Fuel Spray and Mixture Preparation in Split-Cycle …about.brighton.ac.uk/cereev/publications/new/Fuel Spray and Mixture... · Fuel Spray and Mixture Preparation in Split-Cycle Engine

1 Larissa Taylor Nuffield Project Summer 2013

Fuel Spray and Mixture Preparation in Split-Cycle Engine

A Nuffield Project with STEM Sussex

Larissa R Taylor

Park College Sussex Downs

Eastbourne

Supervisors;

Dr Steve Begg

Dr Oyuna Rybdylova

Dr Elena Sazhina

Sir Harry Ricardo Laboratories

Centre for Automotive Engineering

University of Brighton

2013



A Nuffield Project with STEM Sussex

Abstract

A new type of engine is being designed that is aimed to be more efficient by employing the

split-cycle engine concept. The engine operates as a two-stroke by dividing the conventional

four-strokes (the induction, compression, expansion and exhaust strokes) between two

separate cylinders; air at high pressure is fed into the combustor cylinder from the compressor

cylinder via a cross-over passage and valve. Feasibility studies of mixture preparation for a

given mass flow rate of air and fuel injection into the combustion chamber of a split-cycle

engine have been performed by numerical simulation. Fuel spray evaporation, vapour mixing

with air and the onset of combustion are key factors that influence the efficiency of an

engine. If the spray is placed in the right position then the gaseous cloud of fuel will be

formed near the spark plug when the fuel evaporates in the chamber. It is essential that the

fuel evaporates in the chamber in the correct position for complete and stable combustion.

The aim of research is to use a numerical CFD modelling program, ANSYS FLUENT, to test

where the vapour cloud of fuel will end up for ignition. By setting up the desired spray

parameters for realistic engine geometry and operating conditions, it can be determined

where the fuel droplets will end up inside the engine. Three injector locations were

considered, and three spray breakup cases were compared, namely; no breakup model, TAB

and WAVE breakup models. Interaction of spray and in-cylinder flow pattern has revealed

results that can be used for further design improvements to the new engine under the EU

INTERREG CEREEV project. The fuel droplets were considered as a multicomponent

liquid; 95% of iso-octane C8H18 and 5% of n-heptane C7H16 by mass. The study of the

evaporation process of multicomponent fuel droplets is relevant to the EU INTERREG E3C3

project, focusing on biofuels. The rate of evaporation as predicted by ANSYS FLUENT, is

monitored for both the C7H16 and C8H18 components. It is observed that n-heptane evaporates

faster than iso-octane as it can be expected. The results have shown that the optimum location

of the injector and ignition for a combustible mixture is close-spaced and central in the

chamber. The current project has provided a feasibility study of conventional CFD

capabilities for multicomponent fuel droplets. Thus, it lays foundation for CFD

implementation of the newly developed spray models at SHRL. As the SHRL experimental

and modelling work grows in sophistication and depth, newly developed spray models take

into account temperature gradients and species diffusion inside a multicomponent droplet.

Further research on validation of the CFD results against available experimental data is

needed.


Acknowledgements

I would like to thank the members of the Sir Harry Ricardo Labs at the University of

Brighton for their help and support and also the staff members at Park College Sussex Downs

for their support.

I would also like to give special thanks to STEM Sussex staff, in particular, Daniel Hawkins,

Bronagh Liddicoat and Patricia Harwood, and gratefully acknowledge the financial help of

the Nuffield Research Placement grant (http://www.stemsussex.co.uk/index.php/secondary-

fe/school-activities/nuffield-science-bursaries/).


List of Figures

Figure 1 Four stroke spark ignition engine

Figure 2 Four Stroke Petrol Engine and Definition of TDC and BDC

Figure 3 Four Stroke Diesel Engine

Figure 4 Two Stroke Engine

Figure 5 Scuderi Split Cycle Engine

Figure 6 Principal Elements of Split Cycle Engine

Figure 7 Mesh Components for CFDs

Figure 8 TAB break up activated in FLUENT.

Figure 9 Wave break up activated in FLUENT.

Figure 10 Mesh and Initial Conditions

Figure 11 Mesh and fuel injector positioned near inlet valve injector

Figure 12 Mesh and fuel injector positioned near exhaust duct

Figure 13 Mesh and fuel injector positioned near spark plug

Figure 14 Mesh and all fuel injector positions

Figure 15 Percentage of octane in the fuel vapour by mass for no-break up model

Figure 16 Percentage of octane in the fuel vapour by mass for TAB breakup model

Figure 17 Percentage of octane in the fuel vapour by mass for Wave breakup model

Figure 18 In-cylinder mass of octane vapour for TAB breakup model

Figure 19 Mass of octane vapour for TAB and no-breakup models

Figure 20 Mass fraction of octane at 2ms for no-breakup model for near-inlet injector

Figure 21 Mass fraction of octane at 2ms for TAB model for near-inlet injector

Figure 22 Mass fraction of octane at 2ms for Wave model for near-inlet injector

Figure 23 Initial velocities of droplets for injector positioned by the exhaust duct

Figure 24 Initial velocities of droplets for group injectors positioned by the exhaust duct

Figure 25 Velocity magnitude contours for in-cylinder flow for group injectors positioned

by the exhaust duct; no-breakup model for spray

Figure 26 Mass fraction of octane vapour for group injectors positioned by the exhaust

duct; no-breakup spray model

Figure 27 Mass fraction of octane vapour of group injectors by the exhaust duct for Wave

breakup model

Figure 28 Initial velocities of droplets for the single injector positioned by spark plug

Figure 29 Mass fraction of octane vapour for no-breakup model for the spark-plug

location of the injector

Figure 30 Mass fraction of octane vapour for Wave breakup model for the spark-plug

location of the injector

Figure 31 Ratio of octane in fuel vapour by mass for the spark-plug location of the

injector for the injection velocity of 50 m/s; Wave spray breakup model.


