Energy in Systems
5. Energy in Transportation and Engineering
Learning outcomes
- I can describe how energy is transferred and transformed in transportation systems.
- I can analyze the role of power, efficiency, and energy losses in engineering applications.
- I can compare different transportation technologies using energy considerations.
- I can evaluate engineering designs that improve energy efficiency.
- I can apply energy concepts to solve practical problems involving transportation and machines.
Energy in Transportation and Engineering
Transportation systems move people and materials by transferring energy from an energy source into useful mechanical energy.
Whether the system is a car, train, aircraft, bicycle, ship, or electric vehicle, the same physics applies:
energy is transferred → forces produce motion → some energy becomes useful output → some energy is dissipated
Engineers aim to maximize the useful energy transfer while reducing unnecessary losses.
Important concepts include:
- energy stores
- energy transfer pathways
- work
- kinetic energy
- power
- efficiency
- friction
- air resistance
- energy density
- mechanical advantage
- conservation of energy
Energy Flow in Transportation
Every transportation system requires an energy source.
Examples include:
| Transportation System | Main Energy Source |
|---|---|
| Gasoline car | Chemical store in gasoline |
| Diesel truck | Chemical store in diesel |
| Battery-electric vehicle | Chemical store in battery |
| Hydrogen vehicle | Chemical store associated with hydrogen |
| Electric train | Electrical energy supplied through grid |
| Bicycle | Chemical store from food |
| Aircraft | Chemical store in aviation fuel |
| Sailing boat | Kinetic energy of moving air |
The energy must ultimately be transferred into the kinetic energy of the vehicle or used to overcome forces opposing its motion.
Energy Flow in a Gasoline Vehicle
A simplified energy pathway is:
chemical store in fuel
↓
heating during combustion
↓
mechanical transfer in engine
↓
mechanical transfer through transmission
↓
kinetic store of vehicle
However, not all the chemical energy becomes useful kinetic energy.
Energy is also transferred into:
- engine thermal stores
- exhaust gases
- cooling system
- tyres
- road
- surrounding air
- sound
Therefore, the engine and drivetrain are not 100% efficient.
Energy Flow in an Electric Vehicle
A battery-electric vehicle has a different energy pathway:
chemical store in battery
↓
electrical transfer
↓
electric motor
↓
mechanical transfer
↓
kinetic store of vehicle
Energy losses occur through:
- electrical resistance
- battery heating
- motor heating
- friction
- tyre deformation
- air resistance
Electric drivetrains generally have fewer major energy-conversion stages than combustion drivetrains.
Conservation of Energy
Energy cannot be created or destroyed.
For any vehicle:
total input energy = useful energy + dissipated energy
Suppose a vehicle receives:
1000 kJ
of energy.
If:
700 kJ → useful output
then:
300 kJ
must have been transferred elsewhere.
For example:
1000 kJ input = 700 kJ useful + 300 kJ dissipated
The energy has not disappeared.
Useful Energy
The useful energy output depends on the intended purpose.
For a moving vehicle, useful transfers can include:
- increasing kinetic energy
- increasing gravitational potential energy while climbing
- moving passengers or cargo
- operating necessary systems
Some energy may also be required simply to overcome resistive forces.
Energy Losses
Common sources of unwanted energy transfer in transportation include:
- friction
- rolling resistance
- air resistance
- engine heating
- electrical resistance
- vibration
- sound
- braking
- deformation of tyres
These processes reduce the fraction of input energy available for the desired output.
Efficiency
Efficiency measures how much input energy becomes useful output.
efficiency = useful energy output / total energy input
As a percentage:
efficiency (%) = useful output / total input × 100
Efficiency can also be calculated using power:
efficiency (%) = useful power output / total power input × 100
Worked Example: Vehicle Efficiency
A vehicle receives:
5000 kJ
from its energy source.
Useful mechanical output:
3500 kJ
Efficiency:
efficiency = 3500/5000 × 100
efficiency = 70%
Energy dissipated:
5000 − 3500 = 1500 kJ
Comparing Two Transportation Systems
Vehicle A:
Input = 1000 kJ
Useful output = 300 kJ
Efficiency:
30%
Vehicle B:
Input = 1000 kJ
Useful output = 850 kJ
Efficiency:
85%
Vehicle B produces much more useful output from the same input energy.
