Bài giảng hệ thống động cơ đẩy

Lecture Aircraft Propulsion Aircraft Propulsion Systems • categories (invented so far) – Piston Engines • Otto and diesel cycle engines • Normally drive propellers – Jet engines • • • • • Turbojets Turbofans Turboprops Pulse jets Ram jets – Rocket engines • Very high specific fuel consumption • Only practical for hypersonic applications – Electric motors • Quiet • Problems with energy storage Piston Engines PAV TV ηp = = BHP PSHP • • • • T: Thrust V: Velocity PAV: Power Available BHP: brake-horsepower, also called shafthorse-power, SHP • ηp: Propeller efficiency Four stroke Otto Cycle Intake stroke Compression Stroke Power Stroke Exhaust Stroke Constant volume heating Factors Affecting Power Output of Piston Engines • Fuel to Air ratio – Too rich and combustion may not be complete – Too lean and combustion might not take place • Max RPM – Increasing RPM will increase power output, but there are limits – Max values for typical engines are 2200 to 3500 RPM • Temperature – At higher air temperatures (hot day) the SHP will decrease • Altitude 460 + TS SHPT ≈ SHPTS 460 + T – As altitude increases, power output decreases Altitude Effects on Power Output • Charge per stroke – Quantity of air in introduced to cylinder controls amount of heat released – Depends on the intake pressure, also called the manifold absolute pressure (MAP) – Decreases with altitude – Supercharging can increase the MAP and compensate for altitude Air is compressed by an air-pump before entering the cylinders • Combustion – Sufficient quantity of air needed for complete burning – Since density decreases with altitude, power decreases with altitude for a given throttle setting SHPalt ρ alt ≈ 1.132 − 0.132 – For non-supercharged engines: SHP ρ alt =0 alt = Engine Performance • When in operation, engine performance charts can be used to determine the BHP and the required fuel flow: – 1: Find the point on the full throttle altitude curve that corresponds to the indicated RPM and MAP (point A) – 2: Similarly, find the point on the sea-level performance chart corresponding to the RPM and MAP ( point B, then go straight across to C) – 3: Draw a straight line connecting C and A Mark the spot corresponding to the indicated altitude (point D) – 4: Point D represents the available BHP at standard temp conditions Need to compensate for actual temperature: • • 460 + TS SHPT ≈ SHPTS 460 + T Once actual BHP is obtained, required fuel flow can be computed SFC: specific fuel consumption (lbs of fuel per shaft horse-power per hour) – wf SFC = BHP Sea level curve Full throttle – altitude curve Propeller Analysis Methods • Momentum Theory – Simplest method • Blade-element Theory Not today – Used to design propeller blades, such as airfoil geometry – Does not account for the downwash produced by propeller blades and how it affects the thrust • Combined blade element and momentum theory – More complete Momentum Theory • Momentum Theory models propeller as an infinitely thin actuator disc across which the pressure increases discontinuosly • Velocity is constant over disc • Flow is incompressible and can be separated from rest of flow by a streamtube • From Momentum theory, it can be shown that thrust is : T = ρA(V0 + w)w • The power added to the flow is P= [ ρA(V0 + w)(V0 + 2w)2 − V 2 ] P = T (V0 + w) • Propeller efficiency is the ratio between useful power (thrust x velocity) and the total power added to the flow ηp = TV0 V0 = T (V0 + w) (V0 + w) – To keep efficiency high, it is desirable to keep induced velocity low – To keep induced velocity low, need to keep T/A (disc loading) low – To keep T/A low, need to make A large, which means a large diameter propeller However large propellers are not desirable • Large tip speeds • Ground clearance issues • Structural issues Combined Momentum Bladeelement Theory T CT = ρn D P CP = ρn D V J= nD TV0 CT J ηp = = P CP • • • • CT = Thrust coefficient CP = Power coefficient J: Advance Ratio CT and CP functions of advance ratio and propeller design Propeller Efficiency Piston Engines vs Turbojets Otto Brayton • Piston engines are related to the Otto Cycle – Heat (combustion) added at constant volume • Turbine engines can be modeled with the Brayton Cycle – Continuous flow of working gas – Burning occurs at constant pressure Turbojet Overview • Intake funnels air into the engine In general, flow must be subsonic before entering compressor • Compressor made up of series of rotating and stationary blades (stators) that progressively heat and compress the air before entering combustion chamber • Combustion chamber continuously burns fuel with the compressed air • Turbine spins up like a windmill via the hot combustion gases Turbine drives compressor • Nozzle accelerates hot gases such that it exhausts to atmosphere at a high speed Turbojet Overview Fthrust = m& (Ve − V∞ ) + ( pe − p∞ )Ae Propulsive Efficiency • • • Propulsive efficiency: ratio between the work done on the aircraft and the energy imparted on the engine airflow Work done on aircraft is the net thrust multiplied by the aircraft speed Energy imparted on airflow is the work done on the aircraft plus the energy wasted in the exhaust η propulsive Assuming nozzle exhaust fully expands to atmospheric pressure… • • ⎡ ⎤ V∞ ⎢m& (Ve − V∞ ) + ( pe − p∞ )Ae ⎥ ⎣ ⎦ = ⎡ ⎤ V∞ ⎢m& (Ve − V∞ ) + ( pe − p∞ )Ae ⎥ + m& (Ve − V∞ ) ⎣ ⎦ V∞ m& (Ve − V∞ ) 2V∞ = = V∞ + Ve V∞ m& (Ve − V∞ ) + m& (Ve − V∞ ) At low flight speeds, the efficiency is low Efficiency increases as higher speeds Thrust Specific Fuel Consumption • As speed increases, mass flow into engine increases • More fuel is required to sustain combustion t.s.f.c = fuel consumption in lbs/hr thrust output in lbs = lbs/hr lbs • Since thrust decreases with speed, thrust specific fuel consumption will increases with speed Turbofan Bypass Ratio = mass flow rate through fan mass flow through turbine • A turbofan consists of a ducted fan and a turbojet engine behind it that powers the fan • Part of the air goes through the fan and into the engine to be ignited The rest bypasses the engine • For high bypass engines, the majority of the thrust is produced by the air that bypasses the engine Turbofan Propulsive Efficiency = bypassed air = airflow into engine η propulsive = V∞ m& (Ve1 − V∞ ) + V∞ m& (Ve − V∞ ) 1 2 & & & V∞ m1 (Ve1 − V∞ ) + V∞ m (Ve − V∞ ) + m1 (Ve1 − V∞ ) + m& (Ve − V∞ ) 2 • In general, for a given thrust, it is more efficient to move a lot of air at a lower exhaust speed, than to move a small amount of air at a higher speed • Therefore, turbofans have a higher propulsive efficiency than a turbojets • For this reason, most jet engines today are turbofans • Intake duct slows airflow before reaching fan blades This allows turbofans to operate at higher speeds than turboprops Turboprop • Turbojet designed to drive a propeller • Turboprops have a high propulsive efficiency, but since propellers must operate at subsonic inlet speeds, only viable at moderate speeds • Popular among smaller commuter aircraft and military transport aircraft Propfans • Propfans attempt to combine the speeds and performance of a turbofan with the fuel efficiency of a turboprop – Potentially 30% more efficient than turbofan – Propeller blades are swept back to reduce wave drag – Very noisy, and gas prices aren’t high enough to make them worth the added cost and complexity