Real Engines

A real engine converts energy from a fuel or another energy source into useful mechanical work under practical operating conditions.

A common example is the internal combustion engine, where fuel burns inside the engine itself. The hot gases produced by combustion expand and exert forces on pistons or turbine components.

Two important types of internal combustion engine are:

• gasoline engines
• diesel engines

Unlike an ideal Carnot engine, real engines contain friction, rapid heat transfer, turbulence, combustion, exhaust losses and other irreversible processes. Their efficiency is therefore always below the maximum theoretical Carnot efficiency.

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Internal Combustion Engines

In an internal combustion engine, fuel burns inside a combustion chamber.

The general energy pathway is:

chemical energy in fuel

↓

thermal energy from combustion

↓

expanding hot gases

↓

mechanical work

The expanding gases exert pressure on a piston.

The piston moves and turns a:

crankshaft

which produces rotational mechanical motion.

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Main Engine Components

A typical piston engine contains:

• cylinder
• piston
• connecting rod
• crankshaft
• intake valve
• exhaust valve
• combustion chamber
• fuel system
• cooling system
• lubrication system

Gasoline engines also normally use:

spark plugs

Diesel engines rely primarily on:

compression ignition

rather than spark plugs.

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The Piston and Cylinder

The piston moves up and down inside the cylinder.

Gas pressure acting on the piston produces a force.

This linear piston motion is transferred through the connecting rod to the crankshaft.

The crankshaft converts:

reciprocating motion → rotational motion

This rotation can ultimately drive the vehicle's wheels.

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The Four-Stroke Cycle

Most modern gasoline and diesel piston engines operate using a four-stroke process:

intake → compression → power → exhaust

Each stroke corresponds to movement of the piston through the cylinder.

A complete four-stroke cycle requires:

two crankshaft revolutions

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Intake Stroke

During the intake stroke:

• piston moves downward
• intake valve opens
• material enters the cylinder

In a conventional gasoline engine, this is typically an air-fuel mixture or air with fuel added by injection.

In many diesel engines:

air enters first

and fuel is injected later.

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Compression Stroke

The valves close.

The piston moves upward and compresses the contents of the cylinder.

Compression causes:

• volume to decrease
• pressure to increase
• temperature to increase

The compression stage prepares the mixture or air for combustion.

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Power Stroke

During the power stroke, combustion rapidly increases the temperature and pressure of the gases.

The high-pressure gases push the piston downward.

This produces:

mechanical work

The power stroke is the main stage in which chemical energy from the fuel ultimately produces useful mechanical output.

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Exhaust Stroke

The exhaust valve opens.

The piston moves upward.

Combustion products are forced out of the cylinder.

The cycle can then begin again.

The exhaust gases still contain significant:

thermal energy

which represents one important pathway by which energy leaves the engine without becoming useful shaft work.

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Gasoline Engines

A gasoline engine generally uses spark ignition.

During operation:

• air and fuel are introduced
• the mixture is compressed
• a spark plug creates a spark
• combustion begins
• expanding gases push the piston

The timing of ignition is carefully controlled so combustion produces useful pressure at the appropriate point in the cycle.

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Spark Ignition

The spark plug produces an electrical discharge across a small gap.

This initiates combustion of the compressed air-fuel mixture.

The combustion front spreads through the chamber.

Pressure rises rapidly and drives the piston during the power stroke.

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Diesel Engines

A diesel engine normally uses:

compression ignition

Rather than using a spark plug to ignite a premixed charge, the engine strongly compresses air.

This raises the air temperature.

Fuel is then injected into the hot compressed air.

The fuel ignites because the air temperature is sufficiently high.

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Diesel Compression

Diesel engines typically operate with a higher compression ratio than conventional gasoline engines.

Compression ratio compares the cylinder's maximum volume with its minimum volume:

compression ratio = V_max/V_min

A higher compression ratio generally allows greater thermodynamic efficiency, although real engine design involves many additional factors.