However, efficiency alone does not determine which vehicle is best.
Other considerations include:
- energy-storage mass
- energy density
- range
- cost
- charging or refueling time
- infrastructure
- environmental impact
- reliability
Power in Transportation
Power is the rate at which energy is transferred.
P = E/t
where:
- P = power in watts
- E = energy in joules
- t = time in seconds
Recall:
1 W = 1 J/s
A vehicle with a high-power motor can transfer large amounts of energy quickly.
This can allow:
- rapid acceleration
- climbing steep hills
- moving heavy loads
- maintaining high speeds against resistance
Worked Example: Motor Power
A motor transfers:
600,000 J
in:
20 s
Power:
P = E/t
P = 600,000/20
P = 30,000 W
P = 30 kW
Mechanical Power
When a constant force acts on an object moving at constant velocity:
P = Fv
where:
- P = power
- F = force
- v = velocity
This equation is particularly useful for transportation problems.
Worked Example: Car at Constant Speed
A car experiences:
800 N
of total resistance while traveling at:
25 m/s
At constant speed:
driving force = resistance
Therefore:
F = 800 N
Power required:
P = Fv
P = 800 × 25
P = 20,000 W
P = 20 kW
The car must provide at least 20 kW of mechanical power simply to overcome resistance under these conditions.
Why High-Speed Travel Requires Power
Vehicles experience resistive forces including:
- rolling resistance
- mechanical friction
- air resistance
Air resistance becomes particularly important at high speeds.
As speed increases, aerodynamic drag generally increases.
Therefore, a vehicle traveling faster often requires much more power to maintain its speed.
This is a major consideration in:
- cars
- aircraft
- high-speed trains
- bicycles
Aerodynamic Drag
Drag is a resistive force caused by movement through a fluid.
Air is a fluid.
Drag depends on factors including:
- speed
- shape
- frontal area
- air density
- surface characteristics
Streamlined designs allow air to flow around a vehicle more effectively.
Reducing drag can reduce the power needed to maintain speed.
Streamlining
Engineers reduce aerodynamic losses using:
- smooth body shapes
- smaller frontal areas
- carefully shaped vehicle surfaces
- aerodynamic fairings
- optimized underbody designs
- streamlined mirrors and accessories
Examples include:
- aerodynamic cars
- high-speed trains
- aircraft
- racing bicycles
Reducing drag can increase:
- range
- fuel economy
- maximum speed
- energy efficiency
Rolling Resistance
Tyres deform as they roll.
The road surface may also deform slightly.
These deformations transfer energy into thermal stores.
This creates rolling resistance.
Rolling resistance can be reduced through:
- suitable tyre design
- correct tyre pressure
- appropriate materials
- reduced unnecessary vehicle mass
However, tyres must still provide enough grip for safe acceleration, steering, and braking.
Friction in the Drivetrain
Mechanical systems contain moving components such as:
- gears
- shafts
- bearings
- chains
- axles
Friction between these components transfers mechanical energy into thermal energy.
Engineers reduce these losses using:
- lubrication
- efficient bearings
- precise alignment
- appropriate materials
- improved gear design
Vehicle Mass
Mass affects transportation energy requirements.
Recall:
Eₖ = ½mv²
A more massive vehicle requires more energy to reach the same speed.
For example:
Vehicle A:
m = 1000 kg
Vehicle B:
m = 2000 kg
Both travel at:
20 m/s
Vehicle A:
Eₖ = ½(1000)(20²)
Eₖ = 200,000 J
Vehicle B:
Eₖ = ½(2000)(20²)
Eₖ = 400,000 J
Doubling the mass doubles the kinetic energy at the same speed.
Speed Has an Even Greater Effect
Because:
Eₖ = ½mv²
kinetic energy depends on the square of speed.
Consider a 1000 kg vehicle.