Glossary

BDC Bottom dead centre

CA BTDC Crank angle before top dead centre

CFD Computational fluid dynamics

ICE Internal combustion engines

MFR Mass flow rate

SHRL Sir Harry Ricardo Laboratory

SI Spark ignition

SSC Scuderi Split Cycle

TDC Top dead centre



1. Background

An engine is a machine designed to convert chemical energy of fuel into useful mechanical

energy. Heat engines such as internal combustion engines (ICE) in cars and external

combustion engines such as steam engines burn fuel releasing heat in the process, which then

creates motion (Engine 2013).

The fuel in ICE is normally a fossil fuel (hydrocarbons) and the combustion of the fuel occur

using oxygen in air as the oxidizer. An example of a fuel combustion equation for octane is:

C8H18 + 12 ½ O2 9H2O + 8CO2

Improvements in engine design address both better fuel efficiency and less pollution

simultaneously. Fossil fuel is a non-renewable resource and its cost is rising. The product of

combustion is carbon dioxide, a green house gas associated with global warming (Gent

2007). An alternative could be using biofuels, such a biodiesel, which are renewable.

At the Sir Harry Ricardo Laboratory (SHRL) at the University of Brighton, a team of

theoretical and experimental researchers are investigating different engine designs and fuel

injection capabilities for engine efficiency and emissions reduction (SHRL 2013). The

University is actively involved in several research programmes in this field.

Upon arriving at the University of Brighton I met with some of my project supervisors, Dr

Elena Sazhina and Dr Steve Begg. Dr Begg gave me a tour of the SHRL experimental

facilities. The SHRL has a variety of engine test beds and they talked me through some of the

research being carried out at the laboratory. I also met some of the postgraduate students, in

particular K. CK, MSc student.

My supervisors were keen that I should get a university experience. I had an opportunity to

attend a doctoral conference in July (Research Student Conference 2013). I attended the key

note speech on Stem Cell research and even though it was not relevant to my project I found

it very interesting. After that I attended the second stream at the doctoral conference which

was mostly about engines. It was very interesting to see the projects that the PhD students

were doing and how it related not just to my project but to the problems of the current world.

Angad Panesar presented his findings on “fuel consumption improvement by waste heat

recovery for automotive application” which was interesting to me as it showed how making

engines more efficient is a very important subject at the moment, especially with the

regulations regarding emissions This is why the split cycle engine will be very useful as it

will be very efficient. Where the current ICE is maybe only about 40% efficient, the split


cycle engine will be 60% meaning that extra processes to make it more efficient will not be

needed.

After the conference I was given a tour of the Vetronics lab by Dr Panagiotis Oikonomidis

(Vetronics, 2013). I saw the Vetronics laboratory room where they test the control box of a

car and the small systems used for it. What really caught my attention was the car they had

built. They had designed it and built it from scratch. It had this system where the steering

wheel was not directly attached to the wheels but to a control box which was connected to the

wheels. The control box sensed the direction the steering wheel had been turned to and so

made the wheel turn too. This is useful in that sensors on the control box could be used to

make driving safer. The use of the control box also means that the car can be steered using a

joy stick too and because it is not connected directly to the wheels it can be steered from

places like behind the car.

I was also shown a quick tutorial on how to use SolidWorks by Dr Manzanares, a commercial

graphics computer program used by students and researchers for design and development.

In early August I was shown how to use ANSYS FLUENT, a numerical modelling program

that would assist me in calculations for this project.

I also attended an Automotive Engineering research workshop in August on modelling of

droplet and spray dynamics, heating and evaporation. Everyone participating had some really

good ideas and had clearly thought hard about how to solve their problems. Heated

discussion on each presentation showed the interest and the level of quality of work from

everyone involved; it was a learning experience for me. It highlighted how research

collaborations work (Automotive Engineering 2013).

My supervisors have given me the background I needed to carry out a project related to on-

going studies at SHRL. The project focused on numerical simulation studies on the feasibility

of mixture preparation for given mass flow rate of fuel injection into a split-cycle engine.

Fuel spray and mixture preparation for combustion are key factors in the efficiency of an

engine. A numerical modelling program, ANSYS FLUENT, has been used to carry out this

study. FLUENT is a computational fluid dynamics (CFD) software package. A brief

description of CFD is given in Section 2. The outcome of the calculations guides future

design considerations or used by researchers at SHRL as experimental parameters.In what


follows, literature review will be followed by original results of the CFD simulations and

analysis of the results.

Four stroke engines

A four stroke engine has four main stages: intake, compression, power and exhaust (see

Figure 1). The most common ICE engine is the piston-type reciprocating engine where the

crankshaft is turned by piston moving up and down in a cylinder (Heywood 1988). The stroke

refers to the movement of the piston. A four-stroke engine completes a cycle in four strokes

and two crankshaft revolutions.

Figure 1. Four stroke spark ignition engine (reproduced from Internal Combustion Engine

http://authors.library.caltech.edu/25069/6/AirPollution88-Ch4.pdf uploaded August 2013)

Gasoline direct-injection engines

The piston rests at top dead centre (TDC) – stroke position 1 (up). As the piston moves down,

air is forced into the cylinder due to pressure difference. The piston descends down the

cylinder to bottom dead centre (BDC) – stroke position 2 (down). The piston is then pushed

back up to TDC compressing the air – stroke position 3 (up). For direct-injection engines,

fuel spray is injected into cylinder well before TDC. It takes time to evaporate and mix with

air before the combustible gaseous mixture is ignited with a spark plug. Rapid combustion of


the fuel and the sudden increase in pressure causes the piston to be pushed down to the

bottom of the cylinder again – stroke position 4 (down), thus powering the crank shaft. At the

next stroke the piston returns to TDC, the exhaust valve opens and the combustion products

are expelled via exhaust manifold. This process finishes after two complete revolutions of the

crank shaft.

