lesson 3: reciprocating engine theory of operation

Lesson 3: Reciprocating Engine Theory Of Operation

Reciprocating Engine Theory Of Operation

• Engine Operating Principles


• 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?


• Intake

• Compression

• Ignition

• Power

• Exhaust


• Two-Stroke


• Four-stroke

• Four strokes required

to complete the events.

• Two revolutions of the

crankshaft (720°) are

required to complete

the four strokes.


• Engine Power And Efficiency

– Work = Force x Distance

– Horsepower = ft-lb per min or ft-lb per sec

33,000 550


• 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.


• Piston Displacement

• PD = Area of cross section of cylinder multiplied by total distance the piston moves.

• V = A x H


• 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.


• 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.


• 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.


• 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.


• 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.


• 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


• Friction Horsepower

• Indicated horsepower minus brake horsepower.


• 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.


• 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.


• 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.


• Efficiencies

• Volumetric Efficiency

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

lesson 3: reciprocating engine theory of operation

Documents

engine speed

engine compression ratio

pressure of charge

ambient pressure

stroke slide

pressure measurement

manifold pressure times

horsepower horsepower