Simanaitis Says

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RECENT NEWS concerning Peugeot S.A. and Robert Bosch GmbH makes for technical confusion. The key words are “hydraulic hybrid”—whereas, a better term is “pneumatic hybrid.” Even its developers call the technology Hybrid Air.

The term “hydraulics” concerns mechanical properties of liquids. It comes from the Greek ϋδωρ, hydor, water, though the term involves fluids of petrochemical origin as well.

However, it’s odd seeing the word “hydraulic” when the “fluid” is air. We’ve got a perfectly good word for this: pneumatic. Again, thank the Greeks for πνοά, pnoa, breath. And aren’t word origins fun?

What’s more, this distinction is appropriate because there have already been advances with hydraulic hybrids; that is, hybrids using hydraulic fluid as a storage and propulsion medium. The U.S. Environmental Protection Agency and Eaton Corp. have been involved with these.

Misnamed or not, Hybrid Air is an interesting technical development of Bosch and PSA (this French company, the second largest automaker in Europe behind only Volkswagen Group). Briefly, Hybrid Air replaces the electric hardware with pneumatic counterparts.


The PSA Hybrid Air system has high-pressure air storage amidships, air stored at lower pressure at the rear, a pump/motor integrated with the powertrain up front and regenerative braking.

This third-generation powertrain uses a 1.2-liter gasoline engine, a PSA-proprietary transmission integrated with the pneumatic pump/motor and two storage tanks. Residing amidships is the high-pressure tank at 2175 p.s.i. At the rear, there’s a reservoir tank at lower pressure.

There are three modes of operation. A gasoline-only mode is for highway use. An interactive hybrid mode offers compressed-air boost through the pump/motor. Last, in Zero Emissions Vehicle mode, the gasoline engine shuts down for purely air-driven propulsion. As with a conventional hybrid, there’s regenerative braking in any of these modes, in this case replenishing air pressure by running the pump/motor as a pump. Check out for a PSA video on Hybrid Air.

Don’t expect very much ZEV travel, however. PSA notes that the available energy from Hybrid Air is only 0.04 kWh. (To put this in perspective, a conventional Toyota Prius has a 1.3-kWh battery, carrying more than thirty times as much energy). Hybrid Air’s ZEV travel is said to be limited to perhaps several hundred yards.

This analysis also offers a counterexample to those hyping compressed air alone as vehicle propulsion. Succinctly, compressed air doesn’t contain enough energy to handle the task by itself.

Hybrid Air, nevertheless, offers potential benefits. According to PSA, system costs are less than half those of an electric hybrid. Weight savings could amount to as much as 45 lb. Durability has already been established in related applications such as flap actuation on Airbus aircraft.


Comparison of different propulsion technologies in terms of price and CO2. Source: PSA.

PSA sees Hybrid Air’s particular automotive application in cost-competitive hybridization of B-class cars in Europe, the U.S. subcompact category. Plans point to 2016 as an introduction date.

More news is expected at the 2013 Geneva Motor Show in mid-March. PSA’s automotive products include Citroëns as well as Peugeots. It also has a cooperative venture with Chinese automaker Dongfeng.

Like the U.S., Europe has goals that promote hybrids. The U.S. Corporate Average Fuel Economy goal for 2025 is 54.5 mpg. (Because of how this figure is calculated, think a real-world U.S. 39 mpg that year. See for details.)

Europeans prefer to state goals in emissions parlance: an average of 95 grams/kilometer of CO2 by 2020 (think 57.6 mpg, with less computational leeway). PSA estimates a Hybrid Air B-class car being around 69 g/km CO2/81 mpg.

The Euro average was around 136 g/km CO2 for 2011. Hence, European automakers—and others wishing to sell cars there—feel challenged, and for good reason. ds

© Dennis Simanaitis,, 2013

One comment on “HYDRAULIC HYBRID

  1. Pingback: Peugeot-Citroen Hybrid Air Most Significant Intro at Geneva? | AutoInformed

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This entry was posted on February 8, 2013 by in Driving it Tomorrow and tagged , , .
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