FUEL INJECTION AND SPRAY

FORMATION

AIN SHAMS UNIVERSITY

FACULTY OF ENGINEERING

SUBMITTED TO:

DR. Mostafa Abdelkhalek

September 26, 2015

Pressure injection and its effect on performance

Injection system is used to optimize the fuel/air ratio that enters the

engine .

The aim of develop of fuel injection system is:

1-achieve a high degree of atomization to enable sufficient evaporation in a

very short time

2-achieve sufficient spray penetration to utilize the fuel air charge.

We develop this system to get high injection pressure to get more efficiency.

*When injection pressure decrease, droplets size will enlarge and ignition

delay period (formation of mixing of fuel and air) will increase, so pressure of

piston will increase and efficiency will decrease.

*when injection pressure increase, droplets size will be very small and ignition

period will be better so efficiency will increase.

*when injection pressure extremely increase, droplets size will be very very

small and igniton period will decrease more and more , possibility of

homogenous mixing will decrease and efficiency will decrease again.

There is some results from an experiment was made in india :

The engine is started and run the engine at 1500 rated RPM for 16.5:1

compression ratio.

Experiments are conducted starting with no load at the injection timing of 23 o

b TDC and six different injection pressures of 170,180, 190, 200, 210 and 220

bars. Then the load is increased to 25% load, 50% load, 70% load and full

load with diesel fuel.

Data such as the relation between BTE, BSCF, CO emissions, smoke opacity

and IP .

Brake thermal efficiency ( BTE):

Is the break power of a heat engine as a function of the thermal input from

the fuel and used to evaluate how well engine convert the heat from a fuel to

mechanical energy.

It is observed that :

1- as IP decreases, BTE decreases due to coarse spray formation , poor

atomization , poor mixture formation and poor penetration.

2- at BTE at IP of 200 bar for 20BD ,30BD is higher than BTE for diesel.

So 200 bar and 20 BD is the optimum for bio diesel.

Brake specific fuel consumption ( BSFC):

It is the measure of the fuel efficiency and used for comparing the efficiency of

internal combustion engine with shaft output.

It is observed that :

1- when IP increase, BSFC decrease .

2- the lowest BSFC was found at 200 bar and 20 BD =12.67 MJ/KW- hr and it

is better than that for diesel =13.25 MJ/KW-hr.

CO emissions:

It is observed that :

1- as IP increases , co emissions increase for all bio diesel blends. may be

due to improper mixing of fuel particles with air, less penetration of fuel

particles and ineffective combustion of the blend at these pressures.

UHC ( un burnt hydrocarbons):

Is the result of incomplete combustion of fuel.

It is observed that :

1-when IP increase , UHC decrease. But at IP of 210,220 bar , there is an

increase of UHC emissions cause of finar fuel spray and penetration of the

droplets resulting incomplete combustion.

2- the minimum UHC was at IP of 200 bar and 20BD,100 BD.

Smoke opacity:

It is observed that:

1-the highest smoke opacity for blends at 170 bar.

2- as IP increase , the smoke opacity decrease.

3- all blends have less smoke opacity than diesel. The presence of oxygen in

the blends in addition to good atomization of fuel at higher pressure may be

the reason for lower opacity.

4- the better combustion was at IP of 200 bar.

Finally this is the difference between the shape of fuel in diesel and bio diesel:

biodiesel typically produces longer spray tip penetration lengths and narrower

spray angles.

