Unit 2: Introduction to Small Engines

1. Unit 2: Introduction to Small Engines

2. Six Classifications of Engines • Combustion – whether the engine is internal or external combustion. • Ignition – compression versus spark ignition • Number of Strokes – 2 stroke or 4 stroke • Cylinder Design – vertical, horizontal, slant, V, opposed, inline • Shaft Orientation - vertical or horizontal • Cooling System – liquid cooled or air cooled

3. Combustion Engine Types • Two types of Engines based on Combustion • External Combustion Engines • Internal Combustion Engines

4. External Combustion Engines • External combustion engines separate the heat source from the source of power. • The external heat source heats an internal fluid, through a heat exchanger of some type • The heated fluid expands creating pressure • The pressure drives a turbine which provides power for use.

5. Most common types of External Combustion Engines • Steam engines • Stirling engines

6. Steam Engines • first invented by Thomas Newcomen in 1705. • powered all early locomotives, steam boats and factories • acted as the foundation of the Industrial Revolution.

7. How Steam Engines Work • Heat is obtained from fuel burnt in a closed firebox • The heat is transferred to the water in a pressurized boiler, boiling the water and transforming it into saturated steam. • The steam is transferred to the motor unit which uses it to push on a piston sliding inside a cylinder to power machinery. • The used, cooler, lower pressure steam is exhausted to atmosphere

9. Stirling Engines • Invented by Robert Stirling in 1816, • Has the potential to be much more efficient than a gasoline or diesel engine. • Today, Stirling engines are used only in some very specialized applications, like in submarines or auxiliary power generators for yachts, where quiet operation is important

10. How Stirling Engines Work • The Stirling cycle engine uses air as its liquid, which resolves many of the issues with steam. It is not as dangerous as steam and does not lose as much energy in transition, because it does not transition. • The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet.

11. How Stirling Engines Work • The key principle of a Stirling engine is that a fixed amount of a gas is sealed inside the engine. • Stirling cycle involves a series of events that change the pressure of the gas inside the engine, causing it to do work.

12. How Stirling Engines Work • Heat is added to the gas inside the heated cylinder (top), causing pressure to build. This drives the hot piston in its power stroke. This is the part of the Stirling cycle that does the work

13. How Stirling Engines Work • The heated gas expands and pushes the hot piston to the bottom of its travel in the cylinder. • The expansion continues in the cold cylinder, which is 90° behind the hot piston in its cycle, extracting more work from the hot gas.

14. How Stirling Engines Work • The gas is now at its maximum volume. The hot cylinder piston begins to move most of the gas into the cold cylinder, where it cools and the pressure drops

15. How Sterling Engines Work • Almost all the gas is now in the cold cylinder and cooling continues. The cold piston, powered by flywheel momentum (or other piston pairs on the same shaft) compresses the remaining part of the gas

16. How Sterling Engines Work

17. Why aren’t Stirling Engines more common? • Because the heat source is external, it takes a little while for the engine to respond to changes in the amount of heat being applied to the cylinder -- it takes time for the heat to be conducted through the cylinder walls and into the gas inside the engine. This means that: • The engine requires some time to warm up before it can produce useful power. • The engine can not change its power output quickly.

18. Internal Combustion Engines • An internal combustion engine is one in which: • the combustion of a fuel is used to push a piston within an cylinder • the pistons movement turns a crankshaft that provides mechanical power • mechanical power moves the other parts of the drive train

19. Seven main types of internal combustion engines • 2 stroke cycle • 4 stroke cycle • Compression (diesel) • Rotary • Rocket • Gas Turbine • Jet (Hache)

20. Parts of a Two Stroke Engine

21. Sparks Fly • Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. • The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. • As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder.

22. Fuel Intake • As the piston reaches the bottom the intake port is uncovered. • The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel • The piston is shaped so that incoming fuel doesn’t simply flow right over the top of the piston and out the exhaust.

23. The Compression Stroke • the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. • As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. • This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. • Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle.

24. Two-Stroke Animation • It's called a two-stoke engine because there is a compression stroke and then a combustion stroke.

25. Advantages of Two-Stroke • Two-stroke engines do not have valves, which simplifies their construction and lowers their weight. • Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost. • Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.

26. Disadvantages of the Two-stroke • The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear a lot faster so engines don’t last as long. • Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas • Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon. • Two-stroke engines produce a lot of pollution -- so much, in fact, that it is likely that you won't see them around too much longer. The pollution comes from two sources: • The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke. • The second reason is that each time a new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That's why you see a sheen of oil around any two-stroke boat motor

27. Common uses of Two-Strokes • Chain saws • Lawn cutters • Snowmobiles • Outboard motors • Dirt bikes

28. Four-Stroke Cycle Engine Parts

29. Intake Stroke • The cycle begins at top dead center (TDC), when the piston is farthest away from the axis of the crankshaft. • The intake valve opens. • The piston descends from the top of the cylinder, reducing the pressure inside the cylinder. • A mixture of fuel and air is forced (by atmospheric or greater pressure) into the cylinder through the intake (inlet) port.

