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I've had my eye on compressed air hybrid technology for some time as one of the promising developments that could lead to major improvements in fuel economy.

Recently, I noticed that one of the leading developers has filed a raft of new patent applications. The Scuderi Group, a Massachusetts-based engine development company, is now working on and attempting to patent a V design of its split-cycle engine. Let me attempt to explain.

On one side of the V engine are two cylinders that perform the cold side of the four-cycle process, intake and compression. The two cylinders on the other side of the V perform the hot side, power and exhaust.

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The key to split-cycle design is that the cold side compresses the air and blasts it into the combustion cylinder just before it fires. This cylinder fires on every crank shaft revolution, not on every other one as in a normal four-stroke, providing greater torque. The compressed air acts like a super super-charger. This is said to reduce NOx emissions by up to 80 per cent and improve fuel efficiency by up to 50 per cent, compared to a conventional gasoline engines.

Much like a gasoline-electric hybrid, the air hybrid can store kinetic energy produced during braking and coasting; in this case, the energy is stored in the form of compressed air. That means the cold side of the engine can be shut down from time to time when the compressed air tank is full. Scuderi Group wants to licence its technology to engine manufacturers although I haven't noticed that any have signed up yet.

New engine technology like this is critically important to the automotive industry as it must reach an average fleet fuel economy of 35.5 miles per U.S. gallon by 2016. Light trucks, which are about half of all sales, have to get about 30 mpg, which is an increase of at least 20 per cent and a huge challenge to meet.

Trucks - being trucks - need to be big and heavy to handle the big loads they carry and tow. The weight reduction strategy that is such a big deal in getting better fuel economy from cars has less application in big trucks. That's one reason why compressed air hybrid engines are being examined very carefully by truck builders.

An air car or truck isn't just a bunch of hot air. Different variations on the theme have been worked on in the past decade.

A French inventor produced a little urban vehicle powered by an air engine. It had an all-aluminum four-cylinder air engine that used compressed air to drive the pistons up and down. It also had a small gasoline engine to power an onboard air compressor.

Another version from South Korea uses a small motor, powered by a 48-volt battery, to compress air to drive the pistons. The compressed air is used when the car needs a lot of energy such as starting up from a stop and accelerating. The electric motor kicks in once the car has gained normal cruising speed.

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Scuderi has been saying its split-cycle engine has been "almost ready" for sale to auto makers for a few years now. However, the V design is being touted as its "next generation" and greatly improved. The company has raised $65-million (U.S.) in funding while building its patent portfolio to about 476 patent applications filed and 154 issued. Should an OEM licence the Scuderi technology it will take three to five years for a vehicle to make it into production.

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