CAVITATION 1. INTRODUCTION

One of the important methods in reducing the exhaust emissions from

compression ignition engines at source is by improving the spray break up

and introducing smaller droplets inside the combustion chamber. The flow

inside the fuel injector nozzle has effects on the spray formation. Cavitation

occurs if the local pressure drops below the vapor pressure of the liquid at

local temperature. The high pressure drop across the injection nozzle tends to

accelerate the liquid within the small nozzle holes. This acceleration of liquid

inside the nozzle generates a high level of turbulence. At the sharp edges

inside the nozzle holes, such as the inlet of the nozzle hole, the streamlines

are contracted such that the effective cross section of the flow is reduced

leading to accelerated velocity of the liquid. According to Bernoulli principle,

this causes a reduction in the local static pressure and it can reach value to as

low as the vapor pressure of the liquid. bubbles are formed inside the nozzle

hole because of the cavitation phenomenon. These cavitation bubbles are

swept out of the nozzle into the combustion chamber. As the bubbles are

introduced into combustion chamber pressure, they implode and contribute to

further break up of the spray.This leads to finer droplets of fuel which aids in

faster evaporation of fuel.

2. CAVITATION

Cavitation is the formation of vapor cavities in a liquid that are the

consequence of cavitational forces acting upon the cavitational liquid. It

usually occurs when a liquid is subjected to rapid changes of pressure that

cause the formation of cavities where the pressure is relatively low.When

subjected to higher pressure, the voids implode and can generate an intense

shockwave.

Cavitation is a significant cause of wear in some engineering contexts.

Collapsing voids that implode near to a metal surface cause cyclic stress

through repeated implosion. This result in surface fatigue of the metal causing

a type of wear also called "cavitation". Apart from excessive vibrations,

cavitation drastically alters the flow field, reducing the hydraulic efficiency of

the affected hydraulic components.Even if vibration and erosion problems are

avoided by design or operation, it is likely that the performance of the systems

is sub-optimal because countermeasures by design were needed to prevent

cavitation problems.

3. HYDRODYNAMIC CAVITATION

Hydrodynamic cavitation describes the process of vaporization, bubble

generation and bubble implosion which occurs in a flowing liquid as a result

of a decrease and subsequent increase in local pressure. Cavitation will only

occur if the local pressure declines to some point below the

saturated vaporpressure of the liquid and subsequent recovery above the

vapor pressure. In pipe systems, cavitation typically occurs either as the result

of an increase in the kinetic energy (through an area constriction) or an

increase in the pipe elevation.

Hydrodynamic cavitation can be produced by passing a liquid through a

constricted channel at a specific flowvelocity or by mechanical rotation of an

object through a liquid. In the case of the constricted channel and based on

the specific (or unique) geometry of the system, the combination of pressure

and kinetic energy can create the hydrodynamic cavitation cavern

downstream of the local constriction generating high energy cavitation

bubbles.

The process of bubble generation, and the subsequent growth and collapse of

the cavitation bubbles, results in very high energy densities and in very high

local temperatures and local pressures at the surface of the bubbles for a very

short time. The overall liquid medium environment, therefore, remains at

ambient conditions. When uncontrolled, cavitation is damaging; by controlling

the flow of the cavitation, however, the power can be harnessed and non-

destructive.

Orifices and venturi are reported to be widely used for generating cavitation.

A venturi has an inherent advantage over an orifice because of its smooth

converging and diverging sections, such that that it can generate a higher flow

velocity at the throat for a given pressure drop across it.

4. Cavitation in Automotive Applications

Cavitation is known to occur in various automotive components, where high fluid velocities and rapid accelerations develop; for example fuel may cavitate

in high pressure fuel injection systems, or lubricant in piston rings and bearings. As a result cavitation erosion may accumulate, causing damage and affecting engine durability. Cavitation is also known to alter the composition of

Diesel fuel properties. In large Diesel engines it might be possible the coolant fluid to cavitate at cylinder walls, due to vibrations produced by the engine

operation.

On the other hand, cavitation is believed to enhance atomization, thus improving combustion and reducing emissions in Diesel engines.

5. FACTORS AFFECTING CAVITATION

According to the flow configuration (shape and relative motion of the walls limiting the flow field, or physical properties of the liquid), injection condition and injector geometric structure parameters influence the development of

cavitation in nozzles.

