How does an engine work 5

Performance, energy and consumption

The chapter "Electromobility Basics" gives an overview of the units relevant to the electric car. The power of the engine is given in kilowatts (kW). The storage capacity of the battery and the consumption usually have the unit kilowatt hour (kWh). When charging, a distinction is made between alternating current (AC) and direct current (DC). The power of a charging station is also given in kilowatts.

An electric motor has no displacement. There are no cylinders, neither oil pressure nor an injection ratio. The bottom line is that an electric car is much easier to understand, even if the key figures and units are still unfamiliar.


"My new 3-series BMW has six cylinders and 200 hp!" When making this statement, most of them have a picture in their head of what kind of performance the car is doing. Although very few people say "My new Nissan Leaf has 110 kW and a 40 kWh battery!" can start something.

The BMW sounds somehow more potent, and yet it will probably pull the short straw against the Nissan at the traffic light start. Because with electric drives, the numbers take on a new meaning.

A little thin at the bottom

In the case of an internal combustion engine, the power is given in HP or in kW. In the prospectus there is always something like “at 3,000 rpm” next to it. Because the combustion engine - regardless of whether it is gasoline, diesel or gas - has a problem. It needs a certain speed in order to develop its power. If it turns too slowly, it hardly develops any strength. Therefore, he needs a manual or automatic transmission. If you do not pay attention when starting, you will stall the engine. The power developed by the engine is not enough to move the vehicle. Instead of rolling off, the engine stops and goes out.

While the diesel develops its power at relatively low speeds, gasoline engines usually need a higher speed. The electric motor doesn't really care. Because it can develop its full power over almost the entire speed range and even from a standstill. That is why electric cars have neither a clutch nor a manual or automatic transmission.

And that's why the supposedly weaker Nissan leaves the BMW at the traffic lights. Because while the engine of the BMW first comes up to speed and has to bring the power to the road via a clutch or an automatic transmission, the electric motor drives the axle directly with all its power.

Why are there no two-stroke screwdrivers?

The force that a motor can exert on the axle is called torque. The torque is given in Newton meters (Nm). And that is the key value here. This is because an internal combustion engine achieves its full torque in a relatively small speed window. A lot of research and effort has gone into making this window as large as possible for the internal combustion engine. The electric motor has full torque from zero revolutions per minute over almost its entire speed range - simply because it can. This is why you can use a good cordless screwdriver to turn stiff screws in and out very slowly.

If the screwdriver had an internal combustion engine, it would first have to be brought up to speed in order to then get back to the required speed with a clutch and a gearbox. That is why there are cordless screwdrivers and no two-stroke screwdrivers.


It takes energy for something to turn. This energy can come from a horse pulling a cart. We can transfer the energy to the wheel of a bicycle through pedals and a chain. Horses and humans alike get their energy from their food. In a very crude way, one can say that the body burns sugar and oxygen in the muscles to form carbon dioxide and energy. The efficiency in humans is around 25 percent. This means that 25 percent of the energy used - in the form of sugar, for example - is actually converted into movement. The rest is lost as heat. If we make an effort, our cooler starts up and we start to sweat.

If the body is no longer supplied with energy through food and all reserves have been burned, the human machine comes to a standstill. Until the respiratory muscles and the heart muscle run out of fuel.

Humans have learned to use other energy than muscle power. For millennia, wind and water have been driving mills and pumps as natural sources of energy.

Fire to movement

With the invention of the steam engine at the beginning of the 18th century, humans succeeded in converting the energy of a fire into movement. This invention changed everything and was the start of the industrial revolution. In the steam engine, a wood or coal fire heats a kettle. The water vapor of the boiling water expands, so the steam can set a piston in motion. Valves in the piston create a back and forth movement. A planetary gear then turns this movement into a rotary movement.

The efficiency of a steam engine is a disaster. Watt’s steam engine only achieved three percent. 97 percent of the energy used in the form of wood or coal is "lost" as heat.

