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10. Combustion EnginesIntroduction

This section will address the basic features

operating principles of practical combustiotems, mainly internal combustion engines,

dominantly used for propulsion.

External combustion engines will be descr

but will not be discussed.

The distinction between internal and externbustion engines is dependent on the nature


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bustion engines is dependent on the nature

Working fluid, as the name implies,- produces work by pushing on a piston

bine blade that in turn rotates a shaft,

- works as a high momentum fluid thatdirectly for the propulsive force.

In internal combustion engines, the energy

is a combustible mixture, and the combust

products is the working fluid.

In external combustion engines, the combuproducts is used to heat a second fluid tha


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With this definition, these are the most cominternal combustion engines:

- Gasoline engines (also known as spar

ignition, SI): homogeneous/stratified c- Diesel engines (also known as compre

ignition, CI, engines).

- HCCI engines (homogeneous charge csion ignition): currently under-develo

- Gas turbine engines: aircraft propulsi

tionary power production.Ch i l k t


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Examples of external combustion engines:- Steam power plants.

- Home heating furnaces fuelled by gas

- Stirling engines.

What kind of engines are the following?

- Solar power plant.- Nuclear power plant.

- Fuel cells.

- Electrical rocket propulsion.


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Internal Combustion Engines Steady Flow internal combustion engines:

- Gas Turbine

- Ramjet / Scramjet

- Chemical Rockets

Non-steady Flow internal combustion engi

- Non-premixed charge

- Premixed chargeStratified charge


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Gas Turbine Engines Aircraft Jet Engines:

- Turbojet engines: all jet except for w

needed for the turbine that drives the

pressor.

- Turbofan engines: part jet, part shaft drive a fan (in addition to the compre

The fan privides about 5-6 times mor

passing around the engine core.


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- Turboprop engines: same as turbofanrate of airflow through the propeller m

25 to 30 times the airflow through the

engine. Turboshaft Engines:

- Industrial stationary engines used for

production: electricity generation; to pump.

- To drive a rotor (helicopter), or to dri

ships propeller.


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14/56Turbojet internal total pressure


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i i l d f i i


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i i l d f bi


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h i b


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l i b


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h i f b j i h f b


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Combustion in Gas Turbines:

Spray combustion (liquid fuels)

Ignition

Flame stability - combustion noise

Flame propagation

Pollutant formation [CO, unburned HC, Nsoot]

Heat transfer

Cooling / dilution


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Current Combustion Problems in Gas Turbine

NOx control

Combustion noise (hooting)

Soot formation (carbon formation)

Fidelity of CFD of combustion codes

Current Combustion Developments in Gas Tu

Lean-premixed combustion [NOx control,

Hydrogen-enrichment [NOx, efficiency]


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Ramjets

Simplest of air-breathing engines.

A diffuser, a combustion chamber, and an

nozzle.

Most suitable for supersonic speeds.

Compression by ram effect. Fuel injection into compressed flow - flam

ers to stabilize flame.

Combustion gases expand to high velocity


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Chemical Rockets

Liquid Fuel Rockets: Fuel and oxidizer ar

in separate thin-walled tanks at low pressu

fore combustion, they pass through turbine

pumps and are injected to combustion cham

where they burn at high pressure.

Solid Fuel Rockets: Entrire block of prop

(consisting of premixed fuel and oxidizer)

within the combustion chamber. Combusti

ceeds from the surface of the propellant gr

rate that depends on pressure and temperat


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Nonequilibrium expansion:

Equilibrium composition depends on press

temperature, for given fuel and , and may

clude large quantities of dissociated materi In the exhaust nozzle dissociated compoun

to recombine because of temperature drop.

This exothermic recombination reactions mas a heat source in the flow.

Following Fig. illustrates relative importan

dissociation energies before and after an eqrium expansion of a stoichiometric H O


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For products to maintain their equilibrium

position as they expand, recombination rea

should be fast enough to keep pace with ra

pansion.

Since expansion process is very rapid, this

tion is not always met.

In the limit, i.e. recomb >> expan

we have frozen flow at constant compositio

Difference between equilibrium and frozen

can be appreciable for some propellants.


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Current Combustion Problems in Rockets:

Combustion Instabilities: Both liquid- and

propellant rockets are subject to combustio

bilities in the form of large pressure oscillawithin the chamber that may lead to engin

ure.

Low-frequency oscillations (about 100 Hz)

coupling between combustion and feed sys

High-frequency oscillations (several thousathermo-acoustics i e coupling between co


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In solid-propellant rockets, burning rate is

sensitive to pressure and velocity.

Energy release and propellant velocity or p

pattern that causes nonuniformity can interproduce sustained oscillations.

Such oscillations lead to high rates of eros

burning that may change the chamber geomto stable burning or may lead to engine fai

Burning rate of solid propellants

Atomization/mixing in liquid-propellant ro


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Assignment

Visit www.howstuffworks.com and read:

- How car engines work?

- How gas turbine engines work?

- How rockets work?

From links, visit sites related to gas turbinrocket propulsion.


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Spark-Ignition & Compression-Ignition En

In our formal definition of internal combu

engines, we included the gas turbines and

under this classification. Conventionally, hthe term internal combustion engines is u

spark-ignition and compression-ignition en

Spark-ignition engines (Otto cycle engines

gasoline engines, or petrol engines, though

fuels can be used).

Compression-ignition engines (diesel engin


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Flame front Fuel spray fla

Premixed charge

(gasoline)Non-premixed charge

(Diesel)

Spark plug Fuel injector

Fuel + air mixture Air only


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Four-stroke cycle SI engine:

Majority of reciprocating engines operate a

four-stroke cycle.

Each cylinder requires four strokes of its p

- two revolutions of the crankshaft - to com

sequence of events which produces one po

stroke.

Both SI and CI engines use this cycle.

The four strokes are: Intake, compression,and exhaust


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Intake stroke: starts with piston at TC and

with piston BC, which draw fresh mixture

cylinder. To increase mass inducted, inlet

opens shortly before stroke starts and close

it ends.

Compression stroke: both valves are closed

the mixture inside the cylinder is compressa small fraction of its initial volume. Tow

end of the compression stroke, combustion

ated and the cylinder pressure rises more ra


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Power stroke: or expansion stroke:

- starts with the piston at TC and ends

as the high-temp., high-pressure, gase

the piston down and force the crank t- About five times as much work is don

the piston during the power stroke as

ton had to do during compression.- As the piston approaches BC the exha

valve opens to initiate the exhaust pro

and drop the cylinder pressure to clos


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Exhaust stroke: where the remaining burne

exit the cylinder:

- first, because the cylinder pressure ma

substantially higher than the exhaust p- then as they are swept out by the pist

moves toward TC.

- As the piston approaches TC the inletopens. Just after TC the exhaust valv

and the cycle starts again.


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Two-stroke cycle SI engine:

The four-stroke cycle requires, for each en

cylinder, two crankshaft revolutions for eac

power stroke. To obtain a higher output from a given eng

size, and a simpler valve design, the two-s

cycle was developed. The two-stroke cycle (as four-stroke cycle

plicable to both SI and CI engines.

The two strokes are: compression, and pow


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Compression stroke: starts by closing the i

exhaust ports, and then compress the cylin

tents and draws fresh charge into crankcas

the piston approaches TC, combustion is in

Power or expansion stroke: similar to that

stroke cycle until the piston approaches BC

first the exhaust ports and then the intake pare uncovered. Most of burned gases exit

der in an exhaust blowdown process. Whe

ports are uncovered, fresh charge which hacompressed in crankcase flows into cylinde


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Engine knock:

- Fuel octane number

- Engine compression ratio

Pollutant formation:

- Nitric oxides, NOx

- Carbon dioxide, CO- Unburned hydrocarbons, HC

Exhaust treatment:

- Catalytic converters


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Combustion in a CI engine:

Direct in-cylinder injection (large engines)

Prechamber injection (passenger car engine

Spray combustion:

- Compression ignition - ignition delay

- Diesel fuel cetane number

Pollutant formation:

- NOx

, CO, HC, Soot (particulates)P ti l t t d t l ti t


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EXH

EMIS

INJECTION AND SPRAY

CHARACTERISTICS

FUEL-AIR

MIXING

PROCESS

IGNITION

Air Inlet

Inlet Port Design

Chamber Design Turbocharge

AIR MOTION / TURBULENCE

IN THE

COMBUSTION CHAMBER

Fuel Properties

MOSTLY

NON-PREMIXED

COMBUSTION

PARTIALLY

"PREMIXED"

COMBUSTION

Injection Timing

Injection System Design

Injection Duration

Injection Rate

EGR

HEAT RELEASE

RADIATION EXCHANGE BET

HOT AND COLD POCKETS

NOX& SOOT FORMATION

SOOT OXIDATION

Wh t i HCCI?


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August 5-9 2001 DEER Workshop Caterpillar Engine Research

What is HCCI?

hot flame region:

nitric oxideshot flame region:

nitric oxides + smoke

Diesel Engine

(compression ignition)

Gasoline Engine

(spark ignited)

HCCI Engine

(Homogeneous ChargeCompression Ignition)

Low temperature combustion

ultra low emissions

spark plugfuel injector

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