Designs for Local Service Establist National Standards
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By P.A.C. Brockhigton, A.M.I.Meeh.E.
OVER many years, the design and development work of the Birmingham and Midland Motor Omnibus Co., Ltd., has given the company an unexcelled status as producers of passenger vehicles. Road tests of the Midland " Red " C.M.5 motorways coach ( The Commercial Motor, November 27, 1959) and the D.9 double-decker (April 8, 1960), give practical backing to this contention. Both vehicles represent designs which have been continuously developed by testing components in service.
This process has enabled the company to pioneer a large number of new developments and applications with the minimum expenditure on experimental and prototype vehicles, and it ensures that every part, approved for standardization, has been fully tested under the conditions for which it is intended. The vehicle industry generally has benefited from the results of these developments, whilst some of the outstanding mechanical features of c20 more recent designs are of global significance.
At the focal point of design inspirations, Mr. D. M. Sinclair, general manager, presides over a design committee comprising the chief engineer, Mr. E. C. Tuff, and his deputy, Mr. J. L. Ransome, the chief designer, Mr. P. W. Colley, and the development engineer, Mr. J. Pearson. Decision to produce a new component or modify an old type is followed by the preparation of drawings in Mr. Colley's office.
Parts for a prototype assembly or vehicle are normally produced in the central workshops of the company. The development department tests prototype vehicles and components, and is responsible for the progressive improvement of existing vehicles and assemblies, plans for which are initiated by the staff. Routine and special tests are frequently performed at the Motor Industry Research Association's proving ground, followed by further tests in normal service. The company have no regrets that, after four years of experimental work, they initiated the use of underfloorengined single-deckers in 1945, and are confident that applications of this type of power unit to the D.10 doubledecker will be equally successful.
Prototype underfloor-engined singledeckers were based on chassis that were run experimentally with petrol driven rear engines before the war.
"Partially Successful" " Although experiments with rearengined buses were "partially success ful" and many of the drawbacks were associated with the use of petrol engines, underfloor mounting of oil engines is still regarded as preferable.
Underfloor location provides valuable latitude in design and layout, whereas, for example, increasing the length of a rear engine might be impossible or might create major installation difficulties because of the restricted width of the vehicle. Dimensional modifications to an underfloor type can be introduced without regard to space considerations, and there is greater freedom in the layout of auxiliaries.
Engine noise is less concentrated relative to the passengers and, if required, insulation can be more effectively applied. In service, the dirt problem does not normally represent a handicap and, although flood water could immobilize the vehicle if special precautions were not taken, these can be applied at a reasonable cost. Special problems which must be investigated in underfloor location involve a study of underbody airflow characteristics -• to ensure that the intake filter is adeqUately supplied with cool air. In one application tests showed that at .30 m.p.h. there was a forward flow of air parallel with the engine. It is now general practice to locate the air filter (a Tallow staggered-baffle type with oil-soaked felt elements) on the off side of the engine facing forward.
Midland " Red " have 302 singledeckers equipped with disc brakes at the front and rear, and by the end of the year 95 double-deckers will be fitted with them. Developed in conjunction with Girling, the brakes are of the four-cylinder twin-pad type (a segmental pad is employed on each side). The solid discs of close-grained grey iron are about 1 in. wide and 15i. in. in diameter.
Ventilated discs were used experimentally, but the solid type is preferred for normal service, because it has a greater "heat-soak " capacity. This feature can be of greater importance than favourable heat dissipation, because the latter is a function of sPeed.
Vents in the disc reduce heat absorption, and when a large amount of heat is generated at low speed, only a small percentage is absorbed by the disc or liberated to the air. A large proportion is, therefore, transmitted to the hub, the excessive heating of which may cause melting or deterioration of -the grease.
As Midland " Red " technicians emphasize, this shows that heat-flow problems arising from the use of disc brakes are different from those associated with drum brakes. With a high axle loading, heat-dissipation difficulties can result in vaporization of the hydraulic fluid, as well as melting of the hub grease.
This problem led to the decision to fit drum brakes lo the rear wheels of the new D.9 double-deckers. It is, however, considered that a new type of operating mechanism and disc shape may enable a system suitable for heavy duty to be developed.
The success of disc brakes is shown by the mileage of nearly 50m. recorded by the 302 vehicles equipped .With them. The Firodo pads combine adequate friction properties with a long working life.
Avoiding " Over-braking"
Apart from complete freedom from fade, the progressive increase in retardation rate with increases in pedal pressure is of special operational importance. The pressure required for a normal stop is 100-120 lb. The driver can accurately match retardation to traffic requirements and avoid "over-braking."
Turhocharging of the 8-litre engines of the C.M.5 coach has been watched with interest by operators throughout the country. Although the total mileage covered by the 10 vehicles on the motorway service is 'insufficient to provide data for a detailed assessment of turbocharging, results are more favourable so far than were forecast before the service was opened. The fuel-consumption average of the vehicles is 13.81 m.p.g. and has been improving from month to month.
