05a-Combustion in SI Engines

Combustion in SI Engines, CI Engines,

and Gas Turbines

Combustion in SI Engines

The combustion process of SI engines can be divided into three broad regions

ignition and flame development

Generally considered to be the consumption of the first 5% of the air-fuel mixture (some sources use the first 10%)

During this period, ignition occurs and the combustion process starts, but very little pressure rise is noticeable and little or no useful work is produced

flame propagation

Bulk of the fuel and air mass, about 80-90%, is burned during this period

Work produced in an engine cycle is the result of the flame propagation period of the combustion process


Picture of turbulent flame

propagation inside a spark

ignition engine

The pressure trace of an IC

engine experiencing knocking

shows unsteady waves


Pressure in the cylinder is greatly increased which provides the force to produce work in the expansion stroke

flame termination

The final 5% (some sources use 10%) of the air-fuel mass burns in this period

During this time, pressure quickly decreases and combustion stops


Images of the flame propagation process in an HCSI

engine (color scale qualitatively represents burning

intensity.)

The presence of the spark is highlighted in the first image


Combustion in SI engine ideally consists of an exothermic subsonic flame progressing through a premixed homogeneous air-fuel mixture

The spread of the flame front is greatly increased by induced turbulence, swirl, and squish within the cylinder

Combustion in a gaseous fuel-air mixture ignited by spark is characterized by the more or less rapid development of a flame that starts from the ignition point and spreads in a continuous manner outward from the ignition point

When this spread continues to the end of the chamber without change in its speed or shape, the combustion is called normal

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When the mixture appears to ignite and burn ahead of the flame, the phenomenon is called autoignition

When there is a sudden increase in the reaction rate, accompanied by measurable pressure waves, the phenomenon is called detonation

Combustion is initiated by an electrical discharge across the electrodes of a spark plug (piloted ignition) which occurs anywhere from 10to 30 before TDC, depending on the geometry of the combustion chamber and the immediate operating conditions of the engine

Combustion starts very slowly because of the high heat losses to the relatively cold spark plug and gas mixture

Flame can generally be detected at about 6 of crank rotation after spark plug firing

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Autoignition can occur when

critical pressure and

temperature are exceeded in

the engine

Engine knock occurs

when unburned gases

autoignite


Applied potential is generally 25,000 - 40,000 volts, with a maximum current on the order of 200 amps lasting about 10 nsec (1 nsec = 10-9 sec)

This gives a peak temperature on the order of 60,000 K

Overall spark discharge lasts about 0.001 second, with an average temperature of about 6000 K

A stoichiometric mixture of hydrocarbon fuel requires about 0.2 mJ (0.2 X 10 -3 J) of energy to ignite self-sustaining combustion

This varies to as much as 3 mJ for non-stoichiometric mixtures

The discharge of a spark plug delivers 30 to 50 mJ of energy, most of which, however, is lost by heat transfer


The P-V diagram of actual engines differs somewhat from the ideal Otto cycle diagram due to heat losses, friction, and the finite amount of time required for release of the fuel energy

Pressure-volume trace from

a typical IC engine

Pressure trace and heat release rate

versus CAD for a research engine with

a fuel mixture of 70% isooctane and

30% n-heptane


Spark ignition timing has a significant impact on the performance of an SI engineTo produce the maximum torque for a given rpm, the best timing is found when the peak pressure occurs around 510 CAD after TDCThis optimal timing is referred to as the maximum brake torque (MBT) timingWhen the engine speed increases, timing is advanced to achieve the best thermal efficiencyIf timing is advanced too early, an engine may experience knocking


The relation between flame development and pressure depends on many factors

Effect of engine speed

The average flame speed remains nearly proportional to the piston speed

If the spark is advanced as speed increases in such a way as to keep peak pressure at the optimum crank angle (15 to 20 ATC), the apparent time loss will be nearly independent

of speed

Effect of cylinder size

The ratio of flame speed is nearly the same for similar cylinders of different sizes

At a given piston speed, burning time will be nearly inversely proportional to bore, and, since rpm is inversely proportional to bore, the burning angle will be nearly independent of the bore


At a given rpm, average flame speed will be nearly proportional to bore, and effective burning angle will again be independent of the bore

