Well-to-Wheel Efficiency and CO2 Emissions of Cars with different Drivetrain Technologies

Mileage and emissions depend on the Well-to-Wheel Efficiency (WTW) and Weight and Size of the car.

Well-to-Wheel Efficiency is based on Well-to-Tank Efficiency (WTT) - the ‘Production and Transportation of Energy to the Charging Station’ and the Tank-to-Wheel Efficiency (TTW) – the ‘Use of the Energy within the Car’. The car’s drivetrain transfers the energy into the mechanical power for driving. The amount of power needed is determined by the weight (tire rolling resistance) and frontal area (width, height, drag coefficient) of the car.  

The CO2 Emissions depend on the amount and type of fuel burned, and NOx and Soot on the quality of the combustion. A catalytic converter can reduce the emissions.


  1. MIT - Units & Conversion Fact Sheet, Derek Supple, MIT Energy Club  http://web.mit.edu/mit_energy
  2. Wikipedia   Well-to-Tank Wirkungsgrade (German),  Including: Frischknecht /Tuchschmid für esu-services, 18. Dezember 2008:    Primärenergiefaktoren von Energiesystemen
  3. Values (km/MJ) converted from MPG (Reference 6), Gasoline 121.3 MJ/gal, Diesel 135.5 MJ/gal, H2 120 MJ/kg
  4. Product of Well-to-Tank and Tank-to-Wheel. Published data for WTT efficiencies can vary significantly, based on the inclusion of all activities required and standing of the technology at the time of data collection.
  5. Manufacturers, Wikipedia.
  6. US Department of Energy, www.fueleconomy.gov  Hyunday iX35: 0.95 – 1.4 kg H2/100km. (Wirtschaftswoche 37, 2017), Applied 1.1 kg.  ( ) in L/100km
  7. CO2 Content (g/MJ) divided by Well-to-Wheel (km/MJ) consumption.
  8. Current production utilizes: Coal 33%, Natural Gas 33%, Nuclear 20%, Hydro 6%, and Wind 5% (97% of Total) with a CO2 content of 168 g/MJ.

When using current sources of electricity production (Reference 8), the CO2 content increases to 168 g/MJ and the CO2 emissions to 223 g/km (Nissan) and 198 g/km (Tesla) – higher than conventional cars.

An 'electricity mix' of 50% from powerplants (natural gas) and 50% renewable energies (wind) is required to reach the CO2 emissions of the Hydraulic Hybrid, and a 40% / 60% distribution when operating the Hydraulic Hybrid with natural gas (ca. 25 g/km CO2).

The following graph shows the total ‘Well-to-Wheel’ energy in MJ/km as sum of ‘Well-to-Tank’ and ‘Tank-to-Wheel’ consumption. It reflects the requirements for an energy efficient vehicle: Low weight - efficient drivetrain - simple energy supply chain.

  1. ‘Electricity Mix’ for Well-to-Tank (battery charger) consumption:
    0.57 MJ/km with 100% Natural Gas (NG) powerplant. Total 1.18 MJ/km.
    0.11 MJ/km with 75% Renewable Energies (RE) plus 25% NG powerplant. Total 0.72 MJ/km
    (Production and transport efficiencies: RE 95%, NG 51.7%)


The graph shows the efficiency in km/MJ (Mega-Joule), based on WTW (not Tank-to-Wheel) efficiency and the rolling and air resistance – the only energies needed for driving. (km/MJ and MPG have the same graphical proportions)


Remarks:  Hydraulic Hybrid:

Well-to-Wheel Efficiency:  High Well-to-Tank efficiency and established infrastructure. High Tank-to-Wheel efficiency through hydrostatic drivetrain with high degrees of the braking and exhaust heat energy recuperation.

Drivetrain Efficiency:  High thermal efficiency and homogeneous combustion through ultra-high pressure (3,500 bar) peripheral fuel injection. Very high, variable compression ratio, piston charger efficiency, and medium combustion pressure (40 bar) and no piston side-loads. Operation only at most efficient point for fuel consumption and emissions. No idling. See: Section ‘Drivetrain’.

Energy recuperation:  Braking: 75% (100% minus round-trip losses). Exhaust: Doubling of the expansion ratio through the secondary expansion in the exhaust gas driven piston charger.

Weight:  The complete, drivable platform of the Hydraulic Hybrid weighs 164 kg (360 lbs.) less than the battery of a comparable electric car (Tesla Model 3). The accumulator as load carrying backbone of the car, and the crash energy absorbing hydraulic bumper system allow for a light, and less costly car body.

Emissions:  Lower fuel consumption reduces CO2 emissions proportionally. NOx and soot are in addition significantly reduced through the peripheral fuel injection of the Hydraulic Free-Piston Engine. A slightly modified version runs on Natural Gas (CNG) to reduce CO2, NOx, and soot further.

Costs:  The average weight of a medium size 5-seat car (1,540 kg/3,390 lbs) is reduced by 65% (1,007 kg/2,218 lbs) The costs for the 250 kg CFRP (Carbon Fiber Reinforced Plastic) are lower than the 1,257 kg (1.007 + 250 kg) of conventional material they replace. (cost/kg ratio 5:1) The number of parts of the Hydraulic Hybrid platform are nearly proportionally reduced.

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