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Dreel concept

The energy required for a vehicle is a sum of the Fahrwiderstand (Driving resistances), which have to be overcome:

  1. Air resistance or Aerodynamic drag
  2. Rolling resistance
  3. Hill climb resistance
  4. Acceleration resistance

Over about 50 km/h the Aerodynamic drag becomes dominant, because it increases with the square of the speed, the rolling resistance, however, only linear.
Hill climb and acceleration are temporary; their energy is also stored in the vehicle - potential energy or kinetic energy - and after this phase it can be used as a swing or converted back into electrical energy: Regenerative brake

Air resistance

The air resistance is calculated from the front surface - the area of the shadow - multiplied by the air resistance coefficient (cw). The drag coefficient depends on which turbulences the body causes - ideal is a Teardrop with a drag coefficient of 0.02, a penguin has 0.03, a ball 0.45, a rectangular plate 2.0.
The value of cars was steadily improved until the 1990s, after which not much optimization was possible anymore. At the same time the weight increased of each new model generation, while the front surface of about 2 m² remained fairly constant.

The trend towards Sport Utility Vehicle (SUV) brought a new twist. The front area (A) now rose, caused by uncertain socio-psychic motives - from Golf VI with about 2.2 m² front area to Range Rover with 3 m²; a seated person has about 0.6 m². The box shape also creates more turbulence, so that the drag coefficient increases again. The Golf got a cw * A of 0.66 with cw 0.3, the rover pushed through the landscape with 3.0 * cw 0.37 = cw * A 1.1, and even a current Smart got 0.85 cwA, an astonishingly high value.

Fossil fuels

In the case of combustion cars, fuel consumption has long been an important factor because - at least in the case of EU-typical taxes - it accounts for the greatest costs over the life of a car. Thanks to increasingly efficient engines (injection, compression, valve control, electronics), their efficiency - the ratio of chemical energy in the fuel to the drive energy generated - has been steadily increased, from initially ~10 percent to now up to 42 percent under ideal conditions. This development has been under pressure since ~2000 due to the tightening of exhaust gas regulations; see also Diesel Scandal.

In terms of technical design, however, increasing consumption can be easily compensated for - with larger tanks. Because the energy density of gasoline, diesel or natural gas is so high at 40..50 MJ / kg that the mass and volume of the tank are not critical.


The situation is different for electric vehicles (BEV) - the current batteries with the highest energy density, based on Lithium, the element with the greatest normalized ionization potentials in the periodic table, reach about 700 kJ or 200 Wh per kilogram - 1/60 of fossile fuel. This means that the weight and volume of the battery and thus the range become a critical design factor.

Another critical factor of the BEV economy is the limited number of cycles of the lithium cells, i.e. the number of discharge / charge processes, after which a significant decrease in capacity (typically to 80%) occurs. Cell manufacturers traditionally named '1,000', but the recording of the cycles for laptop batteries already showed the optimism of this number, in real terms often just half was reached.
The reason is that due to the high energy density, the chemical compounds are also broken down faster. In addition catastrophic failures of individual cells are unavoidable from time to time. The 'wear and tear' of a LiIon battery also increases if any one of the performance parameters is exhausted: capacity, performance, quick charge and also calendar time.
The limited battery 'cycle life' is therefore an essential cost factor of current BEV. This is further explained in the metacell project: Zahlenspiele.

Battery strategy


The US manufacturer Tesla answered these conditions with a simple idea: Let's put so many cells in one car that the battery is sufficient for the typical total mileage of the first owner. This resulted in 90 kWh, which at roughly 50 ct/Wh until about 2015 meant $45,000 for the cells alone, without the car around it. A Tesla-S thus inevitably moved into the premium segment and was purposefully marketed there. Face area and mass were high, but this corresponds with the segment. The 90 kWh allow at 20 kWh / 100 km (on US highways) about 450 km (300 miles), which leads to 225,000 km or 140,000 miles even if we only assume 500 cycles - within the typical range of fuel cars.
But at lower retail prices of a BEV, the batteries have to shrink - so does the overall lifespan.
BTW, what would be the sales price for a used middle-class BEV with 100,000 km on the odometer, whose 50 kWh battery will soon have to be replaced? Close to scrap value.


