In the article on exhaust systems in the August issue of Motor Sport, I made a nonsense of the description of the four exhaust pipes on the twin turbo-charger layout on the Renault. This was pointed out to me by a reader in Cheltenham, who also offered an interesting reason for one of the turbos packing up while Jabouille was at the pits during the British GP.
In my article I said that the blow-off valve, or waste-gate, expelled highly compressed air out of the small diameter exhaust pipe. This was totally wrong, for the waste-gate is upstream of the turbine, in a branch off from the exhaust collector pipe leading to the turbine. It is set so that if the exhaust pressure is more than the turbine needs the valve opens and it is high pressure exhaust gas that comes out of the small tail pipe, while the large one exhausts gases that have passed through the turbine. This should have been obvious when looking at the system, for both large and small pipes are discoloured, due to passing hot gases, whereas if the small one had been passing compressed air it would not have been discoloured.
When the exhaust gases have spun the turbine up to its working speed, the compressor which is on the same shaft generates the present inlet manifold pressure, and opens the waste-gate so that excess exhaust flow by-passes the turbine. The high pressure exhaust gases pass out through the small pipe while the large pipe is emitting exhaust gases that have expanded to a low pressure through the turbine. This arrangement ensures that the turbine is up to speed at relatively low throttle openings, so reducing the turbine lag, and it also means that the turbine is no larger than it need be. Its size is relative to the needs of the compressor, not the needs of the exhaust back pressure. At idling engine speeds the turbine would be well up to speed, but would have a small mass flow through the compressor, with the throttle closed. As the passage of cold air through the compressor acts as a heat sink for the whole unit, it is possible that this is why it overheated at the pits while Jabouille sat with the engine idling while repairs were done to the front of the Renault.
When Renault first built their turbo-charged V6 engine they used a single turbine/compressor unit made by Garrett-AiResearch of California, whose main work is with big diesel engines. When they went to a twin-turbo layout they changed to the German KKK firm (Kuhnle-Kopp and Kausch), who could offer a smaller unit, as used by Porsche. This provided a much better layout altogether, each bank of cylinders having its own turbine/compressor unit, its own exhaust system, its own intercooler and its own inlet manifold. Pressure balance pipes being the only connection between the two installations, apart from the cables from the butterfly throttles in the inlet pipes to the compressor, these two cables joining with a junction box to a single cable to the accelerator pedal. Air to the two compressors is drawn from sunken ducts, gauze covered, at the rear of the side pods. On the second car built, RS11, these triangular ducts were increased in size over the prototype car RS10, but no advantage was found so the third car, RS12 reverted to the small inlets.
Renault are still working with AiResearch in California, but technical communication is long-winded, and they are getting better results with KKK in Germany, who are more automobile orientated. — D.S. J .