Tank-to-wheel

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Tank-to-Wheel (also: Tank to Wheel , Tank2Wheel or TTW , literally: "from the fuel tank to the wheel", more factually correct: "from the fuel pump / charging point to the wheel") considers the chain of energy absorbed (fuel, electrical Energy) up to conversion into kinetic energy in motor vehicles . The last (mostly calibrated) measuring device before the final energy is transferred to the vehicle is usually installed in the fuel pump / charging column. At TTW, only that part of the entire energy chain ( well-to-wheel ) is considered that directly affects the technology in the motor vehicle, since the provision of drive energy ( well-to-tank ) is excluded.

General

The tank-to-wheel approach corresponds to the manufacturer's information on the respective vehicles. When comparing motor vehicles with different drive systems, care must be taken to ensure that the same functional chains are always compared. Either the energetic chain in the “technical system of the motor vehicle” itself (i.e. tank-to-wheel) to make the vehicle technology comparable. Or as a more comprehensive way of considering well-to-wheel or the life cycle analysis , which can include indirect losses and expenses in addition to direct effort. Manufacturers and vehicle technicians usually compare tank-to-wheel, as they can only influence other factors outside of the vehicle to a limited extent. For comparison purposes, the following are shown in the manufacturer's documents for the motor vehicle:

Only the tank-to-wheel chain can be influenced by the vehicle manufacturer through design measures ( air resistance , weight reduction, better engine technology, recuperation ). Attempts are also made to improve overall tank-to-wheel efficiency by using low-friction tires that reduce rolling resistance.

The tank-to-wheel approach is the basis for the manufacturer's information such as fuel consumption (fuel costs) and emissions ( vehicle tax ). Together with the maintenance and upkeep costs (insurance), assuming a certain mileage or a certain period of time, the basis for comparing the operating costs of different vehicles and types of drive .

The cost of providing the drive energy ( well-to-tank ) is not included in the tank-to-wheel analysis . If this effort is included, one receives a comprehensive consideration ( well-to-wheel ) of the functional chains of different drive systems, mostly for comparisons with environmental influences and resource consumption during the operation of motor vehicles.

A cross-drive comparison is made more difficult primarily by the different information provided by the manufacturers. Therefore it makes sense to refer to a uniform SI unit . Based on the billing unit of the energy supplier, kilowatt hours (kWh) are usually used. But kilojoules (kJ) are also possible. The following applies:

The usual liquid fuels gasoline or diesel (consumption indication liters / km 100 and l / 100 km) can be as well as hydrogen or natural gas (kg / 100 km or l / 100 km or m 3 /100 km) according to their energy content in kWh / 100 represent km.

When comparing fossil consumption of primary energy is the basis of the CO 2 emissions (gCO 2 /100 km) for alternative drive energies often a petrol equivalent is calculated.

Recuperation

The possibility of regenerative braking improves the efficiency of the motor vehicle and thus the “tank-to-wheel” efficiency during the driving cycle and in practical driving. Part of the braking energy that is otherwise lost as thermal energy can be recovered for vehicle operation. Since a purely mechanical recuperation cannot be implemented economically, the kinetic energy of the vehicle is converted into electrical energy and stored. The recuperation brake is always combined with a mechanical brake . Therefore, in addition to the driving profile and vehicle weight, the respective driving style largely determines the use of the brakes and thus the influence of recuperation. While the use of mechanical brakes results in only minor losses in light vehicles and a forward-looking driving style , which improves "tank-to-wheel" efficiency, the influence is greater when the vehicle is heavy and the driving style is not adapted (higher proportion of mechanical brakes are used) the recuperation lower. Particularly in the case of vehicles with an electric drive, this explains the differences between the manufacturer's standard specifications and the various test consumption levels.

In vehicles powered by internal combustion engines , the possibility of recuperation is limited, since the conversion mostly takes place through the modified starter generator and storage in the starter battery and is therefore limited. The recovered energy is used for electrical consumers, not for driving.

In vehicles with (partially) electric drive, the engine (generator) design, engine control and type of traction battery determine the possible amount of recuperation energy.

Tank-to-wheel efficiency in vehicles with internal combustion engines

The efficiency is largely determined by the efficiency of the drive engine, the route profile and the driving style and is generally higher in diesel vehicles than in vehicles with gasoline engines .

Real internal combustion engines are far from the efficiency of an ideal thermodynamic machine, the so-called Carnot efficiency .

Since internal combustion engines only have their maximum efficiency in a small area, the overall efficiency of the engines is well below the maximum values ​​in practical operation in motor vehicles, despite multi-stage gearboxes for speed and torque adjustment. Complex gears with multiple shafts and gear pairings as well as losses in transfer gears and drive shafts also reduce the tank-to-wheel efficiency of the overall system. Wheel hub motors have not caught on due to their complexity and design problems.

Tank-to-wheel efficiency in vehicles with electric drive

The electric motor has a high degree of efficiency in a wide speed range. Therefore, the reduction to the wheel is usually only done by single-stage reduction gear with high efficiency. In the case of electric wheel hub motors, there are even no further mechanical losses caused by gear, drive and distribution shafts. Since more powerful electric motors only increase slightly in mass and volume, but enable a more efficient regenerative brake , electric motors in motor vehicles are often overdimensioned. The maximum power can then only be delivered for a limited time.

The charging losses of traction batteries are around 15% and vary considerably depending on the charging strategy and the design of the battery system. Just like losses in the inverter, they reduce the efficiency of the drive system. In some cases, the components also have to be cooled. Overall, only 80–90% of the electricity used for charging is available to drive the traction motors.

In serial hybrid electric vehicles , for example fuel cell vehicles , additional losses occur when storing and / or converting the second energy carrier, for example the electrical energy from the fuel cell. Since the second energy converter (the combustion engine with generator or the fuel cell ) is usually operated at the best operating point for reasons of efficiency and durability, the electrical energy must be temporarily stored and released in a controlled manner for the necessary power control during driving. This causes additional losses.

Web links

  • Optiresource - program for varying energy sources, fuels and driveconceptsand comparing fuel consumption and CO 2 emissions

Individual evidence

  1. ↑ Joint Research Center - Institute for Energy and Transport (IET), July 2011: Well-to-Wheels Analyzes of future automotive fuels and powertrains in the european context (PDF; 1.6 MB), p. 13 "Scope and methodology" , inserted April 18, 2011
  2. ↑ AM Foley, B. Smyth, B. Gallachoir, 2011: A Well-to-Wheel Analysis of electric Vehicles and greenhouse Gas savings (PDF; 75 kB), inserted April 18, 2012
  3. ↑ Verkehrsrundschau: Tank-to-wheel ( Memento of the original dated August 7, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed August 30, 2012 @1@ 2Template: Webachiv / IABot / www.verkehrsrundschau.de
  4. ^ Agora Verkehrswende (2019): Carbon footprint of electric cars. Influencing factors and potential for improvement. (PDF), accessed July 20, 2019
  5. ↑ ADAC, October 12, 2018: Electric cars in the test: This is how high the power consumption is , accessed July 20, 2019
  6. ↑ WELT.de, November 7, 2018: Charging losses in e-cars , accessed July 20, 2019