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Gasoline vs Diesel

These are broad comparisons; modern engine designs vary considerably.

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Why Diesel Engines Can Be More Efficient

Several factors can contribute to diesel-engine efficiency.

Diesel engines commonly use:

• higher compression ratios
• leaner operation
• reduced throttling losses
• combustion strategies suited to high-pressure operation

Higher compression ratios can increase the amount of useful work obtained from the expanding gases.

However:

diesel engines are still real engines

and therefore remain far below perfect efficiency.

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The Otto Cycle

The idealized thermodynamic model commonly associated with a conventional gasoline engine is the:

Otto cycle

The real gasoline engine does not exactly follow the ideal Otto cycle.

The model simplifies processes such as:

• compression
• combustion
• expansion
• exhaust

It allows physicists and engineers to study the thermodynamic principles without modeling every detail of a real engine.

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The Diesel Cycle

The idealized model commonly associated with a diesel engine is the:

Diesel cycle

Like the Otto cycle, it is a simplified thermodynamic model.

Real diesel engines differ from the ideal model because combustion and gas flow occur over finite times and involve many irreversible processes.

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Real Engines vs Ideal Engines

An ideal engine may assume:

• no friction
• reversible processes
• perfect insulation where needed
• controlled heat transfer
• ideal gas behavior
• no mechanical losses

A real engine contains:

• friction
• turbulence
• rapid combustion
• heat transfer
• exhaust flow
• pumping losses
• incomplete idealization of compression and expansion

Therefore:

η_real < η_ideal

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Efficiency of a Real Engine

Engine efficiency can be expressed as:

η = useful energy output / total energy input

For an internal combustion engine:

η = W_useful/E_fuel

where:

• W_useful = useful mechanical work
• E_fuel = chemical energy supplied by the fuel

Percentage efficiency:

η = (W_useful/E_fuel) × 100%

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Worked Example: Engine Efficiency

An engine receives:

E_fuel = 5000 J

and produces:

W_useful = 1750 J

Then:

η = 1750/5000

η = 0.35

Therefore:

η = 35%

This means:

35% of the input energy becomes useful mechanical output

The remaining 65% has not disappeared.

It has been transferred through other pathways.

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Where Does the Other Energy Go?

Energy leaving an engine may be transferred through:

• hot exhaust gases
• engine cooling system
• friction
• thermal radiation
• sound
• pumping gases
• mechanical accessories

Energy is conserved.

The issue is that much of it becomes:

dispersed thermal energy

rather than useful mechanical work.

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Exhaust Losses

Exhaust gases leave the engine at relatively high temperatures.

Therefore they carry away:

internal energy

This energy is no longer available to produce work in the piston engine unless additional technology is used to recover some of it.

Hot exhaust is often one of the major energy-loss pathways in combustion engines.

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Cooling Losses

Combustion produces extremely high temperatures.

Engine components cannot safely remain at those temperatures.

A cooling system therefore removes energy from:

• cylinder walls
• cylinder head
• engine block

This protects the engine from:

• overheating
• deformation
• lubricant breakdown
• material failure

However, energy transferred to the cooling system is generally unavailable as useful shaft work.

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Engine Cooling System

Many vehicle engines use liquid cooling.

A coolant circulates through passages around the engine.

The coolant absorbs thermal energy and transports it to a:

radiator

The radiator transfers energy to the surrounding air.

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Friction Losses

Engine components move against one another.

Examples include:

• pistons against cylinder walls
• crankshaft bearings
• camshaft components
• valve mechanisms

Friction converts mechanical energy into:

thermal energy

This reduces useful output and produces entropy.

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Lubrication

Engine oil reduces friction by forming a layer between moving surfaces.

Lubrication helps:

• reduce friction
• reduce wear
• remove some heat
• increase component life

Reducing friction can improve engine efficiency.

However, friction can never be completely eliminated in a practical engine.

