Fuel-borne catalyst cleaning up London taxis, but alternative fuels get Mayor's strategy preference

Diesel Fuel News, Nov 11, 2002 by Jack Peckham

San Diego -- The venerable London Black Cab won't be spitting black diesel fumes within this decade, as the Mayor of London and the British government continue to push for clean-up of old diesel taxicabs, buses and heavy-goods vehicles.

According to the Lord Mayor's Air Quality Strategy (published this fall), some 18% of central London's particulate matter ([PM.sub.10]) emissions are attributed to some 20,000 diesel taxis, 45% of which are relatively old Euro-1; another 26% are pre-Euro emissions standards.

However, tests on four London Black Cabs (three Euro-I, one Euro-II) retrofitted with diesel particulate filters (DPFs) and using Associated Octel's iron-strontium fuel-borne catalyst (FBC) for soot-oxidation boost show that PM can nearly be eliminated from such vehicles, as Octel researcher Paul Richards showed in a paper (SAE 2002-01-2 75) to Society of Automotive Engineers Powertrain & Fluids conference here.

Despite this encouraging news, the Lord Mayor's current strategy instead pushes for gradual conversion of these taxis to LP-Gas, rather than DPF retrofits. The rationale: LP-Gas also reduces NOx and vehicle noise compared to filter-equipped diesels. However, Octel argues this wouldn't be the most cost-effective strategy, given that DPF/FBC retrofits are much cheaper than gas conversions, and taxis account for only 7% of central London NOx anyway.

Even today's DPF/FBC conversions will cut NOx about 2%, and possibly more if eventually optimized for vehicles employing exhaust gas recirculation (EGR), selective catalytic reduction (SCR) or possibly some future NOx trap.

As of now, the Mayor's strategy includes a three-phase plan that would require new taxis to meet Euro-III emissions limits, while all existing taxis must meet at least Euro-I (in order to keep their license). Eventually, all taxis must achieve full conversion to at least Euro-III later this decade.

Such a scheme would however require considerable government subsidies to compensate taxi owner costs given typical 15-year lives for taxicabs, the Mayor concedes.

On the other hand, "in the future it may be possible to retrofit taxis with a combination of technologies such as a particulate trap with selective catalytic reduction," the Mayor's clean-air report says. This might be accomplished at a cost of around U.S. $3,900 per vehicle, still cheaper than the estimated $4,700 for an LP-gas conversion or for repowering a vehicle with a new, Euro-III compliant engine, the Mayor's report shows.

FBC Overcomes Low-Temp Duty Cycle

For the Octel tests on the four black cabs, researchers found that DPF/FBC system can be regenerated regularly, such that average fuel economy wouldn't be affected much - although the range was wide, from a 6.8% improvement to a 4.5% fuel penalty.

Regular soot regen was accomplished despite taxis spending less than half of the time at exhaust temperatures above 200[degrees]C, and less than 20% of the time at temps above 300[degrees]C, the researchers found.

Octel also employed a new FBC dosing system to overcome problematic dosing by fuel-tank level. Fuel sloshing at braking/acceleration can lead to over-dosing, so the new system was set to feed FBC continuously at calculated fuel-consumption rates.

"This meant that after refueling, the FBC concentration was lower than the optimum, but as fuel was consumed and FBC introduced, the amount of FBC present in the fuel would gradually increase and ultimately exceed the optimum level prior to the next refueling," the Octel researchers reported.

Target FBC dosage was 20 parts per million, but over-dosing via fuel-tank-level injection "probably" led to 40-60 ppm concentrations, Octel's Richards explained here. The new dosing system meant that average FBC dosage was around 20-25 ppm, but the range could vary between <1 ppm to 50-60 ppm, he said.

FBC average dosage ideally should be minimized to extend the catalyst-ash cleanout intervals for DPFs.

Octel's DPF/FBC system is actually marketed under the trade name, "Adastra." While it's uncertain whether this system will become commonplace among London Black Cabs (due to the Mayor's current bias toward LP-Gas conversions), the Adastra system does indeed qualify for a 75% cost subsidy due to its official listing on the British government's "CleanUp" register (see: www.cleanup.org.uk/download/Web_CleanUp_Register.pdf).

The Adastra registration also applies to vehicles over 7.5 tons (including buses, coaches and refuse trucks).

On another front, Octel is working with Tata Energy Research Institute (TERI) for an FBC/DPF demonstration program in Mumbai, India, Richards told us. For this project, Octel is working with Indian joint-venture partner Dorf-Ketal.

Meantime, more clean-diesel highlights from the Mayor of London's latest clean-air strategy:

* By 2005, all 6,400 Transport for London (TfL) buses will have achieved at minimum Euro-II emissions and be fitted with DPFs, cutting PM emissions by 95% compared to older buses. Over half already have DPFs, and all new TfL buses (Euro-Ill) come with DPFs installed.