Gassing up in the cit
Page 10
If you've noticed an error in this article please click here to report it so we can fix it.
by Brian Weatherley
• At last week's Coach & Bus '93 Volvo demonstrated its low-emission Compressed Natural Gas (CNG) bus, and Commercial Motor was one of the first behind the wheel.
Based on a conventional midengine BlOM chassis, the smart single-decker is one of 20 undergoing extensive field trials in Sweden In place of the normal TD103KF 9.6-litre diesel engine is a THG103 spark ignitim unit. Both are horizontal in-line sixes but there the similarity ends.
The gas engine has been created by Volvo's bus division together with United Turbine, a Volvo subsidiary which has developed the electronically controlled fuelling system following initial conversion work by Marintek in Norway.
Its cylinder head is specifically designed for lean-burn sparkignition operation with each spark plug having its own ignition coil. The conventional engine's turbine has also been replaced by one with a waste,gate.
Power from the CNG engine is matches the diesel's 185kW (245hp) but torque is down from 1,050Nm (7741bft) at 1,000rpm to 950Nm (701Ibft) at a l,2(X)rpm.
Low emissions
What makes the CNG engine so attractive is its exceptionally low emissions.
It's already well inside the Euro-2 emission requirement for diesel engines due for 1995/96, apart from a small quantity of unburned methane.
An oxidising catalytic converter downstream in the exhaust further reduces carbon monoxide and hydrocarbons. The CNG is housed in 15 steel tanks mounted on the roof of the Saffle Swedish single deck body, adding about a tonne to the kerbweight. The roof itself has needed little extra strengthening to take the tanks, which provide enough fuel to cover 400-4501cm (250-280miles).
To find out how the gas bus runs, we took it for a short spin around Warwick and Stratfordupon-Avon. Sitting in the driving compartment, there's nothing to indicate you're in anything other than a conventional single-decker. Switch the fuel on, hit the starter button and the CNG engine rumbles into life. The start-up noise has a distinctive rorty noise compared with a diesel, but while the sound is different it's clearly quieter.
Above all else, the vibrationinducing idle of the diesel is refreshingly absent.
At idle the gas engine normally ticks over at around 600rpm, although on our vehicle the engine tended to hunt a little after switching from drive to neutral.
The smoothness of the CNG engine when pulling away from a bus-stop or traffic lights and the lack of harsh acceleration noise are its strongest selling points.
Around the congested Warwick streets and tight one-way system of Stratford the CNG BlOM kept up well with the traffic. The extra weight on the roof had little perceptible effect on roll stability.
For most of the time the slighter lower torque is well-hidden. The only time we noticed it was when climbing a long hill out of Stratford, although the CNG B1OM's rear-axle ratio (the Gothenburg buses are geared to a maximum 90km/h) is higher for most UK city buses and that affected it as well.
With city bus operators coming under ever-increasing Green pressure, the CNG bus must be attractive. Moreover the fixedroute nature of bus work means there is no problem in refuelling once the infrastructure is in at the depot. Refuelling time is around 15-20 minutes.
While the mid-engine BlOM may not be everybody's chassis choice, Volvo says there's no reason why the conversion couldn't be carried out on a low-floor, rearengined BlOB chassis where the gas tanks could be placed below the floor between the cross members and chassis rails.
"Whatever chassis we bring in we'll offer it to a local bodybuilder", says the company. "Any bodybuilder could handle it, it's just a matter of working with us." Ii For a full report on Coach & Bus '93 see page 8.