Energy Efficiency of Fossil-fuel and Electricity-powered Cars

Andrew Porter takes a closer look at how energy travels from the grid to the road, and argues that electric cars are not necessarily more energy efficient compared to fossil-fuel powered cars. Electricity generation needs to be re-examined to gain greater energy efficiencies.

For many years, the dominant power source applied within motorcars has been the Internal Combustion Engine. This has used either petrol or diesel for fuel. However, this presents multiple problems:

* Energy transfer efficiency is very poor. Thermal efficiency, the amount of energy from combustion converted to mechanical work, is only about 26%.' (a) in a typical engine. If you want to test this, remove the muffler and listen how much noise energy there is, then drain the cooling system and see how long it takes for the engine to seize through the heat generated.

* Toxic waste products are emitted. Even when a catalytic converter is fitted, this means more fuel is consumed. Also, the toxic emissions start to reduce only once the catalytic converter is warmed up.

* Burning oil-based fuels (hydrocarbons) means greenhouse gases are emitted.

* Fossil fuels are a finite source. They will eventually run out.

Assuming that the car will continue to be a popular mode of transportation, changes must take place in terms of how cars are powered to both remove the present dependency upon fossil fuels, the associated pollution problems, and the inevitable global warming impact.

Alternative Fuels
Much has been said about alternative fuels, with a strong suggestion that alcohol fuel derived from biological sources, such as food crops, is a possible solution. However, remembering that up to 74% of the fuel energy is wasted means this is still a very wasteful use of biological energy. It also means if we have as many cars as we do now, there will not be enough land for growing food.

Even the idea of using food crop waste products, presently destroyed as being unfit for human consumption, is unlikely to be a total replacement for petrol and diesel fuels.

It is an attractive option for countries like the UK, where very little investment is needed to introduce biofuels by adding to existing petrol and diesel fuels. This is because of the high level of vehicular fuel duty, which becomes an incentive not to bring dramatic change.

Electric Cars
The most significant element is the electric motor. At the size required by a car, about 100hp, a well designed electric motor can be over 90% efficient, way above an internal combustion engine.

In addition, when the car is stationary - for example at traffic lights – it is not consuming any energy, unlike an internal combustion engine, which is wasting 100% of its fuel energy. Similarly, an electric motor can use regenerative braking to recover energy when the driver brakes. In the case of a petrol engined car braking energy is simply lost as heat.

The limiting factors for an all-electric car have not been the electrical and electronic systems, or even the electric motors. Trains have used these for decades. Until now battery technology has been the problem. This lies in two elements; energy density, and the ability to recharge the battery fast enough.

Luckily Lithium Titanate Batteries have brought a breakthrough. The Lightning Car Company is now taking orders for an all-electric sports car. It offers 0 to 60mph (0 to 97kmh) in 4 seconds, 330 miles (531km) per battery charge, and a 70% recharge in 4 minutes. As to the battery life, expect 12 years or 100,000 miles, (161,000km) before the battery needs replacement. The motors are maintenance free, and ownership costs are very low. It sounds the perfect solution – but is it?

The idea of using electric motors that are 90% efficient may sound good, until the issue of generating electricity and distributing it is considered. In the UK, it’s estimated that by the time electricity arrives at your home, it is 33% efficient. In other words, 67% of the original fuel energy has been lost. So even if you assume battery charging is 90% efficient, and the electric motor is 90% efficient, the overall energy efficiency suddenly drops to 26.73%, only 0.73% better than internal combustion engines (see detailed explanation at end of article).

And as many countries generate most of their electricity from fossil fuels (39% gas, 33.3% from coal in the UK)(b), this means problems are not solved, just shifted. In addition, as most nuclear power stations in the UK are due to close within the next ten years(b), means that for the immediate future, there will be more demand to use fossil fuels, just as these reserves are reaching the point of being depleted, including the UK's North Sea stock of gas and oil. Even the new Combined-Cycle Gas Turbine Power Station can only approach an Energy Transfer Efficiency of 55%, and this is before electrical transmission losses are considered.

Non-fossil Way Forward
Internal combustion engines are inefficient in providing a sustainable, effective energy transfer system for road vehicles. Even if biofuels are used, these will fail to provide any significant efficiency improvement, and it is unlikely that the land area required will be sufficient just for the purposes of producing fuels.

Whilst the all-electric car is now a reality, this will only solve the original problems when electricity generation is on non-fossil fuels. Even then, transmission losses will be significant. So maybe we ought to consider other methods of localised electricity generation to reduce, or even eliminate the present inefficient system.


(a)'s Web Project Folder/EICE-Main.htm

(b) UK Parliamentary Office of Science and Technology, February 2007, Number 280, 'ELECTRCITY IN THE UK.'


The Lightning Car Company.


Additional Notes on Total Energy Transfer in Moving a Car

The best electricity generating stations are just over 50% efficient in terms of the energy transfer from fossil fuels to the electricity transmitted from the station using overhead cables. This is because some of the fossil fuel energy is converted to heat and sound, meaning that losses occur, as not all the heat can be converted to electric energy, and any sound energy is simply lost.  With an energy transfer efficiency of 50%, the electricity output is no greater than 50kW for every 100kW of fossil fuel energy supplied. The other 50kW of energy is wasted.

The electricity is then transmitted using overhead cables, often stretching hundreds of miles. As the cables are not perfect electrical conductors, heat is generated, and some sound, both of which are sources of energy lost. The voltage is also very high, therefore substations containing transformers are used to convert the high voltage into a lower voltage. Again, there is some energy loss as heat and sound. By the time that the electricity arrives at the home, there is only 33kW left, meaning that a total of 67kW has been lost as heat and sound, from the original 100kW provided by the fossil fuels.

If we now take this 33kW to charge a car battery, and the battery charging process is 90% efficient, this means that 90% of 33kW is used, with the result is 29.7kW. The rest is lost as heat and sound. However, when the battery is used to power the car, and the motors in the car are 90% efficient, this means that 90% 0f 29.7kW is used, that is 26.73kW, where once again the electric motors lose energy as heat and sound.

Having started with 100kW in the original fossil fuel at the electricity generating station, we end with 26.73kW in moving the car. Hence the total energy transfer efficiency is only 26.73%.  -Andrew Porter

Related Article - Car Pollution