Effect of Reynolds Index

Flame speed always increases with Reynolds number

Effect of combustion-chamber shape

Flame speed is higher in combustion chamber with squish than in an open type combustion chamber

The more compact the chamber, the greater the rate of pressure rise

Effect of stroke-bore ratio

Flame angles tend to be larger as the stroke is reduced, at a given piston speed, with a given bore

The increase in angle is not as great as the increase in rpm

Abnormal Combustion in SI Engines

The two major abnormal combustion phenomena are knock and surface ignition

These abnormal combustion phenomena are of concern because

when severe, they can cause major engine damage

even if not severe, they are regarded as an objectionable source of noise

Knock is the noise (transmitted through the engin e structure) associated with autoignition of a portion of the fuel-air mixture ahead of the advancing flame front

Autoignition is the spontaneous ignition and the resulting very rapid reaction of a portion or all of the fuel-air mixture


Surface ignition is ignition of the fuel-air mixture by a hot spot on the combustion chamber walls such as an overheated valve or spark plug, or glowing combustion chamber deposit: i.e., by any means other than the normal spark discharge

It can occur before the occurrence of the spark (preignition) or after (postignition)

When autoignition occurs repeatedly, during otherwise normal combustion events, the phenomena is called spark-knock

Spark-knock is controllable by the spark advance: advancing the spark increases the knock severity or intensity and retarding the spark decreases the knock

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NeonNote combustion process which is initiated solely and by a timing spark and in which the flame front moves completely across the combustion chamber in a uniform manner at normal velocity .....

Note that ... the flame shape also does not change

NeonNoteA combustion process in which a flame front may be satrted by hot combustion chamber surfaces either prior so or after spark ignition , or a process in which some part or all the charge may be consumed at extremely high rate

NeonNoteA knock is the recurrent and repeatable in terms of audibility . It is controllable by the spark advance . Advancing the spark invrease the knock intesity and returding the spark reduces the intensity

NeonNoteSurface ignition is the ignition of fuel air charge by any hot surface other than the spark discharge prior to the arrival of the normal flame front. It may occur before the spark ignites the charge (preignition) or after the normal ignition (postignition)


NeonNoteSurface ignition which does not result in knock

NeonNoteKNocking which has been produced by the surface ignition. ITis not controllable by the spark advance

NeonNotecontinution of engine firing after the electrical ignition is shut off

NeonNoteRun away surface ignitionSurface ignition which occurs earlier and earlier in the cycle . It can lead to serious overheating and strutural damage to the engine

NeonNoteRumbleA low pitched thudding noise accompanied by the engine roughness . PRobably caused by the high rates if pressure rise associated with very early ignition or multiple surface ignition

NeonNoteWild ping knocking surface ignition characterised by one or more erratic sharp cracks. It is probably the result of early ignition from deposite patiles


Of all the engine surface-ignition phenomena, preignition is potentially the most damaging

Knock primarily occurs under wide-open-throttle operating conditions thus a direct constraint on engine performance

It also constrains engine efficiency, since by effectively limiting the temperature and pressure of the end-gas, it limits the engine compression ratio

The occurrence and severity of knock depend on the knock resistance of the fuel and on the antiknock characteristics of the engine


Pressure trace in knocking combustion

Schematic

representation of engine

knock

Combustion in CI Engines

CI engines are merited with high engine efficiency (up to 45%) because of (1) higher compression ratios, (2) no throttling, (3) lower running speed than SI engines, therefore less friction losses, and (4) lean air/fuel mixture

At most load ranges, CI engines are more fuel efficient than SI engines

These engines are heavier than spark ignition engines because of the need to support higher internal pressures in the cylinders

They are also noisier because of the spontaneous ignition of the charge

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Advantages of diesel engines as compared to SI engines

Compression ratio (CR) is higher, leading to higher thermal efficiency

Since no throttling valve is needed, intake losses are lower, thus efficiency is higher

Overall equivalence ratio is lean (f ~ 0.70.8), so less unburned hydrocarbons and CO are leftover from the gas phase combustion

Walls and crevices contain air only during the compression stroke, so in principle, no hydrocarbons and CO go unburned due to quenching in the crevices


