Researchers Say Compressed Air Could Pave Way to Smaller Engines

PARIS – Researchers at ETH Zurich, a leading European engineering school, are developing tiny engines that could deliver big-engine performance while reducing gasoline consumption 30% or more.

The trick is to use the engine to compress air during deceleration and employ that air to eliminate turbo lag during acceleration.

The compressed air could move a car for only about 700 ft. (200 m) by its own power, but it could provide a stop-start feature that outperforms a micro-electric hybrid, eliminate turbo lag and allow for radical downsizing.

“Our key idea is to make engines much, much smaller; replace a 3.0L V-6 engine with a 1.0L inline 2-cyl.,” says Lino Guzzella, professor of thermotronics in the department of mechanical and process engineering. “My group has been working on it for three years.”

In a paper Guzzella will present at the SAE World Congress in Detroit in April, his team reviews recent research on the pneumatic hybrids and compares calculated results with those from an experimental 0.75L test-bench engine that is designed to provide the same performance as a 134-hp 2.0L engine.

“With much smaller friction losses and pumping losses, we are close to the maximum efficiency” of a gasoline engine, Guzzella says.

ETH Zurich compressed-air engine on test bench.

Diesels can change chemical energy to propulsion with 40% efficiency, and some gasoline engines can do the same at a 36% rate. But the average car in Europe is only 17%-18% efficient, he adds, so “there is a huge potential for saving fuel.”

The problem with simply offering a small engine is that performance is not good enough for climbing hills or accelerating on a freeway, and while turbocharging can provide the torque required, there usually is a delay in spinning one up to 200,000 rpm.

Having a 20L-30L tank of air compressed to 290 psi (20 bar) connected to the engine means the cylinder can be supercharged within one combustion cycle on acceleration.

The air is compressed during braking, using the engine as a pump. The key element is having a valve that is totally independent of the engine regime, one that can open and close as needed to move air to or from the storage tank.

Such a valve, and the control system, will be the most expensive part of a pneumatic hybrid engine, Guzzella says. The ETH Zurich engine uses an electronically controlled hydraulic valve from Robert Bosch GmbH.

“The hope we have is that we can make a system only 20% more expensive, but reaches 80% of the benefits of an electric hybrid. You pay three times as much for an electric-hybrid engine than a gasoline engine, and this is something that will not work in China or India.”

Guzzella appreciates that electric hybrids are more efficient, but “hybrids run into a cost problem. They are fine for countries like the United States or Switzerland, but the game will be decided in China and India.”

However, he does not believe powering a car exclusively with compressed air is a good solution. Such a system is under development for the India-built Tata Nano at Guy Negre’s Motor Development International SA operation in Nice.

“Compressed air is a bad idea for energy storage,” he says. “The energy density of an air tank is very low compared to batteries, at least 10 times lower. People have always dreamed about air-powered cars…but this won’t work.

“The concept of compressing air and using it back to drive is not very efficient. You lose a lot of capability to do useful work. Electric motors and batteries are better.”

Guzzella’s researchers work loosely with those at other European universities that also will present papers at SAE.

Lund University in Sweden is working with pneumatic hybrid diesels, and the University of Orleans in France is specializing in gasoline combustion, Guzzella says, while his group is working mainly on the energy-management system.

He believes a small pneumatic hybrid engine on a 1,300- to 1,800-lb. (600- to 800-kg) car would provide excellent performance and achieve 95-120 mpg (2.5-2.0 L/100 km).

Research is gaining momentum. Scientific papers have been delivered on the subject regularly in the past 10 years, and this year the SAE World Congress will feature a half-dozen reports.

While ETH Zurich has interested Bosch in the research, no auto maker has yet to come forward.

“We have talked to many car makers, but we (have) hit a bad moment right now,” Guzzella says, referring to the worldwide sales slump. “The car industry is in a panic mode.”

However, he says, it is an advantage to be a university in Switzerland, which does not have a national auto maker. “So we do whatever we think is best,” he says. “I’m sure we will succeed.”