Pistons for high outputs
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IN a Technitopics published many years ago, the author propounded the theory that the use of relatively heavy pistons would improve the performance and fuel consumption of an engine given that the higher inertia forces involved did not produce unacceptable bearing loads. During a recent visit to Wellworthy Ltd., Lyrnington, the company's chief designer, Mr, G. Longfoot, agreed that heavy pistons were now accepted by engine makers when bearing loading and torsional vibrations were suitably controlled. This statement is particularly significant in the light of piston developments that may enable turbocharged road-vehicle diesel engines to be uprated by 100 per cent in the near future, and by up to 200/400 per cent in due course.
On the compression /expansion stroke around top-dead-centre, at least a part of the force of combustion is absorbed in counteracting the inertia of the piston. At a critical speed the combustion force may balance the inertia, force and the piston "floats". Because of the inertia of the piston, a part or the whole of the combustion force is, therefore, operative at a phase in the expansion stroke when there is a considerable angle between the connecting rod and the crank. A higher torque is, therefore, produced for a given combusion pressure without late burning of the fuel which would reduce combustion efficiency.
According to Mr. Longfoot, employing a comparatively heavy steel-crowned piston and connecting-rod assembly of the oil-cooled type would enable the b.m.e.p. of a diesel to be approximately doubled compared with a typical turbocharged roadvehicle engine in current production, that is from around 165 p.s.i. to 300 p.s.i. and 300 p.s.i. is not the ceiling pressure. The use of SG-iron or of all-steel pistons (with a coating of aluminium) might well provide for an increase in b.m.e.p. up to 400 p.s.i. while a b.m.e.p. of as much as 600 p.s.i. might be possible if the unit were fitted with variable-compression-ratio pistons.
Output in bhp is proportional to b.m.e.p. at a given speed, and it can be concluded from Mr. Longfoot's comments that doubling diesel output is "immediately possible" provided that excessive bearing loads are not created by the higher combustion pressures.
It is not anticipated that greater piston weight would create unacceptable loads on the lower bearing surfaces except, possibly, in the case of some higher speed units. High bearing loads result from piston inertia on the exhaust stroke when cylinder pressure is minimal.
The two-stroke has the advantage that piston inertia is counteracted by combustion or compression pressure on every stroke and there is, therefore, no practical limitation on piston weight. A higher piston weight has been specified in the design stage of one or more two-stroke diesels solely to obtain the advantages that a higher weight provides.
According to a statement by Harry Weslake of Weslake and Co. Ltd. quoted in a Technitopics published in 1965, increasing the weight of a piston gives an increase in piston friction. In Mr. Longfoot's view, however, the increase in friction on the lower half of the stroke resulting from a higher inertia force is "balanced" by the reduction of friction on the upper half of the stroke. In a practical application of a piston of one of the types mentioned, it would probably be expedient (in the author's opinion) to reduce peak rpm with some loss of power to give lower-than-normal friction losses (as well as quieter running) and still obtain double the output compared with current types of naturally aspirated diesels.
Maybach diesel engines have been in the news as the power units of the Israeli gunboats, and it is of pertinent interest that the pistons of these German diesels are probably of the type made by Wellworthy for Maybach loco engines when they were built by the Bristol Siddeley company in this country. The units were intercooled as well as turbocharged and operated at a b.m.e.p. of 285 p.s.i. As can be seen in the drawing of the piston, the oil is supplied from a tube (that is part of a telescopic feed line) and circulates through a passage behind the ring belt and thence to an annulus under the crown to a discharge drilling in the centre above the gudgeon pin. The crown is made of EN52 steel and is bolted to a forged aluminium body.
Wellworthy points out that spraying oil onto the underside of the crown may create high temperature gradients in the crown section and correspondingly high thermal stresses, which can be serious in the case of larger pistons. In smaller engines of the road-vehicle type the "pressure barrier" tends to restrict uprating, whereas the "heat barrier" is the more serious with bigger engines. The rate of pressure rise is higher in small engines and this increases bearing loads for a given specific output. But it is pertinent that reducing the compression ratio and turbocharging the unit gives a lower rate of pressure rise.
If an engine maker decided to employ pistons of one of the advanced types described to provide for a high b.m.e.p. output, there would be no complication in fitting the pistons. But if a b.m.e.p. of around 300 p.s.i. were to be achieved it would probably be necessary to have two turbochargers in series and possibly an intercooler. Moreover, if the pistons were of the non-v.c.r. type increased crankshaft bearing areas would normally be required and this could involve increasing the overall length of the engine.
It should, however, be possible to obtain double the horsepower for an increase in weight of about 25 per cent and a relatively small space penalty, which would depend upon accommodating the turbochargers in free-space areas. The important advantage of employing variable-compression-ratio pistons with regard to uprating existing engines, is that they provide a big increase in output without increasing peak combustion pressure and could, therefore, offer a new lease of life to units in current production with modification of the crankcase assembly.
Future developments mentioned by Mr.
Longfoot include the use of crowns of heat-resistant material such as nimonic material or titanium. He said that producing a piston with a crown of a ceramics material, for example silicon nitride, that provided "complete insulation" was a possibility, although employing a piston with sprayed-on ceramics material was still a pipe dream. Complete insulation of the piston crown from the burning gases would be ideal with regard to combustion as it would reduce the amount of heat that was dissipated through the body of the piston and would increase the amount that was converted into useful work. It might give an increase in output of at least 10 per cent.