Vehicle Energy and Efficiency Basics: Fuel, Hybrids, EVs and Hydrogen

Learning goals: follow energy from storage to the road

Every powertrain answers the same physical question: how can stored energy create enough force at the tyres to move a vehicle along a route? Petrol, diesel, LPG and CNG release chemical energy through combustion. A battery-electric vehicle stores electrical energy electrochemically. A fuel-cell car carries hydrogen and converts its chemical energy to electricity. Hybrids combine more than one energy path.

By the end of this lesson, you should be able to:

• distinguish energy from power and attach the correct units;
• read consumption and economy figures without reversing their meaning;
• identify rolling, aerodynamic, climbing, acceleration and auxiliary energy demands;
• draw the energy chain for combustion, hybrid, battery-electric and fuel-cell vehicles;
• separate vehicle-side, charger-side and upstream measurement boundaries;
• calculate energy cost, range and simple real-world consumption;
• explain why standard test results and actual journeys differ;
• compare different powertrains without equating a litre, kilogram and kilowatt-hour.

This is a conceptual and quantitative lesson. It does not declare one powertrain universally best or replace model-specific engineering data.

Energy and power are related, but they are not interchangeable

Energy is the capacity transferred or transformed while work is done. The SI unit is the joule (J). Vehicle electricity is often measured in kilowatt-hours (kWh), while fuel energy may be described in megajoules (MJ) or through a quantity of fuel. One kilowatt-hour equals 3.6 megajoules.

Power is the rate of energy transfer. One watt is one joule per second; one kilowatt is 1,000 watts. A 100 kW motor can transfer energy at a high rate, but the energy used depends on how long and at what actual power it operates.

A charger rated at 100 kW does not necessarily add 100 kWh in every hour. The vehicle, cable, battery temperature, state of charge and charging curve may lower power. Similarly, a 150 kW engine or motor is not consuming or delivering 150 kW continuously during gentle cruising.

Fuel quantity is not automatically energy quantity

Litres measure volume; kilograms measure mass; kilowatt-hours measure energy. A litre of petrol, a litre of LPG, a kilogram of CNG and a kilogram of hydrogen do not represent equal energy. Composition, density, temperature and whether higher or lower heating value is used also affect a technical conversion.

This explains why raw consumption numbers cannot rank powertrains. An LPG vehicle may use more litres per 100 km than it uses on petrol because the energy per litre differs. A CNG vehicle is commonly measured in kg/100 km, not LPG litres. An FCEV may use less than one kilogram of hydrogen per 100 km, but the small number does not mean that one kilogram equals one kilowatt-hour.

For household spending, a consistent comparison is simpler:

For engineering efficiency, convert each carrier using a documented energy value and the same measurement boundary. Do not choose a convenient heating value for one fuel and a different convention for another.

Fuel blends and specifications vary. Use current certified values for a formal energy analysis rather than memorising one approximate table.

Consumption and economy point in opposite numerical directions

Consumption states how much energy carrier is used for a fixed distance. Lower is better when the carrier and boundary are the same. Common forms are L/100 km, kWh/100 km and kg/100 km.

Economy states how far the vehicle travels per unit. Higher is better. Common forms are km/L and miles per gallon. Consumption and economy are reciprocals after units are made consistent:

US and imperial gallons are different volumes, so US mpg and UK mpg are different even for the same car. Approximate reciprocal conversions are:

Because the relationship is reciprocal, a 5 mpg improvement saves more fuel when moving from 15 to 20 mpg than from 45 to 50 mpg over the same distance. The conversion table makes that non-linearity visible.

Five road-load demands explain most journey energy

The powertrain must create tyre force to overcome resistance and change the vehicle’s motion or height. A useful learning model separates five demands:

On a level road at steady speed, acceleration and climbing terms are momentarily zero, but rolling, drag, auxiliary and conversion demands remain. In stop-start traffic, repeated acceleration and braking become important. In mountain travel, climbing can dominate a segment.

The route determines the mixture. This is why one combined consumption number cannot perfectly describe city, motorway, winter and towing use.