5. DETERMINATION OF THE CAVITATION NUMBER

Typical situations in which cavitation can appear and grow in a liquid flow:

The case of a sharp inlet, where the flow separates at the corner, the flow experience a vena contracta. A diagram of the sharp entrance flow. Point 1

would be downstream of the injector needle that the local velocity would be small, such as in the sac ofthe injector. Point c is downstream of the inlet,

where the vena contracta effect is amaximum. In the case of a sufficiently rounded nozzle this point is nonexistent.For convenience a ratio between the area at the contraction and the nominalnozzle area, known as the coefficient

of contraction, is defined:

(Ac( represents the effective flow area through the contraction and )A

(represents the nominal nozzle area. The value of the contraction coefficient

varies with the nozzle geometry and cavitation characteristics. For a very rounded

entrance, the flow will not separate and the coefficient of contraction will be unity. Another relevant integral property of the flow is the coefficient of discharge,

Cd. The coefficient of discharge represents the efficiency of the nozzle between points 1 and 2 and thus is a measure of whatever losses occur in the

nozzle The definition of the coefficient of discharge is:

useful cavitation parameter and is referred to as K .

* The shearing between two neighboring flows having very different velocities entails large turbulentfluctuations of the pressure.

Results from experiments Badock et al. (1999) showed experimentally that increasing the radius of

curvature at the nozzle inlet could reduce cavitation. Desantes et al. (2010) also reported that the cone angle of the fuel spray was found to be increased

due to the formation of vapor inside the nozzle. Payri et al. (2004 and 2005) also reported that the spray cone angle and outlet speed increased with the cavitation .Suh et al. (2008) also studied experimentally and reported that

cavitation enhanced the fuel spray characteristics and the primary fuel breakup due to the turbulence created inside the nozzle. Recently, bio-diesel

fuels promise clean, alternative and renewable source of energy. Biodiesels have a number of properties that make it an excellent alternative fuel for diesel engines, particularly because of its low

emissions compared with the diesel fuel (Buyukkaya, 2010; Moser et al.,

2009). Moreover, the distinct properties of bio-diesel may influence the

cavitation phenomenon in the nozzle hole.

DROPLET SIZE Droplet size measurement

Atomic transmission microscope ATM model JEM-1230 is used for nano

emulsion fuel particle size measurement. It is also used for measuring the

histogram of the obtained nano emulsion blend. Max magnification of the

used ATM is 600Kx with max resolving power about 0.3nm per line. Energy

intensity was in the range from 40kv up to 120kv on steps.

Calorific value determination

The calorific value of the sample was measured in the Egyptian Petroleum

Research Institute using

Parr 6200 calorific value tester.

Determination of viscosity

Brook filed model DV-II+ viscometer was used to determine the viscosity and

sample was kept in the water thermostat bath until it reaches the equilibrium

temperature of 18°C. After reaching the equilibrium temperature, the

viscometer tip was inserted to the sample and the reading was taken from the

controller.

Reaction Mechanism

The above equation presents the estimated slow reaction mechanism of

nonylphenol and oleic acid in which neutralization reaction was obtained

showing the formation of one molecule of water and condensation of

nonylphenol and oleic acid resulting viscous dispersing agent of water in

biodiesel. Also this high viscosity may be is the reason for the long stability of

the obtained biodiesel nanoemulsion fuel.

Effect of nonylphenol concentration

Fig.1 shows the effect of nonylphenol surfactant concentration on the ignition

time at biodiesel

concentration 80%, and 1% oleic acid. The results show that as the

nonylphenol concentration increases from 1.82% to 6% no significant change

in ignition time is observed and the ignition times were 15.33, 15, and

15mins., respectively, while at 8% dose nonylphenol no ignition is obtained.

Also the observations showed that as the nonylphenol concentration

increases the flame length decreases and no flame at all was obtained at 8%

nonylphenol concentration. Based on the above results 1.82% nonylphenol

surfactant was selected as the optimum surfactant concentration.

