reciprocating engines, superchargers,propellers lecture 9 chapter 4
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Reciprocating engines, Superchargers,Propellers
Lecture 9
Chapter 4
Reciprocating Engines
• Internal combustion recip. (piston) engine
• Four stroke cycle engine credited German Nickolaus Otto in 1876.
• Steam was not successful. Why?
• Figure 4-13 p. 94 Wright Brother’s engine– How did it work?
Reciprocating Engines
• 1st reciprocating engines were liquid cooled.• 1st airplanes were so slow that air cooled was not
practical• The rotary engine was cooled by air. • The crankshaft of a rotary is mounted to the
airframe & cylinders revolve around it.• The cylinders are air cooled even at low speeds.
Air Cooled Engines
• Pro: Cylinders placed radially around the crankshaft for equal cooling for many cylinders
• Con: This arrangement allows for high drag with the large frontal area.
• Horizontally opposed configuration worked well with 2/4 cylinders– Horizontally opposed recip. engines with as many as 8
cylinders producing up to 450 horsepower.
Reciprocating EnginePerformance
• Horsepower- English unit
• One Horsepower = 550 foot-pounds– It would take one horsepower to move
something requiring a force of 550 lbs over a distance of one foot every second
• Mean Effective Pressure- average pressure throughout a stroke {force on the piston}
Power
• Power is the time rate at which work is done• The number of power strokes per minute is ½
times the rpm because there is a power stroke every other revolution
• The power in one cylinder is proportional to average cylinder pressure times the length of the stroke times piston area times rpm.
Terms
• Brake horsepower- horsepower delivered at the shaft
• Pony Brake- classical method of measuring power output.
• Shaft horsepower- power delivered to the propeller.
• Thrust horsepower- amount of power that actually gets converted into thrust.
Terms
• Rated brake horsepower- the power output at a rpm at standard sea level density.
• Figure 4-15 p. 98 typical variation of brake horsepower with altitude
• Economy cruise- 55% power
• Performance cruise- 75% power
• Good compromise- 65% power
Superchargers
• Superchargers utilize a small compressor in the intake manifold that compresses the air received from the atmosphere to a higher pressure.
• Turbosuperchargers are more efficient because they use exhaust gas pressure.
Turbosuperchargers
• The exhaust drives the turbine, which is connected to the compressor in the intake (like the compressor & turbine function in a turbojet)
• The effect of supercharging/turbocharging is that sea level, or rated, power can be maintained up to a certain altitude.
Figure 4-16 p. 100
• This shows the effect of supercharging
• The dotted line represents the power available at a constant rpm with turbocharger.
• The solid line represents the original unsupercharged engine.
Propellers
• Propeller (airscrew) is essentially a small wing rotated in a plane perpendicular to the path of flight & developing thrust in the same way that a wing develops lift.
• Figures 4-18 – 4-25
Propeller Efficiency
• The efficiency of the propeller is really how much brake horsepower it converts into thrust power-dependent on the ratio to forward speed to rotational speed.
• Figures 4-26- 4-28• What are the different types of props?• Pros/Cons?• Figures 4-29 & 4-30
Turboprop & TurbojetPerformance
• Figure 4-31
Quiz on Lecture 9Chapter 4
Please take out a sheet of paper
Include today’s date & your name
Quiz on Lecture 9Chapter 4
• Compare and contrast superchargers and turbosuperchargers.
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