City buses, refuse trucks and delivery vans are perfect candidates for hybrid powertrains, which use less energy and cause less pollution than conventional combustion engines, especially in stop-and-go traffic.

But while image-conscious drivers, especially in the United States, have embraced hybrid cars, truck operators find the new technology too expensive compared with the potential fuel savings -- and hybrid diesel-electric trucks are struggling to catch on.

Hybrid trucks operate along the same principle as their smaller hybrid cousins in the car industry, reclaiming kinetic energy when the vehicle brakes, transforming it into electric energy and storing it for later use.

When driving in urban environments with stop-and-go traffic or on a construction site where there is repeated acceleration and braking, manufacturers estimate hybrid trucks can save anywhere from 20 to 35 percent in fuel.

While consumers may decide what car to buy based on emotions, truck fleet operators think strictly in terms of total cost of ownership. How much cargo can it haul? How often will it break down? And how much fuel will it consume?

Added weight from a second powertrain can limit a trucker's haul -- increasing per ton transport costs -- and with most hybrid models still at the development and field-testing stage, prices are set to stay high until there's enough of a demand to warrant industrial-scale production.

The main obstacle to growth is the truck industry's low volume compared with cars, according to Stefano Chmielewski, president of Renault Trucks, the French unit of Volvo.

Take Europe. About one third of the overall truck market is distribution vehicles, divided up over 7 brands so about 15,000 units each. Economies of scale begin at 150,000 for trucks, Chmielewski told Reuters ahead of last month's European Road Transport Show in Amsterdam.

Volvo, the world's second-largest truck maker after Daimler, has global sales of nearly 220,000 units -- a fraction of the 9 million vehicles sold last year by carmaker Toyota.

Toyota has sold over 1 million hybrid petrol-electric cars since 1997, with the Prius alone accounting for roughly three-quarters of that figure. It estimates its hybrids have led to carbon dioxide emissions dropping by about 3.5 million tons compared with conventional engines.

If hybrid trucks are ever going to take off, then manufacturers are going to have to find a lot more people like John Formisano, vice president for global vehicles of package delivery company FedEx.

Formisano recently bought 10 of the diesel-electric version of the Daily large delivery van from Iveco, the truck arm of Fiat.

We have the largest fleet of hybrids in the world with 95 vehicles at present. Right now it's a developmental technology, Formisano said, declining to reveal the actual price paid.

If we can get hybrid trucks at the same premium that hybrid cars are at ... the manufacturers will be able to sell as many as they can make, he said.

Hybrid cars like the Toyota Prius sell for about 25 percent more than comparable vehicles running on conventional powertrains in the United States.

Iveco sales boss Stefano Sterpone said the potential of the hybrid truck market was enormous, after registering interest from logistics companies following recent hybrid deals with both FedEx and its competitor, Dutch mail and parcel group TNT.

Now we have six customers with a potential of 100 each ... but this is the beginning, Sterpone told Reuters.

Chmielewski, who showed off his hybrid truck concept Hybrys at the show, wants governments to step in and help stimulate demand through tax breaks and subsidies.

If there are no incentives from the government, a transport company will not (buy hybrids) unless they do it for marketing, he said.

Lars Stenqvist, head of vehicle definition and quality at Sweden's Scania, expects hybrids will be regular business in a couple of years if the industry can demonstrate that the technology is a sensible buy.

For a commercial vehicle operator it will come back to the profit potential -- cost versus savings. The payback time will decide the success of hybrids, he said.

Stenqvist went so far as to recommend incorporating the technology into trucks built by Scania, which generates the bulk of its business with very heavy vehicles for long haulage.

If we want a global CO2 reduction with hybrids, it must be in haulage, he said, defying a general belief in the industry that fuel savings in long-distance transport would be minimal.

Meanwhile, FedEx's Formisano said he is willing to expand his hybrid fleet, which consists mainly of custom assembled Freightliners using an Eaton electric drivetrain.

We're depending upon the producers to develop the technology, bring the cost down and find new customers, he said.

(Editing by Eddie Evans)