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City buses battle for pollution prize
Page 46
Page 47
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The Nice event included a class 'for city buses and these entries, plus several other non-participating exhibits, represented all the leading European marques.
The British flag was shown almost singlehanded by a solitary Leyland National. This vehicle was finished to a French specification. It now appears that an order for six is near completion with one of the French municipalities. This would be the first European order. However, all is not quite plain sailing because the French -and several other Eurpean operators — are demanding a number of changes to the stan
dard design. It now seems likely that a significant amount of work will need to be done locally on buses imported to the Common Market countries.
Major changes will involve seating, rear step height and heating. All these requirements will eventually be met by production modifications. At present, hand finishing is necessary. The bus on show at Nice had 40 seats to GRSA specification. These are basically Leyland's safety frame trimmed to local requirements.
French municipalities are demanding Webasto heating and the "quiet bus" specification, items which cannot be "productionized" at present.
Berliet had three versions of its PR 100 entered. So far 300 PR 100s have been ordered for use in French cities, 174 of which have already been delivered. Seventeen French cities have placed orders for the PR100 and Marseille, with an order for 167, is easily the biggest operator. Agreement has been reached between Berliet and POLVIOT, the Polish manufacturing industry, to supply 250 complete PR100s and a further 1450 sets of parts for assembly, so its future looks assured.
In Germany two separate bus standards have developed — the VOV, a true city bus, and the Stuelb (standard line bus) an inter-urban stage carriage bus designed for the German Post and Railways authorities. Both Daimler-Benz and Magirus-Deutz have produced models to suit the standards. As such items as the area of the windscreen, size of the doors, floor height and so on are laid down in the standard it is not surprising to find that equivalent models look very much alike.
Both manufacturers, spurred on by an environmentally-conscious populous, have produced very quiet vehicles powered by low-exhaust-emission engines. The MagirusDeutz 200 SM110U city bus and the 230 L117 inter-urban bus both use V8 versions of the air-cooled Deutz FL413 engine range and by using a two-stage injection system have achieved a significant reduction in exhaust emission levels, sufficient in fact to reach the limits required by the 1975 Californian regulations. This two-stage injection does not involve a new engine but is a variant of the 413 engine; only the pistons and the heads are different and these parts are directly interchangeable with the standard engine.
In the two-stage combustion cycle, fuel is injected into a hot swirl chamber with only about half of the air needed for combustion, with the result that a low level of oxides of nitrogen is produced. The increase of pressure produced by the precombustion then pushes the incompletely burnt gases towards a double swirl pattern recessed into the crown of the piston. Here the second part of the combustion takes place with an excess of air, but at a relatively low temperature and much turbulence, which leads to a low unburnt hydrocarbon level.
As one would naturally expect, these changes have a derogatory effect on fuel and output. The basic FL413 V8 produces 232 bhp but is normally used in a 200 bhp derated form for buses, but output is reduced further to 180 bhp by the changes to heads and pistons. A spokesman for Magirus told us, however, that the company's engineers were confident they would eventually get 200bhp while maintaining the Californian 1975 exhaust gas levels. At the moment, specific fuel consumption is some eight per cent poorer for the indirect-injection engine and that is the price we must pay for environmental cleanliness. The same engine, incidentally, has good noise levels and complete vehicles can reach 76dBA.
Daimler-Benz currently builds two basic city bus types: the standard city and regular service buses 0.305 and 0.307. Since February 1973 the 0.305 has been supplied with a horizontal version of the new 400 series engine, the 210 hp direct-injection 11-litre OM 407, a new front axle, the newly developed planetary hub reduction axle arid larger brakes.
Buses fitted with Daimler-Benz automatic transmission are becoming increasingly popular, as the production figures show. All 0.305 city buses exported in 1972 were fitted with automatic transmission and in Germany the share in 1972 was 42 per cent, while for 1973 an increase to approximately 70 per cent is expected.
The newest city bus on show, and a major prize-winner, was the Daimler-Benz standard regular Stuelb inter-city service bus the 0.307 which went into production Environmental variants Both 0.305 and 0.307 models are available with a factory fitted engine compartment noise insulation package to reduce exterior noise by 10 dBA. The noise level has reached the point where at speeds above 36 mph practically only the rolling noise of the tyres is audible.
Daimler-Benz say that approximately 40 per cent of all standard buses of the type 0.305 will be supplied with the lowpollution and encased engine in 1973.
Standard Mercedes-Benz diesel engines using a spiral air flow in the inlet port satisfy the most stringent standards valid at the present time, i.e. the California Test 1973 for diesel engines. For example, typically they produce only 12.5 per cent of the permitted carbon monoxide level and 50 per cent of the unburnt hydrocarbons and oxide of nitrogen levels.
The present engines are also far below the standards for carbon monoxide specified to become effective in California in 1975. Only the emission of nitrogen oxide and hydrocarbons had to be dealt with to achieve the 1975 level. This has been done by a 20 per cent reduction in output combined with other minor changes.
The already low smoke emissions are reduced by more than half when the engine output is reduced and, due to the considerable excess dir and thus more complete combustion, the emission of unburnt hydrocarbons and carbon monoxide is insignificant. Nitrogen oxides, on the other hand, are at their highest in the presence of excess air, and they increase further with the combustion temperature.
Emissions of nitrogen oxides are reduced by increasing the air volume and reducing the peak temperatures but both these measures result in reduced output. Peak temperatures can be reduced by retarding the fuel injection, and firther reductions are possible by lowering the piston pressure. It is possible for the direct injection diesel to have the same favourable emission characteristics as a pre-chamber diesel engine.
Daimler-Benz have already tested such engines and, compared with conventional standard engines, their output is reduced and fuel consumption is increased by 4 to 5 per cent. At the moment the changes necessary to reach the 1975 limits mean a loss in engine outpUt of about 20 per cent. During the week at Nice, Daimler-Benz gave us a chance to sample the ride of its new 0.307 design. For some tastes there was a little too much "float" after passing bumps but damping was generally good. We were sitting near the rear of the vehicle during some extensive climbing and the silenced engine never became obtrusive. In fact there seemed to be more noise from the transmission and axle than the engine proof that engine silencing has been effective. While it is tricky to judge noise by ear without a meter, it appeared that the 0.307 is well in line with its silenced competitors. '
The Nice vehicle was fitted with particularly comfortable seats. One wonders whether these will make the production model or if they were an eye-catching item designed for the judges' benefit. Even so, comfort for passengers near the wheel arches was reduced by inadequate leg-room.
The 0.307 is the third major component in Mercedes' bus line-up. Like the 0.305 and long-established 0.302 it will be offered in frame-only form to bodybuilders. The 0.302 incidentally, sold more than 5,000 units last year to bring the total production of the model since it 1965 introduction to nearly 25,000. The 0.302 is due to be replaced by a new model next year when, we are told, more strenuous efforts may be made to break into the British market in a major way.