Rolling resistance links tyres, load and distance

A simplified rolling-resistance force is:

C_rr is a rolling-resistance coefficient, m is mass and g is gravitational acceleration. The energy needed over a level distance d is approximately force multiplied by distance.

This model shows why mass and distance matter. It does not mean every tyre has one constant coefficient. Pressure, temperature, construction, road texture, speed, alignment and deformation change actual losses.

Lower rolling resistance is only one tyre property. Braking, wet grip, load capacity, temperature, wear, noise and approved fitment matter. Do not alter pressure or specification solely to chase an efficiency number; follow the vehicle and tyre requirements for the actual load.

Aerodynamic power rises rapidly with air-relative speed

A simplified aerodynamic drag force is:

ρ is air density, C_d drag coefficient, A frontal area and v speed relative to the air. At unchanged conditions, drag force is approximately proportional to speed squared, while the power required is proportional to speed cubed.

If air-relative speed rises from 100 to 120 km/h, the simple drag-force ratio is 1.2² = 1.44 and drag-power ratio is 1.2³ = 1.728. That does not mean total vehicle consumption rises by exactly 72.8%, because rolling, accessories and powertrain efficiency also contribute. It shows why motorway speed can have a strong effect.

Wind changes air-relative speed. A 100 km/h road speed into a 20 km/h headwind exposes the vehicle to approximately 120 km/h air-relative speed. Roof boxes, open windows, bicycles and trailers can alter effective drag area.

Climbing and acceleration store mechanical energy

Increasing height adds gravitational potential energy:

A 1,600 kg vehicle gaining 250 m of elevation increases potential energy by approximately 3.924 MJ, or 1.09 kWh, before powertrain and rolling or aerodynamic losses. On descent, some of that energy can be dissipated in brakes or partly recovered by a hybrid or EV.

Increasing speed adds kinetic energy:

Because speed is squared, doubling speed gives four times the kinetic energy. That energy must come from the powertrain during acceleration and must be removed during braking or coasting. Smooth traffic flow can reduce repeated conversion, but safety and traffic rules always control driving.

Mechanical energy at the vehicle is not the same as energy purchased. Conversion and accessory losses mean more stored energy is required uphill or during acceleration than the ideal mechanical result.

Efficiency depends on where the boundary is drawn

Efficiency compares useful output with energy input across a defined system:

The result is meaningless without the boundary. Common boundaries include:

• tank-to-wheel: energy in onboard fuel to mechanical work at the wheels;
• battery-to-wheel: traction-battery energy to mechanical work;
• wall-to-wheel: electricity from the supply through charging and vehicle to motion;
• well-to-wheel: upstream production and delivery plus vehicle operation;
• lifecycle: may also include vehicle and infrastructure production, maintenance and end-of-life under a stated method.

A battery-to-wheel figure excludes charger losses. A tailpipe statement excludes upstream energy production. Neither is wrong if labelled, but comparing two unlike boundaries is wrong.

Cost boundaries differ too: dashboard kWh may explain range, while meter kWh determines the electricity bill. Choose the boundary that answers the question.

Combustion powertrains transform chemical energy through heat

Petrol, diesel, LPG and CNG engines release chemical energy through combustion. Expanding gases create mechanical work, which passes through the transmission and driveline to the wheels. A large share of input energy leaves as exhaust heat, cooling-system heat, friction, pumping and accessory demand.

Efficiency changes with engine speed, load, temperature and design. A cold engine and aftertreatment system behave differently from a fully warmed vehicle. Idling consumes fuel while covering no distance, so L/100 km can become extremely high over very short or stationary operation.

Diesel and petrol engines have different combustion and aftertreatment systems, but a fuel label alone does not determine vehicle efficiency. Vehicle size, gearing, aerodynamics, route and operating point matter. LPG and CNG may use dedicated or bi-fuel systems; conversion quality and calibration also affect results.

When comparing fuels technically, use the measured quantity and a consistent energy value. When comparing household cost, use actual consumption and the correct price unit for each fuel.

