lecture16_F11 by wuzhengqin


									8.2 Automobiles
What is a car engine?

  • Physicists call it a “heat engine”
  • The specific design is an
    “internal combustion engine”
• Heat engines
• Car engines
• Knocking
The Heat Engine

                             Air outside
                  cylinder   the tail pipe

 Heat Engines
 As heat flows naturally from hot to cold,
 a heat engine diverts some heat and
 converts it into useful work.

• Natural heat flow increases entropy
• Converting heat to work decreases entropy
• If more entropy is created than destroyed, the
  overall entropy doesn’t decrease and some heat
  can become work!
Heat Pump
 (last lecture)

Heat Engine
  (this lecture)
As the temperature difference between
hot and cold increases:
 – A heat pump becomes less efficient   Th  Tc
 – A heat engine becomes more efficient Th  Tc

  Cold     Heat                Cold     Heat
                      Hot                          hotter
  region   engine              region   engine
                      region                       region

               work                         work
It’s all about entropy
                          • More entropy is created
                            (by heat moving) when
                            the temperature
                            difference is increased
“You down with entropy?
Yeah, you know me!…”
                          • That means more heat
                            can be diverted into
                            work without violating
                            the 2nd law!
Stirling Engines

                               ’79 AMC spirit

Hard to start quickly, poor acceleration response
Internal Combustion Engine
• Burns fuel and air in enclosed space
• Produces hot burned gases
• Allows heat to flow from hot engine to cold
  outside air
• Converts some
  of this heat into
  useful work                      Nikolaus Otto
                                    patented the 4-
                                    stroke engine when
                                    he was only 34!
The Four Stroke Engine
1. Induction Stroke
• Engine pulls piston out of
• Low pressure inside
• Atmospheric pressure
  pushes fuel and air mixture
  into cylinder
• Engine does work on the
  gases during this stroke
Engine – Step 1

   Fuel and air mixture
   after induction stroke:
    Pressure      = Atmospheric
    Temperature   = Ambient
2. Compression Stroke
 • Engine pushes piston into
 • Mixture is compressed to
   high pressure and
 • Engine does work on the
   gases during this stroke
Engine – Step 2

   Fuel and air mixture
   after compression stroke:
   Pressure       = High
   Temperature    = Hot
3. Power Stroke
 • Mixture burns to form hot
 • Gases push piston out of
 • Gases expand to lower pressure
   and temperature
 • Gases do work on engine during
   this stroke
Engine – beginning of Step 3

 Burned gases after ignition:
   Pressure       = Very high
   Temperature    = Very hot
Engine – end of Step 3

 Burned gases after power stroke:
   Pressure      = Moderate
   Temperature   = High
4. Exhaust Stroke
• Engine pushes piston into
• High pressure inside cylinder
• Pressure pushes burned gases
  out of cylinder
• Engine does work on the gases
  during this stroke
Clicker Question:

 What directly effects the
 efficiency of a car engine?

 A) Octane of the gas
 B) Weight of the car
 C) The temperature of the gas when it burns
 D) The number of cylinders
Efficiency is all about
 compression ratio

Most cars: 8:1 – 12:1

High-performance cars: 15:1

Diesel: 20:1
Efficiency Limits
• An ideal engine is never perfect
  (never has 100% efficiency)
   – Not all the thermal energy can become work
   – Some heat must be ejected into the
• However, ideal efficiency improves as
   – the burned gases become hotter
   – the outside air becomes colder
• Real engines never reach ideal efficiency
The 4-stroke cycle is a myth…
                 -4° TDC

                      +30 ° TDC
  • Engine is inefficient
  • Pressure wave forms – knocks!
  • Use higher octane fuel!
Octane     Ignition temperature
 87             750 °C
 90             800 °C
 93             850 °C
Multiple cylinders

At least one cylinder is always
  executing power stroke
Where does the spark come from?

More later when we talk about magnetism!
Tricked-out cars

                             Joseph Priestly

• Higher density of oxygen
• Can burn more fuel (more power)

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