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Towards Higher Pressures

DEALING with long-term targets it is appropriate first to appraise current development work relative to heavier industrial oil engines. The design of automotive units is complicated by the wide speed-load range, changes in operating conditions, the uncertainty of good maintenance and the necessity to cater for a high rate of acceleration.

When making comparisons between the two types, outputs are quoted in terms of brake mean effective pressure, as this indicates thrust produced on the firing stroke. Although larger industrial units have a b.h.p. output per litre that compares unfavourably with the output of smaller automotive units, by virtue of the lower operating speed, the b.m.e.p. produced by naturally aspirated and turbocharged industrial units of advanced design is substantially higher than that of typical road-vehicle engines.

In the case of automotive units the average b.m.e.p. is 100-I10 p.s.i., and the b.m.e.p. of a turbocharged unit rarely exceeds 130-140 psi. The industrial unit corresponding most nearly to the vehicle engine, with regard to conditions of service, is the type that is employed in railcars. Common requirements are small size, low weight and an ability to operate satisfactorily through a wide speed range. The great majority of railcar engines are turbocharged and the average b.m.e.p. is about 155 p.s.i.

The target of research and development engineers is an increase in b.m.e.p. to about 220 p.s.i. within three years and to 300 p.s.i, in the foreseeable future.

Because of its relatively high operating speed it is unlikely that design advances in the vehicle unit will allow running at the higher pressure for many years. There is, however, a prospect of proportionate gains in the same periods. In addition 'to further development of the turbocharger for the automotive unit it will be necessary to produce more robust crankcase assemblies and cater for increased loads on the gudgeon pin and bearings.

Although many manufacturers and the majority of operators do not have complete confidence in turbocharging, a comment an the subject by a wellknown engine maker indicates that resistance to the technique may well be overcome in the reasonably near future. This manufacturerconsiders that if a gain of 30 per cent were required it would be preferable to raise the output available by increasing bore size. But, he thinks, turbocharging is an inevitable development if research progress gives 100 per cent. improvement, because of the great saving in space and weight.

It is reasonable to divide overseas users of British engines into two sections. One welcomes turbocharged engines of up-to-date design, and the other considers improvements in naturally aspirated engines to be the most fruitful line of research.

Turbocharger research is, in any case, essential to the progress of the industry because even if it does not result in the general use of turbocharged units, it will provide evidence of limiting factors which will be of great importance to fundamental research. Moreover, if there are basic deficiences it is well that British manufacturers should discover them first.

For naturally aspirated and turbo

charged engines the B.I.C.E.R.A. variableratio piston offers a number of advantages with regard to performance and mechanical stressing. It may represent an essential part of an engine if brake mean effective pressures in excess of, say, 200 p.s.i. are used.

It is generally accepted that peak bear

ing pressures represent a limiting factor in power-output improvement, but it is not practical to reduce the compression ratio of an oil engine below a minimum level. The function of the B.I.C.E.R.A. variable-ratio piston is to restrict automatically the maximum cylinder pressure to any predetermined value. This may be selected as a basis of engine design having regard to all relevant factors.

These pistons have been used in a pressure-charged engine of 8f-in. bore. The application allowed an output three times greater than that achieved by the unit in its original naturally aspirated form. No overstressing was recorded.

Automatic ratio control is accompanied by a small reduction in maximum output and a slight increase in specific fuel consumption above the critical speed— normally in excess of the speed at which the engine is run continuously. Below the critical speed the piston increases cylinder pressure, which reduces specific fuel consumption by virtue of the higher compression ratio and gives easier starting.

This is particularly important for turbocharged :nines operating at a ores;tire ratio in excess of, say, 2f to I. Higher pressures will be difficult to apply if it is not possible to reduce the :ompression ratio under naximum-load conditions.

According to a director of 1. large engine manufacturer, he use of variable-ratio Astons could enable power )utputs to be increased by ;0 per cent. without raising nak bearings pressures. He Os° claims that a similar reduction in naximum loading could be achieved if in efficient heat exchanger were leveloped for turbocharged engines.

A long-term project of importance to he industrial engine industry is the potential use of a refrigerator to cool the charge air. Although it is unlikely that refrigeration as such will be applied to automotive units, a similar principle using a high velocity air jet might be employed.

Recent developments of interest to users of lighter vehicles have generally been related to the production of smaller units to replace petrol engines. The staff of Ricardo consider that production of smaller oil engines with comparable

outputs to conventional petrol engines of similar capacity should be regarded as an economic necessity for the future.

The Ricardo Comet compression-swirl combustion system is particularly suited to engines of small cylinder capacity operating at high speeds. It has been successfully applied to engines running at more than 4,500 r.p.m.

A notable example in this category is a Peugeot engine of 1.7-litre capacity which develops 45 b.h.p. at 4,000 r.p.m. As a private-car unit, it replaces ea

litre engine and while it does not provide the same peak output as the sparkignition engine it develops the same torque throughout the speed range.

