Exhaust Brake! 'rove Themselves
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This Analysis of a Controversial Braking System Indicates That. it Has Many Proven • Advantages, Particularly Saving on Maintenance: No Damage is Done to Engine or Other Units
by P. A. C. Brockington, A.M.I.Mech.E.
REPORTS by operators that the use of an exhaust brake does not cause damage to the power unit or increase wear and tear, have failed to convince many potential users that they can employ such equipment with impunity. They remain convinced that "there is a snag in it somewhere."
Before providing detailed evidence on behalf of exhaust brakes, it is appropriate to cite some aspects of design and performance which are not fully understood by typical operators.
When the engine of a road vehicle is being overrun, a large proportion of the work done in compressing the air in the cylinder is restored on the following stroke, despite the absence of fuel and the lack of combustion force. A certain amount of heat is wasted, but this is small and represents the extent of the energy absorbed. The extra braking effort afforded by an exhaust brake is a function of compressing the gas in the manifold and pipe external to the cylinder. In this case, closure of the exhaust valve during the following stroke prevents restoration of energy.
Pumping Energy Loss
A certain proportion of the total pumping energy is lost by virtue of the overlap of the inlet and exhaust valves, which results in blow-back through the inlet valve over a limited number of degrees when the exhaust valve is nearing its closed position. In a conventional engine with average overlap the wastage is, therefore, small but abnormal overlap can be a decisive disadvantage.
Tension of the exhaust-valve springs in relation to valve area is also important. In the majority of standard engines, spring tension is sufficient to resist opening of the valves of the remaining cylinders when one or more of the pistons is
BIG.
pumping gas into the exhaust system, but springs that are too weak to oppose back pressure are the most common reason for an engine being unsuitable for the application of an exhaust brake. Although equipping the engine with stronger springs is relatively simple, this may not be acceptable to the makers.
During a substantial proportion of the compression stroke of a two-stroke cycle, the incoming air is helping to expel the exhaust gas and this is further assisted, in a typical application, by a scavenge blower. The maximum exhaustgas pressure that can be obtained is thereby limited, and the increased load on the blower may cause damage. Clayton-Dewandre, makers of Clayton-Oetiker exhaust brake equipment, state that two-strokes are definitely unsuitable for exhaust brake applications.
Minimum Gas Pressure
According to this company, the minimum gas pressure for efficient operation of an exhaust brake is 25 p.s.i. This may be unobtainable for the reasons given, or because of the low power-to-weight ratio of the vehicle, the pressure obtainable being directly related to the pumping action of the engine, which indirectly determines its output. Average back pressure is about 40 p.s.i.
Obviously the mechanical condition of the engine is also important, faulty sealing at the valves or pistons resulting in loss of pressure. Mechanical faults that reduce brake efficiency will normally, however, result in a substantial reduction in the performance of the engine and represent the outcome of sub-standard maintenance.
Engines listed by the Clayton-Dewandre Co., Ltd., Lincoln, as unsuitable for exhaust brake application include Foden FDA, FD.6 and ED.12 units, the Ford 4D, the
Leyland 0.350, Perkins P3(V), p4(v) and P6(V) engines, the Rootes TS.3 arid the Thornycroft KRN6/S engine.
Clayton-Dewandre confirm that an exhaust brake can be fitted to engines equipped with a turbocharger (between the manifold and blower), but point out that obviating loss of blower efficiency depends on reducing the distance between the engine and turbocharger to a minimum. If the brake is applied to a supercharged engine, it is essential that a pressure-relief valve be fitted in the inlet manifold to obviate excessive loads on the working components including the gear drive.
Brakes are offered . with control equipment based on air-pressure or vacuum-operated linkage.
Application to a petrol engine is also practical, but involve's the use of a separate air intake, which is opened to atmosphere simultaneously with operation of the brake butterfly to provide fuel cut-off. Production of such equipment by the company has not, however, been started because of a number of technical difficulties, the main objection to the application being that the inlet manifold cannot be employed to provide power for a vacuum braking system.
With regard to the possibility of lubricating oil being drawn into the inlet manifold, technicians of the company state that this could only result from excessive clearance between the piston and bore, such as may be found in a badly worn engine. Concerning valve life, it is emphasized that the temperature of the air being compressed in the engine is very much lower than the working temperature of the exhaust gas. It is claimed that the life of the valves should be increased, particularly in the case of vehicles that are regularly used on routes with long descents.