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Pumping Losses

An engine must move gases:

into and out of cylinders

This requires work.

Energy is required to:

• draw in air
• compress gases
• push exhaust gases out
• overcome restrictions in intake and exhaust systems

These are called:

pumping losses

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Gasoline Engine Throttling Losses

Many gasoline engines control power partly using a:

throttle

At partial load, the throttle restricts incoming air.

The engine must work to draw air past this restriction.

This creates:

pumping losses

Diesel engines often operate with less intake throttling, which can contribute to better efficiency under some operating conditions.

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Combustion Losses

Real combustion is not an ideal reversible process.

Combustion occurs:

• rapidly
• at finite temperature differences
• with turbulent gas motion
• through complex chemical reactions

These processes produce:

entropy

Therefore some of the fuel's potential to perform useful work is lost through irreversibility.

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Incomplete Combustion

Ideally, hydrocarbon fuel burns completely to form primarily:

CO₂ + H₂O

In real engines, combustion may not always be complete.

Incomplete combustion can produce substances such as:

• carbon monoxide
• unburned hydrocarbons
• soot

This means some chemical energy may leave without being fully converted through combustion.

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Sound and Vibration

Engines produce:

• sound
• vibration

These energy transfers usually represent only part of the total energy balance, but they are still energy pathways that do not contribute to useful vehicle motion.

Eventually much of this energy becomes:

thermal energy in the surroundings

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Accessories

Some engine output is used to operate components such as:

• oil pump
• coolant pump
• alternator
• air-conditioning compressor
• power-assisted systems

This means not all mechanical work produced at the crankshaft reaches the wheels.

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Drivetrain Losses

In a vehicle, mechanical energy must pass through components such as:

• transmission
• differential
• bearings
• drive shafts

These components also have friction and other losses.

Therefore:

engine efficiency

and:

overall vehicle efficiency

are not necessarily the same thing.

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Why Real Engines Are Less Efficient Than Carnot Engines

A Carnot engine is:

perfectly reversible

A real engine is:

irreversible

Real engines produce entropy through:

• combustion
• friction
• heat transfer across finite temperature differences
• turbulent flow
• mixing
• exhaust expansion
• pressure losses

Therefore real engines cannot reach Carnot efficiency.

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The Carnot Limit

For any heat engine operating between hot and cold reservoir temperatures:

η_Carnot = 1 − T_c/T_h

This gives the theoretical maximum efficiency for a reversible engine operating between those temperatures.

Temperatures must be in:

kelvin

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Worked Example: Carnot Limit

Suppose an engine operates approximately between:

T_h = 1000 K

and:

T_c = 300 K

Then:

η_Carnot = 1 − 300/1000

η_Carnot = 0.70

η_Carnot = 70%

If the real engine operates at:

35%

then:

35% < 70%

which is consistent with the Second Law.

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Carnot Efficiency Is Not the Expected Real Efficiency

If the Carnot limit is 70%, this does not mean engineers should expect the real engine automatically to reach 70%.

Carnot efficiency assumes:

completely reversible operation

A real combustion engine cannot meet those conditions.

Therefore Carnot efficiency is best viewed as:

an upper theoretical benchmark

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Temperature and Efficiency

In general, increasing the maximum operating temperature can increase the theoretical opportunity for higher efficiency.

However, high temperatures create engineering problems.

Materials may:

• weaken
• oxidize
• deform
• melt
• lose protective coatings

Lubricants may also degrade.

Therefore practical engine temperature is limited by:

materials and design

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Compression Ratio and Efficiency

Increasing compression ratio can improve the theoretical efficiency of certain engine cycles.

A greater compression ratio can allow more useful work to be extracted during expansion.

However, compression ratio cannot simply be increased without limit.

Potential problems include:

• abnormal combustion
• high mechanical stresses
• excessive temperatures
• emissions
• material limitations

Engine design requires compromises.

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Knock in Gasoline Engines

In gasoline engines, excessive pressure and temperature can cause portions of the air-fuel mixture to ignite in an uncontrolled manner.

This is associated with:

engine knock

Knock can:

• reduce performance
• increase mechanical stress
• damage components

Fuel properties, compression ratio, ignition timing and engine controls are therefore important.

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Octane Rating

Gasoline's octane rating describes its resistance to unwanted autoignition under specified test conditions.

Higher-octane fuel can support engine designs or operating conditions with greater resistance to knock.

It does not simply mean:

the fuel contains more energy

Octane rating primarily relates to:

knock resistance

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Diesel Fuel and Cetane

Diesel fuels can be characterized using a:

cetane number

This relates to ignition behavior in compression-ignition engines.

A diesel engine requires fuel to ignite reliably after injection into hot compressed air.

Thus gasoline and diesel fuels are optimized for different combustion processes.

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Turbochargers

A turbocharger uses energy from exhaust gases to compress incoming air.

Hot exhaust drives a turbine.

The turbine drives a compressor.

The compressor forces more air into the engine.

This can allow:

• more fuel to be burned
• greater power from a smaller engine
• improved performance
• recovery of some otherwise unused exhaust energy

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Turbocharging and Efficiency

A turbocharger does not recover all exhaust energy.

However, it can use some energy that would otherwise leave directly with the exhaust.

Modern engine designs may combine:

• turbocharging
• direct fuel injection
• variable valve timing
• advanced combustion control

to improve performance and efficiency.

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Waste Heat Recovery

Some systems attempt to recover additional energy from:

hot exhaust

For example, exhaust energy can potentially be used to:

• drive turbochargers
• generate electrical energy
• heat other fluids
• support combined thermodynamic cycles

Waste-heat recovery can increase overall system efficiency.

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Engine Performance

Efficiency is only one measure of engine performance.

Engineers may also consider:

• power
• torque
• fuel consumption
• emissions
• reliability
• mass
• durability
• cost
• noise
• operating range

A high-performance engine must often balance several competing requirements.

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Power

Power is the rate at which work is done.

P = W/t

where:

• P = power
• W = work
• t = time

SI unit:

watt (W)

An engine that performs more work each second produces greater power.

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Efficiency vs Power

A more powerful engine is not necessarily:

more efficient

For example:

Engine A may produce:

200 kW

at 30% efficiency.

Engine B may produce:

100 kW

at 40% efficiency.

Engine A produces more power.

Engine B converts a larger fraction of its input energy into useful output.

Therefore:

power ≠ efficiency

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Torque

Torque describes the turning effect of a force.

For a force acting perpendicular to a lever arm:

τ = Fr

where:

• τ = torque
• F = force
• r = perpendicular distance from the axis

SI unit:

N·m

Engines produce torque at the crankshaft.

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Torque and Power

For rotational motion:

P = τω

where:

• P = power
• τ = torque
• ω = angular velocity

This means engine power depends on both:

torque

and:

rotational speed

An engine can produce high torque at relatively low rotational speed or lower torque at high rotational speed and still produce substantial power.

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Fuel Consumption

Fuel consumption provides another way of evaluating an engine.

A more efficient engine generally requires less fuel to produce a given amount of useful work.

However, actual vehicle fuel consumption also depends on:

• vehicle mass
• speed
• aerodynamic drag
• tire resistance
• driving behavior
• road gradient
• transmission
• accessories

Therefore engine efficiency and vehicle fuel economy are related but not identical.

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Evaluating Two Engines

Suppose Engine A receives:

100 MJ

of fuel energy and produces:

35 MJ

of useful work.

η_A = 35/100

η_A = 35%

Engine B receives:

80 MJ

and produces:

32 MJ

of useful work.

η_B = 32/80

η_B = 40%

Engine B is more efficient even though Engine A produces more total useful energy in this example.

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Energy-Flow Evaluation

Suppose a gasoline engine receives:

100 units

of fuel energy.

Imagine:

• 35 units → useful mechanical work
• 30 units → exhaust
• 25 units → cooling
• 7 units → friction and accessories
• 3 units → other transfers

Total:

35 + 30 + 25 + 7 + 3 = 100

Efficiency:

η = 35/100

η = 35%

This type of energy-flow analysis helps engineers identify where improvements may be possible.

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Sankey Diagrams

A Sankey diagram can represent engine energy transfers.

The width of each arrow represents the amount of energy transferred along that pathway.

A Sankey diagram can show:

fuel energy → useful work + exhaust + cooling + friction

It provides a useful visual tool for evaluating engine performance.

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Evaluating Improvements

Suppose engineers reduce friction losses from:

10% → 6%

If other energy transfers remain unchanged, more input energy may become available for useful work.

But improving one component can sometimes affect others.

For example:

• higher temperatures may improve thermodynamic efficiency
• but increase cooling requirements
• lighter components may reduce mass
• but reduce durability
• emissions controls may affect exhaust flow

Engineering involves:

trade-offs

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Emissions and Performance

Engine performance cannot be judged solely by efficiency.

Combustion engines can produce pollutants including:

• carbon monoxide
• nitrogen oxides
• unburned hydrocarbons
• particulate matter

They also commonly produce:

CO₂

when carbon-containing fuels burn.

Therefore engine evaluation increasingly considers both:

energy performance and environmental impact

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Catalytic Converters

Gasoline vehicles commonly use catalytic converters to reduce harmful exhaust pollutants.

They promote chemical reactions that reduce emissions such as:

• carbon monoxide
• unburned hydrocarbons
• nitrogen oxides

Catalytic converters improve emissions performance but do not eliminate all environmental impacts of fuel combustion.

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Diesel Emission Controls

Modern diesel vehicles may use technologies such as:

• diesel particulate filters
• oxidation catalysts
• selective catalytic reduction

These systems help reduce pollutants.

However, they add:

• complexity
• cost
• maintenance requirements

Again, real engineering involves balancing multiple objectives.

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Gasoline vs Diesel Performance

Gasoline engines often offer:

• relatively light construction
• high rotational speeds
• smooth operation
• strong power-to-mass performance

Diesel engines often offer:

• higher thermal efficiency
• strong low-speed torque
• durability
• efficient operation under heavy loads

Neither engine type is automatically "better."

The appropriate choice depends on the:

application

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Passenger Cars

Passenger cars may prioritize:

• low mass
• smooth operation
• acceleration
• low noise
• low emissions
• efficiency

These factors historically contributed to widespread gasoline-engine use in many passenger vehicles.

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Trucks and Heavy Equipment

Heavy vehicles often require:

• high torque
• long operating life
• efficiency under sustained load
• durability

These characteristics have made diesel engines common in:

• trucks
• buses
• construction equipment
• agricultural machinery

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Ships and Generators

Large diesel engines are also widely used in:

• ships
• stationary generators
• industrial machinery

Large engines can achieve high efficiencies compared with many smaller internal combustion engines, although they remain subject to Second Law limitations.

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Hybrid Vehicles

Hybrid vehicles combine an internal combustion engine with:

• electric motor
• battery
• electronic control systems

The electric system can help operate the combustion engine closer to efficient conditions.

It can also recover some vehicle kinetic energy through:

regenerative braking

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Regenerative Braking

Traditional friction braking converts kinetic energy mainly into:

thermal energy

Regenerative braking instead converts some kinetic energy into:

electrical energy

which can be stored in a battery.

This improves overall vehicle energy efficiency.

However:

not all kinetic energy can be recovered

because real systems contain irreversible losses.

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Real Engines and Entropy

Real engines produce entropy.

Important sources include:

• combustion
• friction
• mixing
• heat transfer across temperature differences
• exhaust flow
• turbulence

Therefore:

ΔS_total > 0

for real engine operation.

This entropy production is one fundamental reason why not all fuel energy can become useful work.

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The First and Second Laws

The First Law tells us:

energy is conserved

If fuel supplies 100 MJ, all 100 MJ must eventually be accounted for.

The Second Law tells us:

not all of that energy can be converted into useful work

Some becomes dispersed in forms that are less useful for performing mechanical work.

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Real Engines vs Carnot Engines

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Evaluating an Engine

When evaluating a real engine, ask:

What is the energy input?

How much useful work is produced?

What is the efficiency?

Where does the remaining energy go?

How much power does it produce?

What torque does it produce?

How much fuel does it consume?

What pollutants are produced?

How reliable is it?

How expensive is it to operate?

A complete evaluation considers much more than a single number.

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Worked Example: Engine Energy Balance

An engine receives:

E_in = 20 MJ

It produces:

W = 7 MJ

Efficiency

η = W/E_in

η = 7/20

η = 0.35 = 35%

Energy transferred elsewhere

E_other = 20 − 7

E_other = 13 MJ

Therefore:

13 MJ

is transferred through exhaust, cooling, friction and other pathways.

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Worked Example: Power

An engine produces:

600 kJ

of useful work in:

4 s

Use:

P = W/t

P = 600,000/4

P = 150,000 W

Therefore:

P = 150 kW

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Worked Example: Fuel Comparison

Engine A:

• fuel input = 50 MJ
• useful output = 15 MJ

η_A = 15/50

η_A = 30%

Engine B:

• fuel input = 50 MJ
• useful output = 20 MJ

η_B = 20/50

η_B = 40%

Engine B produces:

5 MJ more useful work

from the same fuel-energy input.

Therefore Engine B is more efficient.

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Worked Example: Real vs Carnot

A real engine operates between approximate temperatures:

T_h = 1200 K

T_c = 300 K

Carnot limit:

η_Carnot = 1 − 300/1200

η_Carnot = 0.75

η_Carnot = 75%

Suppose the real engine efficiency is:

40%

This is physically reasonable because:

40% < 75%

The difference reflects the fact that the real engine contains substantial irreversibility.

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Common Mistakes

Mistake: Saying gasoline and diesel engines ignite fuel in exactly the same way.
Gasoline engines normally use spark ignition; diesel engines use compression ignition.

Mistake: Saying diesel engines have spark plugs for normal ignition.
Normal diesel combustion relies on fuel injection into hot compressed air.

Mistake: Saying unused energy disappears.
Energy is conserved and transferred through exhaust, cooling, friction and other pathways.

Mistake: Saying all heat leaving an engine is simply caused by bad engineering.
Some losses can be reduced, but the Second Law imposes fundamental limits on heat-engine efficiency.

Mistake: Saying the most powerful engine is the most efficient.
Power and efficiency measure different things.

Mistake: Saying higher compression is always better without limits.
Material stresses, combustion behavior, emissions and other factors impose practical constraints.

Mistake: Saying a turbocharger creates free energy.
It recovers some energy from exhaust flow to drive a compressor.

Mistake: Assuming the Carnot efficiency is the expected efficiency of a real engine.
It is the maximum theoretical limit between two temperatures.

Mistake: Treating engine efficiency and overall vehicle efficiency as identical.
The drivetrain and vehicle itself introduce additional energy transfers and losses.

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Key Terms

Internal combustion engine — engine in which fuel burns inside the engine's combustion chamber.

Gasoline engine — internal combustion engine normally using spark ignition.

Diesel engine — internal combustion engine normally using compression ignition.

Spark ignition — initiation of combustion using an electrical spark.

Compression ignition — ignition caused when fuel is injected into sufficiently hot compressed air.

Piston — moving component that receives force from expanding gases.

Cylinder — chamber in which the piston moves.

Crankshaft — rotating shaft that converts piston motion into rotational motion.

Compression ratio — ratio V_max/V_min describing the change in cylinder volume during compression.

Otto cycle — idealized thermodynamic cycle commonly used to model spark-ignition engines.

Diesel cycle — idealized thermodynamic cycle commonly used to model compression-ignition engines.

Efficiency — fraction of input energy converted into useful output.

Power — rate at which work is performed.

Torque — turning effect of a force.

Pumping loss — energy required to move gases through an engine.

Friction loss — conversion of mechanical energy into thermal energy through friction.

Waste heat — thermal energy transferred away rather than becoming the desired useful output.

Turbocharger — device using exhaust flow to drive a compressor that increases intake-air pressure.

Sankey diagram — diagram showing energy-transfer pathways using arrow widths proportional to energy amounts.

Regenerative braking — recovery of some vehicle kinetic energy into stored electrical energy.

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Key Takeaways

• Internal combustion engines burn fuel inside the engine.
• Chemical energy from fuel ultimately produces mechanical work.
• Most piston engines use intake, compression, power and exhaust strokes.
• A four-stroke engine completes one operating cycle in two crankshaft revolutions.
• Gasoline engines normally use spark ignition.
• Diesel engines normally use compression ignition.
• Diesel engines generally operate at higher compression ratios than conventional gasoline engines.
• Higher compression ratios can contribute to greater thermodynamic efficiency.
• Diesel engines are often more efficient than comparable gasoline engines, although actual performance depends on design and operating conditions.
• Real engines are always less efficient than ideal reversible engines.
• Real engines contain irreversible processes that produce entropy.
• Major energy-transfer pathways include exhaust heat, cooling, friction and pumping losses.
• Energy lost from useful output is not destroyed.
• The First Law requires all input energy to be accounted for.
• The Second Law limits how much input energy can become useful work.
• Engine efficiency is η = useful output/input energy.
• Carnot efficiency provides an upper theoretical benchmark.
• Real engines cannot achieve Carnot efficiency.
• Higher operating temperatures can increase theoretical efficiency but create material and cooling challenges.
• Friction converts useful mechanical energy into dispersed thermal energy.
• Lubrication reduces friction and wear.
• Turbochargers recover some energy from exhaust gases.
• Waste-heat recovery can improve overall system efficiency.
• Power measures the rate of doing work and is not the same as efficiency.
• Torque measures rotational turning effect.
• Engine evaluation should consider efficiency, power, torque, fuel consumption, emissions, reliability and cost.
• Gasoline and diesel engines have different strengths for different applications.
• Hybrid systems can improve overall vehicle efficiency by combining combustion and electric technologies.
• Regenerative braking recovers some energy that would otherwise become thermal energy through friction.
• Real engine design involves trade-offs between efficiency, performance, emissions, durability and cost.

Check Your Understanding

1. Define an internal combustion engine.
2. Name the four strokes of a four-stroke engine in order.
3. What happens during the power stroke?
4. How is piston motion converted into rotational motion?
5. Explain how fuel is normally ignited in a gasoline engine.
6. Explain how fuel is normally ignited in a diesel engine.
7. Give two differences between gasoline and diesel engines.
8. Why can higher compression ratios contribute to improved engine efficiency?
9. Identify four major energy-loss pathways in a real engine.
10. Explain why hot exhaust represents an energy loss from useful engine output.
11. Why does an engine require a cooling system?
12. Explain how friction reduces engine efficiency.
13. An engine receives 5000 J of fuel energy and produces 1800 J of useful work. Calculate its efficiency.
14. Where does the remaining energy in Question 13 go?
15. Explain why a real engine cannot reach Carnot efficiency.
16. Distinguish between engine power and engine efficiency.
17. Explain how a turbocharger can improve engine performance.
18. How can regenerative braking improve the overall efficiency of a vehicle?
19. Compare a real internal combustion engine with an ideal Carnot engine.
20. Two engines have different efficiencies, power outputs, fuel consumption and emissions. Explain what information you would consider when deciding which engine has the better overall performance.