Figure 2. Four Stroke Petrol Engine and Definition of TDC and BDC (reproduced from a screenshot

of https://www.youtube.com/watch?v=4vWoSXYnTa0, uploaded by codene on Jul 20, 2007)

Diesel Engines

Diesel engines differ to the gasoline engines in that they use hot compressed air to ignite the

fuel rather than a spark plug. The fuel is injected as small droplets. The hot compressed air

vaporises fuel from the surface of the droplets and then the heat from the compressed air

ignites the fuel vapour. The droplets continue to get smaller as the surface fuel is vaporised

and then ignited until all the droplets are used up. This increase in pressure due to the rapid

combustion of the gases causes the piston to be pushed down and power the engine. This is

illustrated in Figure 3

Figure 3 Four Stroke Diesel Engine (reproduced from

http://carenginecooling.blogspot.co.uk/2012/07/diesel-engines-diesel-engine-also-known.html, seen

on 12/08/2013)

TDC

BDC

https://www.youtube.com/user/codene?feature=watch


A four-stroke engine completes a power cycle in four strokes, this means that it requires 2

revolutions of the crankshaft to complete the process. Another engine has been designed to

only need one crankshaft revolution to complete the power cycle. It is a two-stroke engine. It

only requires two strokes, or up and down movement, of the piston to fully complete to

process.

Two Stroke Engines

Two stroke engines complete the power cycle in just one crankshaft revolution. It does this

by having the end of the combustion stroke happen simultaneously with the start of the

compression stroke, and having exhaust and intake (otherwise known as scavenging) happen

at the same time too.

Figure 4 Two Stroke Engine (reproduced from

http://en.wikipedia.org/wiki/Two-stroke_engine seen on August 2013))

The disadvantage of two stroke engines is that some fuel is lost to the exhaust duct when it

enters the chamber and so can cause pollution when it leaks out of the exhaust. Two stroke

engines are also not very fuel efficient. (How Stuff Works 2013)

Scuderi Split-cycle Engine

The Scuderi Split Cycle (SSC) engine is designed so that it is more efficient. It is similar to

the gasoline cycle in that it uses a spark plug not hot compressed air. It is different in that it

does intake and compression in one cylinder, known as the compression cylinder and then

expansion and exhaust in another, known as the power cylinder. The two cylinders are

connected by a “crossover port”, through which high pressure gas is transferred from the

compressor cylinder to the expander cylinder between the compression and power strokes.

(Philips et al 2011) This means that it only requires one complete revolution of the crank

shaft for the process to be completed. A normal 4 stroke engine would have to do two

complete crankshaft revolutions, as shown in Fig. 1, to match the power of the Scuderi

engine. The use of a turbocharger (a device with a turbine powered by the kinetic energy of

the exhaust to improve volumetric efficiency) with this Scuderi engine also comes in useful

as it allows one compression cylinder to provide air flow to multiple power cylinders.

Intake

Exhaust


(Scuderi 2013; Meldolesi 2012) High turbulence in the engine means that the fuel and air

mixes quickly and this prevents knock, giving successful combustion.

Figure 5a and 5b. Scuderi Split Cycle Engine (reproduced from

http://www.scuderigroup.com/technology/ SCUDERI™ Group, Inc 2013)

EU INTERREG project CEREEV

The EU INTERREG project CEREEV (CEREEV, 2012) establishes collaboration between

University of Brighton, IRSEEM (Institut de Recherche de l’ESIGELEC), and Jules Verne

University of Picardie. The research under the CEREEV aims to overcome problems of

volumetric efficiency and combustion phasing. A major functional benefit of the split cycle

engine is the separation of the compression and power cylinders. This allows optimum

conditions for each can be achieved (Meldolesi 2012). Figure 6 shows a schematic

representation of the cylinders in split cycle engine.

Figure 6 Schematic of Split Cycle Engine (reproduced from Meldolesi and Badain, 2012)

The CEREEV project aims to develop a new type of split-cycle engine when intake and

compression are done in a separate cylinder. The proposed two-stroke process requires rapid

filling in the power cylinder, where combustion will take place, while simultaneously

injecting fuel and air from different places.


2. Computational Fluid Dynamics (CFD)

Computational Fluid Dynamics (CFD) is the term used to describe numerical codes that can

calculate the properties of a fluid, such as temperature, velocity, chemical composition

throughout a region of space (ANSYS 2013).

CFD breaks down geometries into cells that make up a mesh, and algorithms based on

conservation of energy, mass and momentum are applied to each individual cell to compute

the fluid flow, species and temperature (Fig. 7). Boundary conditions are set by the user.

Partial differential equations that describe the flow are converted into simultaneous algebraic

equations which are set up for each cell. These equations are solved using numerical methods

and when the answers found for each cell coincide with the specified tolerance then they

converge and the solution is found. (ANSYS 2013)

Figure 7. Mesh Components for CFDs (reproduced from

http://willem.engen.nl/uni/fluent/documents/external/fluent-overview.ppt)

ANSYS FLUENT

ANSYS FLUENT is a CFD code. It is a numerical simulation program that allows the user to

model flow, chemical species and temperature for complex geometries; the calculation starts

by reading a mesh from its case and data file or from creating one in the program itself. By

setting the properties, boundary conditions and any desired conditions to be tracked,

FLUENT can calculate a solution for the given parameters and give a variety of data outputs.