Disadvantages of diesel engines as compared to SI engines

The liquid spray flame burns in diffusion flame mode, causing high temperatures that result in high NOx

At high loads, soot/particles are formed

Cost of diesel engines is high due to the high-pressure injection system

Engines must be heavier to withstand the higher pressures

Maximum operable engine speed (RPM) is lower than in SI engines, so peak power output is lower


Diesel spray consists of several

processes in sequence including

evaporation, mixing with air, and

ultimately combustion

Diesel spray consists of three distinct

zones

(1) spray evaporation

(2)mixing with surrounding hot air

(3) combustion

An estimate of the total physical

time required to complete the entire

spray combustion process in a diesel

engine is:

ttotalphysicaltime = tevap + tmix + tcomb

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The total physical time places an upper limit on how fast the engine can run

Usually the injection timing is set around 30 Before Top Dead Center (BTDC) with a total burn duration of 70Crank Angle Degrees (CAD)

When the engine is run at 3,000 rpm, the total time available for spray combustion is about 3.9 ms

For reference, droplets of size of 10 mm can be vaporized at 900 K and 4 MPa (40 bar) within 0.5 ms

Current diesel engines employ high boost pressures, high injection pressures and high exhaust gas recirculation (EGR) rates than ever used before to pursue better fuel economy and meet stringent emissions standards


The combustion processes themselves are principally governed by mixture formation, auto-ignition and turbulent diffusion

The basic feature of a CI engine is operation with heterogeneous mixture

This enables operation with extremely lean overall fuel-to-air (F/A) ratio, since local values can be kept well within the flammability limits

The consequences are the two persistent problems with the diesel engine emissions: formation of nitric oxides (NOx) and soot particles


Mixture preparation and in-cylinder motion have a critical impact on autoignition, combustion, and formation of pollutants in a CI engine

Over a period of time, the direct injection concept has achieved absolute dominance over the divided chamber (prechamber or swirl-chamber) owing to the significant efficiency advantages


In CI engines, fresh air enters the cylinder during the intake process and mixes with whatever amount of exhaust residual might be present

The air often enters the cylinder at pressures higher than the ambient pressure owing to turbocharging

After the intake valve (or port) closes, the fresh charge is compressed by the piston to very high pressures and temperatures

The fuel is injected at high velocities through small holes on the injector nozzle just before the piston reaches the TDC

The piston top is shaped in a way that allows development of the spray, fuel atomization, and good mixing with air


Fuel evaporates and mixes with air, and owing to very high gas temperatures, autoignites after a delay of only a few crank-angles

Fuel/air mixture prepared during the ignition-delay period burns rapidly and this is referred to as a premixed phase of burning

The injection continues after ignition, and the subsequent stage of the process controlled by mixing rates is called a diffusion phase

The premixed burning is much more dominant at low loads (relatively small amount of fuel injected), and diffusion burning is more dominant at high load (large amount of fuel injected)


Spray combustionCombustion processes in a

typical diesel engine

Temporal trajectories

of local f and T values in the

combustion chamber


Rate of heat release obtained in a conventional CI engine

(a) the typical profile demonstrating a

premixed spike followed by a diffusion

burning phase

(b) sequence of rate of heat-release

profiles obtained during a fueling change,

from low to high. Lower loads display

relatively more premixed burning (back),

while at high loads diffusion part

becomes more dominant (front).


The rate of heat release is defined as the rate at which the chemical energy of the fuel is released by the combustion process

The overall compression-ignition diesel combustion process can be defined as:

Ignition delay (ab): The period between the start of fuel injection into the combustion chamber and the start of combustion [determined from the change in slope on the p-q diagram, or from a heat-release analysis of the p(q) data, or from a luminosity detector]

Premixed or rapid combustion phase (bc): In this phase, combustion of the fuel which has mixed with air to within the flammability limits during the ignition delay period occurs rapidly in a few crank angle degree


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When this burning mixture is added to the fuel which becomes ready for burning and bums during this phase, the high heat-release rates characteristic of this phase result

Mixing-controlled combustion phase (cd): Once the fuel and air which premixed during the ignition delay have been consumed, the burning rate (or rate of heat release) is controlled by the rate at which mixture becomes available for burning

