1. 52 RCACS Ground School Engines PO 407 EO 1 “Basic Construction and Four Stroke Cycle”

2. Introduction Teaching Points Teaching Points Types of combustion engines Types of combustion engines Construction of reciprocating engines Construction of reciprocating engines The Four Stroke Cycle The Four Stroke Cycle Timing Timing Valve Clearances Valve Clearances Review Review Reference: Reference: FTGU Chapter 3 FTGU Chapter 3 Section A Pages 51-53 Section A Pages 51-53

3. Types of combustion engines There are three types of piston engines currently used. They are: There are three types of piston engines currently used. They are: Horizontally opposed Horizontally opposed Two rows of cylinders directly opposite on a flat plane. Two rows of cylinders directly opposite on a flat plane. One crankshaft is driven by both rows. One crankshaft is driven by both rows. Advantages: Advantages: Less drag because of its small frontal shape Less drag because of its small frontal shape Good pilot visibility. Good pilot visibility. It is the most common type of engine used. It is the most common type of engine used.

4. Horizontally opposed

5. Types of combustion engines Radial Radial Cylinders are arranged around the crankshaft similar to bicycle spokes around a hub. Cylinders are arranged around the crankshaft similar to bicycle spokes around a hub. Advantages: Advantages: the crankshaft is short, compact and light. the crankshaft is short, compact and light. Disadvantages: Disadvantages: causes considerable parasite drag causes considerable parasite drag obstructs pilot visibility. obstructs pilot visibility.

6. Radial

7. Types of Combustion Engines In-line In-line All cylinders are arranged in a row side by side. similar to most cars. All cylinders are arranged in a row side by side. similar to most cars. Usually there is a maximum of six cylinders. Usually there is a maximum of six cylinders. If more are required they are arranged as V, X and H type, with two crankshafts side by side. If more are required they are arranged as V, X and H type, with two crankshafts side by side. Advantage: Advantage: Little drag is produced Little drag is produced Disadvantage: Disadvantage: The engine is heavier and size is limited. The engine is heavier and size is limited.

8. In-line

9. Construction of Reciprocating Engines Piston Piston Cylinder shaped object that moves up and down. Cylinder shaped object that moves up and down. Piston rings Piston rings wrap around the piston and provide a seal between the piston and cylinder. wrap around the piston and provide a seal between the piston and cylinder. Connecting rod Connecting rod joins the piston to the crankshaft, which turns the propeller. joins the piston to the crankshaft, which turns the propeller. Cylinder head Cylinder head contains the inlet (intake) valve, exhaust valve and two spark plugs. contains the inlet (intake) valve, exhaust valve and two spark plugs. Camshaft Camshaft turned by the crankshaft and operates the push rods and rocker arms. turned by the crankshaft and operates the push rods and rocker arms. The camshaft turns at half the speed at which the crankshaft turns. The camshaft turns at half the speed at which the crankshaft turns.

10. Construction of Reciprocating Engines Magnetos Magnetos Provides electrical current to ignite fuel/air mixture through the distributor. Provides electrical current to ignite fuel/air mixture through the distributor. Intake valve ports Intake valve ports Allow air to enter the cylinder and is connected to the carburetor where the air and fuel are mixed Allow air to enter the cylinder and is connected to the carburetor where the air and fuel are mixed Exhaust valve Exhaust valve Connected to the exhaust pipe, which vents the exhaust fumes away from the cabin. Connected to the exhaust pipe, which vents the exhaust fumes away from the cabin.

11. Construction of Reciprocating Engines Piston Cylinder shaped object that moves up and down Piston rings wrap around the piston and provide a seal between the piston and cylinder Connecting rod joins the piston to the crankshaft, which turns the propeller. Cylinder head contains the inlet (intak) valve, exhaust valve and two spark plugs. Camshaft turned by the crankshaft and operates the push rods and rocker arms. The camshaft turns at half the speed at which the crankshaft turns. Magnetos /Spark plugs Provides electrical current to ignite fuel/air mixture. Intake valve ports Allow air to enter the cylinder and is connected to the carburetor where the air and fuel are mixed Exhaust valve Connected to the exhaust pipe, which vents the exhaust fumes away from the cabin.

12. Construction of Reciprocating Engines Intake Intake During intake During intake inlet valve is open inlet valve is open the piston moves down the cylinder. the piston moves down the cylinder. air/fuel mixture is drawn in. air/fuel mixture is drawn in. One half revolution of the crankshaft has been made. One half revolution of the crankshaft has been made. Piston engines used on aircraft work on the four stroke cycle. The cycles are:

13. The Four Stroke Cycle Compression Compression During Compression During Compression Both valves are closed Both valves are closed The piston moves up the cylinder, compressing the air/fuel mixture. The piston moves up the cylinder, compressing the air/fuel mixture. One complete revolution of the crankshaft has now been made. One complete revolution of the crankshaft has now been made. Compression ratio: Compression ratio: the ratio of the cylinder volume when the piston is as low as it will go ("Bottom Dead Center" BDC) to the volume when it is as high as it will go ("Top Dead Center" TDC). the ratio of the cylinder volume when the piston is as low as it will go ("Bottom Dead Center" BDC) to the volume when it is as high as it will go ("Top Dead Center" TDC). Example Example 80 cubic inches to 10 cubic inches is a compression ratio of 8:1 80 cubic inches to 10 cubic inches is a compression ratio of 8:1

14. The Four Stroke Cycle Power Power During Power During Power The spark plug ignites the fuel/air mixture just as the piston reaches the top of the cylinder. The spark plug ignites the fuel/air mixture just as the piston reaches the top of the cylinder. Resulting expansion forces drive the piston down during this power stroke. Resulting expansion forces drive the piston down during this power stroke. The crankshaft has made one and one-half revolutions. The crankshaft has made one and one-half revolutions. Both valves are closed. Both valves are closed.

15. The Four Stroke Cycle Exhaust Exhaust During Exhaust During Exhaust The exhaust valve opens The exhaust valve opens the piston again moves to the top of the cylinder. the piston again moves to the top of the cylinder. Burned gas is driven out of the cylinder. Burned gas is driven out of the cylinder. The second revolution of the crankshaft is now completed. The second revolution of the crankshaft is now completed. The cycle starts again. The cycle starts again.

16. 4 Stroke Animation

17. Timing Valves must be timed to open and close at the proper time. Valves must be timed to open and close at the proper time. Performance is improved by the use of valve lead, lag and overlap. Performance is improved by the use of valve lead, lag and overlap. Valve lead: Valve lead: Timing the valve to open early. Timing the valve to open early. Valve lag: Valve lag: Timing the valve to close late. Timing the valve to close late. Valve overlap: Valve overlap: Allowing both valves to remain open at the same time. Allowing both valves to remain open at the same time.

18. Valve Clearances Valve clearances (or Tappet Clearance) Valve clearances (or Tappet Clearance) A space provided between the valve stem and rocker to allow for heat expansion of the metal. A space provided between the valve stem and rocker to allow for heat expansion of the metal. Clearances too wide cause a loss of power and excessive wear. Clearances too wide cause a loss of power and excessive wear. Clearances too close can warp the valves. Clearances too close can warp the valves.

19. Questions? ?