Published: 27.01.09
Pneumatic Hybrid Engine

Simply save gasoline

A pneumatic hybrid engine could be used to power vehicles in the future. The benefit of this technology: it is much cheaper than today’s electric hybrids and almost just as economical.

Peter Rueegg
Physics laboratory technician Till Coester works on the new hybrid engine being checked thoroughly on the test stand. (Photo: P. Rüegg / ETH Zurich)
Physics laboratory technician Till Coester works on the new hybrid engine being checked thoroughly on the test stand. (Photo: P. Rüegg / ETH Zurich) (large view)

The Japanese automobile manufacturer Toyota has built a car, the “Prius”, which is seen as a concept for the future. It uses an electric hybrid engine to save fuel while still offering the usual drivability standards. However, this vehicle has a serious disadvantage. It is expensive and is unaffordable especially for consumers in up-and-coming countries such as China and India that are increasing rapidly their mobility demands. In addition, the battery needed by the electric hybrid as an energy storage device is heavy and expensive. Last but not least, the technology in the coupling between the gasoline engine and the electric drive is very complicated.

Simpler and cheaper

This is why Lino Guzzella, Professor of Thermotronics, does not think the electric hybrid is the only solution. As an experienced engineer, he therefore looked for an approachthat was simpler than an electric hybrid but remained affordable even for people with less purchasing power. Guzzella explains that “The apple must be ripe but still hang just low enough to stay within reach.” The fruit currently ripening in his group is the pneumatic hybrid drive. The concept is simpler than that of an electric hybrid: the new hybrid engine has a compressed air tank connected to the engine instead of a battery unit. When required, e.g. when starting from rest or after changing gear, compressed air flows into the engine through an electronically controlled valve. If fuel is also injected, the engine responds quickly. Although the system used to control the valve is also technologically complex, this challenge can be mastered nowadays thanks to powerful algorithms and computer systems.

The compressed air supply also allows the engine constructors led by Lino Guzzella to achieve extreme downsizing. Conventional car engines can have peak powers of 150 hp or more, but usually need no more than 30 hp for everyday driving. Downsizing the engine halves the number of cylinders from four to two. This also halves frictional losses and increases the engine’s average efficiency. To keep the maximum power and thus satisfy the consumer’s drivability demands, the engine is highly supercharged by a turbocharger – which exploits the exhaust gas enthalpy as an energy source, and which boosts the to the desired levels..

Efficiency up by a third

Initial tests on the test stand in the ETH Zurich Machinery Laboratory show that Guzzella and his group are on the right track. They were able to increase the engine’s average efficiency in the European Test Cycle from 18 to 24 percent. This corresponds to a fuel saving of one third. Energy savings of up to 50 percent are achievable in purely urban traffic, because the engine can pump air into the compressed air tank during braking, thus recovering the kinetic energy.

Although the fuel saving achieved by the pneumatic hybrid is not as large as that of an electric hybrid, it still amounts to 80 percent of the latter. In return, the price-performance ratio is distinctly better. So good, in fact, that Guzzella can imagine the pneumatic hybrid also being suitable for use in poorer countries. He estimates the additional costs compared to a conventional gasoline engine to be approximately 20 percent. On the other hand, the additional costs for an electric hybrid are calculated to be at least 200 percent.

Motor manufacturers interested

The new engine concept has aroused the interest of several major motor companies and automitive suppliers, who have obtained information on-site. Some of the ideas of the new concept have already been patented. Only the financial crisis and the global recession worry Guzzella slightly. He says that these are difficult times in which to launch a new drive concept. Nevertheless, he is convinced that he will find people interested in adopting this system, since no other technology is on the horizon that could replace the internal combustion engine, even in the next two decades. This is why the way leads via hybrid concepts, which remain affordable while retaining the advantages of a gasoline or diesel engine.

Literature reference

Dönitz et al. Realizing a concept for high efficiency and excellent driveability: The downsized and supercharged hybrid pneumatic engine. Soc. of Automotive Engineers, April 2009.