FLUENT also allows you to see pictures showing things like fluid flow, giving the user a

wide variety of results to see from the calculated solution. (ANSYS 2013).

http://willem.engen.nl/uni/fluent/documents/external/fluent-overview.ppt


Breakup models

Within FLUENT, the user can choose to activate atomization models for their run. The term

breakup refers to fragmentation and splitting of the droplets injected.

If each individual droplet is broken up into smaller droplets then this will create more surface

area for the same amount of fuel and so it should evaporate quicker. The idea of breakup is

similar to what actually happens in an engine so is a helpful tool for realistic modelling.

There are different models of breakup:

Taylor analogy breakup (TAB) model.

This model uses oscillations of certain amplitude to break up the droplet. External forces

acting on the droplet are caused by the motion of it.

The droplet has both surface tension, which stops it from

falling apart, and a damping force, which stops the

oscillations from getting too big (Turner 2012).

The droplet will break up if the distortion reaches a critical

level. The distortion should to be equal to half the radius of

the droplet in order for breakup to succeed. The child

droplets are assumed to be neither oscillating nor distorted.

(ANSYS 2013).

From Fig. 8, you can see that the number of breakup parcels

is 2 meaning that the parent droplet will breakup into 2

similar child droplets.

Wave break up.

Otherwise known as stripping breakup, it relies on the

relative velocity between the liquid and gas phases.

The Kelvin–Helmholtz instability occurs when there is a

velocity difference across the interface between liquid fuel

and gas; instability of the fuel jet phase causes ‘child

droplets’ to be stripped from liquid core of the jet, resulting

in the gradual decrease in size of the injected fuel. The flow

travelling in the opposite direction to the fuel spray can also

help with the stripping of the core liquid (ANSYS 2013).

The CFD code provides a breakup constant for the value of

stable droplet sizes. This constant will depend on the type of

injector used (Turner 2012)

Figure 9 Wave breakup model

activated in FLUENT.

Figure 8 TAB breakup model

activated in FLUENT.


3. Aims and Objectives

It has been decided to focus research on optimization of injector location for a good mixture

preparation in this study. The objectives are:

To setup CFD simulation for a realistic engine geometry and boundary conditions

To explore spray tracking for several injector locations

To assess mixture preparation for each case

To formulate recommendations for the engine design

For my study, a mesh of the engine to work with was taken from 2-ACE project. An EPSRC

project, “A Fundamental Study of the Novel Poppet Valve 2-stroke Auto-ignition

Combustion Engine” has been carried out at SHRL. As part of the EPSRC study, a CFD

simulation using FLUENT was set and explored for a realistic engine geometry (2-ACE

2012).

The focus of my study is to set the injector position and run cases for different breakup

models to test where the vapour cloud of fuel will end up for ignition. Three injector

positions (Fig. 14) were investigated, namely:

fuel injection by the inlet valve;

fuel injection by the outlet valve

fuel injection near the spark plug.

Three injector types:

flat fan

single injection

group injections

were modelled. Three breakup models were explored, namely: no breakup, TAB and Wave.

An accompanying study by CK, 2013 is focusing on volumetric efficiency for a range of inlet

pressures. This facilitates team work for my research.


4. Methodology

Input parameters. Each CFD case had the same base parameters set, whilst varying the

position of the fuel injector and its type (single, group, flat-fan). The following base input

parameters for each CFD case were used:

Inlet Pressure = 10bar at T= 400°K (1 bar = 100 kilopascals)

Initial in-cylinder

condition

Pressure = 1bar at T= 300°K

In-cylinder initial air

composition

Taken as mixture of nitrogen and to 0.23 of oxygen by mass

Liquid fuel Multicomponent fuel consisting of 95% octane (C8H18) and

5% heptane (C7H16) by mass.

Injector flowrate 0.0076 kg/s

The inlet duct pressure was set to about 9.9 bar as initial condition. Figure 10 shows the mesh

and initial conditions. For the first CFD case, the injector used was a flat-fan-atomizer that

had 80 streams of multicomponent droplets. The width of the orifice was 0.147mm. The

droplets had an initial speed of 200m/s. After evaporation, the in-cylinder charge shall be a

combustible mixture of air with fuel, namely iso-octane and n-heptane vapours. The ANSYS

FLUENT model was set to “Discrete Phase – On” for spray calculation, and interaction of

gas with droplets was activated to make it more realistic. Mass flow rate was 0.0076 kg/s.

This is a historical value taken from the 2-ACE project. This means that stoichiometric ratio

will be achieved at about 4ms for in-cylinder mass of 0.4g. This corresponds to the volume of

around 5.05e-05m3 at 40 CA BTDC (crank angle before top dead centre) and in-cylinder

pressure of 10 bar at T = 400°K. The CFD run was set to monitor pressure, temperature,

speed, cylinder mass, cylinder volume, temperature, density, mass fraction of octane, mass

fraction of heptane averaged over in-cylinder volume, thus giving a large amount of output

data files.

Figure 10 Mesh and Initial Conditions (reproduced from 2-ACE EPSRC project, 2012)

10bar

In-cylinder air composition 23% O2

9.9 bar

1bar


All runs used these same parameters for the base case but the position of the injector

changed. Three positions were tested:

A) Injector near inlet valve

Figure 11 Mesh for 2-ACE engine and fuel injector positioned near inlet valve

B) Injector near exhaust duct

Figure 12 The 2-ACE mesh and fuel injector positioned near exhaust duct

Fuel injector near inlet

valve:

Direction in which fuel was sprayed


Fuel Injector near exhaust duct: 95% octane (C8H18) and 5% heptane (C7H16).