30. Compression Stroke • When the piston reaches the lower limit of its travel, it begins to move upward. • The intake (inlet) valve (or valves) close • As the piston moves upward, the air/fuel mixture is compressed • The compression stroke compresses the fuel–air mixture. • The compression process also causes the air/fuel mixture to increase in temperature.

31. Power Stroke • As the piston reaches the top of its travel on the compression stroke, a high voltage electric spark is produced at the spark plug. • The air–fuel mixture is then ignited near the end of the compression stroke: • by a spark plug (for a gasoline or Otto cycle engine) • by the heat and pressure of compression (for a Diesel cycle or compression ignition engine). • The resulting pressure of burning gases pushes the piston through the power stroke. • The power impulse is transmitted down through the piston, through the piston rod (connecting rod), and to the crankshaft. The crankshaft is rotated due to the force.

32. Exhaust Stroke • As the piston reaches the bottom of its travel, the exhaust valve opens. • In the exhaust stroke, the piston pushes the products of combustion from the cylinder through an exhaust valve or valves. • When the piston reaches the top of its travel, the exhaust valve closes, and the intake valve opens. • The cycle repeats again with the intake stroke.

33. Four-Stroke Animation • These four strokes require two revolutions of the crankshaft. The process continuously repeats itself during the operation of the engine. • Thus the engine only fires once every four strokes or every second time the piston reaches the top of its travel. • http://www.youtube.com/watch?NR=1&v=GwFB3RcVcHI

34. Advantages of Four Strokes Over Two Strokes • use much less fuel than 2 strokes • produce less pollution • have a wider power band (the engine RPM range over which the engine produces its most power. • have a dedicated lubrication system which means that • They usually last longer • They do not burn oil

35. Disadvantages of Four Stroke Engines • They are heavy, more complicated and more expensive to build • They do not create as much power as a same size 2 stroke. • They cannot operate in a non-vertical position.

36. Common uses of Four-Strokes • Automobiles • ATV’s (4 wheelers) • Snowmobiles • Snowblowers • Lawnmowers • Motorcycles

37. Rotary Engine • Rotary engines use the four-stroke combustion cycle, which is the same cycle that four-stroke piston engines use. But in a rotary engine, this is accomplished in a completely different way. • The heart of a rotary engine is the rotor. This is roughly the equivalent of the pistons in a piston engine. The rotor is mounted on a large circular lobe on the output shaft. This lobe is offset from the centerline of the shaft and acts like the crank handle on a winch, giving the rotor the leverage it needs to turn the output shaft. As the rotor orbits inside the housing, it pushes the lobe around in tight circles, turning three times for every one revolution of the rotor.

38. Intake Stroke • The cycle starts when the tip of the rotor passes the intake port. • When the intake port is exposed to the chamber, the volume of that chamber is close to its minimum. • As the rotor moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber.

39. Compression Stroke • As the rotor continues its motion around the housing, the volume of the chamber gets smaller and the air/fuel mixture gets compressed. • By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum. This is when combustion starts.

40. Combustion Stroke • When the spark plugs ignite the air/fuel mixture, pressure quickly builds, forcing the rotor to move. • The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in volume. The combustion gases continue to expand, moving the rotor and creating power.

41. Exhaust Stroke • Once the peak of the rotor passes the exhaust port, the high-pressure combustion gases are free to flow out the exhaust. • The rotor continues to move forcing the remaining exhaust out of the port. • When the rotor passes the intake port and the whole cycle starts again.

42. Rotary Engine Animation

43. Common use of a Rotary Engine • http://www.youtube.com/watch?v=QRiPSlx8PxA

44. Engine Ignition Types • A small engine is a Spark Ignition or Compression Ignition engine based on how the fuel is ignited. • Spark ignition • the fuel mixture is ignited with an electrical spark • Commonly use gasoline • Compression • The fuel mixture is ignited by compressing the fuel mixture under pressure and heat • Commonly use diesel fuel

45. Engine Stroke Types • A stroke is one complete travel of the piston from top dead center to bottom dead center or vice versa. • Two (2) Stroke • Utilizes two strokes to complete the intake, compression, power (ignition) and exhaust cycle • Fuel intake and compression on one stroke • Power (ignition) and exhaust on the other stroke • Four (4) Stroke • Utilizes four strokes to complete the intake, compression, power (ignition) and exhaust cycle • Intake, compression, power (ignition) and exhaust occurs on a different stroke.

46. Engine Cylinder Design • Cylinder Design • Vertical – pistons travel up and down vertically • Horizontal - pistons travel back and forth in the horizontal plane • Slant – pistons are oriented at an angle to the vertical • V – pistons are divided into two banks at an angle to the vertical forming a v-shape • Inline – pistons are all oriented in the same direction

47. Vertical Cylinder Design • the cylinders are arranged inline in a single bank that move vertically:

48. Horizontal Cylinder Design • also known as horizontally opposed or a boxer • the cylinders are arranged in two banks on opposite sides of the engine:

49. Slant Cylinder Design • the cylinders are arranged inline and specifically designed such that the cylinders are inclined at a 30-degree angle from vertical.

50. V Cylinder Design • V - the cylinders are arranged in two banks set at an angle to one another: