Heat Engines
5. Efficiency and Sustainability
Learning outcomes
- I can calculate thermal efficiency.
- I can identify factors affecting efficiency.
- I can compare different energy technologies.
- I can explain how thermodynamics influences sustainability.
- I can evaluate methods of improving energy efficiency.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
Gasoline vs Diesel
| Gasoline Engine | Diesel Engine |
|---|---|
| Usually spark ignition | Compression ignition |
| Uses spark plugs | Uses fuel injectors; no ignition spark required in normal diesel combustion |
| Generally lower compression ratio | Generally higher compression ratio |
| Fuel ignited by spark | Fuel ignites in hot compressed air |
| Often lighter | Often heavier and more robust |
| Common in passenger vehicles | Common in trucks and heavy equipment |
| Often capable of higher rotational speeds | Often strong low-speed torque |
| Typically lower thermal efficiency | Typically higher thermal efficiency |
These are broad comparisons; modern engine designs vary considerably.
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.
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.
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.
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
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%
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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.
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.
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
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.
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
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
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.
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.
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
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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
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
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.
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.
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
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.
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
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.
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
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.
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.
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.
Real Engines vs Carnot Engines
| Real Engine | Carnot Engine |
|---|---|
| Physical machine | Idealized model |
| Irreversible | Reversible |
| Friction present | No friction |
| Rapid combustion possible | Ideal controlled processes |
| Entropy produced | No entropy produced |
| Finite power | Reversible limit tends toward very slow operation |
| Efficiency below theoretical maximum | Maximum possible efficiency |
| Practical | Benchmark |
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.
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.
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
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.
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.
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.
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.
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
- Define an internal combustion engine.
- Name the four strokes of a four-stroke engine in order.
- What happens during the power stroke?
- How is piston motion converted into rotational motion?
- Explain how fuel is normally ignited in a gasoline engine.
- Explain how fuel is normally ignited in a diesel engine.
- Give two differences between gasoline and diesel engines.
- Why can higher compression ratios contribute to improved engine efficiency?
- Identify four major energy-loss pathways in a real engine.
- Explain why hot exhaust represents an energy loss from useful engine output.
- Why does an engine require a cooling system?
- Explain how friction reduces engine efficiency.
- An engine receives 5000 J of fuel energy and produces 1800 J of useful work. Calculate its efficiency.
- Where does the remaining energy in Question 13 go?
- Explain why a real engine cannot reach Carnot efficiency.
- Distinguish between engine power and engine efficiency.
- Explain how a turbocharger can improve engine performance.
- How can regenerative braking improve the overall efficiency of a vehicle?
- Compare a real internal combustion engine with an ideal Carnot engine.
- 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.