Effect of Water content

The effect of water content on the ignition time of the obtained biodiesel

nanoemulsion fuel at

nonylphenol concentration of 1.82% was illustrated in Fig.2. Biodiesel

percentage normally changed with the variation of water content at 0.5ml oleic

acid was added for each sample. The results showed that as the water

percentage increases the ignition time decreases where it was 18min utes for

5ml raw biodiesel and decreases to 15min., 15min, and 10min. at water

content 15%, 24.5% and 35% respectively. The observation shows that the

flame length was the highest at raw biodiesel and slightly decreases at the

rest of the sample at different water percentages. Based on the above

obtained results water content of 24.5% which is balanced with 72.7%

biodiesel was selected as the optimum water concentration.

Nanoemulsion stability

Fig. 3 shows the effect of water content on the observed viscosity of the

obtained biodiesel

nanoemulsion. It was found that as the water content increases the viscosity

of the obtained biodiesel

nanoemulsion increases too. Fig. 4 shows the change of the viscosity of the

optimum selected composition with storage time. Biodiesel nanoemulsion

prepared in this work exhibited good stability without phase separation for

more than 6 months but with dramatic decrease in the viscosity. The results

show that the raw biodiesel has a viscosity of 6.81MP at 18°C, while the

obtained viscosity of prepared nanoemulsion at the beginning of preparation

was decreased to 18 MP at 18°C then after 4 months the viscosity of the

same nano emulsion reached 3.5 MP at 18°C. The decrement of the nano

emulsion fuel viscosity with regard to storage time may be attributed to the

continuous slow decomposition of the reaction product of nonylphenol and

oleic acid.

From the above obtained data and according to the emulsion satiability,

ignition time, and flame length, the following composition was selected as the

optimum biodiesel nanoemulsion fuel. This composition was water percentage

24.5%, biodiesel 72.7%, nonylphenol 1.82%, and oleic acid 0.98%. This

sample has a density of 0.92g/mL, viscosity of 3.5MP at 18°C at the beginning

of preparation.

ATM and Droplet size results

The optimum sample was subjected to ATM analysis for obtaining the

biodiesel nanoemulsion fuel particle size and its histogram.

Fig.5 shows the ATM analysis for the measurement of selected optimum

composition particle size.

The analysis shows that the obtained particle sizes were ranged from 18nm to

240nm. Fig. 6 shows the

histogram of the optimum sample of biodiesel nanoemulsion fuel prepared

using nonylphenol and oleic acidsurfactant at water percentage of 24.5%.

The major particle size was 18nm (73%) and the minor particle size was

124nm (8.5%).

Calorific value

The prepared biodiesel nanoemulsion fuel under optimum conditions was

subjected to calorific value

tester and it is found 35.94KJ/Kg.

CONCLUSIONS

Novel formulation of water in biodiesel nanoemulsion form is successfully

prepared with the

composition of 72.7% biodiesel, 1.82% nonylphenol, and 0.98% oleic acid

with water balance of 24.5%.

Resulted biodiesel nanoemulsion, prepared under optimum conditions, has

major particle size 18nm

and the minor particle size was 124nm, density of 0.92g/mL, viscosity of

3.5MP at 18°C and calorific value of

35.94 KJ/kg.

Recommendations

It is recommended to decrease surfactant dose to ensure maintaining low

viscosity by time and also

studying the effect of oleic acid concentration.

Also it is recommended to apply the prepared biodiesel nanoemulsion on

the engine and investigate

its effect on the engine performance, fuel consumption and so o

SPRAY FORMATION Spray Regimes

Diesel engine sprays are usually of the full-cone type. This means that in the

idle mode the fuel is blocked from the upstream side of the nozzle and during injection the core of the spray is more dense than the outer regions.

The liquid spray can be characterized by distinguishing five regimes. Starting

from the nozzle exit first there is an intact liquid core. A few nozzle diameters further downstream in the so-called churning flow the liquid consists of

ligaments .These liquid parts are like large droplets with sizes comparable to the nozzle diameter. Then the ligaments breakup into many smaller droplets in the thick zone where the volume and mass fraction of the liquid phase is

high. Further downstream the breakup process of droplets goes on and in the same time more and more of the surrounding gas is entrained into the spray

area. The regimes after the thick zone are the thin zone (low volume but still high mass fraction of liquid) and the dilute zone (negligible volume and low mass fraction of liquid), respectively.