At 10 m/s:
Eₖ = ½(1000)(10²)
Eₖ = 50,000 J
At 20 m/s:
Eₖ = ½(1000)(20²)
Eₖ = 200,000 J
Doubling speed increases kinetic energy by a factor of:
4
This has major implications for:
- acceleration
- braking
- collision energy
- energy consumption
Climbing Hills
Vehicles climbing hills gain gravitational potential energy.
Recall:
ΔEₚ = mgΔh
Suppose a:
1500 kg
car climbs:
100 m
Using:
g = 9.8 N/kg
Increase in gravitational potential energy:
ΔEₚ = 1500 × 9.8 × 100
ΔEₚ = 1,470,000 J
ΔEₚ = 1.47 MJ
Additional energy must also be supplied to overcome resistance.
Descending Hills
When a vehicle travels downhill:
gravitational potential → kinetic
Without braking, the vehicle may accelerate.
Conventional brakes transfer much of this energy into thermal energy.
Electric and hybrid vehicles may recover some of it using regenerative braking.
Regenerative Braking
During conventional braking:
kinetic energy → thermal energy
During regenerative braking:
kinetic energy → mechanical transfer → electrical transfer → chemical store in battery
Some energy is still dissipated, so regenerative braking is not 100% efficient.
However, it allows part of the vehicle's kinetic energy to be recovered.
Worked Example: Regenerative Braking
A vehicle has:
600 kJ
of kinetic energy before braking.
Its regenerative system recovers:
420 kJ
Recovery efficiency:
efficiency = 420/600 × 100
efficiency = 70%
Energy not recovered:
600 − 420 = 180 kJ
This energy is transferred into other stores.
Internal Combustion Vehicles
Internal combustion engine vehicles store energy in fuels such as:
- gasoline
- diesel
- biofuels
Advantages can include:
- high fuel energy density
- long driving range
- rapid refueling
- established infrastructure
Challenges include:
- substantial thermal losses
- exhaust emissions
- mechanical complexity
- friction
- noise
- dependence on combustible fuels
Electric Vehicles
Battery-electric vehicles store energy chemically in rechargeable batteries.
Potential advantages include:
- efficient electric motors
- regenerative braking
- fewer major moving drivetrain components
- no direct exhaust emissions
- quiet operation
Challenges can include:
- battery mass
- charging time
- charging infrastructure
- raw material requirements
- battery manufacturing
- battery degradation
- lower stored energy per kilogram compared with liquid hydrocarbon fuels
A complete comparison therefore requires more than simply comparing motor efficiency.
Energy Density and Transportation
Energy density describes the energy stored per unit mass or volume.
For transportation, high energy density can be particularly valuable because the vehicle must carry its energy source.
A high-energy-density fuel can provide:
- longer range
- lower fuel mass
- smaller storage volume
This is particularly important in aviation.
Batteries and Energy Density
Modern batteries have much lower stored energy per kilogram than gasoline or aviation fuels when comparing raw energy content.
However, electric motors can convert a high proportion of battery energy into useful mechanical output.
Therefore, transportation comparisons should consider:
energy density + drivetrain efficiency
rather than either quantity alone.
Hydrogen Transportation
Hydrogen can be used in:
- fuel-cell vehicles
- some experimental combustion systems
Hydrogen has high energy content per kilogram.
However, hydrogen gas has low density, so storage may require:
- high-pressure tanks
- cryogenic liquid storage
- specialized infrastructure
The complete system must include the mass and volume of the storage equipment.
Trains
Rail transportation has an important mechanical advantage:
low rolling resistance
Steel wheels rolling on steel rails can produce relatively low rolling resistance compared with rubber tyres on roads.
Trains can also carry large numbers of passengers or large quantities of cargo.
Electric trains can receive energy continuously from:
- overhead wires
- electrified rails
This means they may not need to carry a large onboard energy store.
High-Speed Trains
At lower speeds, rolling resistance may be important.
At very high speeds, aerodynamic drag becomes increasingly significant.
High-speed trains therefore use:
- streamlined noses
- smooth surfaces
- covered equipment
- carefully designed connections between cars
Reducing drag lowers the power required to maintain high speed.
Aircraft
Aircraft present one of the most demanding energy-engineering problems.
