Drivetrain Technologies: Efficiencies and CO2 Emissions

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 (rolling resistance), frontal area (width, height), and 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 and filter can reduce the emissions.


  1. MIT - Units & Conversion Fact Sheet, Derek Supple, MIT Energy Club
  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,  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.


The 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 following graph shows the CO2 emissions and efficiency in km/MJ (km/Mega-Joule), based on WTW (not Tank-to-Wheel) efficiency.  

The CO2 emissions for the electric cars are based on powerplants operated with 100% Natural Gas (NG). 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.

For the Tesla, an 'electricity mix' of 50% from powerplants (NG) and 50% renewable energies (wind) is required to reach the same CO2 emissions as the Hydraulic Hybrid. A 40% (NG) and 60% (wind) distribution is required when operating the Hydraulic Hybrid - Ingocar with natural gas (ca. 25 g/km CO2). NG also reduces the NOx emissions significantly.

MPG and km/MJ have the same graphical proportions.



Remarks:  Hydraulic Hybrid:

Well-to-Wheel Efficiency:  High Well-to-Tank efficiency and an established infrastructure. (Gasoline, Diesel, Natural Gas) High Tank-to-Wheel efficiency through highly efficient hydrostatic drivetrain with energy storage, full braking energy recovery and high degree exhaust-heat-and-pulsation energy recuperation.

Drivetrain Efficiency:  The Hydraulic Free-Piston Engine has high thermal efficiency and homogeneous combustion through ultra-high pressure (3,500 bar) and peripheral fuel injection. Very high, variable compression ratio and piston charger efficiency result in high medium combustion pressure (40 bar +) and power density (kW/kg). No piston side-loads, valve train, and bearing losses. Operation only at the most efficient point for fuel consumption and emissions at nearly constant speed and power. No idling. (See: Section ‘Drivetrain’)

Energy recuperation:  Braking: 75% (100% minus round-trip losses) (Electric cars 20%). Exhaust: Doubling of the expansion ratio through the secondary expansion in the highly efficient exhaust gas driven piston charger - charging the combustion chamber  with air and the accumulator with pressurized fluid. Shock absorber: Dampening energy recuperation.

Weight:  The complete, drivable platform of the Hydraulic Hybrid weighs 293 kg (654 lbs.), that is 120 kg (265 lbs.) less than the battery for a comparable electric car (Tesla Model 3, 310 miles travel). In addition, the accumulator as load carrying backbone of the car, and the crash energy absorbing hydraulic bumper system allow for a very light and less costly car body. The system includes active bumpers at the front, rear and both sides.

Emissions:  Low fuel consumption reduces the CO2 emissions proportionally. NOx and soot are in addition significantly reduced through the ultra-high pressure peripheral fuel injection ring with 24 micro-slots, creating an ultra-fine air-fuel mixture for homogeneous combustion conditions with low emissions for the Free-Piston Engine. A slightly modified version runs on Natural Gas (NG) to reduce CO2, NOx and soot further. (See: Section 'Emissions')

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) components are lower than the 1,257 kg (1.007 + 250 kg) (2,768 lbs.) of conventional material they replace. (Cost/kg ratio 5:1) The number of parts for the platform are nearly proportionally reduced.

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