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Who wants torque back-up?

16th February 1968
Page 43
Page 43, 16th February 1968 — Who wants torque back-up?
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Which of the following most accurately describes the problem?

• The difficulty of forecasting the type of torque characteristics operators will require was discussed at a joint meeting in London last week of the Institution of Mechanical Engineers and the Diesel Engineers and Users Association.

A paper, "Investigation of diesel engine and turbocharger interaction", by Mr. E. Kellett, Mr. J. F. Betteridge and Mr. M. Mistovski of CAV was read by Mr. Kellett.

Mr. I. W. Goodlet, of the Holset Co., asked whether operators wanted a constanthorsepower engine and quoted the views of a Swedish authority that it was horsepower and not torque that enabled a vehicle to climb hills. An exhaust-gate turbocharger provided a torque back-up without undue complications. If turbochargers in series were used, the compressors rather than the turbines should be of the variable-geometry type.

What could be done, asked Mr. Goodlet, about "the acceleration smoke" of a turbocharged engine? Employing a fuel supply governor controlled by inlet manifold pressure was a promising possibility.

Would the operator rather have a torque back-up of, say, 10 per cent over a reduced speed range or a back-up of five per cent over a larger speed range combined with automatic control of the transmission, asked Mr. D. Q. Martin, of Perkins. What did the user want?

In support of a good torque back-up Mr. V. H. F. Hopkins, of the DEUA, mentioned experiments in America which had shown that turbocharging gave improved economy as well as better average speeds and less driver fatigue. He was referring to an inline engine that was turbocharged and run at

a lower speed to produce the same horsepower as the naturally aspirated version, but a higher torque. It was used in conjunction with a five-speed gearbox, whereas a 10speed transmission was fitted to the standard vehicle and greatly reduced the number of gear changes required on a 300-mile test route.

Mr. W. S. May, of the British International Combustion Engine Research Institute, said that a single-stage turbocharger could give similar results to those described in the Paper (obtained from turbochargers in series). There was big scope for a variable-nozzle type which had a better potential than a bypass (exhaust-gate) type. Two-stage turbocharging could profitably be applied to larger engines and should incorporate inter-stage cooling. Pulse energy should be fully utilized. Lowering the compression ratio reduced smoke; a variable compression ratio piston prevented thermal overloading and controlled the rate of pressure rise.

Mr. R. M. Appleby, of Dorman (a Dorman 6LBT engine was the test unit mentioned in the Paper), said that a compression ratio of less than 12 to I would be required if a b.m.e.p. of 240 p.s.i. were exceeded. A variable compression ratio piston enabled the compression ratio to be automatically reduced according to cylinder pressure from any desired maximum ratio to as low as 8 to 1.

Tests of exhaust manifold pulse converters applied to four-stroke engines were mentioned by Mr. M. S. Janata, of Queen Mary's College, London University. Mr. Janota said that a pulse converter enabled the turbocharger performance of a four-cylinder engine to be improved to that of a six-cylinder unit. This had been shown in a test of a Foden four-cylinder engine; the pulse converter had reduced the specific fuel consumption to 0.38 lb/bhp/hr. which was as good as the consumption of the company's six-cylinder engine.

In reply to a question Mr. Betteridge described the two-stroke as the ideal unit for turbocharging. The basic problem of the two-stroke, he said, was that it was difficult to sell.

It was a pity, said Mr. Betteridge, that the American turbocharger experiment described by Mr. Hopkins had not been performed in this country ahead of the Americans. The use of two-stage turbocharging was not claimed as a serious economic possibility for the time being, but it could be preferred to employing a singlestage unit if pressure ratios in excess of 4 to 1 were required. Torque back-up rather than horsepower might well be appropriate for motorway running which did not include the ascent of severe gradients. Both Bosch and Hartridge smoke meters gave good repeatability.


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