C) Near spark plug

Figure 13 The 2-ACE mesh and fuel injector positioned near spark plug

Three different atomization models: no-breakup, TAB breakup and Wave breakup were

tested on the three different injector positions; near the inlet valve, near the exhaust and by

the spark plug (see Fig. 14). The runs were performed for 800 times steps (8ms).

Figure 14 Mesh and all fuel injector positions (reproduced from 2-ACE Project, School of

CEM. University of Brighton, Brighton, 2012)


Fuel Injector near spark plug: 95% octane (C8H18) and % heptane (C7H16).

Near inlet

valve

Near exhaust duct

Near spark

plug


As described in the previous section, each case for each injector had been set up first before

any calculations were made. This case and data files with the desired parameters then had to

be read into FLUENT. The time step size has been set as 0.01ms.

On convergence, for post-processing of the results, FLUENT allows the user to create and

view velocity vectors and contours of scalar variables. The output data files created from the

monitors set. The breakup models for each injector were tested consecutively with the data

being gathered and analysed after each individual run.

No-breakup atomization model did not require any change to the set up. So after checking

that all the parameters were correct, the only thing needed to be done was to initialise, patch

and then calculate the solution. No-breakup calculation ran until 300 time steps (3ms) where

it was stopped due to length it was taking but still provided sufficient data for analysis.

TAB breakup required TAB breakup menu option to be turned on from the Models menu of

FLUENT. Figure 8 (in Breakup Models section) shows the set-up of the “Discrete Phase –

On” editing menu where breakup can be turned on. Once changed to TAB breakup on, it is

initialised by patching initial conditions, and then transient run is launched to calculate the

solution. The TAB breakup run was calculated for 800 time steps (8ms).

Wave breakup needed the Wave breakup menu option to be turned on. The same process as

TAB break up was used to do this; in the Models menu with “Discrete Phase – On” , go to

“Edit” and click on the “Physical Models” tab. At the bottom of the menu is the breakup

options. Figure 9 (in Breakup Models section) shows the activation setup for Wave breakup.

Wave breakup calculation also ran to 800 times steps (8ms).

After each calculation was finished the data were post-processed. FLUENT allows images of

filled contours to be made, showing the output of the calculation on any part of the mesh and

can be easily adjusted to the desired needs. Images of mass fraction of octane and heptane

vapour, and vectors of velocity were useful in showing where the vapour cloud ended up.

There are images that show tracks of fuel droplets making it useful to see what has actually

happened with the spray inside the chamber.

Due to setting up monitors, output files are generated by FLUENT. This data was

transferrable to Microsoft Excel and was manipulated to give lots of graphical representation

of the data. This makes all three models easier to compare quantitatively.


5. Results:

Injector near Inlet Valve:

The first scenario used was to test the fuel spray without breakup. This meant that the droplet

stayed the same size they were when they were injected.

The graph below shows the percentage of octane in the fuel vapour for the no-breakup model.

The octane vapour mass fraction is density-averaged over in-cylinder charge by ANSYS

FLUENT code and it is written to an output text file at each timestep. This gives the in-

cylinder mass of octane vapour. The output files are further processed in Excel.

The liquid fuel is injected at time = 0, and in few timesteps the octane percentage in the

vapour rises to 80%. Timestep is set to 0.01ms.

Percentage of octane in the fuel vapour is defined as the ratio of vapour mass of octane to the

fuel vapour mass. The fuel vapour mass is the sum of vapour mass for octane and heptane.

Heptane evaporates quicker than octane so there is less percentage of gaseous octane at initial

period; it stays at 80% for about 0.2ms (20 time steps). Once reaching this time step the

evaporation process seems to be completed, and octane percentage rapidly rises to 95%,

where it stays constant for the remaining time.

Figure 15 Percentage of octane in the fuel vapour for no-break up model

The next graph shows the percentage of octane but with TAB break up on.


Figure 16 Percentage of octane in the fuel for TAB breakup.

Figure 16 shows that initially it rises to 80% due to the liquid fuel evaporation. But instead of

staying constant like the breakup model did, it continues to rise unsteadily until about 0.4ms

(40 time steps), showing how gradually it evaporates. It reaches the 95% level for mass

fraction of octane in fuel vapour, and remains there for the rest of the time. This shows that

evaporation started almost instantly upon injection but did not develop as rapidly as without

break-up.

The next graph shows the octane percentage for the Wave break up model.

Figure 17 Percentage of octane in the fuel for Wave breakup


Figure 17 is similar to the no-break up model. When the liquid fuel is injected the percentage

increases to 80% after one time step. It then gradually increases very slightly for about the

next 0.1ms (10 time steps) perhaps showing some slight evaporation before rapidly

increasing to 95%. The Wave break up model reaches a constant of 95% octane faster than

the other models. The plots above have shown percentage of octane in fuel vapour by mass.

The next figure shows transient behaviour of in-cylinder octane vapour mass.

Figure 18. In-cylinder mass of octane vapour (kg) for all breakup model upto 8ms

Figure 19. In-cylinder mass of octane vapour: magnified view

In Fig 18, the oscillations in mass of octane vapour are observed that settle down with time.

The mass of octane vapour varies with in-cylinder mass. The variations of in-cylinder mass


are ascribed to reflected pressure waves causing backflow of in-cylinder charge into inlet

manifold.

As you can see from Figure 19 the evaporation of fuel for TAB model begins almost

immediately and slowly increasing. After a certain delay, evaporation for Wave and no-

breakup model rises steeply reaching up to just under 1.80E-08 kg. During the first 0.1ms, the

octane mass fraction for the TAB model is still increasing. It reaches a maximum of around

1.7E-08kg, then vapour mass start decreasing for all breakup models, as in-cylinder charge

mass is decreasing due to the flow up the inlet duct because of reflected pressure waves.