Hybrid systems shift energy through more than one path

A mild hybrid can assist the engine and recover some energy but generally cannot provide sustained electric-only driving. A full hybrid has a stronger motor and battery and can move electrically for limited periods without plugging in. A plug-in hybrid adds externally supplied electrical energy and a larger usable battery window.

Regenerative braking uses the motor as a generator during deceleration. Kinetic energy becomes electrical energy and is stored for later use. The chain contains conversion and storage losses, and recovery may be limited by battery state, temperature, traction and motor power. Regeneration cannot create energy or recover rolling and aerodynamic losses already dissipated.

Full hybrids can reduce idling and move the combustion engine toward more favourable operating points. Their benefit is route-dependent: frequent deceleration offers more recovery opportunity than steady travel. PHEV results depend heavily on how often the car is charged and how much distance is electric.

For a PHEV, record fuel and wall electricity together. Reporting only litres can make a frequently charged car appear to use almost no energy; reporting only electricity hides fuel after the battery window.

Battery-electric vehicles add charging and auxiliary boundaries

A BEV stores electrical energy in a traction battery. The inverter controls electrical power for the motor, and a reduction gear and driveline deliver torque to the wheels. Motor and inverter operation is efficient across many conditions, but losses remain in charging, battery, electronics, motor, gearing, tyres and auxiliaries.

Dashboard consumption commonly represents traction-battery energy under the manufacturer’s method. A home meter or public charger measures supply-side energy. The difference can include charger conversion, battery conditioning, balancing, standby and other loads. It varies with temperature, power, state of charge and system.

Do not call every difference a charger fault. First align time window, state-of-charge change, preconditioning and meter boundaries. For cost, multiply billed kWh by each tariff. For range, use available battery energy and route kWh/100 km.

The EV Charging Cost Calculator focuses on billed energy, tariffs and losses. This lesson explains why those inputs are separate.

Hydrogen fuel cells power an electric motor through onboard conversion

A fuel-cell electric vehicle stores hydrogen in high-pressure tanks. The fuel-cell system converts hydrogen’s chemical energy to electricity; a traction battery buffers power and captures some regenerative energy; an electric motor drives the wheels.

Vehicle consumption is commonly shown in kg/100 km or distance per kilogram. That mass measurement is appropriate for onboard fuel use. A well-to-wheel analysis additionally needs the energy and emissions associated with producing, conditioning, compressing, transporting and dispensing the actual hydrogen.

Fast refuelling and vehicle range are operating characteristics, not measures of whole-chain efficiency. Hydrogen source and station energy matter upstream; fuel-cell, battery and motor losses matter onboard.

Do not compare “1 kg/100 km” with “20 kWh/100 km” by the printed numbers. Convert to a shared energy boundary or compare price per 100 km and state that the latter is an economic, not physical-efficiency, comparison.

How to measure real consumption without mixing boundaries

For liquid fuel, a repeatable fill-to-fill method is:

1. Use a consistent fill endpoint at a suitable station and park condition.
2. Reset the trip record or note the odometer.
3. Drive a meaningful distance across the intended use.
4. Refill consistently and record litres and distance.
5. Calculate L/100 km and repeat across several tanks.

One tank can be distorted by pump cut-off, slope, short distance and unusual use. Never top beyond safe manufacturer or station instructions merely to make the fill “more exact.”

For CNG, hydrogen and billed electricity, use the seller’s mass or energy record over the matching distance. For an EV, label wall, charger or dashboard energy. For a PHEV, record both charged kWh and fuel plus electric or total kilometres. A single combined cost per 100 km may be useful, but it must not erase the underlying quantities.

From consumption to range and cost

Once quantity per 100 km is known, simple models follow:

For a tank, usable quantity should include a reserve rather than assuming the full nominal capacity. For an EV, use the planned battery window, not gross battery size. For a PHEV, calculate the electric segment and fuel segment separately.

Cost per 100 km is not total ownership cost. Purchase price, depreciation, finance, insurance, tax, maintenance and tyres can outweigh a small energy difference. The powertrain calculator compares recurring assumptions; the Car Cost Calculator adds the wider ownership boundary.