Turbocharged 8-litre engines are also being employed experimentally in double-deckers on stage services, taking the place of 10.5-litre units. Giving an output comparable to the larger engine, they provide a marked saving in fuel consumption. It is considered that the long-term acceptability of turbocharging will, in the main, depend upon unit replacement costs. the difference between these charges and the cost of fuel saved being the determining factor.
The initial development of the company's air-cooled engine has been completed and the information gained will be applied to a new design which may incorporate turbocharging.
Oil consumption of turbocharged engines is normal, and in the case of the M1 vehicles the average is about 1,700 m.p.g. Experiments have been made with almost every type of pressure-charger, but it was not until more efficient types of exhaust turbocharger were developed that the system was accepted. Its only shortcoming operationally, compared with the positively driven type, is its limited low-speed torque.
Slightly Convex Base
Experiments with modified forms of combustion chamber some four or five years ago involved the production of 85 patterns, many of which were tried in service, in addition to being tested in a laboratory engine. Ultimately, an open-top straight-sided shape with a slightly convex base was developed. It enabled a higher output to be obtained, and a more favourable fuel consumption, despite a reduction in compression ratio from 17 to 1 to 16 to 1. One of the main objects of this exercise was to reduce the peak bearing loads without loss of power, and it was successfully achieved.
A more recent development has been the use of Brimochrome alloy valve-seat inserts, which have doubled the useful life of the valve assemblies before grinding-in is required.
Midland " Red " have been using rubber suspension since June, 1949, and the 390 vehicles now equipped with it have covered a total of 56m. miles. The merits of the types of rubber suspension employed, with regard to both operation and maintenance, are now well known, and the company's engineering staff believe that the advantages of rubber springs have established a design lead that cannot be challenged by conventional forms of air suspension. Vehicles with air springs have been extensively tested.
The work of the development department is of particular interest, because it actively demonstrates the principle of testing in service. Of the 1,800 vehicles in the fleet, a large c22 number is equipped with one or more experimental items, so that the total test time accounts for a daily average of 'iiiany thousands of hours.
:About half the experimental items ' are -dictated by, practical experience, whilst the remainder comprise mainly the application of modified parts to investigate ad-vantages claimed •by accessory makers.
Tests of prototype and modified vehicles at the Motor Industry Research AssociatiOn's proving ground are directly controlled by the department, who organized the high-speed tests of the C.M.5 coach before it was passed for service on Ml. Regular work at M.I.R.A. includes life and fade tests of brake linings and pads, in which the anti-squeal properties of the material are also assessed. All track trials are followed by service tests on routine runs.
Although the company do not envisage the development of multi-fuel engines for normal applications, the use of " different " fuels is under constant review, and practical experiments have been made with various fuel mixtures. The latest of these is a mixture comprising five parts of fuel oil to one part of Coalene, which is a product of low-temperature carburization. The mixture gives a mileage per gallon and performance comparable with that of normal fuel. Tests are being continued to evaluate its effect on wear rates, particularly wear ofthe fuel pump.
Because of the company's outstanding success over the past 20 years in the application of new principles, the opinions of their leading engineers on future developments are of special interest. For example, hydrostatic transmission is regarded as the " ultimate " type if its efficiency is improved. Keen interest is being taken in related research work sponsored by the National Research Development Corporation.
Of the many potential merits of hydrostatic transmission, ease of control and the benefit to engine performance of infinite ratio variation are thought to be most valuable. Elimination of the clutch, and differential and the availability of additional braking power are also noted favourably, as well as the almost infinite'latitude regarding location of the engine without complication of the transmission_ Hydrostatic transmission was cited as a possibility by Mr. Sinclair in a paper to the Public Transport Associa tion as long ago as 1947.
A transmission project in which the company are taking
a more active interest is the development of the Gyreacta kinetic-energy storage unit. This basically comprises an epicyclic gearbox and a ".frictionless flywheel" with a vertical axis-, in which power can be stored indefinitely.
Energized by the engine when power is not required for traction or by vehicle overrun inertia, the flywheel can be employed to aid acceleration, to drive the vehicle in the garage without engine power, and to start the engine. It is claimed that its use could enable a smaller engine to do the work of a much larger unit, cutting fuel consumption by more than 50 per cent.
The form of control employed gives clutchless operation • and a smooth take-up from rest. Energy absorption on the overrun also reduces wear and tear of the braking system.
An active investigation is being made of the possibilities of a new type of pressure exchanger, which can be employed for supercharging without absorption of power in the same way as the turbocharger. The basic operating principle of this unit is an engine-driven rotor (positive drive is required only to overcome friction and to control the speed) having longitudinal vanes in a housing with two sets of intake and outlet ports.
Use is made of pressure waves to create a boost pressure, which is available at relatively low revolutions. This picture shows the installation of a turbocharged engine in a double-decker. Such a unit gives a performance comparable with that of a larger normally aspirated engine, but with a marked saving in fuel.