These oscillations are believed to be caused by pressure waves inside the cylinder; they are

slowly damping with time as can be seen on Figure 18.

The following images show fuel vapour distribution as predicted by FLUENT.

No-breakup model, mass fraction of octane at 2ms

Figure 20 Mass fraction of octane vapour at 2ms for near-inlet valve injector; no-breakup

The bulk of the evaporated fuel remains by the injector. Some of it goes up the inlet duct,

meaning that fuel is wasted; the ignition will be hard as the bulk of the fuel is far from the

spark plug.


Although the maximum is almost 4 times smaller than the Wave model (see Fig. 22), a larger

portion of fuel has ended up spread around the chamber. The mass fraction near spark plug is

about 4e-05 with the highest points being around 5e-05.

With TAB model, you can see that the fuel particles did not travel very far as the whole

section around the injector contains most of the evaporated fuel. Again some escapes up the

inlet valve. Lower values of mass fractions of octane get around the chamber but like the

Wave model, it is quite close to zero.

TAB model mass fraction of Octane shows at 2 ms:

Figure 21 Mass fraction of octane vapour at 2ms for TAB model for near-inlet valve injector

Wave breakup model mass fraction of Octane for 2ms:


Figure 22. Mass fraction of octane vapour at 2ms for Wave model for near-inlet valve injector

Although there is fuel all around the chamber, the main part of the evaporated fuel is centred

around the injection point. This is not good as it’s too far away from the spark plug. Part of

the fuel also ends up going up the inlet valve, again this is not good as fuel is wasted by

settling up there rather than being ignited. A very small proportion of the fuel ends up around

the chamber for evaporation.

From these results, we conclude that the fuel is evaporating too quickly and because of this

we end up with a large portion of fuel vapour right by the injector, not by the spark plug.

Fuel is also being lost due to going up the inlet duct. If the inlet valve is to remain open for

the duration of the combustion process then relocating the injector could help to overcome

this problem. This will be explored below.

Injector near exhaust duct

The injector was moved to the opposite side of the chamber near the exhaust duct. Spray was

aimed towards the spark plug. A new type of injection, single injector was setup. Thus

required setting a new location of the injector and initial droplet size. The chemical

composition of fuel and mass flowrate reamined the same.


Figure 23 Initial velocities of droplets for single injector positioned by the exhaust duct

This test unfortunately diverged so instead a group injection of 10 injectors was tested,

without breakup activated. The mass flow rate (MFR) was decreased to 10% of the original

for each injector to provide the same overall mass injected. Thus, each injector had a MFR of

0.0007642 kg/s.

Figure 24 Initial velocities of droplets for group injectors positioned by the exhaust duct

New position of injector


Figure 25 Velocity magnitude contours for in-cylinder flow for group injectors positioned by

the exhaust duct; no-break up model for spray

The results from the group injector (no breakup) show that the fuel vapour overshoots the

spark plug and ends up going into the inlet duct. The point at which the fuel vapour is fastest

is already past the optimum position near the spark plug for successful combustion. Thus

indicates that the gas cloud of fuel will continue to travel further and end up quite far away

from the spark plug. The Fig 25 shows the fastest point to be quite near the inlet duct. From

this point the fuel vapour could either continue to the other side of the chamber or go up the

inlet valve like before. Neither of these options is good as both positions will be too far from

the spark plug. Next Figure shows that the vapour does in fact go up the inlet valve again.


Figure 26 Mass fraction of octane at 1.6ms for the group injectors positioned by the exhaust

duct; no-break up spray model

All the fuel vapour is accumulated near the inlet valve. This causes the same problem as

before; not enough fuel vapour by the spark plug and it will mean that there won’t be a

successful combustion.

Figure 27 Mass fraction of octane at 0.5ms for the group injectors by the exhaust duct for

WAVE break up model

For Wave breakup model, the results for the first steps after fuel injection have shown that

the fuel evaporated far too quickly (Fig. 27). Though vapour was not lost to the inlet valve it

was still too far away from the spark plug. Another location of the injector was to be tested to

try to solve this problem.


Injector near spark plug:

The new single injector was placed at the top, near where the spark plug is. It was set to have

interaction with continuous phase. Two cases are explored:

The spray is not atomized (no-breakup model is selected). Injection velocity 100 m/s

Wave breakup model for spray atomization. Injection velocity 50 m/s

Figure 28 Initial droplets’ velocities for the case of the injector positioned near spark plug

Due to the position of the injector near spark plug, and the fact that liquid fuel spray goes

directly down towards the piston, for fast-enough evaporation the vapour cloud could be

formed near spark plug, thus facilitating a successful combustion.

This is true if spray is not deflected by the crossflow of intake air.

New injector location


Figure 29 Mass fraction of octane vapour for no-breakup model. The injector is near spark plug

For no-breakup option, the vapour cloud overshoots and is formed on the surface of the

piston. This is clearly undesirable as fuel film on the piston does not evaporate quickly and it

is still far from the spark plug.

Figure 30. Mass fraction of octane vapour for Wave breakup model. The injector is near spark plug

For Wave breakup, the fuel evaporates quickly and thus ends up around the spark plug due to

the position of the injector. This is the optimum place for the vapour cloud to be and hence

this is the best place to position the injector. It shall be observed though that the fuel cloud is

significantly deflected towards the wall by the inlet air flow


6. Discussion

For the injectors near the inlet valve and near the exhaust duct, all models showed that whilst

the injector was directed towards the spark plug, the flow pattern was not appropriate to

create a fuel vapour cloud under the spark plug. Fuel vapour either remains mostly around the

injector or is lost up the inlet valve. To overcome this loss of fuel, a solution could be to keep

the inlet valve closed during fuel injection, ensuring that all the fuel remains in the chamber.