While several processes are involved liquid fuel atomization, vaporization, mixing of fuel vapor with air, preflame chemical reactions the rate of burning is controlled in this phase primarily by the fuel vapor-air mixing process

The rate of heat release may or may not reach a second (usually lower) peak in this phase; it decreases as this phase progresses


Late combustion phase (de): Heat release continues at a lower rate well into the expansion stroke

Several reasons for this are:

A small fraction of the fuel may not yet burned

A fraction of the fuel energy is present in soot and fuel-rich combustion products and can still be released

The cylinder charge is non-uniform and mixing during this period promotes more complete combustion and less-dissociated product gases

The kinetics of the final burnout processes become slower as the temperature of the cylinder gases fall during expansion


Spray Evaporation

The fuel injected into the engine cylinder through orifices of an injector undergoes breakup, atomization and evaporation, and simultaneously mixes with air entrained into the spray plume

The initial droplet size depends on the orifice diameter, injection pressure and air density, and ranges generally from 10 to 20mm in diameter

The cavitation bubbles generated in the nozzle and orifice flow collapse instantly when released in the high pressure ambient air

When the injection pressure is over some 200MPa, the injection velocity exceeds the sound velocity in the in-cylinder air, and shockwaves originating from the orifice exit is observed


The spray droplets transfer their momentum to the entrained air and decrease rapidly their relative velocities, and simultaneously receive heat from the entrained air

The increased vapor pressure on the hot surface of the droplets drives molecular mass transport, i.e., evaporation

With the progress of droplets evaporation inside the spray plume, both local mixture temperature and vapor pressure approach to their adiabatic-saturation conditions which depend on the local fuelair ratio and initial air temperature

When the ambient air pressure exceeds twice the fuel critical pressure and the ambient air temperature is higher than around 1.5 times the fuel critical temperature, the fuel droplet reaches critical temperature during evaporation and turns instantly into gas phase


Auto Ignition

Fuel air mixtures formed during ignition delay period burn explosively when combustion starts and therefore, ignition delay together with the fuel injection rate and air motion plays a key role in determining the initial heat release rate

As the extent of homogeneity of fuel air mixtures formed during ignition delay is responsible for the spatial and temporal distributions of and T in the flame during the early stage of combustion, ignition delay affects largely the formation of NO and soot

Auto-ignition of diesel sprays depends essentially on two processes that progress simultaneously, the physical process governing mixture formation and the chemical process leading to exothermic reactions


The physical properties of a fuel such as density, surface tension, viscosity, and volatility concern closely atomization, evaporation and mixture formation in the spray of this particular fuel

The cetane number is also an important index that relates closely chemical process

The in-cylinder air conditions such as pressure, temperature and oxygen concentration are all involved in both processes

The effect of orifice diameter on ignition delay attracts attention because it tends to become smaller with the increase in injection pressure

Ignition delay decreases with the decrease in orifice diameter but remains unchanged when the orifice diameter is smaller than 0.05 mm


At temperatures above 1000 K, the difference between ignition delays for large and small orifices is bigger than that at lower temperatures, because the physical delay occupies a major portion in the total ignition delay at high ambient temperatures


Instead of one state for the premixed flame, two boundary states are considered for diffusion flames: fuel (which may be diluted in other gases) and oxidizer (diluted or not)

Fuel and oxidizer diffuse towards the reaction zone where they burn and generate heat

Temperature is maximum in this zone and diffuses away from the flame front towards the fuel and oxidizer streams

Diffusion flame structure


The figure illustrates some important considerations:

Far away on each side of the flame, the gas is either too rich or too lean to burn

Chemical reactions can proceed only in a limited region, where fuel and oxidizer are mixed adequately

The most favorable mixing is obtained where fuel and oxidizer are in stoichiometric proportions: a diffusion flame usually lies along the points where mixing produces a stoichiometric mixture

The flame structure is steady only when fuel and oxidizer streams are pushed against each other at given speeds

In a pure one-dimensional unstrained case, the flame spreads with time t, gets choked in its combustion products and the reaction rate slowly decreases as 1/t

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A diffusion flame does not exhibit a reference speed as premixed flames: the flame is unable to propagate towards fuel because of the lack of oxidizer and it cannot propagate towards oxidizer stream because of the lack of fuel accordingly, the reaction zone does not move significantly relatively to the flow field