Aircraft must carry:
- passengers
- cargo
- structure
- engines
- fuel or energy storage
Every additional kilogram must be supported in flight.
This makes energy density by mass particularly important.
Improving Aircraft Efficiency
Engineers can improve aircraft efficiency through:
- improved aerodynamics
- efficient engines
- lightweight materials
- optimized flight paths
- winglets
- reduced unnecessary mass
- improved surface design
Small improvements can produce large energy savings across thousands of flights.
Ships
Ships can carry enormous loads.
Their large size means mass energy density may be less restrictive than for aircraft.
However, ships must overcome:
- water resistance
- air resistance
- mechanical friction
Engineers improve ship efficiency through:
- hull design
- efficient propellers
- engine improvements
- route optimization
- reduced friction
- improved loading
Bicycles
Bicycles are highly efficient transportation machines.
Energy flow:
chemical store in food
↓
muscle contraction
↓
mechanical transfer through pedals
↓
chain and gears
↓
wheel rotation
↓
kinetic energy of bicycle and rider
Energy losses include:
- air resistance
- rolling resistance
- chain friction
- bearing friction
At higher speeds, aerodynamic drag becomes increasingly important.
Gears and Transportation
Gears allow engineers to trade:
force for speed
or:
speed for force
Low gears provide:
- greater force at the wheel
- lower vehicle speed for a given motor or engine rotation
High gears provide:
- greater wheel speed
- less force multiplication
This allows an engine, motor, or cyclist to operate effectively under different conditions.
Machines and Mechanical Advantage
Mechanical advantage is:
MA = output force / input force
Machines can reduce the input force required by increasing the distance over which that force acts.
For an ideal machine:
input work = output work
Therefore:
Fin × din = Fout × dout
Machines multiply force, not energy.
This principle appears throughout transportation engineering.
Power and Hill Climbing
Suppose a vehicle climbs vertically at a steady rate.
The power required to increase gravitational potential energy is:
P = E/t
Since:
ΔEₚ = mgΔh
we can write:
P = mgΔh/t
If vertical speed is:
vvertical = Δh/t
then:
P = mgvvertical
This gives the minimum useful power required to gain gravitational potential energy.
Worked Example: Truck Climbing a Hill
A:
10,000 kg
truck gains:
20 m
of height in:
40 s
Using:
g = 9.8 N/kg
Increase in gravitational potential energy:
ΔEₚ = 10,000 × 9.8 × 20
ΔEₚ = 1,960,000 J
Useful power:
P = 1,960,000/40
P = 49,000 W
P = 49 kW
This ignores air resistance, rolling resistance, and drivetrain losses.
The actual engine or motor power required would be higher.
Including Efficiency
Suppose the drivetrain is:
70% efficient
and the useful mechanical power required is:
49 kW
Input power:
efficiency = useful power / input power
Therefore:
input power = useful power / efficiency
input power = 49/0.70
input power = 70 kW
The energy source must therefore supply approximately 70 kW.
Combined Resistance and Hill Climbing
A vehicle climbing a hill may need power for both:
- increasing gravitational potential energy
- overcoming resistive forces
Suppose:
Power for climbing = 40 kW
Power against resistance = 20 kW
Total useful mechanical power:
Ptotal = 40 + 20
Ptotal = 60 kW
If the drivetrain is:
80% efficient
Input power:
Pin = 60/0.80
Pin = 75 kW
Engineering for Lower Mass
Reducing vehicle mass can reduce the energy required for:
- acceleration
- climbing hills
Engineers may use:
- aluminum
- high-strength steels
- composites
- optimized structures
However, reducing mass must not compromise:
- safety
- strength
- durability
- cost
- crash protection
Engineering decisions involve trade-offs.
Recovering Energy
Engineers increasingly attempt to recover energy that would otherwise be dissipated.
Examples include:
- regenerative braking
- waste-heat recovery
- energy recovery from exhaust
- regenerative elevators
- hybrid power systems
Energy recovery can improve overall system efficiency.
Waste Heat
Engines, motors, batteries, brakes, and electronic systems can become warm during operation.
This indicates energy transfer into thermal stores.
