Peugeot’s “car that runs on air” was never a car powered by the air around it. Hybrid Air, unveiled by PSA Peugeot Citroën in early 2013, was a petrol hybrid that stored energy in pressurised gas instead of a battery. The idea was to recover energy from braking, keep it in tanks under the floor, and use it to move the car through town without burning fuel. PSA promised strong fuel savings at a lower price than battery hybrids, but it shelved the project in January 2015 and the system never reached showrooms.
Image: PSA Peugeot Citroën
Here is how the concept worked, what it promised, why it stalled, and what it still teaches about storing energy in a car.
What Hybrid Air was
PSA presented the system in early 2013 and showed it at the Geneva Motor Show that March. It was developed with the supplier Bosch and the parts maker Faurecia, and PSA filed more than 80 patents on it worldwide. The company said it planned to offer the technology on small cars such as the Citroën C3 and Peugeot 208, with an eventual aim of covering all its car and light commercial vehicle ranges. French reporting at the time put the expected availability at around 2016.
The pitch was simple. Battery hybrids such as the Toyota Prius were already selling well, but their battery packs were heavy and expensive. A system built mostly from tanks, a hydraulic unit and a gearbox could, PSA argued, bring hybrid-style savings to cheaper, smaller cars. The company also said the parts would fit under the passenger compartment, leaving boot space intact.
How the compressed-air hybrid worked
The main parts were an ordinary three-cylinder petrol engine, a hydraulic pump-motor unit, two pressure tanks and a control system that chose between them. There was no plug and no large battery.
When the car slowed down, or when the petrol engine was running at its most efficient point and had spare power, the pump-motor pushed hydraulic fluid into a high-pressure tank. That squeezed a volume of gas, storing the energy much as a coiled spring does. When the driver wanted to move off again, the gas pushed the fluid back through the unit, which then worked as a motor and turned the wheels.
The car switched between three modes:
- Air mode: the hydraulic motor alone drives the car from stored pressure, with no fuel burned and no exhaust. This was intended for town driving at up to about 70 km/h.
- Petrol mode: the engine drives the car on its own, mainly on open roads and motorways.
- Combined mode: engine and hydraulic motor work together, for example when accelerating hard or joining traffic.
The point that is easy to miss is that the energy in the tanks came from the petrol engine or from braking. Air did not replace fuel; it replaced the battery as the place where recovered energy was kept. For a closer look at where usable energy really comes from, see our piece on whether an hour of cycling could power a home.
Image: PSA Peugeot Citroën
The fuel savings PSA claimed
PSA said a C3- or 208-sized car with Hybrid Air would use about 2.9 litres of fuel per 100 km in the combined test cycle and emit 69 grams of CO2 per kilometre, compared with about 104 g/km for its equivalent three-cylinder petrol manual. In city driving it expected savings of up to 45 percent, and it said the car could run on stored pressure alone for a large share of urban driving, depending on traffic.
These were manufacturer figures from the official test cycle of the time. That cycle was later replaced in Europe because its results were widely seen as more optimistic than real driving, so the numbers are best read as targets rather than proven everyday results. The French government had set carmakers a goal of 2 litres per 100 km by 2020, and PSA presented Hybrid Air as a step towards it. A 208 HYbrid Air 2L demonstrator was later built to show that target.
Strengths and weaknesses of storing energy as pressure
Hydraulic storage has real advantages. It can absorb and release a lot of power very quickly, which suits the stop-start rhythm of city traffic, where a car brakes and accelerates constantly. Tanks and hydraulic parts are made from common materials, avoiding the lithium, cobalt and other metals in battery packs. Pressure vessels also tend to tolerate many charge cycles without the gradual wear that batteries suffer.
The weakness is how little energy the tanks can hold for their size. A gas accumulator stores enough to move a car a short distance, which is why Hybrid Air was designed for short bursts in town rather than long electric-only driving. The tanks also lose usable output as pressure drops, so the control system had to manage when to draw on them and when to bring in the engine. On motorways, where there is little braking to recover, the advantage largely disappears.
Why Hybrid Air was shelved
In January 2015 PSA wound down the project. According to Automotive News Europe, the company said it was still looking for a partner to share development costs, and no such partner emerged. The system never went into a production car.
Timing worked against it. PSA was in financial difficulty in the early 2010s, and the industry was moving decisively toward batteries. Battery costs were falling year after year, plug-in hybrids and fully electric cars were gaining customers, and emissions rules were increasingly written around electrified powertrains. A system that only helped in town, and still depended entirely on petrol, had a narrowing role.
Common misunderstandings about “air cars”
“It runs on air instead of fuel”
Not quite. Every joule stored in Hybrid Air’s tanks came from the petrol engine or from braking. The air did not supply energy; it held it.
“Air cars are free to run”
Separate from PSA’s project, other inventors have promised cars running purely on compressed air filled at a station. Compressing air takes energy, usually from the electricity grid, and a lot of it is lost as heat. Such vehicles have struggled with very limited range, and none has become a mainstream product.
“The idea is dead”
Hydraulic hybrid systems have been tested in heavy vehicles such as delivery trucks and refuse lorries, which stop and start hundreds of times a day. That is where storing braking energy as pressure makes the most sense.
What the concept still teaches
Hybrid Air was a clever answer to a real problem: how to bring hybrid savings to affordable small cars. Its story shows that a sound idea also needs the right moment, a partner willing to share the cost, and a market that is not moving the other way. When you see claims about vehicles that run on air, water or other “free” sources, the useful question is always the same: where does the energy actually come from, and how much is lost on the way?
Sources: Institution of Mechanical Engineers, February 2013; Connaissance des Énergies, March 2013; Automotive News Europe, 22 January 2015; images by PSA Peugeot Citroën; earlier coverage by IFLScience.
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how about a air/electric hybrid?
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