Use of electric car
Efficiency: Electric cars are far more efficient
01.07.2024
The efficiency of an electric drive is so high that it is predestined for road traffic alone. We explain how electric cars, combustion engines, and fuel cell vehicles compare. And where the differences derive from.
From the point of view of energy, an electric motor is the most efficient drive technology a car can have. No other drive technology offers a higher degree of efficiency. The term “efficiency” indicates the percentage of the energy supplied that can actually be put to use. This is never 100%, because there are always losses – due to friction and waste heat. Nevertheless, an electric motor on its own achieves an outstanding 80% efficiency.
In order to calculate this correctly, it is important not only to consider energy losses in the vehicle itself – This is because when it comes to electric cars in particular, the major losses occur during the generation and provision of energy, not in the vehicle. The term “well-to-wheel” has become established for this overall consideration – clearly a term which originates from the internal combustion engine.
Applied to electric cars it would be more correct to say “electricity generation to wheel”.
The balance sheet Well-to-wheel sees for electric vehicles as follows:
36% of the energy originally used are lost for the generation and provision of the electricity, and losses also occur when charging the battery.
64% of the energy used power the wheels of the electric car.
How efficient is a combustion engine?
The efficiency of a petrol engine is 20%, while the efficiency of a diesel engine is 45%. In the case of an internal combustion engine, the situation is exactly the opposite of that of an electric motor: the major energy losses occur in the vehicle, not during fuel production. In an internal combustion engine, a chemical reaction takes place that uses oxygen from the air which involves the fuel being combusted. Combustion generates a lot of waste heat. In addition, valves and gearwheels have to be moved before the useful energy reaches the wheels.
The balance sheet Well-to-wheel sees for petrol engines as follows:
80% of the energy originally used is lost in a petrol engine during conveying, generation, transport and combustion.
20% of the energy used drive the wheels of a petrol engine.
Diesel perform better because they are burned more efficiently. Yours sincerely Well-to-wheel-Balance sheet is:
55% of the energy originally used is lost with diesel.
45% of the energy used drive the wheels of the diesel.
Why is an electric drive so efficient?
Electric motors convert electricity into motion, producing very little waste heat. By contrast, petrol and diesel engines burn fuel, thereby generating a lot of waste heat. Electric drives are extremely efficient by design and because of the existing technical solutions. There are electric motors that even achieve efficiencies of more than 90%!
And electric cars offer additional advantages as compared to cars powered by a combustion engine. For example, electric motors have a less complex “transmission chain” for bringing useful energy to the wheels. This reduces friction losses. This is not currently being done in any production electric car, but because they are smaller than combustion engines, electric motors can be installed close to the wheels. Differential gears and drive shafts are still installed. However, a gearbox is usually missing.
Electric motors generate a constant torque from a standstill: They put their horsepower on the road immediately. By contrast, a combustion engine can only develop sufficient tractive force to power the car from 800 to 1000 revolutions per minute. And a gearbox is required to ensure that rotational speed and output are always perfectly matched. The transmission causes friction losses, too.
Electric drives can also generate electric power.
But electric motors can not only convert electrical energy into kinetic energy, they can generate electrical energy from motion, too. In this case, the motor becomes a generator – a power plant. In the case of electric cars, manufacturers take advantage of this when braking: instead of simply allowing the waste heat generated to dissipate, the electric motor recovers electrical energy, which is stored in the drive battery. This increases the range – and therefore the degree of efficiency, too.
One problem that arises with electric drives due to their high level of efficiency is heating the interior in winter. This problem can be alleviated by an in-vehicle heat pump, which – as in the case of a building – extracts heat from the surroundings and then directs it to the occupants. This takes the strain off the traction battery, whose electricity would otherwise have to be used for heating, too. Nevertheless, the heat pump also needs electricity from the battery – just a little less than if you would convert the electricity directly into heat. In summer, the energy requirements for the air conditioning unit of electric vehicles and combustion engines are roughly the same.
What is the efficiency of a fuel cell vehicle?
The efficiency of a fuel cell drive is between that of a petrol engine and a diesel engine, i.e. far less than that of an electric car. Although a fuel cell vehicle has an electric motor and a (relatively small) traction battery, it ultimately converts electrical energy into motion just like an electric car. Fuel cell vehicles have the advantage of a very long range. But in order to achieve a greater range than an electric car, a fuel cell and a hydrogen tank are on board as well. To charge the battery for the electric drive, the fuel cell uses oxygen from the air and hydrogen to generate electricity.
Viewed in isolation, the fuel cell achieves an overall efficiency of more than 80%. But in terms of well-to-wheel efficiency, hydrogen spoils this high figure. In nature, hydrogen doesn’t occur in isolation but has to be produced by way of an energy-intensive process. This is achieved sustainably by means of electrolysers that use renewable electricity to break down water into oxygen and hydrogen. The next step is to store the hydrogen gas at high pressure and low temperatures. Otherwise, the footprint of this type of propulsion would be even worse. The pressure is up to 700 bar. By comparison, a car tyre has a pressure of between two and three bar. The temperature is around –250 degrees Celsius. All of this requires a lot of energy.
As the battery is very small compared to an electric car, recycling is much less time-consuming. In addition, surplus electricity (e.g. generated by the PV systems in the summer) can be converted into hydrogen and stored.
The balance sheet Well-to-wheel sees for fuel cell vehicles as follows:
70% of the energy originally used is lost when hydrogen is generated and stored and when electricity is generated in the vehicle.
30% of the energy used drives the wheels of a fuel cell vehicle.
This also explains why fuel cells are considered a niche technology for road traffic: This is because there is a much less complex and at the same time more energy-efficient solution for many applications: CO2-neutral: the electric car.