In many systems, this thermal energy is difficult to recover.
Engineers therefore try to reduce unnecessary heating using:
- lubrication
- low-resistance conductors
- efficient motors
- improved cooling systems
- better materials
Cooling itself may require energy, so preventing unnecessary heating can be especially valuable.
Engineering Design and Efficiency
An energy-efficient design tries to maximize:
useful output / total input
Possible improvements include:
- reducing friction
- reducing drag
- reducing unnecessary mass
- improving motor efficiency
- improving engine efficiency
- optimizing gears
- recovering braking energy
- improving bearings
- using appropriate tyres
- reducing electrical resistance
- improving thermal management
No single modification solves every problem.
Whole-System Efficiency
Engineers must consider the complete system.
Suppose energy passes through three stages:
Battery discharge = 95% efficient
Motor = 90% efficient
Transmission = 95% efficient
Overall efficiency:
0.95 × 0.90 × 0.95
= 0.812
Approximately:
81%
This is why efficiencies of different stages must be multiplied rather than added.
Worked Example: Multi-Stage Vehicle
A battery initially supplies:
20 MJ
of energy.
Battery output efficiency:
95%
Energy leaving battery:
20 × 0.95 = 19 MJ
Motor efficiency:
90%
Mechanical output:
19 × 0.90 = 17.1 MJ
Drivetrain efficiency:
95%
Useful output:
17.1 × 0.95 ≈ 16.25 MJ
Overall useful fraction:
16.25/20 ≈ 0.8125
Overall efficiency:
≈ 81%
Approximately:
3.75 MJ
is transferred into less useful forms.
Comparing Transportation Technologies
No transportation technology is automatically "best."
Engineers must compare many factors.
| Factor | Important Question |
|---|---|
| Efficiency | How much input energy becomes useful output? |
| Power | How quickly can energy be delivered? |
| Energy density | How much energy can be carried per kg or litre? |
| Mass | How much energy is required to move the vehicle? |
| Drag | How much energy is required to move through air or water? |
| Rolling resistance | How much energy is dissipated at wheels? |
| Range | How far can the vehicle travel? |
| Refueling/charging | How easily can energy be replenished? |
| Infrastructure | What support systems are required? |
| Emissions | What environmental effects occur during operation? |
| Life cycle | What happens during manufacture and disposal? |
| Cost | Is the technology economically practical? |
Operational Versus Life-Cycle Energy
Transportation systems should not be evaluated only during operation.
A complete analysis may consider:
raw materials → manufacturing → energy production → operation → maintenance → recycling/disposal
For an electric vehicle, this can include:
- mineral extraction
- battery manufacturing
- electricity generation
- charging
- driving
- battery recycling
For a combustion vehicle:
- oil extraction
- fuel refining
- fuel transportation
- vehicle manufacturing
- fuel combustion
- maintenance
This is called life-cycle analysis.
Designing for the Intended Purpose
Different vehicles have different requirements.
City Car
Priorities may include:
- efficiency
- regenerative braking
- compact size
- low-speed performance
Long-Distance Truck
Priorities may include:
- range
- payload
- energy density
- reliability
Aircraft
Priorities include:
- very low mass
- high energy density
- aerodynamic efficiency
Train
Priorities may include:
- low rolling resistance
- large passenger or cargo capacity
- continuous electrical supply
Therefore, the most appropriate energy technology depends strongly on the application.
Practical Problem: Electric Car
An electric car has a mass of:
1600 kg
and accelerates from rest to:
25 m/s
Increase in kinetic energy:
Eₖ = ½mv²
Eₖ = ½(1600)(25²)
Eₖ = 500,000 J
Eₖ = 500 kJ
Suppose the drivetrain is:
80% efficient
Required battery energy:
input energy = useful energy / efficiency
input energy = 500/0.80
input energy = 625 kJ
Energy dissipated:
625 − 500 = 125 kJ
Practical Problem: Delivery Truck
A 5000 kg truck climbs 50 m while traveling a route.
