Car makers will show off their latest efforts to fight pollution at Frankfurt's autoshow next week by unveiling models with improvements rather than substitutes to the standard internal combustion engine.

Under pressure to reduce harmful emissions produced by their vehicles amid fears of global warming, they are keen to show the greenest of intentions with the latest line of engines that consume fuel more efficiently.

For anything more advanced, like a hybrid engine, PSA Peugeot Citroen, Mercedes Benz and others will need another two years before they can show anything worthy of the road, far behind their Japanese rivals.

Toyota, whose Prius hybrid car has been out for 10 years, is announcing a partnership with French power utility EDF to set up a network of plug-in points where hydrids can recharge their batteries. A hybrid car has a combustion engine twinned with an electric battery.

As for engines that run on a blend of gasoline and a biofuel like ethanol, several of them including Fiat already sell them in countries like Brazil.

But the cost of biofuel is seen by analysts as too expensive to have them fitted in cars sold on a mass scale in Europe.

So automakers have been working to make the internal combustion engine more fuel efficient so as to emit less carbon dioxide (CO2), seen as a contributor to global warming.

'There's not going to be a revolution (this year),' said West LB Panmure analyst Horst Schneider of the cars to be on display at Frankfurt's biannual IAA autoshow.

'What we are going to see is a tweaking of existing technology,' added Al Bedwell at JD Power.

This kind of tweaking will be shown by Germany's Volkswagen with the unveiling of BlueMotion versions of five models including the Golf, whose CO2 emissions have been reduced to 119 grams per kilometre (0.42 pounds per mile) from 135.

Ford is also coming out with greener versions of existing models under the ECOnetic label. A new Ford Focus, for example, will emit 115 grams per kilometre.

One of the regions where car makers have felt the most pressure to cut CO2 emissions is Europe.

The European Commission plans to introduce legislation next year to have them cut emissions from new cars to an average 130 grams per km by 2012.

That would contribute to an overall target of 120 grams per kilometre by 2012 compared to current levels of roughly 163.

Car makers have criticised the move as unfair and unrealistic, and called on the executive body of the European Union to push the deadline back to 2015 to give them more time to prepare.

Worried that strict curbs could hurt competitiveness and cost thousands of jobs, some of them have suggested makers of heavy cars pay compensation if they exceed the new CO2 limits.

But the Commission refuses to consider it.

The fact that European manufacturers are seen missing a voluntary target to cut the average CO2 output from new cars to 140 grams per kilometre by 2008 has strengthened the Commission's resolve.

Small car makers such as Renault and Fiat are expected to have less trouble meeting these targets than those that make bigger cars because their vehicles pollute less due to their size and fuel efficiency.

It is an advantage that they have exploited in the advertising for their cars.

Renault will present a new Laguna with a 110 horsepower diesel engine whose CO2 emissions are under 140 grams per kilometre.

General Motors will unveil a 1.3 litre Opel/Vauxhall Corsa with 75 horsepower, which emits 119 grams per kilometre.

As for next generation technology, GM will be unveiling a hydrogen fuel cell vehicle.

Cars that use biofuels will also make an appearance, like PSA Peugeot Citroen's 308 BioFlex compact car.

But Transport and Environment, a Brussels-based environmental lobby group, was not impressed with the line-up.