Exhausting exercise

A new exhaust treatment system to reduce nitrous oxide (NOx) emissions will improve fuel efficiency in diesel engines, according to its UK developer.

The Sigma system, designed by engineers at IMI Vision, injects ammonia gas directly into the engine’s exhaust, instead of using a liquid urea spray as in existing systems.

Harmless vapour Current ‘wet’ Selective Catalytic Reduction (SCR) systems spray AdBlue - a water and urea solution - into the exhaust system, where the urea is converted into ammonia. This reacts with NOx in the catalytic converter to produce harmless nitrogen and water vapour.

When urea is added to the exhaust pipe the water evaporates and the remaining solid pellets of AdBlue melt, leaving equal amounts of ammonia and isocyanic acid. The acid is only converted to ammonia once it enters the converter, a process which uses up to 30 per cent of the converter’s capacity.

Martin Johnson, IMI Vision’s managing director, claimed such a system is inherently inefficient. ‘When the engine is idle you are trying to atomise a tiny amount of liquid, only about three grams,’ he said. ‘How can you distribute that evenly over 60 seconds efficiently?’

As in existing SCR systems, the AdBlue solution is stored in a standard 50 litre tank in the lorry. The solution is then pumped through to the Sigma system, which is built directly into the exhaust pipe.

The Sigma generator converts the urea into a gas, which is then stored in the system’s small reservoir to be injected into the exhaust as needed.

Johnson said: ‘What we are doing is creating something like a pressure cooker. We raise the temperature of the urea to above the critical conversion point so it now makes ammonia directly with no other products. This makes it easier to control and regulate than small quantities of liquid because there is a larger volume of it.’

Using gas rather than a liquid to reduce emissions also means that diesel engine manufacturers will be able to tune their engines more aggressively and increase fuel savings, according to Johnson.

‘By having a more efficient system, engines can run hotter, the engine can be tuned leaner and you get more fuel efficiency,’ he said.

‘Sigma will improve fuel efficiency by over 10 per cent,’ he claimed.

‘Wet’ systems have to wait until the catalytic converter’s temperature reaches 260ºC before the addition of AdBlue. Storing the Sigma generator and reservoir in the exhaust pipe means it is available to use much more quickly, as any gas left over is stored in the reservoir ready to inject when the engine is re-started.

The system’s smaller size also means that it could be scaled down for use in almost any diesel engine.

Sigma is currently undergoing testing, and IMI Vision, the advanced technology arm of engineering group IMI, hopes it can be integrated into heavy trucks by 2008, when tough new EC regulations on NOx emissions come into force. Niall