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Evolving jet just after ignitionFully developed reacting jet with

dark zones indicating high-soot

concentrations in the head

vortex


Schematic representation of spray dispersion

CIE Injection Methods and Systems

There are two general types of injection systems:

Cam-operated injection system pressure increase and fuel metering are coupled mechanically

The cam moves the tappet of the injection pump, which for its part "compresses" the fuel volume

The resulting climbing pressure opens a valve and thus releases the feeding pipe for the injection nozzle

The return line is opened via a trimming edge, and so the fuel pressure falls, the valve closes, and injection is over


Common-rail injection system pressure increase and fuel metering are completely separated

By means of a mechanically or electrically operated high pressure pump, fuel is continually delivered into a high pressure reservoir (common rail)

With an electronically controlled injector, fuel is taken from the common rail and sprayed into the combustion chamber

In the case of the distributor injection pump (DIP), only one pump unit exists for all cylinders

During one engine revolution, the DIP piston makes as many strokes (2-stroke engine) or half as many strokes (4-stroke engine) as there are cylinders, and via one rotation of the distributor head, the fuel is added to the single injection pipes


Functional diagram of a distributor injection pump


For smaller engines, DIP is less expensive than inline injection pumps or unit pump systems (UPS)

Modern distributor injection pumps can create a maximum pump pressure of 800 1,000 bar

However, through purposeful exploitation of the pressure waves spreading in the injection pipe, a heightening of this maximum pressure to approx. 1,500 bar at the nozzle orifice is possible with existing systems

The UPS system is a modularly built high pressure injection system consisting of an injection pump, a short high pressure pipe and an injector-nozzle combination

Injection start and the injection amount are measured by means of a solenoid valve for every cylinder


With the help of the solenoid valve, access to a compensating volume is opened/closed

An opening of the valve causes a rapid decline in pressure in front of the injection nozzle and thus leads to a closing of the nozzle


The injection pump and injection nozzle form a unit in unit injection system (UIS), which is installed at every cylinder separately

A fast-switching solenoid valve controls the injection start and finish

It receives its shift signal from an electronic control unit, in the electronic module of which an injection map is stored

In the UIS, injection pressures up to 2,000 bar can be represented which makes low fuel consumption and emission levels possible


In the electronically controlled common rail system (CR), fuel is led to the common rail, a high-pressure reservoir built as a "pipe", with pressures in the area of 1,200 < p < 2,000 bar, from there the fuel is regulated and led to each cylinders


With common rail injection system, almost any injection path can be represented, whereby the advantages of CR injection systems can only be realized when current solenoid valve/piezo valve controlled injectors are replaced with electronically controlled, directly activating piezo common rail injectors whereby pressure-modulated piezo CR injectors represent a highly promising advance

As opposed to cam-operated injection systems, the operating speed of the high pressure pump does not have to be rigidly coupled to the engine speed because of the common rail systems disassociation of pressure production and control functions


Through this, higher injection pressures can be realized even at smaller engine speeds, which causes better mixture formation and thus improved emission behavior

The common rail injection system should become universally successful in the near future due to its important advantages with regard to both pollutant reduction as well as constructive performance, whereby injection pressures well over 2,000 bar are under consideration


Fuel is injected into the combustion chamber through the bores in the injection nozzle

In the injection process, the fuel should be atomized to the highest possible degree in order to achieve a good air-fuel mixing

For varying combustion processes and fuels, varying nozzle designs are utilized


Pintle nozzles are employed in pre- and swirl chamber engines

They have a stroke dependent opening cross-section, are advantageous with respect to combustion noise, tend however towards carbonization (bung-hole nozzle)

Multi-hole orifice nozzles are employed in direct injection diesel engines sac hole nozzles typically for conventional injection systems and mini-sac hole nozzles as well as seat-type nozzles for common rail injection systems

The injection nozzle is integrated into an injector, which is screwed into the cylinder as a structural group


In case of two-spring injectors, varying spring constants are employed

At injection start, the weaker spring only allows a restricted needle lift and thus a limited delivery rate

Only when the injection pressure exceeds the spring force of the second spring does full needle lift and the maximal injection rate become possible

