the light falling on the photo sensor decreases, hence the output of oscilloscope also changes.
- Thus we read three distinct events on the screen of oscilloscope viz. Start of injection, completion of injection and ignition.
- After the completion of ignition delay, the fuel ignites and a characteristic yellow flame is formed, which activates the sensor.
- As soon as the sensor detects the formation of the flame, after the completion of ignition delay, the oscilloscope shows he corresponding event.
- The time difference of event of injection and event of ignition gives the ignition delay of the tested fuel.
- The process is repeated two times for a given temperature to minimize the error.
- The process is repeated for the temperature range of 583-663 k. Thus a set of observation is completed for a particular fuel.
- For all the fuel tested, the mass of the fuel injected per stroke is kept constant so that at a particular temperature the air-fuel ratio is constant for the comparison.

RESULTS AND DISCUSSION

Experiments for the measurement of ignition delay were conducted on pure diesel for generation of base line characteristics. These base line characteristics were then with blends of diesel with n-Pentane, n-Hexane, and DEE, as given in the tables 3:4-3.7 (Appendix)

Since in the market, commercial HS diesel is used, therefore the same fuel was as base line fuel for the study. All samples were prepared after careful filtration and accurately measured blending. Result of ignition delay were obtained and compared with that in the literature. Most of the studies were conducted at elevated pressure; however we conducted our study at various high pressures.

- When we supplied air in the combustion chamber at high pressure the ignition delay is decreases for all blends of diesel with n-pentane, n-hexane and DEE and increases the temperature of air the ignition delay is also decreases.
- As operating speed of modem low emission high performance engines is increasing. Therefore the classical concept of ignition delay is required to be seen with a new technique based on optical methods, which was used in our study.

Basic studies in the constant volume bomb, in steady flow reactors, and in rapid compression machines have been used to study the auto ignition characteristics of the fuel air mixture under controlled conditions. In some of these studies the fuel and air are premixed; in some, fuel injection was used. Studies with fuel injection into constant temperature and pressure environment have shown that the temperature and pressure of the hot air are the most important parameters for a given fuel composition. Ignition delay data from these experiments have usually been correlated by Arrhenius equation given below:

[...]