Impact_of_the_Media_and_Internet_on_Modern_Youths

Propulsion Aircraft Engine Classification • Aircraft engines are classified in 2 broad categories: • Airbreathing: air is used as the fuel oxidizer; this eliminates the need to carry oxidizer (you only need to carry the fuel), but air of sufficient density is required - this leads to altitude limitations » Reciprocating (piston engines) » Gas turbine (includes turbojet, turboprop, & turbofan) » Ramjet & scramjet • Non-airbreathing: no air is needed, as the motor carries both fuel and oxidizer (not necessarily oxygen!) - this increases the vehicle weight, though » Rockets IC Propulsion Systems - Process Focused Internal Combustion Engines Steady Gas Turbine Uses compressor and turbine combination, turbine drives compressor Turboshaft All shaft work to drive propeller, generator, rotor (helicopter) Turbofan Part prop, part jet "ducted propeller" Turbojet All jet except for work needed to drive compressor Ramjet No compressor or turbine Use high Mach # ram air for compression Rocket Carries both fuel and oxidant Jet power only, no shaft work Solid fuel Fuel and oxidant are premixed and put inside combustion chamber Liquid fuel Fuel and oxidant are initially separated and pumped into combustion chamber Non-steady Premixed-charge (gasoline) Fuel and air are mixed before/during compression Usually ignited with spark after compression Two-stroke One complete thermodynamic cycle per revolution of engine Four-stroke One complete thermodynamic cycle per two revolutions of engine Non-premixed charge (diesel) Only air is compressed, fuel is injected into cylinder after compression Two-stroke One complete thermodynamic cycle per revolution of engine Four-stroke One complete thermodynamic cycle per two revolutions of engine Pulse Jet Piston equivalent of jet engine air is not compressed, just mixed & combusted Aircraft - Mission Focused Propulsion Thrust Generation Jet Propeller Flapping Unducted Ducted Power Generation Reciprocating Turbine (Gas) Electric - Battery - Fuel Cell - Solar - Chemical Turbojet Rocket Turbofan Turboprop Thrust Generation • Propulsion devices can also be divided into categories based on the way that thrust is created • Propeller » Piston powered propeller » Electric propeller, ducted or unducted fan » Turboprop (95% of thrust from propeller) • Jet » Turbojet and turbofan (note that the fan is a ducted propeller) » Ramjet and scramjet » Rocket • Note that this convention isn’t as useful (or clean) as we’ll see later once the thrust equation is derived Airbreathing Engines • Most engines are airbreathing, whereby ambient air is used as both oxidizer and a source of mass flow - efficient combustion requires that the air be compressed in some manner either by a piston (reciprocating engine) or fan (turbine) » Piston (reciprocating) o Piston engine drives shaft that turns prop » Turbojet (classical jet engine) o Compressor is used to provide compression; turbine drives compressor » Turbofan o At lower speed, ducted fan is used to increase mass flow rate & efficiency » Turboprop o No duct, free propeller, for even lower speed; prop provides 95% of thrust » Ramjet/scramjet o No moving parts, air is compressed as it is slowed down, mixed w/ fuel & burned Propulsion • Effect of propulsive device on air » Thrust is produced to the left, air feels equal and opposite force to the right U air = 0 U∞ Propulsion Unit Engine Wake U j −U∞ Mass flow rate in streamtube m T = m(U j − U ∞ ) (Simplified version) » What about pressure' We’ll see that best performance is when the exit pressure is equal to the ambient pressure, so take pe=po Propulsive Efficiency • Total power required is equal to the thrust generated (times velocity) plus wasted kinetic energy added to the air PR = PA + Pwasted = TU ∞ + 1 m(U j − U ∞ ) 2 2 = m(U j − U ∞ )U ∞ + 1 m(U j − U ∞ ) 2 2 • The propulsive efficiency then is just the ratio between available power and required power ηP = = P useful power = A required power PR TU ∞ ηP = 2 1+U j U∞ TU ∞ + 1 m(U j − U ∞ ) 2 2 Propulsive Efficiency • At 100% efficiency, thrust is 0; at high thrust, efficiency is low 10 9 8 7 100% ηP = 2 1+ U j U∞ T / m = (U j − U ∞ ) 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 6 5 4 3 2 1 0 1 2 3 4 5 U Vj /V U ∞ / j inf 6 7 8 9 10 11 Compromise • The low efficiency stems from the Uj-U∞ term - at 100% efficiency, all of the energy goes to moving the device and none to moving the air; to a stationary observer, it appears that the air comes out of the engine at the same speed that it goes in • Propellers have a high efficiency because their diameter is large, thus mass flow rate is large, but Uj is small - it is the most efficient of the thrust generating devices; however, it is limited to low speeds due to the tip speed of the prop. • Jet engines, however, act on a smaller mass of air but have very high Uj, thus lower efficiency Propulsive Efficiency Specific Thrust Propulsive, thermal, overall efficiency • Note on propulsive efficiency ηp ≈ » 2U ∞ / U j 1+ U∞ /U j = 2 1+U j U∞ ηp → 1 as U∞/Uj → 1 Uj is only slightly larger than U∞ » But then you need large mass flow to get required » but this is how commercial turbofan engines work! » In other words, the ideal propulsion system accelerates an infinite mass of air by an infinitesimally small ∆U » Fundamentally this is because T ∝ (Uj - U∞), but energy put into flow required to get that thrust ∝ (Uj2 - U∞2)/2 » Overall efficiency is determined by propulsive and thermal efficiency T = m(U j − U ∞ ) ηo ≡ Thrust power Thrust power ∆(Kinetic energy) = = ηthη p Heat input ∆(Kinetic energy) Heat input Total Thrust Equation • What about the addition of fuel' T = ma (1 + FAR )U j − U ∞ + ( p e − p a )Ae [ ] where FAR = m f / ma = Fuel to air mass ratio = f /(1 − f ) ( f = fuel mass fraction) • When FAR