Increase in gravitational potential energy:
ΔEₚ = mgΔh
Using:
g = 9.8 N/kg
ΔEₚ = 5000 × 9.8 × 50
ΔEₚ = 2,450,000 J
ΔEₚ = 2.45 MJ
If the drivetrain is 70% efficient:
input energy = 2.45/0.70
input energy = 3.5 MJ
This ignores resistance, acceleration, and other energy requirements.
Practical Problem: Train
A train experiences:
20,000 N
of total resistance while traveling at:
30 m/s
Required mechanical power:
P = Fv
P = 20,000 × 30
P = 600,000 W
P = 600 kW
If the motor system is:
90% efficient
Electrical input power:
Pin = 600/0.90
Pin ≈ 667 kW
Practical Problem: Energy Used During a Journey
Suppose a vehicle motor operates at an average useful power of:
40 kW
for:
30 minutes
Convert time:
30 min = 1800 s
Energy transferred:
E = Pt
E = 40,000 × 1800
E = 72,000,000 J
E = 72 MJ
If the system is 80% efficient:
input energy = 72/0.80
input energy = 90 MJ
Therefore:
18 MJ
is transferred into less useful forms.
Safety and Energy
Transportation engineering must also consider the large quantities of kinetic energy carried by moving vehicles.
Recall:
Eₖ = ½mv²
Because speed is squared, increasing speed greatly increases kinetic energy.
This affects:
- braking distance
- collision energy
- structural design
- restraint systems
- crash protection
Energy management is therefore important not only for efficiency but also for safety.
Crash Engineering
During a collision, kinetic energy must be transferred.
Vehicle structures may be designed to deform in controlled ways.
Energy can be transferred into:
- deformation
- thermal energy
- sound
- movement of other objects
Crumple zones increase the distance and time over which a vehicle slows down.
This can reduce the average force experienced by occupants.
The energy still has to be transferred, but engineers control how the transfer occurs.
Common Mistakes
Saying Vehicles "Use Up" Energy
Energy is not destroyed.
It is transferred between stores and eventually becomes increasingly dispersed.
Confusing Energy and Power
Energy tells us how much.
Power tells us how quickly.
P = E/t
Assuming a More Powerful Vehicle Is More Efficient
High power does not necessarily mean high efficiency.
A machine can transfer energy quickly while wasting a large fraction of it.
Ignoring Air Resistance
At high speeds, aerodynamic drag can become one of the most important energy losses.
Ignoring Vehicle Mass
More massive vehicles require more energy to accelerate and climb.
Thinking Speed and Kinetic Energy Are Directly Proportional
Kinetic energy depends on:
v²
Doubling speed produces four times the kinetic energy.
Assuming Electric Vehicles Have No Energy Losses
Electric vehicles still experience:
- motor losses
- electrical resistance
- battery heating
- rolling resistance
- air resistance
- friction
Assuming Regenerative Braking Recovers All Energy
Regenerative braking is not 100% efficient.
Some energy is still dissipated.
Comparing Technologies Using Only Efficiency
Efficiency is important, but so are:
- energy density
- range
- mass
- infrastructure
- cost
- environmental impacts
Adding Stage Efficiencies
Overall efficiency is found by multiplying decimal efficiencies.
For example:
90% × 80%
means:
0.90 × 0.80 = 0.72
Overall efficiency:
72%
Check Your Understanding
- Describe the main energy transfers in a gasoline-powered vehicle.
- Describe the main energy transfers in a battery-electric vehicle.
- State the principle of conservation of energy.
- Give four examples of unwanted energy transfers in transportation.
- Define efficiency.
- A vehicle receives 4000 kJ and produces 3000 kJ of useful output. Calculate its efficiency.
- Calculate the dissipated energy in Question 6.
- Define power.
- A motor transfers 900,000 J in 30 s. Calculate its power.
- A car experiences 700 N of resistance while traveling at 20 m/s. Calculate the power required to overcome resistance.
- Explain why aerodynamic design is particularly important at high speeds.
- Explain how tyre deformation causes energy losses.
- Describe two ways engineers can reduce drivetrain friction.
- A 1200 kg car travels at 20 m/s. Calculate its kinetic energy.
- Calculate the kinetic energy if its speed increases to 40 m/s.
