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Unit 2: Introduction to Small Engines

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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. 3RcVcHI

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. Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were only one plug.

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

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
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:

51 Inline Cylinder Design
the cylinders are arranged in a line in a single bank. Can be arranged vertically or slanted.

52 Shaft Orientation Vertical Horizontal
The shaft extends from the bottom of the engine. common applications of a vertical engine include: walk-behind rotary lawnmowers yard tractors Horizontal The shaft extends from one side of the engine and rotates parallel to the ground. Generators snow throwers water pumps pressure washers.

53 Shaft Orientation Vertical Horizontal

54 Engine Cooling Systems
Air Cooled Air is circulated around the cylinder block and cylinder head to maintain the desired temperature Liquid Cooled Liquid is circulated through cavities in the cylinder block and cylinder head to maintain the desired temperature Heat is also removed through the exhaust system and radiant heat from the engine components.

55 Air-Cooled Engines

56 Air-Cooled Engines Most small, single-cylinder engines are cooled by a stream of air developed by fan blades on the flywheel. The air stream is deflected around the cylinder and cylinder head by a metal or plastic cover called a shroud. Additional engine heat is dissipated through cooling fins around the cylinder.

57 Liquid Cooled

58 Liquid Cooled The cooling system on liquid-cooled cars circulates a fluid through pipes and passageways in the engine. As this liquid passes through the hot engine it absorbs heat, cooling the engine. After the fluid leaves the engine, it passes through a heat exchanger, or radiator, which transfers the heat from the fluid to the air blowing through the exchanger.

59 Engine Definition A machine that converts a form of energy into mechanical force. Combustion engines generates heat from an internal or external source and converts that energy into rotation force on the crankshaft. A small engine is an internal combustion engine that converts heat energy from the combustion of a fuel into mechanical energy generally rated up to 25 horsepower.

60 Energy Conversion Energy is the resource that provides the capacity to do work Two forms of energy are: Potential energy Stored energy due to its position, chemical state or condition Water behind a dam, because of its position Gasoline based on its chemical state Kinetic energy Is energy of motion (released potential energy) Water falling over a dam A speeding automobile

61 Small Engines and Energy
Small gasoline engines convert the stored potential energy of gasoline into kinetic energy of the rotating shaft The rotating shaft of the engine is used to do work required by the engine application ie. Mow grass, throw snow etc.

62 Other Energy Principles
All internal combustion engines operate by utilizing basic principles of : Heat Force Pressure Torque Work Power Chemistry

63 Heat All matter is composed of atoms that are in constant motion.
Heat is the kinetic energy caused by atoms and molecules in motion within a substance Heat added to a substance causes the particles velocity to increase resulting in a higher internal energy Heat removed from a substance causes particle velocity to decrease resulting in decreased internal energy.

64 States of Matter A substance can be a liquid, solid or gas
The state of a substance depends upon the intensity of vibration of the molecules To change to or from one state to another requires the addition or removal of heat energy. When heat is added to ice it changes to water, when heat is added to water it changes to steam (vapor) As the compression in a cylinder increases, heat increases, the liquid gasoline droplets in the fuel mixture change to a gaseous state preparing the fuel for more efficient combustion.

65 Heat Transfer Heat is transferred (flows) from one substance to another when a temperature difference exists. Heat is always transferred from a substance with a higher temperature to a substance with a lower temperature Heat transfer rates are proportional to the temperature difference between the two substances

66 Methods of Heat Transfer
There are three methods of heat transfer: Conduction Occurs when particles of a substance come in direct contact with each other Convection Occurs when heat is transferred by currents in a fluid Radiation Occurs when radiant heat travels without a material carrier

67 Heat Transfer

68 Conduction Transfer of heat using direct contact
Heat one end of a metal rod, kinetic energy is passed from one to another, heat is transferred from one end to the other. In a small engine: Engine oil is in direct contact with the hot engine parts. Heat is transferred from the hotter parts to the oil As the oil moves to the oil reserve the cooler crankcase assemble conducts the heat from the oil to the air contacting the outside of the engine

69 Convection Heat is transferred by currents in a fluid
As air is warmed by a fire, the warm air rises and is replaced by cooler air. The movement of air continues as long as the air is heated by the fire. In a Small engine convection occurs in a liquid cooled radiator: A radiator is a multi-channeled container that allows air to pass around the channels to remove heat from the liquid within Warm liquid is pumped into the top of the radiator cooled in the radiator and moves back into the engine

