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Published byBetty McBride Modified over 4 years ago

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**Lesson 3: Reciprocating Engine Theory Of Operation**

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**Reciprocating Engine Theory Of Operation**

Engine Operating Principles

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**Reciprocating Engine Theory Of Operation**

Operating Cycles Two-Stroke Requires only one upstroke and one downstroke of the piston to complete the required series of events in the cylinders. Completes the operating cycle in one revolution of the crankshaft. What are the sequence of events that take place to convert the chemical energy into mechanical energy?

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**Reciprocating Engine Theory Of Operation**

Intake Compression Ignition Power Exhaust

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**Reciprocating Engine Theory Of Operation**

Two-Stroke

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Four-stroke Four strokes required to complete the events. Two revolutions of the crankshaft (720°) are required to complete the four strokes.

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Engine Power And Efficiency Work = Force x Distance Horsepower = ft-lb per min or ft-lb per sec 33,

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Piston Displacement The greater the piston displacement the greater the horsepower an engine will develop. The volume displaced by a piston is its displacement. Expressed in cubic inches.

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**Reciprocating Engine Theory Of Operation**

Piston Displacement PD = Area of cross section of cylinder multiplied by total distance the piston moves. V = A x H

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**Reciprocating Engine Theory Of Operation**

Compression Ratio Comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. The higher the engine compression ratio, the higher the engine efficiency (horsepower output) will be.

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**Reciprocating Engine Theory Of Operation**

Manifold Absolute Pressure (MAP) Average absolute pressure of the fuel/air charge in the intake manifold. Measured in units of inches of mercury (Hg). Dependent mostly on ambient air pressure, engine speed, and throttle setting. Supercharging increases MAP.

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Compression Ratio & Manifold Pressure. Determines the pressure in the cylinder when both valves are closed. Pressure of charge before compression is determined by the manifold pressure. Pressure at the height of compression is determined by manifold pressure times the compression ratio.

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**Reciprocating Engine Theory Of Operation**

Absolute & Gauge Pressure. Absolute Pressure Identifies the pressure measurement as one that is based on a comparison of the pressure in the manifold with pressure at absolute zero. Gauge Pressure Compares the pressure being measured against ambient pressure.

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Indicated Horsepower Horsepower calculated from the indicated mean effective pressure and the other factors which affect the power output of an engine. The power developed in the combustion chamber without reference to the friction losses within the engine.

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**Reciprocating Engine Theory Of Operation**

Brake Horsepower (BHP) The power delivered from the engine to the propeller for useful work. Total horsepower lost due to friction is subtracted from indicated horsepower. The measurement of an engine’s BHP involves the measurement of torque. Torque = Force times distance

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Friction Horsepower Indicated horsepower minus brake horsepower.

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**Reciprocating Engine Theory Of Operation**

Friction And Brake Mean Effective Pressure Indicated Mean Effective Pressure Pressure used to create frictionless power. Friction Mean Effective Pressure The pressure used to overcome internal friction. Brake Mean Effective Pressure The pressure used to produce useful work.

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**Reciprocating Engine Theory Of Operation**

Thrust Horsepower The result of the engine and propeller working together. The ratio of thrust horsepower and brake horsepower delivered to the propeller shaft will never be equal. Determines the performance of the engine-propeller combination.

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**Reciprocating Engine Theory Of Operation**

Efficiencies Thermal Efficiency The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units. Mechanical Efficiency The ratio that shows how much of the power developed by the expanding gases in the cylinder is actually delivered to the output shaft.

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**Reciprocating Engine Theory Of Operation**

Efficiencies Volumetric Efficiency A comparison of the volume of fuel/air charge inducted into the cylinders to the total piston displacement of the engine.

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