Breakup Regimes

The disintegration of liquid jets is described by two main mechanisms. The First mechanism is the breakup of the intact liquid core into droplets and is

called primary breakup. This mechanism is characterized by the droplet size and the breakup length, which is defined as the length of the intact liquid core. The second mechanism is the breakup of droplets into smaller ones, which

is called secondary breakup. Both breakup length and droplet size are dependent on the properties of the liquid and the surrounding gas. At least as

important is the relative velocity between the liquid and the surrounding gas. The primary breakup is the most important mechanism in fuel injection systems, because it determines the size of the droplets that separate from the

liquid core, hence therefore also determines evaporation behavior and it marks the starting point for further breakup into smaller droplets (secondary

breakup).

Primary Breakup

The primary breakup mechanism concerns the breakup of the intact liquid core and can be divided into four regimes. Namely, the Rayleigh regime, the

first and second wind-induced regimes and last but not least the atomization regime. In order to make a quantitative classification of the regimes the Ohnesorge number Oh is introduced:

Herein the Weber number Weland the Reynolds number Relare defined as:

The Weber number is the ratio between inertial (or aerodynamic) and surface tension forces. The Reynolds number is the ratio between inertial and viscous

forces.

The Ohnesorge number is a ratio between viscous forces and surface tension forces.

the various regimes can be classified in the space Oh as function of the jet velocity, or alternatively Rel.In this figure the four regimes and also the relevant zone for diesel injection applications are indicated.

Rayleigh regime Breakup at low jet velocity due to axis symmetric

oscillations initiated by liquid inertia and surface tension forces. (D) droplet>(D) nozzle, the breakup length L jet is long and by increasing jet velocity( u) also L jet increases.

First wind-induced regime Liquid inertia and surface tension forces are

amplified by aerodynamic forces. The relevant Weber number for this regime is:

Here urelis the relative velocity between liquid and surrounding gas and the subscript g

denotes the gas properties. Ddroplet = Dnozzle, Ljet>Dnozzleand by increasing jet velocity

uthe breakup length Ljetdecreases. Second wind-induced regimeThe flow in the nozzle is turbulent.

Ddroplet<Dnozzleand by increasing jet velocity u the breakup length Ljetdecreases.

Atomization regime Breakup at surface directly at the nozzle hole, so the

intact corelength Ljetgoes to zero. Conical spray develops immediately after

leaving the nozzle. Ddroplet is more less than Dnozzle.

Secondary Breakup

The secondary breakup mechanism concerns the breakup of droplets due to

aerodynamic forces that are induced by the relative velocity between the droplets and the

surrounding gas. Similar to the first wind-induced regime for the liquid core the gas Weber number is the relevant dimensionless quantity to identify the process, with the only difference that the nozzle diameter D in equation is

replaced with the droplet diameter before breakup d :

Decreasing the droplet diameter d raises the surface tension force. This

means that the critical relative velocity, the relative velocity at which breakup takes place, must be higher.

Wegin equation is used to separate the droplet breakup regimes. The values at which

transitions from one regime to another occur, are determined experimentally.

In engine sprays all droplet breakup regimes occur at the same time. Near the nozzle the Weber number is high, so most of the breakup takes place at the

nozzle exit. Further downstream the Weber number is lower due to smaller droplet diameters and lower relative velocities. Therefore the breakup far from the nozzle is much less.

Atomization in Diesel Sprays Modern injectors for diesel engines have nozzle diameters of 200 Mm or less,

and the length of the nozzle hole is approximately 1 mm. Injection pressures up to 200 MPa are used and therefore the jet velocity u reaches values of 500 m/s and more.

