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English   Alternative drive systems in Buses of Daimler AG – Part II
30.12.2008 von admin

The Olympic approach: First tests with natural gas in 1971

As the 1972 Olympics approached, commercial vehicle manufacturers were involved in a neck-and-neck race to develop new, environment-friendly drive systems whose effectiveness had not been proven in the least. “We refer to articles published recently about the development of natural gas engines by MAN,” we read in a letter of commercial vehicle and engine designers Rubi, Hartmann and Müller-Berner to Daimler-Benz Development Chief Hans Scherenberg dated July 23, 1971. “They report on a new development proposed by MAN involving the use of liquefied natural gas for vehicle drive systems.”

The engineers learned from a telephone conversation with the MAN engineering department that the news about the reduction of nitrogen oxide emissions by up to 80 percent came from American publications. “Unlike MAN, which has not carried out any tests to date, for the past several weeks we have a gas engine running on a test stand on compressed natural gas. We have not managed as yet to beat the emission figures of a diesel engine, which are a great deal better than those of the comparable gasoline-powered four-stroke engine.”

Just six days later a meeting took place with two representatives of Munich’s municipal authorities after it became known, again through the press, that the local public transport operators there had begun to convert a few vehicles to a liquefied petroleum gas drive system (propane-butane mixture). An internal memo recorded the results of the meeting: “Operation on propane/butane is viewed in Munich only as a temporary solution and is to be superseded in the medium term by liquefied natural gas.”

Finally, on November 26, the press department suggested going public with the Mercedes-Benz natural gas bus after newspapers and television again had reported about forthcoming tests at MAN and about a liquid gas tank truck that was supposed to deliver the fuel for the propane-butane buses. The natural gas bus was to be presented to a “small group of ten to 15 commercial vehicle journalists” and then demonstrated before running television cameras. For after all, Daimler-Benz “currently is the only company that can immediately come out with a ready-to-operate natural gas bus.”

Test bus of 1971: The six-cylinder spark-ignition engine of the Mercedes-Benz OG 305 operated on natural gas – with very low pollutant emissions.

Soot- and irritant-free: The OG 305 natural gas bus

A press release dated May 31, 1972, described the first natural gas-powered bus: “This natural gas bus has the advantage of low-noise, low-odor combustion, and its exhaust emissions are almost entirely free of irritants and soot. However, this environment-friendly design requires making certain concessions. As natural gas only can be carried along in liquid form in the vehicle, thermally insulated tanks that permit a storage temperature of minus 162°C for natural gas had to be developed. In the OG 305 they are arranged underfloor and naturally result in a considerable increase in weight.”

The four insulated tanks made by Linde held 286 liters of natural gas, enough for a range of 400 kilometers. Depending on pressure state, the tanks emitted the natural gas in a liquid or gaseous state of aggregation. If the pressure were to exceed 4.2 bar at some point, two pressure relief valves blew the gas off into the open air. The system, tested by the German technical inspection authority (TÜV), was licensed for road use without reservations. The engine relied on spark ignition. It was a modified six-cylinder, model M 407 hG, with a compression ratio of 1:11, an output of 172 hp and maximum torque of 677 Nm.

The press release remarked in detail on the emission figures: “Today the Mercedes-Benz natural gas engine undercuts the limits for hydrocarbons and nitrogen oxides by more than 20 percent and gives off only a twelfth of the permissible carbon monoxide gas. As natural gas contains no lead and sulfur compounds, this engine cannot produce any such pollutant emissions. However, the price for these advantages is higher operating costs.” Hans Scherenberg also came to a rather skeptical assessment in 1977: “Since natural gas is not available in adequate quantities in our country, realization of this drive system does not appear very feasible here. But it may be very interesting for other countries.”

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