Some fuel might be lost due to impinging on a cylinder wall and piston.

The injector near the spark plug was directed towards the piston. The no-break up model

shows that the spray overshoots impinging on the piston, and the vapour cloud was too far

from the spark plug.

Wave breakup models gave the desired outcome for the case under consideration; the fuel

evaporated almost immediately and so the vapour cloud was adjacent to the spark plug,

though it was deflected to the wall of the cylinder head by strong crossflow of inlet air.

The injector near the spark plug with Wave breakup model was losing fuel to the inlet duct

after 1.8ms. This shows the complexity of the interaction between fuel spray and air jet.

Guiding the flow so that the fuel vapour ends up under the spark plug could help solve this

problem and give a successful combustion. Methods on how to guide the flow should be

further investigated.

Dissemination of knowledge and recommendations for further research

CEREEV workshop

I gave a presentation of my findings to the CEREEV group on 12th

September 2013 at the

University Brighton (CEREEV, 2012). The title of the workshop was: CEREEV Researcher

exchange, technical workshop and project review (Appendix 1). The minutes of the meeting

relevant to this project, read: ‘CFD: Simulations by E. Sazhina, O. Rybdylova and L. Taylor

showed that high-pressure intake conditions produced choked sonic flows conditions in the

poppet valve inner seat area. Tests were carried out with three spray options: no breakup

model, TAB and WAVE breakup. Comparisons of spray velocity and fuel mass distributions

in the cylinder were made between three injector locations; central, inlet and exhaust. The

central position was considered the preferred solution however further work will need to be

undertaken to investigate the pressure waves generated across the chamber.’


From the feedback I received, I got some insight into the direction of further research for the

split-cycle engine. The meeting gave much motivation; it was great to meet the people my

project had actually been helping, and talking with the team was a real joy.

Contribution to the E3C3 EU INTERREG project

The study of evaporation process of multicomponent droplets is relevant for the EU

INTERREG E3C3 project (E3C3, 2013). One of the tasks set in the E3C3 project focuses on

evaporation of biofuels containing many hydrocarbon components. Hence the investigation

of heating and evaporation of multicomponent fuel droplets is relevant for feasibility studies

of CFD capabilities in this area.

The rate of evaporation as predicted by ANSYS FLUENT, is monitored for both C7H16 and

C8H18 components. It is observed that, as expected, at the initial stage n-heptane evaporates

faster than iso-octane. This can be seen from Fig. 31 showing the transient behaviour of

octane concentration in fuel vapour (on mass basis). In other words, the plot shows the ratio

of mass fraction of octane vapour, to the mass fraction of fuel vapour. It is less than the value

of 0.95 until 0.5ms. This demonstrates faster evaporation rate of n-heptane, as it can be

expected based on physical properties.

Figure 31. Ratio of octane vapour to fuel vapour by mass, for the spark-plug location of the

injector with injection velocity of 50 m/s; Wave spray breakup model.

7.50E-01

8.00E-01

8.50E-01

9.00E-01

9.50E-01

1.00E+00

0 0.2 0.4 0.6 0.8 1

Ratio of octane in fuel vapour by mass


The results of the CFD simulations are used as a feasibility studies for multicomponent spray

modelling under the E3C3 EU INTERREG research. The newly developed CFD spray

models will take into account temperature gradients and species diffusion inside a

multicomponent droplet. Whilst current project lays foundation for CFD implementation of

the newly developed spray models at SHRL, further research on validation of the CFD results

against available experimental data is needed.

Conclusions

The project focused on modelling of spray injection and fuel evaporation for studies of

mixture preparation in a split-cycle engine. The modelling has been performed by numerical

simulation using CFD code ANSYS FLUENT. The fuel droplets were set as multicomponent

liquid, 95% of iso-octane C8H18 and 5% of n-heptane C7H16 by mass.

Fuel spray evaporation and vapour mixing with air is calculated by the CFD code, and it can

be monitored by post-processing of the results. CFD runs for three injector locations and

various spray atomization options: No-Breakup, Wave and TAB breakup models, have been

performed for realistic engine conditions. This has a direct relevance to EU INTERREG

CEREEV project (CEREEV, 2012).

The study of evaporation process of multicomponent droplets is relevant for the EU

INTERREG E3C3 project (E3C3, 2013). One of the tasks set under the E3C3 project, focuses

on biofuels containing many hydrocarbon components. Hence the investigation of heating

and evaporation of multicomponent fuel droplets is relevant for feasibility studies of CFD

capabilities in this area. The rate of evaporation as predicted by ANSYS FLUENT, is

monitored for both C7H16 and C8H18 components.

From my work and all the results I got, my conclusion would be that further research is

needed to explore realistic flow patterns and injection strategies. Evaporation process for

multicomponent droplets must be further explored. Engine data obtained by experimental

methods shall be compared with CFD predictions. As a summary of current project, at this

stage of development the recommended position for the injector would be by the spark plug.


References

ANSYS (2013) Theory Guide,

http://www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics/Fluid+Dynamics+

Products/ANSYS+Fluent as seen on 01/08/2013.

Automotive Engineering (2013) Modelling of droplet and spray dynamics, heating and

evaporation, Research Workshop, Centre for Automotive Engineering, Watts Bldg,

University of Brighton, 16th

August 2013 .

CEEREV (2012) http://www.interreg4a-

manche.eu/index.php?option=com_sobi2&sobi2Task=sobi2Details&catid=3&sobi2Id=3110

&Itemid=&lang=en

E3C3 (2013), Energy Efficiency and Environment: a Cross-Channel Cluster

http://www.brighton.ac.uk/shrl/projects/E3C3/

Engine (2013) Definition of engine, http://en.wikipedia.org/wiki/Engine as seen on

01/08/2013.