Some possible sources for atomization are shortly treated in the following.

Aerodynamic shear forces amplify the surface waves created by the

turbulence in the nozzle hole. The waves separate from the jet and form droplets. There are two reasons why this aerodynamic source is less important. First, this process is time dependent, but it is known from

experiments that jets break immediately at the exit of the nozzle. Second, aerodynamic breakup is a surface effect, so it cannot explain disintegration of

the inner structure. Relaxation of velocity profileAt the wall inside the nozzle a no-slip

boundary conditions exists. When the liquid exits the nozzle, the velocity profile will transform into a uniform one. In order to realize that the outer

region of the liquid accelerates, which may cause instabilities and ultimately result in breakup into droplets. However, in modern diesel engines the length to diameter ratio of the nozzle hole is typically small ([ L/D]nozzle = 5), so

probably the flow in the nozzle has no time to develop. Turbulence The presence of radial turbulent velocity fluctuations in the jet

results, if strong enough to overcome the surface tension, in formation of droplets. Turbulence-induced primary breakup is considered one of the most

important mechanisms in high pressure applications.

Fuel Spray Characteristics

The most important diesel fuel spray characteristics may be classified as

• Macroscopic quantities such as: – Spray tip penetration

– Cone angle • Microscopic quantities such as: – Droplet size

All fuel spray characteristics influence the combustion process and engine performance.

-Fuel spray tip penetration(Lp (is defined as the maximal distance measured

from the injector to the spray tip. -Spray cone angle(θ) is defined as the angle between two straight lines

originating from the orifice exit of the nozzle and being tangent to the spray outline. This angle usually ranges from 5 to 30.

-Droplet size is usually measured on an average basis by the medium diameter of the droplets, called the Sauter mean diameter.

Fuel spray penetration is determined by the equilibrium of two factors: the linear momentum of the injected fuel and the resistance of the working fluid in

the control volume. Due to friction, the kinetic energy of the fuel is transferred to the working fluid. This energy transfer decreases continuously the kinetic

energy of the droplets until their movement depends solely

on the movement of the working fluid. Diesel fuel spray penetration depends on injection pressure, fuel

properties, and nozzle geometry. By increasing the injection pressure the fuel

penetration velocity is increased. This means increased fuel momentum and larger spray penetration. Fuel properties like density, viscosity, and surface tension also affect spray penetration. However, when making raw estimates,

fuel density is often used as the only influencing property. In this context fuel density of a given fuel may vary, for example, due to variations in fuel

temperature. An increase of fuel temperature typically reduces the fuel density, which results in shorter spray penetration.

The cone angle is mainly affected by the geometric characteristics of the

nozzle, the fuel and air density. Furthermore, the cone angle increases by increasing the injection pressure and by decreasing the working fluid temperature.

The diameters of the droplets depend on injection pressure, on working

fluid temperature, and on fuel properties. The diameters of the droplets tend to become smaller as the injection pressure raises. Furthermore, the working fluid temperature and fuel properties influence the evaporation rate, which

also affects the droplet size. Namely, by increasing the temperature the rate of evaporation increases. Consequently, the droplets with small diameters

tend to evaporate completely within a quite short time interval. On the other side, the droplets with greater diameters maintain a stable geometry for some time until they also evaporate completely.

In a fuel spray, fuel droplets evaporate as they travel away from the nozzle. The maximal distance, reached by the droplets before they all evaporate,

is called the liquid length. After the liquid length is reached, the evaporated

fuel continues to penetrate the surrounding gas and its range is denoted as the vapor length. It was found out that the liquid length tends to stabilize after

a short spray development time and then remains constant. On the other hand, in a typical

diesel injection timeframe (a few milliseconds) the vapor length does not reach a steady state.

Liquid spray formation is a rather sophisticated physical process, starting from

the breakup of the liquid core into droplets, shortly after the nozzle exit, called the

primary breakup. In the second stage the formed droplets break up into smaller droplets, which is called the secondary breakup .