Through the so-produced pre-injection of a smaller quantity of fuel, a softer pressure increase in the combustion chamber and thus a lower level of noise is achieved


Standard and two-spring injectors

Combustion in Gas Turbines

Chemiluminescence images of a turbulent CH4/H2/N2 jet flame (Red =

15,200)

The long exposure image (far left) indicates the mean flame structure,

and the six shorter exposures to the right illustrate the instantaneous

turbulent structure


The long-exposure image on the left of the figure shows the mean envelope of the reaction zone, which is distributed across the mixing layer of the jet and the coflow

The six short-exposure images illustrate the complex instantaneous structure of the turbulent flames

As the jet exit-velocity increases, the flame becomes increasingly turbulent


Diagram and photograph of

a model gas turbine

combustor operating on

CH4/air at atmospheric

pressure

Fuel is injected from an

annulus separating two

swirling air streams


In this combustor, two annular swirling flows of air surround a ring that injects fuel

The turbulent flame spreads out as a cone, and there are inner and outer recirculation zones

The central theme in nonpremixed combustion is that the structure and stability of a given flame depend on the coupling between turbulent mixing and chemical reactions

Reference velocity

The theoretical velocity for flow of combustor-inlet air through an area equal to the maximum cross section of the combustor casing (25 fps (8 mps) in a reverse-flow combustor; 80-135 fps (24-41 mps) in a straight-through-flow turbojet combustor)


Profile factor

The ratio between the maximum exit temperature and the average exit temperature

Traverse number (temperature factor)

The peak gas temperature minus mean gas temperature divided by mean temperature rise in nozzle design

The difference between the highest and the average radial temperature

Pressure drop

A pressure loss occurs in a combustor because of diffusion, friction, and momentum

The pressure drop value is 2-10% of the static pressure (compressor outlet pressure) the efficiency of the engine will be reduced by an equal percent


The fundamental pressure loss from combustion is proportional to the air velocity squared

Since compressor discharge velocities can be on the order of 500 ft/sec (152.4 m/sec), the combustion pressure loss can be up to one-quarter of the pressure rise produced by the compressor

For this reason, air entering the combustor is first diffused to lower the velocity

Even with a diffuser, velocities are still too high to permit stable combustion

With flame speeds of a few fps, a steady flame cannot be produced by simple injection into an airstream with a velocity one to two orders of magnitude greater

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Even if ignited initially, the flame will be carried downstream and cannot be sustained without continuous ignition

A baffle of some type needs to be added to create a region of low velocity and flow reversal for flame stabilization

The baffle creates an eddy region in the flow continually drowning in gases to be burned, mixing them, and completing the combustion reaction

It is this steady circulation that stabilizes the flame and provides continuous ignition

The problem in combustion then becomes one of producing only enough turbulence for mixing and burning, and avoiding an excess, which results in increased pressure loss


A simple bluff body, such as a baffle placed in the flow stream, is the simplest case of flame stabilization

There are various ways to create flame stability in the primary zone

In one, a strong vortex is created by swirl vanes around the fuel nozzle


Another flow pattern is formed when combustor air is admitted through rings of radial jets

Jet impingement at the combustor axis results in upstream flow

The upstream flow forms a toroidal recirculation zone that stabilizes the flame


Velocity is an important factor in primary zone design

A fixed velocity value in the combustor creates a limited range of mixture strength for which the flame is stable

Also, different flame stabilizing arrangements (baffles, jets, or swirl vanes) exhibit different ranges of burnable mixtures at a given velocity


In the primary zone fuel-to-air ratios are about 60:1; the remaining air must be added somewhere

About 15-20 percent air is introduced around the jet of fuel in this zone to provide the necessary high temperature for rapid conbustion

Some 30 percent secondary, or dilution, air should only be added after the primary reaction has reached completion

Dilution air should be added gradually so as not to chill the flame locally and quench the reaction

The addition of a flame tube as a basic combustor component accomplishes this


Finally, in the tertiary or dilution zone the remaining air is mixed with the products of combustion to cool them down to the temperature required at inlet to the turbine

Sufficient turbulence must be promoted so that the hot and cold streams are thoroughly mixed to give the desired outlet temperature distribution

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05a-Combustion in SI Engines