- By what factor did the kinetic energy increase?
- Explain why reducing vehicle mass can reduce energy consumption.
- A 2000 kg vehicle climbs 50 m. Calculate its increase in gravitational potential energy using g = 9.8 N/kg.
- Explain regenerative braking.
- A vehicle has 800 kJ of kinetic energy and regenerative braking recovers 600 kJ. Calculate the recovery efficiency.
- Explain why energy density is important in transportation.
- Why is energy density especially important for aircraft?
- Give two advantages and two challenges associated with battery-electric vehicles.
- Explain why trains can be energy-efficient transportation systems.
- A system has stages that are 90%, 80%, and 95% efficient. Calculate the overall efficiency.
- Explain why a more powerful motor is not necessarily a more efficient motor.
- Describe three engineering changes that could improve vehicle energy efficiency.
- Explain why transportation technologies should be evaluated using life-cycle analysis.
- Explain how energy concepts are important in crash safety.
- Evaluate the statement: "The most efficient transportation technology is automatically the best transportation technology."
Key Terms
Transportation system — System designed to move people or materials from one location to another.
Energy transfer — Movement of energy between stores or systems.
Drivetrain — Components that transfer mechanical power from an engine or motor to a vehicle's wheels or other propulsion system.
Efficiency — Fraction of input energy or power converted into useful output.
Power — Rate at which energy is transferred.
Mechanical power — Rate at which mechanical work is performed.
Drag — Resistive force acting on an object moving through a fluid.
Air resistance — Drag caused by movement through air.
Rolling resistance — Resistance associated with deformation and interaction between rolling objects and surfaces.
Streamlining — Shaping an object to reduce fluid resistance.
Regenerative braking — System that recovers part of a vehicle's kinetic energy during braking.
Energy density — Amount of energy stored per unit mass or volume.
Drivetrain loss — Energy transferred into less useful forms as energy passes through drivetrain components.
Life-cycle analysis — Evaluation of environmental and energy impacts throughout production, operation, and disposal.
Crumple zone — Vehicle structure designed to deform during a collision and control energy transfer.
Mechanical advantage — Ratio of output force to input force.
Energy dissipation — Transfer and spreading of energy into less useful stores.
Key Takeaways
- Transportation systems transfer stored energy into useful mechanical energy.
- Energy is conserved throughout every transportation system.
- Some input energy is inevitably transferred into less useful forms.
- Efficiency measures the fraction of input energy reaching the desired useful output.
- Power measures how quickly energy is transferred.
- P = E/t.
- For constant-velocity motion against a force, P = Fv.
- Vehicle mass affects the energy required for acceleration and climbing.
- Kinetic energy is calculated using Eₖ = ½mv².
- Because kinetic energy depends on speed squared, speed has a major effect on energy requirements.
- Gravitational potential energy changes according to ΔEₚ = mgΔh.
- Friction, rolling resistance, air resistance, heating, sound, and vibration can reduce transportation efficiency.
- Streamlining reduces aerodynamic drag.
- Lubrication and efficient bearings reduce mechanical losses.
- Regenerative braking can recover some energy that would otherwise be dissipated.
- Electric and combustion vehicles have different energy-transfer pathways and different engineering trade-offs.
- High energy density is particularly valuable when a vehicle must carry its energy source.
- Aircraft are especially sensitive to energy-storage mass.
- Rail systems can benefit from low rolling resistance and external electrical supplies.
- Engineers often reduce mass to decrease energy requirements, but safety and strength must still be maintained.
- Overall efficiency in a multi-stage system is calculated by multiplying the efficiencies of the individual stages.
- A high-power machine is not necessarily an efficient machine.
- The best transportation technology depends on the intended application.
- Energy efficiency must be considered alongside range, energy density, infrastructure, safety, cost, reliability, and environmental impact.
- Life-cycle analysis provides a more complete comparison of transportation technologies.
- Energy concepts are important not only for transportation efficiency but also for braking and collision safety.
The central engineering goal is not simply to provide more energy or more power. It is to transfer the available energy to the desired output as effectively as possible while controlling losses, mass, cost, safety, and environmental impacts.