FLUENT (2013) http://willem.engen.nl/uni/fluent/documents/external/fluent-overview.ppt

as seen on August 2013.

Gent (2007) Gent, D. and Ritchie, R. OCR AS Chemistry, Heinemann, Essex UK , 2007

How Stuff Works (2013) http://science.howstuffworks.com/transport/engines-

equipment/two-stroke1.htm as seen on August 2013

Nuffield (2013) http://www.nuffieldfoundation.org/nuffield-research-placements#1

Research Student Conference (2013) Science Accessible, Huxley Building, The Faculty of

Science and Engineering Doctoral College, The University of Brighton, July 8 - 9 2013

Heywood (1988) Heywood, J.B., Internal Combustion Engine Fundamentals, McGraw-Hill

Book Company, Singapore, 1988.

http://www.interreg4a-manche.eu/index.php?option=com_sobi2&sobi2Task=sobi2Details&catid=3&sobi2Id=3110&Itemid=&lang=en



http://www.brighton.ac.uk/shrl/projects/E3C3/


ICE (2013) Internal Combustion Engine,

http://authors.library.caltech.edu/25069/6/AirPollution88-Ch4.pdf as seen on August 2013.

Meldolesi (2012), Meldolesi, R., and Badain,N. 2012, Scuderi Split Cycle Engine: Air Hybrid

Vehicle, SAE 2012-01-1013, doi: 10.4271/2012-01-1013

2-ACE (2012), A fundamental study of the novel poppet valve 2-stroke auto ignition

combustion engine (2-ACE), http://www.brighton.ac.uk/shrl/projects/2-ace.php and

http://gow.epsrc.ac.uk/ViewGrant.aspx?GrantRef= EP/F058276/1 seen on 12/07/2013

Scuderi (2013) SCUDERI™ Group, Inc http://www.scuderigroup.com/technology/ as seen

in August 2013.

Philips et al (2013), Phillips, F., Gilbert, I., Pirault, J., and Megel, M., Scuderi

Split Cycle Research Engine: Overview, Architecture and Operation,

SAE Int. J. Engines 4(1):450-466, 2011, doi:10.4271/2011-01-0403.

SHRL (2013) http://www.brighton.ac.uk/shrl/ seen on 01/07/2013

Turner (2012), M. R., Sazhin, S.S., Healey, J.J., Crua, C. and Martynov, S.B. A breakup

model for transient Diesel fuel sprays Fuel, 97 , pp. 288-305. ISSN 0016-2361

Vetronics (2013), http://www.brighton.ac.uk/cem/research/groups/vetronics.php) as seen in

August 2013.

http://www.brighton.ac.uk/cem/research/groups/vetronics.php


Appendix 1:

Attendance of research workshops and laboratory tours

Sir Harry Ricardo Laboratories

July 2013, Dr Steve Begg

http://www.brighton.ac.uk/shrl/

‘The University of Brighton and Ricardo UK jointly opened the Sir Harry Ricardo

Laboratories on 14 November 2006.

The SHRL are one of the largest UK research teams dedicated to internal combustion

engines, the development of laser-based measurement techniques, fundamental modelling

and computational simulation. It is regarded as one of the foremost centres for automotive

engine research in Europe. The group's international esteem is demonstrated by its breadth of

collaboration with over 40 academic institutions and industrial partners across the world.’

Tour of Vetronics laboratories

July 2013, Professor E. Stipidis and Dr Panagiotis Oikonomidis

http://www.brighton.ac.uk/cem/research/groups/vetronics.php as seen on 12/09/2013

‘The Vetronics Research Centre (VRC) is the only Academic Centre of Excellence in the

UK conducting research and training in the subject area of Vetronics (Vehicle Electronics),

sponsored by the UK Ministry of Defence (MOD) and supported by Defence Science

Technology Laboratory (DSTL) and Defence Equipment and Support (DE&S).

We focus on specialised and targeted research to investigate new technologies and

methodologies that can be applied in the immediate and near future. Almost all of our

research programmes include a functional demonstrator deliverable that can give a practical

hands-on experience of the output. These systems are designed in a modular fashion to make

them re-usable for an extension of the programme or even in other related applications,

allowing us to keep re-development costs down and have access to an ever increasing set of

testing environments.’

http://www.brighton.ac.uk/shrl/shrl/events/news-SHRL.php


SHRL Research workshop, 16 August 2013:

Modelling of droplet and spray dynamics, heating and evaporation

Centre for Automotive Engineering

Research Workshop

Friday, 16th

August, 2013

Room W623, Watts Building, University of Brighton

Modelling of droplet and spray dynamics, heating and evaporation

14.00: Dr Natalia Lebedeva and Prof Alexander Osiptsov

Application of the full Lagrangian and viscous-vortex methods to modelling of impulse two-phase jets

14.45: Dr Oyuna Rybdylova

Numerical modelling of two-phase vortex ring flow

15.10 Prof Vlad Gun’ko

Quantum chemical approach to study evaporation of Diesel fuel droplets

15.55: Dr Rasoul Nasiri

Quantum chemical studies on n-alkane droplets

16.15: Mansour al Qubeissi

Biodiesel fuel droplets: modelling of heating and evaporation processes


CEREEV Researcher exchange, technical workshop and project

review

12 September 2013

i) Minutes

ii) The presentation by the author

Fuel Spray and Mixture Preparation in Split-Cycle …about.brighton.ac.uk/cereev/publications/new/Fuel Spray and Mixture... · Fuel Spray and Mixture Preparation in Split-Cycle Engine

Documents