Electrically Controlled Brakes
Commenting on the same aspect of engine wear and tear, it is claimed by Thomas Ash and Co., Ltd., 19 Rea Street South, Birmingham, 5, makers of Ashanco electrically controlled equipment, that exhaust braking on long descents prevents "air quenching" of the engine and thus reduces thermal stressing of the valves, head and block. The claim is also made that the reduction in vacuum on the intake stroke is favourable with regard to oil pumping, and that governor overrun is prevented on down grades.
Some form of fuel cut-off device is required if a pneumatically operated governor is fitted. Back pressure from the exhaust system causes air-pulsation during the period of valve overlap, and this can result in the delivery of excess fuel if no cut-off is fitted. If the engine is equipped with a mechanical or hydraulic type of governor. regulation of fuel supply is not, however, affected by changes in inlet-manifold pressure.
Both companies state that it is sometimes necessary (Ashanco refer to small engines specifically) to open the butterfly in the induction manifold of engines with pneumatically controlled governors simultaneously with the closing of the exhaust brake. According to ClaytonDewandre, this is necessary to permit free pulsations in the manifold, and Ashanco say that it is essential as a means of augmenting the air supply.
In addition to a special air cylinder designed to operate the linkage of the fuel cut-off and of the manifold butterfly valve, Ashanco produce a voltage cut-out. The purpose of this is to prevent stalling at idling speed when pressure is applied to the brake pedal. Current is supplied to the brake solenoid through the cut-out, which is connected to the output terminal of the dynamo. When the output of the generator is reduced to a predetermined current corresponding to the idling speed of the engine, the cut-out automatically opens the main brake-solenoid circuit and this releases the exhaust butterfly.
No damage has resulted in the application of exhaust brakes to many hundreds of Leyland engines according to reports by technicians of Leyland Motors, Ltd., who state that they know of no service problems created by the use of this type of brake.
This statement by the Leyland company has particular significance because the concern also manufactures exhaust brakes, and it should entirely dispel any doubts of operators regarding the possibility that employing a well-designed type of brake equipment could damage the power unit or increase maintenance costs.
For many years a Leyland-designed exhaust brake has
been fitted to the Worldmaster chassis, notably to vehicles operating on long-distance Continental tours, many of which involve crossing the Swiss Alps. This brake is an air-operated unit, which in the case of the Worldmaster chassis, actuates the pump-rack to eta off the fuel supply as well as a butterfly-valve in the exhaust system. The airoperated control mechanism is operated by a switch on the horn bracket mounted on the steering column.
Driver fatigue is considerably reduced and a more favtiurable wear rate of brake lining is obtained. It is pointed out that relieving the drum brakes of load on a
long descent ensures that the drums remain cool. Full braking is therefore immediately available for an emergency stop.
Leyland buses for Huddersfield Corporation are equipped with a similar type of exhaust brake, but in this case the use of C:A.V. S.F.-type of engine governor (described in the February 3 issue of The Commercial Motor) eliminates the need for a fuel-supply cut-off device.
No Increased Wear and Tear
Although no special tests of exhaust brakes have been made by A.E.C., Ltd., technicians of the company are completely satisfied that their use does not cause damage and does not increase wear and tear of the engine. This view has been mainly derived from the favourable opinion of operators, and it is notable that Glenton Tours, Ltd., 397 Queen's Road, New Cross Gate, London, S.E.14, state that outstandingly successful results have been obtained in the case of an A.E.C. Regal Mk. IV coach.
In a detailed and laudatory review of exhaust brake applications to seven Dennis Lancet, six A.E.C. Reliance and two A.E.C. Regal Mk. IV coaches, the garage manager of this company states that the engine of the first vehicle to be equipped with this type of brake (an A.E.C. Regal in 1954) showed negligible bore weir after 106,000 miles. Of a total Of 136,000 miles covered by the coach, 131,000 were completed on Continental tours.
