Reciprocating engines, Superchargers,Propellers

1. Reciprocating engines, Superchargers,Propellers Lecture 9 Chapter 4

2. 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?

3. 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.

4. 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.

5. 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}

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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

13. 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

14. Turboprop & TurbojetPerformance • Figure 4-31

15. Quiz on Lecture 9Chapter 4 Please take out a sheet of paper Include today’s date & your name

16. Quiz on Lecture 9Chapter 4 • Compare and contrast superchargers and turbosuperchargers.