unit 23: internal combustion engines (thermo-mechanical power)

itp 150 - prof. djassemi - cal poly

internal combustion (IC) engine definition

a machine that converts fuel energy → thermal energy → fluid power → mechanical power (rotation)

chain of power flow in IC engine

fuel burns → heat generated → gas expands → piston moves → crankshaft rotates → mechanical power

input/output actions of an IC engine

• input

  • mixes fuel + air

  • compresses mixture

  • ignites mixture

• output

  • moves piston linearly

  • converts reciprocating motion → rotary motion (crankshaft)

engine block

• the foundation of the engine where combustion occurs & components are housed

• 4 parts:

  • 1) cylinder

  • 2) piston

  • 3) connecting rod

  • 4) crankshaft

piston & crankshaft assembly

• function of piston: moves up & down (reciprocating) due to gas pressure

• function of crankshaft: converts reciprocating piston motion → rotational motion

• function of connecting rod: connects piston → crankshaft

4-stroke engine cycle (repeating Otto cycle): stroke 1 - intake

• piston moves down

• air + fuel enter cylinder

• intake valve open

• exhaust valve closed

stroke 2 - compression

• piston moves up

• fuel-air mixture compressed

• both valves closed

stroke 3 - power (combustion)

• spark plug ignites mixture

• gas expands

• piston pushed down

• produces mechanical power

stroke 4 - exhaust

• piston moves up

• burned gases expelled

• exhaust valve open

crankshaft rotation

• 2 full rotations in 4-stroke cycle

• each stroke = ½ rotation

camshaft function

• opens & closes intake & exhaust valves

• camshaft vs. crankshaft rotation: camshaft rotates once for every 2 crankshaft rotations

timing belt

synchronizes crankshaft & camshaft rotation

2-stroke engine

• engine that completes the 4 engine events in 2 strokes

• stroke 1: intake + compression

• stroke 2: combustion + exhaust

2-stroke vs. 4-stroke

• 2-stroke advantages:

  • lightweight

  • fewer parts

  • higher power-to-weight ratio

  • higher speed

• 2-stroke disadvantages:

  • more pollution

  • louder

  • more vibration

• 4-stroke advantages:

  • quieter

  • smoother

  • longer life

  • lower pollution

engine power

• small: <25 hp

• average: ~180 hp

• large: upto 600 hp

engine displacement

• total volume of all cylinders as pistons move through one stroke

• 3.0 V6: 3.0L total displacement, 6 cylinders arranged in V shape

fuel ignition: gas vs. diesel

• gasoline engine ignition: spark plug ignites fuel-air mixture

• diesel engine ignition: fuel ignites from high compression (no spark plug)

engine auxiliary systems: fuel delivery

fuel injector: sprays atomized fuel into combustion chamber under high pressure

engine auxiliary systems: spark plug (ignition system)

creates an electric spark to ignite compressed fuel-air mixture

engine auxiliary systems: cooling system (w/ parts)

• why IC engines need cooling: burning fuel creates excess heat that must be removed

• parts:

  • water jackets

  • radiator

  • water pump

  • hoses

  • thermostat

  • cooling fan

engine secondary functions

• drives vehicular systems like:

  • alternator

  • AC compressor

  • power-steering pump

  • water pump

  • oil pump

power transfer from engine → wheels

fuel combustion → piston → crankshaft → transmission → differential → wheels

forms of energy in IC engines

1. thermal energy

2. pneumatic energy (gas pressure)

3. mechanical energy

efficiency of IC engines vs. EVs

• IC engines: ~25% efficient

• EVs: ~90% efficient

  • more efficient b/c of recapturing of energy thru regenerative braking unlike IC engines