When the engine was overhauled at 136,000 miles, the bearings were in a perfect condition and the engine was reassembled with standard piston rings. The original frontbrake linings were refitted after examination at this mileage, the thickness of the linings having been reduced from in. to 12‘ in. Whilst the rear linings were changed because they were impregnated With oil from leaking seals, the rate of actual wear compared favourably with that of the front linings.
Ashanco exhaust brakes, of the type actuated by a micro-switch on the brake pedal, are employed throughout. In addition, a cut-out switch is fitted to the instrument panel, with which the system can be made inoperative when the vehicle is travelling incongested traffic and so on. Comparable "pro rata" results have been obtained from the second Regal coach, which has completed 116,000 miles.
Four Valves per Cylinder
Application of exhaust brakes to the Dennis coaches was deferred until 1957, because it was doubted whether the four-valve-per-cylinder feature of the engine would be favourable, but all doubts on this score were later removed. The engine of the first vehicle to be equipped with an exhaust brake was examined after 24,000 miles, and its condition was comparable to that of a similar engine that was operated without an exhaust brake. The seven Lancets have now completed individual mileages of 45,000 with about 50 per cent. of the normal brake wear.
A measure of the brake's value, operationally, is shown by the descent of Porlock Hill with a full load of passengers and luggage. This was completed in bottom gear without the use of the brake drums, even on the steepest section. In a general appraisal of exhaust brakes it is stated that their use does not, apparently, increase valve-guide wear and that the condition of the engine oil filters when checked during oil changes indicates that it is not detrimental to the lubricant. Employing an independent switch is considered to offer decided advantages with regard to both safety and cost.
Bradford's Gradients
Gradients steeper than 1 in 10 are frequently encountered in the area covered by public service vehicles of Bradford City Transport, and the application of exhaust brakes to over 100 vehicles in the fleet has provided an outstanding test of the equipment. An Ashanco brake is employed, controlled by an electrical switch on the brake pedal. It is reported that the use of exhaust brakes has reduced the cost of relining the front drum brakes by more than 28 per cent., and by around 17 per cent, in the case, of rear brakes. The practice has also provided an appreciable reduction in the wear of the drums.
It is pointed out that the greater reduction in the wear ,4 the front linings can be explained by the fact that a, high rate of retardation with the application of the drum brakes imposes a relatively heavy load on the front units by virtue of weight transference and gives a corresponding• increase in wear; because the exhaust brake affords a limited rate of retardation and results in negligible weight transference to the front axle, a greater proportion of the load is imposed on the rear wheels. The main saving is, therefore, derived from a reduction in front-lining wear.
According to a report issued by the Ashanco concern, outstanding advantages have been obtained in an application to dumpers with a carrying capacity of 18 tons and a gross laden weight of nearly 324 tons. In one operation these vehicles negotiated a 200-yd. section with a gradient of I in 7 when running between the quarry face and the crushing plant, the surface being of rough stone which was frequently covered with slime. Power units were of RollsRoyce, A.E.C. and Cummins manufacture, with a capacity of about 11.3 litres in every case.
Drums for Stops Only
Before exhaust brakes were fitted to the vehicles the drum brakes had to be serviced weekly and the axle oil seals had to be replaced at similar intervals. The vehicles have now covered mileages of 9,500 to 25,000 since they were equipped with exhaust brakes, use of the wheel brakes being limited to stopping the vehicle. No brake maintenance or oil seal renewals have been required.
Commenting on operator reaction to the use of exhaust brakes, Mr. B. Goodfellow, general manager of Sheffield United Tours, Ltd., agrees that it is regarded by many as a potential source of engine trouble. Clayton-Oetiker exhaust brakes have been fitted to a number of vehicles in the fleet since 1956, and there has been no engine failure that could be attributed to their use. The first vehicles to be equipped with exhaust brakes have covered. between 120,000 and 140,000 miles and no major attention has been required. Mr. Goodfellow believes that a mileage of 200,000 will be attained without any disadvantage accruing from this source. Exhaust brakes are regarded as invaluable, particularly for tours on the Continent, and he observed that they are fitted to the majority of Continental passenger vehicles of all sizes.
Although applicable to operations over a limited period of 12 to 18 months, there is no evidence that the ClaytonOetiker exhaust brakes fitted to a number of vehicles in the fleet of the East Kent Road Car Co., Ltd., Canterbury, have caused damage to the engine or excessive wear.