General Motors Europe's Opel unit will show a new hybrid 'concept' version of its top-selling B-segment Corsa at the Frankfurt show next month.

It uses a combined starter generator and stop/start automatic transmission and consumes only around 3.75 litres of diesel per 100 km while emitting 99g of CO2 per kilometre - for comparison, the EC is calling for average emissions of 120g in a few years' time.

The car also has a lithium-ion battery to power the belt-alternator starter while the battery itself is recharged by the concept car's regenerative braking function - when decelerating, the fuel is shut off and kinetic energy is transformed into electrical energy.

The diesel engine is switched off by the hybrid system when the car is stopped, and restarted when the brake pedal is released. When extra power is required, such as when accelerating or overtaking, the starter motor/generator provides "significantly" higher torque, GM said.

Opel will also introduce a Vectra 'Flexpower' with a two-litre turbocharged engine designed for operation on a bioethanol/petrol mix at the IAA show.

ecoFLEX

In addition, Opel will soon start selling new ecoFLEX variants, which have the lowest CO2 emissions in each model line.

The most fuel-efficient variant - the 55kW/75hp Corsa 1.3 CDTI ecoFLEX with a diesel particulate filter - makes its European debut at the show and will be available to order from December. The five-door version emits 119g of CO2 per kilometre, and uses 4.5 litres (roughly one imperial gallon) of diesel per 100km (62 miles) - ergo, it does 62 miles per gallon.

GM also said it would start selling 'ecoFLEX' variants of its other high-volume model lines at the end of this year - Meriva and Astra models with the 1.3 CDTI diesel (the world's smallest common-rail unit developed with and shared with Fiat) plus natural gas-powered Zafira and (Corsa) Combo models.

GM said these CNG version produce almost no soot particles, 20% less carbon dioxide than a comparable petrol model and an up to 80% reduction in other harmful emissions while their low operating costs also qualify them for the ecoFLEX league.

The 69kW/94hp 1.6-litre engine runs on natural gas, biogas or any mix of the two and a 14 litre (roughly three imperial gallons) reserve petrol tank ensures continued mobility when no natural gas is available. The cars have a range of around 500km (300 miles).

GME claims its innovative 'monovalentplus' concept makes it the first automaker to successfully optimise an engine for natural gas operation and still enable the use of conventional petrol - the unit has special pistons, valves and valve-seat inserts, as well as separate injection banks for natural gas and petrol.

Its new 'Eco-Turbo' concept is the basis for its other diesel and petrol engines. These engines follow the downsizing principle - turbocharged units with relatively small cylinder volumes - and successively replace units with larger displacements - Volkswagen is also doing this.

GM said the approach results in reduced engine friction and higher thermal efficiency as its latest engines run at more economical operating points, which significantly reduce fuel consumption and CO2 emissions.

For example, the 1.6 turbo Ecotec motor (132kW/180hp) replaced the two-litre turbo (125kW/170hp) in spring 2007 and the smaller engine uses 14% less fuel than the two-litre unit, and also produces less CO2 emissions. The new 1.7 CDTI turbo-diesel (81kW/110hp and 92kW/125hp) consumes 7% less fuel than the earlier 1.9-litre unit (74kW/100hp and 88kW/120hp).

GME is also replacing large displacement petrol engines with units with petrol direct injection.

Other CO2 emission and fuel consumption reduction measures include introducing electric power steering (EPS) - already on the Corsa - on all models as it only draws exactly as much energy as it needs from the generator.

The variable oil pump operates in a similar way, only moving as much lubricant around the engine as is actually required in the prevailing driving situation. Other fuel-saving measures include low-resistance tyres and detail improvements to the vehicle's aerodynamics.