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GENEVA – Turbocharging is boosting the efficiency of car engines running on compressed-natural gas in Europe, which has helped lead to a mini sales boom.

Gasmobil, the Swiss association promoting natural gas and biogas for vehicles, displays a half dozen turbocharged CNG vehicles, from brands Fiat, Opel and Volkswagen, on its stand at the auto show here, including a concept car the association developed in Switzerland with Fiat Automobiles SpA.

The Fiat 500 1.4 Natural Power Turbo Concept makes 115 hp running on CNG and emits only 109 g/km of carbon dioxide.

The market for CNG vehicles in Europe is swiftly rising from a relatively low base. The low CO2 emissions puts them near the top of the list for green vehicles and in some countries qualifies them for special government subsidies.

In addition, taxes on the fuel generally are lower than for gasoline or diesel, providing a second market driver.

In Italy, compressed natural gas has been a popular consumer choice for years, and Fiat leads the market there. Fiat Switzerland built the 500 turbo concept using the 1.4L engine technology from the Fiat Panda Natural Power.

Hans Wach, director general of Gazmobile SA, a Swiss natural-gas company, says there are more than 1 million CNG vehicles in Europe, half of them in Italy. Switzerland had about 8,700 on the road at the end of 2009, and 120 filling stations serving the fuel. Sales of CNG cars increased last year in France and Spain, as well.

Europe’s CNG cars are flex-fuel capable, with a gasoline tank that takes over feeding the engine when the natural-gas supply is exhausted. The Fiat 500 turbo concept, for example, holds 29 lbs. (13 kg) of CNG, good for a range of 199 miles (320 km), and 8 gallons (30 L) of gasoline for another 280 miles (450 km).

The difference with modern turbocharged engines is they are tuned for the CNG fuel, which has a higher octane rating of 130.

“With a dedicated solution, you get much better performance and efficiency,” says Wach. He points to the attributes of a Volkswagen Passat with a 1.4L engine using both a supercharger and turbocharger, rated at 150 hp and emitting 119 g/km of CO2, the equivalent of 51 mpg (4.6 L/100 km) with gasoline.

Wach says CNG is gaining in popularity for two reasons: “The ecological aspect, and the fuel is cheaper.” The cars on Gasmobil’s display are among the 39 different light vehicles using the fuel offered in Europe.

In addition to the high efficiency that results in low CO2 output, CNG exhaust has no particulate matter. The fuel also can be mixed with methane (biogas) made from waste, which further reduces CO2 emissions.

Wach’s company, Gazmobile, produces biogas and mixes it with CNG in storage wells.

Using a mixture of 10% biogas in the Fiat 500 concept would lower its CO2 output to 98 g/km, the equivalent of 57 mpg (4.1 L/100 km), compared with the 149 g/km, 37 mpg (6 L/100 km) the car gets when running on gasoline alone.