8.2 Automobiles8.2 Automobiles

What is a car engine?What is a car engine?

• Physicists call it a “heat engine”• The specific design is an “internal combustion engine”

Today:Today:• Heat engines• Car engines• Knocking• NOS

The Heat EngineThe Heat Engine

cylinderAir outside the tail pipe

Internal combustion engine

Heat EnginesHeat 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 PumpHeat Pump(last lecture)

Heat Heat EngineEngine

(this lecture)

EfficiencyEfficiencyAs the temperature difference between hot and cold increases:

– A heat pump becomes less efficient– A heat engine becomes more efficient

Heat engine

Cold region

Hot region

work

Heat engine

Cold region

Even hotter region

work

h c

h

T T

T

c

h c

T

T T

It’s all about entropyIt’s all about entropy

“You down with entropy? Yeah, you know me!…”

• More entropy is created (by heat moving) when the temperature difference is increased

• That means more heat can be diverted into work without violating the 2nd law!

Stirling EnginesStirling Engines

’79 AMC spirit

Hard to start quickly, poor acceleration response

Internal Combustion Internal Combustion EngineEngine• 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 EngineThe Four Stroke Engine

1.1. Induction StrokeInduction Stroke• Engine pulls piston out of

cylinder• Low pressure inside

cylinder• Atmospheric pressure

pushes fuel and air mixture into cylinder

• Engine does work on the gases during this stroke

Engine – Step 1Engine – Step 1

Fuel and air mixture after induction stroke:

Pressure = AtmosphericTemperature = Ambient

2.2. Compression StrokeCompression Stroke• Engine pushes piston

into cylinder• Mixture is compressed

to high pressure and temperature

• Engine does work on the gases during this stroke

Engine – Step 2Engine – Step 2

Fuel and air mixture after compression stroke:

Pressure = High

Temperature = Hot

3.3. Power Power StrokeStroke

• Mixture burns to form hot gases• Gases push piston out of

cylinder• Gases expand to lower pressure

and temperature• Gases do work on engine during

this stroke

Engine – beginning of Step Engine – beginning of Step 33

Burned gases after ignition:Pressure = Very highTemperature = Very hot

Engine – end of Step 3Engine – end of Step 3

Burned gases after power stroke:Pressure = ModerateTemperature = High

4.4. Exhaust StrokeExhaust Stroke• Engine pushes piston into

cylinder• High pressure inside cylinder• Pressure pushes burned gases

out of cylinder• Engine does work on the gases

during this stroke

Clicker Question:Clicker Question:

What directly effects the efficiency of a car engine?

A) Octane of the gasB) Weight of the carC) The temperature of the gas when it burnsD) The number of cylinders

Efficiency is all about Efficiency is all about compression ratiocompression ratio

Most cars: 8:1 – 12:1

High-performance cars: 15:1

Diesel: 20:1

Efficiency LimitsEfficiency 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

atmosphere• However, ideal efficiency improves as

– the burned gases become hotter– the outside air becomes colder

• Real engines never reach ideal efficiency

KnockingKnockingThe 4-stroke cycle is a myth…

-4° TDC

+30 ° TDC

• Engine is inefficient• Pressure wave forms – knocks!• Use higher octane fuel!

OctaneOctane Ignition temperatureIgnition temperature879093

750 °C800 °C850 °C

Multiple cylindersMultiple cylinders

At least one cylinder is always executing power stroke

Where does the spark come from?Where does the spark come from?

More later when we talk about magnetism!

Tricked-out carsTricked-out cars

+

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

Joseph Priestly