  • Whoever wants to save the climate should suffer.
  • Comfort, space, performance, design and other traditions are overturned, and instead everything is geared towards lowering air resistance, the essential energy sink for individual transport.
  • Two people can be transported - one behind the other, because length costs nothing when it comes to air resistance.
  • The frontal area becomes significantly smaller when you lie down, your head just high enough that you can see over your feet - see Bobsleigh. Target values: Front surface 0.8 m², cw 0.25 = cw * A 0.20.
  • Three wheels - two in front, one driven in the back - are ideal for aerodynamics (teardrop shape).
    A two-wheeler would be narrower, but has to build the center of gravity higher in order to be acceptable to corner - this increased front surface as well as aerodynamic turbulences.
  • Performance - a Dreel should be able to drive on the highway, because many destinations in Germany can only be reached this way reasonably. 90 km/h are common in the right-hand lane if you do not want to become an obstacle - even on slopes (maximum 8% on motorways).
    This Dreel's maximum speed is limited to 110 km/h, enough on the flat route to overtake trucks - power requirement increases with the cubic (^ 3) of the speed.
  • Engine power - for 80 km / h at cw * A 0.20 modest 2 kW are sufficient, at 90 it is 2.65 kW.
  • The weight is critical for hills - an additional 5.9 kW is then required, at 300 kg weight and 8% inclination.
  • 10 kW engine power should therefore be sufficient for above applications.
  • Lightweight construction is necessary because of the hills, but with the small space that has to be covered and correspondingly small levers and forces, it can also be easily implemented - target value without passengers: 150 kg
  • Range - 100 km is enough for commuters - at 90 km/h it only takes 2.65 kWh. 4 kWh battery capacity provide enough buffer and enable a smaller charging stroke, thus a higher cycle count.

Low battery capacity complements synergistically with low air resistance:

  • 4 kWh need - for example with Samsung 21700 cells - only 15 kg and ~10 liters, at a cost of € 800
  • Dreel can thus drive 200 km at 80 km / h, or further at lower speeds

This allows the switch to LiFePO4 technology which is hardly an option for larger BEVs. These battery technology has better safety because there is no danger of burning electrolyte, and at least twice the number of cycles compared to Li-Cobalt cells. This outweighs the approximately 30% higher price for the user.
The disadvantage of the low energy density are double weight and volume. An exclusion criterion for large cars, this is not a problem with the Dreel because of the low energy requirement in relation to weight and volume:

  • 4 kWh LiFePo with brand cells weigh 31 kg and have 22 liters, at a cost of around € 1,000
  • For € 1,600 at 36 kG and 26 l you can get LiFePO cells with a high power density of 20 C. Here you would have enough power for the mountain even with only 500 Wh capacity.
    Practical middle ground: 2.4 kWh LiFePO at 23 kg and € 1,000.


Dreel is not a startup :

  • For startups, the inevitable top-or-flop strategy has already brought many good ideas down
  • Large quantities are hardly realistic at Dreel because of the unusual layout - little space and comfort
  • Therefore, it is not worthwhile to manufacture, weld and paint a unibody in the sheet metal press - which offers important cost advantages at large quantities
  • Dreel therefore uses a ladder frame that acts as a seat (frame) and protection zone for the passengers; the slightly higher weight is acceptable given the small volume
  • The frame is screwed together from pipes, pipe connectors and sheets - no welding
  • These structural elements are generally made of common 6060 aluminum, widespread and inexpensive, which also makes the strength calculation easier
  • The fuselage is screwed on in frame-plank construction made of aluminum sheet or polymer, it only has to withstand the air pressure and weather
  • The 'Dreel' brand and the construction rights are rented to workshops for small series (franchise)
  • Because of the simple construction, they do not need any equipment or training beyond a car workshop
  • The workshops work according to specific orders (Build to order) from their local customers; they therefore only bear economic risk for their work and, if necessary, the purchase of parts for their local series.
  • Dreel is produced in small series of a maximum of 20 or 75 pieces each
  • The learning processes from the production and operation of a series flow into the next series
  • Workshops and other partners can develop their own components and manufacture (possibly larger) series that they offer to the Dreel network
  • These parts can be used as standard in the following series
  • The quality of all parts is tracked online via a transparent Quality Management (QM)
  • The QM also tracks the quality of the vehicles from the customer's perspective; with restricted access
  • Assemblies that cannot be manufactured from standard parts:
    • Front axle, engine, wheels, brakes
      For this, available series parts are used for the time being, possibly from other cars or ATVs with approval.
  • A StVZO - compliant approval is valid for small series of 20 pieces and costs around € 12,000
    • This includes approvals for a) EMV (electromagnetic compatibility), b) electrics and c) driving noises
    • These tests could be carried out in cooperation with universities as semester or bachelor theses,
      advantage: no costs - disadvantage: time frame
    • Each vehicle must also be individually approved (STVZO §21) (~ 200 €) before it receives a license plate for the specific holder
    • This registration is only possible in Germany, but the vehicles may drive in the EU
    • With such an approval, individual approvals in non-EU countries may be possible, tbd
  • EU small series: Here, certification as a "vehicle manufacturer" at the Kraftfahrtbundesamt is necessary, as is ISO-900x certification.
    • Costs are an additional ~ 8,000 €
    • This covers EU-approved series of 75 pieces
  • A EU model approval that allows unlimited production numbers without individual approval would be significantly more complex - hardly worthwhile, also considering the ongoing development, which would require new approvals for each change.
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