Bus sets out the case for gas
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EXCLUSIVE by John Kendall • Diesel engines might have much lower emissions than their petrol counterparts, but elderly soot-belching buses on our crowded city streets have focused attention on diesel smoke and particulate emissions. Natural gas runs under almost every urban street in the UK. It is not only readily available but offers the potential to cut harmful emissions quickly and easily.
Benefits
Mercedes-Benz launched its 0405 low-floor natural gas-powered city-bus at last year's Hanover Show. It shares the same body options as its dieselpowered stablemate but the diesel engine has been extensively modified to run on compressed natural gas (CNG). Mercedes-Benz has brought one to the UK to demonstrate the environmental benefits of gas power and CM has driven it.
Mercedes' 12-litre in-line engine is dedicated for bus use. Modifications include spark ignition with electronic engine management. The combustion chamber has been designed for gas use and different pistons provide a lower compression ratio than the diesel engine.
The bus carries 825 litres of fuel in five cylinders. This is sufficient for around 300km on one fill. The cylinders are constructed from high alloy steel wrapped in aramide fibre-reinforced plastic to keep weight to a minimum. This allows the gas to be carried on the roof.
Although the alloy steelfaramide construction is not yet approved by British Gas, the tanks have been rigorously tested. Mercedes claims they can withstand pressures in excess of 500 bar—more than two and a half times their operating pressure of 200 bar. In the event of severe damage, the cylinders will not explode but are designed to crack and release gas slowly.
The gas is stored at 200 bar pressure so enough can be carried on boai-d to give a realistic operating range. From the tanks the pressure is reduced progressively in three stages to 0.4 bar, when it is ready for use.
There are numerous safety valves in the stainless steel fuel lines on board the vehicle. These pipes are encased in vented jackets so in the unlikely event of a leak, the danger of a gas build up is minimised.
We started our drive at British Gas's Slough natural gas vehicle (NGV) fuelling station. It took about 10 minutes to fill up from the high-pressure pump. From here, we set off for Marble Arch where Westmin-ster City Council was carrying out roadside emissions testing.
As usual with CNG-powered vehicles, the low noise levels are the first obvious difference from a diesel-powered vehicle, Driveby noise is reduced and interior sound levels are noticeably lower than in a diesel-powered bus. From the driving seat, noise from the rear mounted engine is barely perceptible.
Compared to a 17-tonne rigid truck, the 0405 N has generous reserves of power. Operating in stop/start traffic is further simplified by the ZF automatic gearbox with integral retarder. The smooth shifting ZF slurs changes almost imperceptibly— good news for passengers used to being thrown around by jerky automatic transmissions. Air suspension adds to the cosset factor—heaps better than the spine-jarring ride of too many buses on our city streets.
Tempting car drivers away from the comfort of their cars will not be helped by the seating arrangements in the Mere, typical of city buses-32 seats and room for 68 standing? Probably the surest way to ensure that commuting cars will continue to clog city streets.
Westminster City Council's emissions test recorded so little carbon monoxide (CO), that it registered as 0.00% by vol ume in the exhaust. It also recorded 0.12 parts per million (ppm) of hydrocarbons (11C) and an air/fuel ratio of 1.86:1. A welltuned petrol engine would show around 1 to 2% CO and below 500ppm HC. Unfortunately there was no data on diesel engines but the figures would be lower than for petrol.
Credentials
The case for gas is clear from these results, but any government keen to show its green credentials would have to help. Fuel duty rates on natural gas are high in the UK preventing widespread take-up of the fuel for road use. As natural gas carries less energy in a litre of fuel than diesel, more of it needs to he burnt to produce the same power. Any adjustment in duty rate needs to reflect that too. It's up to Parliament to seize the cleaner air initiative.