Energy remains energy

Why is "lost" in quotation marks? Because when you look closely, energy is not lost. Energy is only converted. Humans convert the chemical energy stored in sugar into kinetic energy and thermal energy. The chemical energy stored in wood and coal turns the steam engine into a little movement and a lot of heat. But since the steam engine is about movement, the thermal energy is largely useless and so it is said colloquially that the energy is "lost".

Otto and Diesel

At the end of the 19th century, Nicolaus August Otto came up with the idea of ​​using the power of fire directly to create movement. So without going through the steam. Instead of slow combustion, it needs an explosion. The fuel only has to expand suddenly enough in a chamber to set a piston in motion. Wood and coal were out of the question. However, if you mix luminous gas with air and ignite it with a spark, it burns suddenly in an explosion. Alcohol, kerosene, and gasoline also burn this way.

Rudolf Diesel discovered that certain fuels do not need an ignition spark to explode. All that was needed was a glow plug and sufficient compression of the fuel-air mixture.

Rolling heaters

These internal combustion engines were not only more powerful than a steam engine, they could also be made much smaller, were more reliable and much more flexible and easier to operate. There were already steam-powered vehicles on the streets. But getting in and driving off was not possible. After all, steam first had to be generated in the boiler. So you had to make a fire under the kettle hours before you left. Then in 1886 Gottlieb Daimler had the brilliant idea of ​​installing an explosion engine in a road vehicle - the birth of the car.

The explosion engine was also much more efficient than the steam engine. But still over 90 percent of the energy was lost as heat. In addition, the explosion engine initially had a power problem. Because the early engines could not generate nearly as much power - that is, power - as a steam engine. For this reason, ocean liners and locomotives continued to run with the power of steam for the time being.

In the meantime, more than 130 years of research and development have gone into the combustion engine. As a result, the combustion has become cleaner and more effective. One liter of petrol, diesel or gas gives you a lot more exercise today than you did then. Even so, combustion engines still primarily produce heat. On the test bench, some engines achieve an efficiency of 40 percent - but in everyday life it is only 20 percent on average. Cars with internal combustion engines are therefore primarily rolling heaters.

Movement from electricity

At the beginning of the 19th century, the Danish physicist Hans Christian Ørsted discovered the magnetic effect of electric current. When a current flows through a conductor, a magnetic field is created. This magnetic field attracts or repels other magnetic fields or magnetic materials such as iron. It still took a bit of experimentation and research, but just a few years later in 1832 the first electric motor was driving a vehicle. Then it happened in quick succession. Inventors and inventors constantly improved the electric motor and used new functional principles. In 1888, just two years after the Benz patent motor car number 1, the Coburg machine factory A. Flocken built the first well-known German electric car.

But the electric drive could not prevail on the road against the internal combustion engine. It is true that the electric car has repeatedly tried to achieve a breakthrough over the past 130 years, but ultimately the batteries were not powerful enough and gasoline and diesel were much more easily available.

But what is the advantage of the electric motor over the combustion engine? The answer is that it is superior to the internal combustion engine in many ways. As we have already learned above, the electric motor can develop its power much better and more easily. It is comparatively compact and easy to build. Instead of hundreds of moving parts, there is actually only one moving part in the engine. But it shows its greatest advantage in its efficiency. Because unlike the steam engine or the combustion engine, the electric motor has an efficiency of over 90 percent. The motor converts the energy used almost completely into motion, and only little waste heat is generated.

And the problem of energy storage is also being solved better and better. There are now electric cars like the Tesla Model S 100D that can travel 450 kilometers in everyday life and, thanks to its own fast charging network ‘, can recharge enough electricity for 270 kilometers in 30 minutes. Even smaller cars like the Renault ZOE can cover 300 kilometers in everyday life. The cars only need a fraction of the energy that a car with a combustion engine needs.

How does that look in numbers?

Petrol has a calorific value of around 8.5 kWh per liter. In the case of diesel, the calorific value is around 9.8 kWh per liter. A Golf needs 7.3 liters of petrol or 5.6 liters of diesel per 100 kilometers. An eGolf with a comparable performance needs 16.6 kilowatt hours for 100 kilometers (source: Spritmonitor).

  • Energy consumption Golf Diesel for 100 km: 5.6 * 9.8 kWh = 54.88 kWh
  • Energy consumption Golf petrol engine for 100 km: 7.3 * 8.5 = 62.05 kWh
  • Energy consumption Golf Elektro for 100 km: 16.6 kWh

The electric drive is therefore much more energy efficient than the combustion engine.

And what about hydrogen?

One often hears that the fuel cell, which turns hydrogen and oxygen into water and electricity, can be an alternative to the battery electric car. But does it really make sense to install a fuel cell in a car when battery technology now enables more and more ranges that are suitable for everyday use? To do this, we want to take a look at the energy balance of the fuel cell.

A fuel cell in a car has an efficiency of 60 percent. So only 60 percent of the energy stored in hydrogen is actually converted into electricity. The rest is warmth here too.

In practice, the Toyota Mirai needs around one kilogram of hydrogen for 100 kilometers. The hydrogen has a calorific value of 33.33 kWh.

All figures only consider consumption from the tank or battery. To produce gasoline or diesel, additional energy is required. Exact numbers of how much energy fossil fuels need from the borehole to the tank are hard to find. The ADAC gives a CO2 equivalent of 425 grams per liter for gasoline and 525 grams per liter for diesel for one liter of gasoline.

Bio and e-fuels

Petrol and diesel can also be obtained as so-called bio or e-fuels. In the case of bio-fuels, gasoline or diesel is now usually obtained from plants with a high starch content, such as corn. On the one hand, fuel production is of course in competition with food production. On the other hand, intensive agriculture releases CO2 and nitrogen oxides and consumes valuable soil.

Attempts to obtain fuel with the help of bacteria are still at the experimental stage today.

In the case of electric fuels or eFuels, the fuel is made from electricity, water and carbon dioxide from the air. Using electrolysis, water is split into its elementary components, hydrogen and oxygen. The hydrogen is then reacted with carbon dioxide from the air to produce gasoline. The process is also called Power to Liquid (P2G, PtL).

This process is very energy-intensive. At the moment, the production of liquid fuels for cars is neither economically nor energetically meaningful. In Germany we couldn't produce enough electricity to meet the demand for liquid fuels. So we would be dependent on imports again here.

Where does the electricity come from?

Of course, if we switch our mobility from fossil fuels to electricity, we shouldn't just shift the exhaust from the car to the power plant. So it is important where the electricity comes from.

In Germany, as in many other countries, electricity comes from different sources. In Germany, the most important sources are fossil fuels such as hard coal and lignite or natural gas, nuclear power and renewable energies.

In Baden-Württemberg, the electricity mix consists primarily of hard coal, nuclear power and renewable energies. It can be observed in both Baden-Württemberg and Germany that the share of renewable energies in the electricity mix is ​​increasing.

More and more electricity is green

In 2007 only 14 percent of the electricity in Germany came from renewable sources such as solar, wind and water power, in 2015 it was 30 percent. Above all, the share of hard coal and nuclear power in the electricity mix has decreased.

In Baden-Württemberg, for historical reasons, the share of renewable energies is somewhat lower. Whereas in 2007 the south-west was still at the same level as the federal government at 14 percent, the proportion rose to just under 24 percent in 2015. This is also due to the fact that Baden-Württemberg only started developing wind power very late.

Make your own energy transition

Through the liberalization of the electricity market, everyone can make their own contribution to the energy transition. Everyone can freely choose their electricity provider. There is an almost unmanageable range of different tariffs. This also includes many green electricity tariffs. Here, however, not all green electricity is the same as green electricity.

Real green electricity is only available from providers who, on the one hand, generate renewable electricity themselves and, on the other hand, actively expand renewable energies. The most famous representatives are the Energiewerke Schönau (EWS), Lichtblick, Naturstrom, Greenpeace Energy and Bürgerwerke. Regardless of whether you have an electric car or not - if you want to drive the energy transition forward, you should switch to one of these providers.

Of course, the energy transition is even more direct from your own roof with your own solar system. Those who do not have their own roof can participate in wind turbines or solar parks through a citizens' cooperative. This is how you invest sustainably in the energy transition and benefit from profit sharing.