MILLBROOK, UK - Tests at GM-owned Millbrook proving grounds revealed unexpected results when comparing London buses running on five different fuels. A catalyst that had performed well in traditional dynamometer tests did poorly when tested at Millbrook with a more realistic test cycle.
The new test cycle was based on data gathered from a real London bus route. The bus typically spent 40 percent of its run accelerating, 30 percent decelerating and 30 percent idling. Steady running was negligible. The average city-center speed was 10kph.
The test compared buses using CNG, LPG, low-sulfur diesel and specially formulated, ultra-low-sulfur 'city' diesel, combined with different after-treatment systems.
The Millbrook tests showed that the catalyst was only 40 percent efficient using this real-life cycle.
'The catalyst was cooling off during the long periods of idle,' said Andrew Eastlake, manager of emissions engineering. 'Using this data, the catalyst was redeveloped by the manufacturer, and a lagged downpipe was fitted to retain heat. When it was retested it gave conversion levels of around 90-95 percent throughout the test.'
The tests also showed that a diesel engine using 'city'-quality fuel and fitted with continuously regenerating trap exhaust after-treatment is almost as clean as a gas-fueled engine.
As a result, London Transport Buses, will adopt 'city' diesel fuel and has fitted 300 of its older buses with continuously regenerating trap after-treatment.