Mention should be made of the Ricardo method of analysing oil engine combustion by colour photography. The camera is incorporated in the cylinder head of a research engine and takes photographs through a transparent window at a rate up to 15,000 frames a second. The film can he stopped for examination of detailand the employment of colour is of particular benefit to accurate analysis.

Reverting to turbocharger developments for vehicle applications, operators of vehicles in mountainous areas overseas should welcome the ability of a turbocharger to compensate automatically for losses of output with increases in altitude.

Ricardo technicians and others agree that casual applications of turbocharging should be avoided. Accurate matching of turbocharger with power unit is essential, they say. Although there is scope for developing automatic fuel controls, exhaust gates and so on, they take the view that the full development of

turbocharging in its simplest form should be the first objective.

. They also emphasize that an allimportant consideration is the elimination of hot-spots and steep temperature gradients to obviate the creation of high thermal .stresses. It is pointed out that thermal distortion can create higher stresses than those resulting from peak combustion pressures and inertia forces.

While hiet atmospheric dust concentrations are comparatively rare in this country they are normal hazards in routine operation in many parts of the world. Engines of farm tractors probably suffer most and the research work of the National Institute Of Agricultural Engineering will probably have an increasing influence on the development of efficient air-filters for commercial vehicles.

Radioactive Tests

Closely observed tests overseas by the East African Tractor and Implement Testing Unit of the N.I.A.E. and analysis of wear rates with the aid of radioactive techniques in the Institute's laboratory at Silsoe, have shown the importance of testing a filter, when fitted to the service engine, to ensure correct matching.

Small differences in throat form and in the design of the element and container can substantially reduce the efficiency and, contrary to general assumptions, excess capacity can be a disadvantage. Current research work includes an invesVgation of air pulsation characteristics with the aid of a pressure transducer mounted in the engine inlet and connected to a cathode-ray tube. The importance of small differences in efficiency is shown by comparing the results of using a 99 per cent, efficient air filter, with those obtained when the efficiency is increased to 991 per cent. This will give an increase in engine life of 100 per cent. If two filters, each having an efficiency of 99 per cent, when used separately, are mounted in series, the second filter will usually operate with an efficiency of about 50 per cent., thereby doubling the life of the engine compared with a single-filter installation.

Altitude Settings Also of interest to road vehicle operators are engine tests by the East . African Unit to determine. optimum fuel settings for altitudes up to 9,500 ft. For these, a mobile dynamometer truck was used in conjunction with a fuel consumption meter and smoke-density analyser. Tests show that the output of an engine which is not de-rated at sealevel is reduced by about 31 per cent. per 1,000 ft. and that adjustment of the fuel pump setting to give an acceptable smoke-density reading, corresponding to 6 on the Bacharach scale, is necessary at approximately 3,000 ft., 6,000 ft., and 9,000 ft. Taking the case of a Fordson Major diesel tractor, output is reduced from about 41 belt h.p. at sea-level to .31.8 h.p. at 9,400 ft. Simulated altitude tests are performed at Silsoe up to 6,000 ft

tr6 Photo-elastic analysis has been used in a number of investigations and has been proved an effective method of solving problems of stress distribution. In some cases this method has been employed to check mathematical solutions and it has provided information on problems which were virtually insoluble mathematically.

In this method, a scale model is made of the component to be tested using a suitable transparent plastics material. Representative loads are dial applied to the model, and by viewing it in polarized light a pattern of light-and-dark bands or fringes is seen superimposed on the image of the model. This is photographed This schematic arrangement of the 13.1.C.E.R-A. Mk. II hydraulic RETURN fuel-injection TO TANK system indicates its general opera tion. It can operSUPPLY ate at Orate of FROM TANK

over 3,000 injections per minute.

for detailed analysis.

Alternatively, the stresses may be "locked-in" by a heat cycle whilst under load, and the fringes indicating the stress distribution can then be viewed in the polarized light at leisure without maintaining the load.

Advanced applications of strain-gauge techniques are shown by a recent investigation of an oil-engine connecting-rod failure. Stress analysis of a redesigned small-end has also been made by the same method.

Possibly the most important long-term potential of any equipment developed by this Association is offered by the hydraulically operated pump-injector system, the use of which eliminates many of the costly mechanical components of

a conventional type. Moreover, it is much quieter in operation than the jerk pump system and is extremely compact, an 8 mm. plunger performing the work of the 20 mm. plunger of a mechanically operated pump. A high rate of injection is obtainable and both the timing and rate of injection are readily variable. The absence of mechanical drive simplifies fitting and maintenance.

A reference to investigations of engine performance by the Motor Industry Research Association is a fitting footnote to the oil-engine section • of this symposium, because the Association are comparative newcomers to this line of research and current practical work and plans are, therefore, strictly related to up-to-date developments in the industry.

Investigation of the basic problems in theapplication of turbocharging to vehicle engines and detailed analyses of power output factors are included in the research programme. A decompression chamber nearing completion will allow altitude tests to be simulated up to 12,000 ft. in the case of a 400 b.h.p. engine and to higher altitudes with smaller units. Routine investigations also are in progress with standard tests of air and oil filters.