Lesson 3 Reciprocating Engine Theory Of Operation Reciprocating

Lesson 3: Reciprocating Engine Theory Of Operation

Reciprocating Engine Theory Of Operation • Engine Operating Principles

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?

Reciprocating Engine Theory Of Operation • Intake • Compression • Ignition • Power • Exhaust

Reciprocating Engine Theory Of Operation • Two-Stroke

Reciprocating Engine Theory Of Operation • Four-stroke • Four strokes required to complete the events. • Two revolutions of the crankshaft (720°) are required to complete the four strokes.

Reciprocating Engine Theory Of Operation • Engine Power And Efficiency – Work = Force x Distance – Horsepower = ft-lb per min or ft-lb per sec 33, 000 550

Reciprocating Engine Theory Of Operation • 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.

Reciprocating Engine Theory Of Operation • Piston Displacement • PD = Area of cross section of cylinder multiplied by total distance the piston moves. • V=Ax. H

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.

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.

Reciprocating Engine Theory Of Operation • 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.

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.

Reciprocating Engine Theory Of Operation • 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.

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

Reciprocating Engine Theory Of Operation • Friction Horsepower • Indicated horsepower minus brake horsepower.

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.

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 enginepropeller combination.

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.

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.