Compressed natural gas (CNG) has long been seen as a cleaner alternative to petrol and diesel but questions remained about how to get the most out of this fuel.
Now, researchers from the Institute of Internal Combustion Engines and Automotive Engineering at the University of Stuttgart and FKFS believe they have found a solution.
They presented a paper at the SAE 2010 World Congress describing the optimisation of a natural gas engine with a parallel hybrid powertrain through the use of EGR and an oversized turbocharger with active waste gas control. It’s a goal they have been working on since 2006 with the aim to develop a prototype of a CNG fuelled parallel hybrid based on an Opel Astra Caravan with less than 90g/km of CO2 emissions.
By minimising the pumping mean effective pressure, the researchers increased the engine’s efficiency. They used a hybrid powertrain layout and active waste gas control to bring improvements at all engine speeds. The base four cylinder engine was replaced by a turbocharged 1.0litre, three-cylinder run on natural gas and by adding an exhaust gas turbocharger the team boosted rated torque from 80Nm in petrol mode to 160Nm in natural gas mode.
With the highest levels of efficiency achieved with exhaust gas turbocharging, the researchers replaced the exhaust manifold and upstream catalytic converter with a turbocharger flange.
The result was that CO2 emissions were lowered by 1.9 per cent due to the use of an oversize turbocharger; and by a further 1.0 per cent with the active waste gas control strategy. The greatest reduction however – 4.5 per cent – was achieved by combining all the methods examined. This brought overall emissions below the 90g/km threshold.