Worked measurement: petrol and EV over the same 720 km

A petrol car travels 720 km and takes 49.0 L to return to the same fill condition. Consumption is:

At $1.72/L, fuel costs $84.28, or about $11.71/100 km.

An EV travels the same 720 km and the matching supply meters record 151.2 kWh. Supply-side consumption is:

Suppose 70% of energy is charged at home at $0.30/kWh and 30% publicly at $0.58/kWh. Cost is $31.75 + $26.31 = $58.06, or about $8.06/100 km.

The EV’s raw 21.0 is not “worse” than 6.806 because the units differ. Cost can be compared after prices are applied. Physical efficiency would require energy conversion and aligned boundaries. Ownership cost would require many more lines.

The prices are illustrative, not current market data or a claim about specific models.

For engineering, transport and automotive students: practise the energy chain

Solve each exercise with units before opening the answer. The purpose is to identify the boundary and equation, not merely produce a number.

Why test-cycle and real-world values differ

A standard laboratory or controlled road procedure sets test conditions, speed pattern, temperature treatment, vehicle configuration and measurement rules so results can be compared. It does not reproduce every owner’s route. Its value is consistency under the specified protocol.

Real consumption changes with:

• trip length and frequency of cold starts;
• city, rural and motorway speed pattern;
• temperature, wind, rain, snow and air density;
• elevation and direction of travel;
• tyre type, pressure, alignment and wheel size;
• payload, trailer, roof box and external accessories;
• heating, cooling, battery conditioning and electrical loads;
• traffic, road surface and driving decisions;
• PHEV charging share and EV charging boundary.

Compare a measured result with a test value only after describing those differences. A single high tank or low winter range does not isolate a fault; a stable multi-period deviation under comparable conditions is more informative.

Common errors in vehicle-energy calculations

• Using kW as though it were battery capacity in kWh.
• Calling L/100 km and mpg a linear conversion.
• Comparing US mpg directly with UK mpg.
• Ranking litres, kilograms and kWh by raw numerical size.
• Using a dashboard EV figure for cost while billing uses wall energy.
• Reporting only fuel for a PHEV and omitting charged electricity.
• Assuming regenerative braking returns all kinetic energy.
• Treating the full tank or gross battery capacity as usable range.
• Applying an aerodynamic cube relationship to total consumption without separating other loads.
• Calling tailpipe, tank-to-wheel and lifecycle results interchangeable.
• Using one unusual trip as a permanent efficiency value.
• Comparing vehicles of different size, load and route as if powertrain were the only variable.

A correct unit with the wrong boundary can still produce a misleading answer. Write both beside every result.

Learning path for vehicle energy and efficiency

1. Understand quantities: use this Academy lesson for energy, power, consumption, road load and conversion boundaries.
2. Measure liquid fuel: enter fill-to-fill distance and litres in the Fuel Consumption Calculator.
3. Convert economy units: use the table when mpg or L/100 km is the only difference.
4. Model one journey: use the Trip Cost Calculator with route-specific consumption and price.
5. Model EV range: connect usable battery, consumption and reserve in the EV Range Calculator.
6. Compare powertrains: keep LPG, CNG, PHEV, BEV and hydrogen units separate in the Powertrain Cost Comparison.
7. Make a vehicle decision: use How to Choose a Car Powertrain for infrastructure, duty and ownership context.

Each resource serves a different purpose: the Academy teaches mechanisms, calculators process scenarios, the table converts units and the guide structures a choice.

Limits of this Academy lesson

The physics equations are simplified learning models. Real tyres, airflow, powertrains, batteries, fuels, roads and control systems vary with operating conditions. Certified engineering, safety, emissions and homologation work requires the relevant detailed standards and measured data.

Illustrative energy and price calculations are not guarantees of range, consumption, cost or environmental performance. Fuel composition, tariffs, charging losses, hydrogen source, weather and vehicle configuration differ. Use current vehicle documentation and appropriate qualified analysis for a material decision.

This Academy page is for education. It does not recommend a fuel, vehicle, conversion, driving speed or modification.