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.