TECHNITOPICS
Page 45
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By P. A. C. Brockington, AM1 MechE
Astringent Comment from Coventry
New Daimler chief engineer sees stratifiedcharge petrol engine as a future probability
L'ORM ERLY chief designer of Coventry
Climax Engines Ltd.. and chief engineer of Transport Vehicles (Daimler) Ltd. since August, 1965, Mr. P. WindsorSmith disagrees with the many dieselengine authorities (see " Technitopics " for June 11 and September 24) who consider that it would be profitable to reduce the compression ratio of diesels by several ratios, given that some means could be found to enable the unit to be started. But he agrees that sometimes there is a case for a reduction of the ratio of turbocharged units or for multi-fuel applications.
Mr. Windsor-Smith points out that the control of combustion lag represents one of the major problems in the design of combustion chambers and injection equipment and that the rate of burning is a critical factor. When burning is delayed because of comkustion lag it "gets out of control" and the resultant high rate of burning is wasteful and destructive in that less of the potential force is converted into mechanical energy, a proportion of this force being' expended in the form of impact on the surfaces of the chamber, including the piston crown.
Whilst combustion lag cannot be eliminated unless pilot injection is employed (experimental designs of which have increased fuel consumption), reducing the compression ratio below an " optimum " figure increases the lag period because it reduces compression temperature. Mr. Windsor-Smith emphasizes that there are examples of the ratio being increased to give more uniform combustion rather than easier starting. Apart from the improved efficiency that higher ratios provide, smoother running is obtained in a typical ease, stresses are reduced and there may well be a gain in fuel economy. Moreover, the peak pressure may also be reduced.
These views are of particular interest to the author because they highlight the anachronism that thermal efficiency of both diesel and petrol engines is dependent on the completion of burning around top dead centre after compression of the mixture or air into the smallest space consistent with uniform combustion. The incongruous point is that, in the interests of efficiency, combustion must be concentrated into a part of the cycle during which the connecting rod and crank are virtually in a straight line.
At lower crankshaft r.p.m., a considerable part of the force of combustion must, therefore, be wasted in the form of internal strain losses, the saving grace of the piston engine relative to this phenomenon being that piston inertia promotes efficiency at higher speeds. This is because a substantial part of the combustion pressure is absorbed in reversing the piston, the force being restored to the cycle lower in the stroke at a more favourable connecting rod/crank angle. In a very high speed engine, combustion force may match inertia force at peak r.p.m.
Computer exercises have shown that on an indicated basis, a so-called delayedaction piston can afford a considerable increase in low-speed torque, the essential characteristic of the piston being that, in operation, reversal at top dead centre is delayed until the crank has rotated some 15-25' past t.d.c. No extra working points are required but certain friction difficulties and a weight problem remain to be overcome. An arcuate-ended crosshead takes the place of the gudgeon pin and moves relative to the piston body with angular displacement of the connecting rod.
The claim that piston inertia is (within limits) advantageous is, of course, completely contrary to accepted thinking of a few years ago and is of academic interest only because a heavy piston has excessive inertia at high speeds. Research engineers have proved, however, that at low speeds increasing the weight of the piston is favourable to fuel economy.
On the general subject of turbocharging, Mr. Windsor-Smith attributes the lack of progress on the part of the majority of engine makers in this country to difficulties with gasket sealing and instances Scania-Vabis, Fodens ' and Cummins as notable examples of those companies that have successfully dealt with the problem. In his view, turbocharging can play a very valuable part in extending the outputs covered by a basic range of engines and has "virtues of its own" in terms of fuel economy and reduced smoke emission.
A major break-through in petrol-engine design is dependent, in Mr. WindsorSmith's view, on the development of an
efficient, stratified-charge combustion system that will enable the engine to operate without throttling of the air, and therefore at the rated compression ratio, throughout the load-speed range of the unit.
This would enable a very substantial increase in part-load economy to be obtained, and whilst this view is in line with the opinions of other leading authorities it is noteworthy that Mr. Windsor-Smith believes the development of a commercially applicable stratifiedcharge system is a " probability " in the not-too-distant future as distinct from a "possibility ". Such a development could greatly increase the scope of the petrol engine for stop-start work and possibly for long-distance trunking operations.
Of more immediate petrol-engine interest, Mr. Windsor-Smith points out that an overhead camshaft engine enables a more efficient type of combustion chamber to be employed and that the cost of such a unit need not exceed that of a conventional push-rod type. Location of the valve gear above the block also gives more uniform heat distribution and thermal stresses and if the cams act directly on the valves, the life of the valves and valve-gear is greatly improved. Although adjustment of clearances offers difficulties, this is more than offset by the extended mileage the unit can operate between adjustments.
Petrol injection could also be exploited, Mr. Windsor-Smith believes, materially to reduce fuel consumption. Substance is given to this view by the experimental application of a Tecaletnit system to a Coventry-Climax 1,000 c.c. fire pump unit having a compression ratio of 10 to I.
When operating at its maximum power of 75 b.h.p. at 6,000 r.p.m. the specific fuel consumption was 0-49 pints b.h.p./hr. and was of this order throughout the load-speed range down to half-load at 2,000 r.p.m. At this rating, the engine produced 10-7 b.h.p. and returned a consumption of 0-514 pints b.h.p.thr. The most favourable consumption •of 0-437 pints b.h.p./hr. was achieved at 75 per cent load at the same speed, when the unit had an output of 16 b.h.p.