70 Radiation Occurs when radiant heat travels without a material carrier
Radiant energy travels through space without producing heat. Heat is produced when the waves strike an opaque object. Heat is produced on earth by radiant energy from the sun In a small engine radiant energy is produced by the heated metal that radiates heat away from the parts where there is little air flow

71 Radiator Example The primary function of the radiator is to transfer waste heat The processes that accomplish this are: Convection Conduction Radiation These processes are dependent upon 3 variables: The existence of temperature differences between liquid and air The existence of temperature differences between coolant and air flow The design of the heat transfer surfaces to maximize their potential

72 Measuring Heat Temperature is the measurement of the degree or intensity of heat Temperature can be measured by a glass thermometer A glass tube is filled with alcohol or mercury The tube has a scale Based on the known expansion rate of the material

73 Quantity of Heat The quantity of heat is the amount of heat required to produce an accepted standard of physical change in matter A British Thermal Unit (BTU) is the amount of heat energy required to raise the temperature of one pound (lb) of Water 1oF (Fahrenheit) A Calorie is the amount of heat energy required to raise the temperature of one gram (g) of Water 1oC (Celsius)

74 Force Force is anything that changes or tends to change the state of rest or motion of a body Force is measured in pounds (lb) in the English system and newtons (N) in the metric system One or more forces can act on body Force acting on a body does not always produce motion Force applied in different ways can produce pressure, torque or work

75 Pressure Pressure is a force acting on a unit of area
Area is the number of unit squares equal to the surface of the object In an internal combustion engine force is exerted by combustion pressure applied to the area of the piston head P = F/A Where P – pressure (in lb/sq. in.) F – Force (in lb) A – Area (in sq. in.)

76 Pressure Example What is the pressure exerted on the top of a 4.91sq. in. piston if the combustion exerts a force of 2000 lb ? P = F/A P = 2000 lb/ 4.91sq. in. P = lb/sq. in. P = psi

77 Torque Torque is a force acting on a perpendicular radial distance from a point of rotation The result is a twisting or turning force expressed in pound-feet (lb-ft) or newton- meters (Nm) that may or may not result in motion Torque is found by applying the formula: T = F x r Where T – torque (in lb-ft or Nm) F - force (in lb or N) r – radius (in ft or m)

78 Torque Example How much torque is produced by a 60 lb force pushing on a 2’ lever arm ? T = F x r T = 60 lb x 2 ft T = 120 lb-ft

79 Work Work is the force applied through a parallel distance causing linear motion. Work is measured in lb-ft or Nm Torque and work are very similar. The real difference is that torque may not produce motion. Work is found by applying the formula W = F x D Where W – work (in lb-ft or Nm) F - force (in lb or N) D – distance (in ft or m)

80 Work Example What is the amount of work performed if a horse pulled a container that weighed 330 lb a distance of 100’ ? W = F x D W= 330 lb x 100 ft W= 33,000 lb-ft

81 Power Power is the rate at which work is done
Power ratings include horsepower, the watt (W) or kilowatt (kW) Power is found applying the formula: P = W/T Where P – power (in lb-ft/min or W) W – work (in lb-ft or Nm) T - time (in min)

82 Power Example What is the power output of an engine that produces 100,000 lb-ft of work in 6 min. ? P = W/T P = 100,000 lb-ft / 6 min. P = 16, lb-ft/ min.

83 Horsepower Horsepower (HP) is a unit of power equal to:
746 watts (W) 33,000 lb-ft/min 550 lb-ft/s Horsepower is commonly used to rate power produced by and engines at a finite speed The formula for Horsepower is: HP = W/(T x 33,000) Where HP – horsepower (in HP) W – work (in lb-ft/min) T - time (in min)

84 Horsepower Example What is the horsepower rating of an engine that produces 412,500 lb-ft in 2.5 minutes ? HP = W/(T x 33,000) HP = 412,500 lb-ft /(2.5 min x 33,000) HP = 412,500 lb-ft /(82,500 min) HP = 5 HP

85 Chemistry All internal combustion engines utilize some form of fossil fuel (hydrocarbon) Combustion chemistry involves the combining of hydrocarbon fuel with oxygen from the atmosphere. When ignition occurs in the engine a chemical reaction between the hydrocarbon molecule and atmospheric oxygen causes an exchange of elements which releases heat energy 2 C8H O N CO H2O + 94 N2 Fuel Mixture Exhaust Gasses

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