10. Combustion Engines Introduction • This section will address the basic features and operating principles of practical combustion sys- tems, mainly internal combustion engines, pre- dominantly used for propulsion. • External combustion engines will be described, but will not be discussed. • The distinction between internal and external com- bustion engines is dependent on the nature of the working fluid. 10. Combustion Engines 1 AER 1304–ÖLG
Transcript
10. Combustion EnginesIntroduction
• This section will address the basic features andoperating principles of practical combustion sys-tems, mainly internal combustion engines, pre-dominantly used for propulsion.
• External combustion engines will be described,but will not be discussed.
• The distinction between internal and external com-bustion engines is dependent on the nature of theworking fluid.
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• Working fluid, as the name implies,- produces work by pushing on a piston or tur-bine blade that in turn rotates a shaft, or
- works as a high momentum fluid that is useddirectly for the propulsive force.
• In internal combustion engines, the energy sourceis a combustible mixture, and the combustionproducts is the working fluid.
• In external combustion engines, the combustionproducts is used to heat a second fluid that acts asthe working fluid.
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• With this definition, these are the most commoninternal combustion engines:- Gasoline engines (also known as spark-ignition, SI): homogeneous/stratified charge.
- Diesel engines (also known as compression-ignition, CI, engines).
- HCCI engines (homogeneous charge compres-sion ignition): currently under-development.
- Gas turbine engines: aircraft propulsion; sta-tionary power production.
- Chemical rockets.
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• Examples of external combustion engines:- Steam power plants.- Home heating furnaces fuelled by gas or oil.- 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
• Aircraft Jet Engines:- Turbojet engines: all jet except for workneeded for the turbine that drives the com-pressor.
- Turbofan engines: part jet, part shaft work todrive a fan (in addition to the compressor).The fan privides about 5-6 times more airpassing around the engine core.
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- Turboprop engines: same as turbofan, but therate of airflow through the propeller may be25 to 30 times the airflow through the coreengine.
• Turboshaft Engines:- Industrial stationary engines used for powerproduction: electricity generation; to drive apump.
- To drive a rotor (helicopter), or to drive aship’s propeller.
• Simplest of air-breathing engines.• A diffuser, a combustion chamber, and an exhaustnozzle.
• Most suitable for supersonic speeds.• Compression by ram effect.• Fuel injection into compressed flow - flame hold-ers to stabilize flame.
• Combustion gases expand to high velocity in thenozzle.
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Schematic diagram of a ramjet engine.
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Chemical Rockets• Liquid Fuel Rockets: Fuel and oxidizer are storedin separate thin-walled tanks at low pressure. Be-fore combustion, they pass through turbine-drivenpumps and are injected to combustion chamber,where they burn at high pressure.
• Solid Fuel Rockets: Entrire block of propellant(consisting of premixed fuel and oxidizer) storedwithin the combustion chamber. Combustion pro-ceeds from the surface of the propellant grain at arate that depends on pressure and temperature andgeometry of the burning surface.
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Schematic diagram of a rocket engines.
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Nonequilibrium expansion:• Equilibrium composition depends on pressure andtemperature, for given fuel and Φ, and may in-clude large quantities of dissociated material.
• In the exhaust nozzle dissociated compounds tendto recombine because of temperature drop.
• This exothermic recombination reactions may actas a heat source in the flow.
• Following Fig. illustrates relative importance ofdissociation energies before and after an equilib-rium expansion of a stoichiometric H2-O2 mixture.
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• For products to maintain their equilibrium com-position as they expand, recombination reactionsshould be fast enough to keep pace with rapid ex-pansion.
• Since expansion process is very rapid, this condi-tion is not always met.
• In the limit, i.e. τrecomb >> τexpan
we have frozen flow at constant composition.• Difference between equilibrium and frozen flowcan be appreciable for some propellants.
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Current Combustion Problems in Rockets:
• Combustion Instabilities: Both liquid- and solid-propellant rockets are subject to combustion insta-bilities in the form of large pressure oscillationswithin the chamber that may lead to engine fail-ure.
• Low-frequency oscillations (about 100 Hz) due tocoupling between combustion and feed system.
• High-frequency oscillations (several thousand Hz):thermo-acoustics, i.e., coupling between combus-tion and acoustics (and flow field).
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• In solid-propellant rockets, burning rate is verysensitive to pressure and velocity.
• Energy release and propellant velocity or pressurepattern that causes nonuniformity can interact toproduce sustained oscillations.
• Such oscillations lead to high rates of erosiveburning that may change the chamber geometryto stable burning or may lead to engine failure.
• Burning rate of solid propellants• Atomization/mixing in liquid-propellant rockets.
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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 turbines androcket propulsion.
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Spark-Ignition & Compression-Ignition Engines
• In our formal definition of internal combustionengines, we included the gas turbines and rocketsunder this classification. Conventionally, however,the term internal combustion engines is used forspark-ignition and compression-ignition engines.
• Spark-ignition engines (Otto cycle engines, orgasoline engines, or petrol engines, though otherfuels can be used).
• Majority of reciprocating engines operate as thefour-stroke cycle.
• Each cylinder requires four strokes of its piston- two revolutions of the crankshaft - to completesequence of events which produces one powerstroke.
• Both SI and CI engines use this cycle.• The four strokes are: Intake, compression, power,and exhaust.
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• Intake stroke: starts with piston at TC and endswith piston BC, which draw fresh mixture intocylinder. To increase mass inducted, inlet valveopens shortly before stroke starts and closes afterit ends.
• Compression stroke: both valves are closed andthe mixture inside the cylinder is compressed toa small fraction of its initial volume. Toward theend of the compression stroke, combustion is initi-ated and the cylinder pressure rises more rapidly.
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• Power stroke: or expansion stroke:- starts with the piston at TC and ends at BCas the high-temp., high-pressure, gases pushthe piston down and force the crank to rotate.
- About five times as much work is done onthe piston during the power stroke as the pis-ton had to do during compression.
- As the piston approaches BC the exhaustvalve opens to initiate the exhaust processand drop the cylinder pressure to close to theexhaust pressure.
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• Exhaust stroke: where the remaining burned gasesexit the cylinder:- first, because the cylinder pressure may besubstantially higher than the exhaust pressure;
- then as they are swept out by the piston as itmoves toward TC.
- As the piston approaches TC the inlet valveopens. Just after TC the exhaust valve closesand the cycle starts again.
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Two-stroke cycle SI engine:• The four-stroke cycle requires, for each enginecylinder, two crankshaft revolutions for eachpower stroke.
• To obtain a higher output from a given enginesize, and a simpler valve design, the two-strokecycle was developed.
• The two-stroke cycle (as four-stroke cycle) is ap-plicable to both SI and CI engines.
• The two strokes are: compression, and power orexpansion.
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Two-stroke engine operating cycle.
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• Compression stroke: starts by closing the inlet andexhaust ports, and then compress the cylinder con-tents and draws fresh charge into crankcase. Asthe piston approaches TC, combustion is initiated.
• Power or expansion stroke: similar to that in four-stroke cycle until the piston approaches BC, whenfirst the exhaust ports and then the intake portsare uncovered. Most of burned gases exit cylin-der in an exhaust blowdown process. When inletports are uncovered, fresh charge which has beencompressed in crankcase flows into cylinder.
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Four-stroke CI engine operating cycle.
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Wankel engine operating cycle.
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Wankel engine operation:
• Wankel rotary engine operates with the four-strokecycle.
• As the rotor makes one complete rotation, the ec-centric shaft rotates through three revolutions.
• As the rotor makes one rotation, each chamberproduces one power stroke.
• Three power pulses occur for each rotor revolu-tion; thus for each eccentric shaft revolution thereis one power pulse.
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Combustion in SI Engines:• Mixture preparation:
- Carburation (no longer used in North Ameri-can markets).
- Port injection - fuel is sprayed into the airstream just before the inlet valve.
- Direct injection - fuel is injected into thecylinder (DISI).
• Ignition: spark plug.• Flame kernel development and flame propagation.
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• Engine knock:- Fuel octane number- Engine compression ratio
