Modern Engine Testing
Page 40
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THE author of the paper " Modern Engine Testing," Mr. S. G. Mundy, M.I.Mech.E., M.I.E.E., managing director, Crypton Equipment, Ltd., which was read before the Institute of Road Transport Engineers in London, on March 16, confined his remarks mainly to the service testing of petrol engines before their repair.
He pointed out that engine service must be judged by the extent to which it fulfills its proper purpose--to restore performance to the maximum extent attainable as rapidly and at as low a cost as possible. The time taken to diagnose troubles is often excessive, and the results frequently inaccurate and incomplete. As a consequence, good after-service performance is not always achieved.
The procedure must start by considering the factors responsible -for engine performance. These are the limits, clearances and specifications established by the maker and built into the unit at the factory. Normal wear causes these standards to alter. In addition, terminals corrode, cable insulation deteriorates, springs weaken and passages clog, with a gradual decline in performance, resulting in a loss of acceleration, increased oil and fuel consumption and specific troubles such as missing, pinking and difficult starting.
Diagnosis Comes First
Before attempting the restoration, a complete and accurate diagnosis should he made to discover the alterations which have occurred since the original standards.
Engine service can be divided into three phases: (I) initial diagnosis of the whole engine; (2). necessary reconditioning, which may comprise adjustments, repairs or replacements; (3) final inspection and tuning after shop service to ensure that the work has been properly done and the unit tuned for best performance and economy.
For diagnosis, every part of the engine should be tested and inspected, and this must include the components, such as the battery, cables, starter and its circuit, ignition, comprising condenser, breaker points, the automatic advance mechanism, coil, distributor cap and rotor, h.t. cables, sparking plugs and ignition timing. The test must also cover compression, valves, piston rings, gaskets, manifolds and exhaust system, the dynamo cut-out and voltage regulator, fuel pump, fuelsupply, cylinders and carburetter.
The Human Element
Under present conditions, it would he commercially impracticable to make a complete and thorough test of all these. Consequently, the mechanic is able to make only a partial test and inspection, and his diagnosis is mainly deductive and experimental. He may deal largely with suspected components and take certain things for granted.
During recent years efforts have been made to overcome this problem by developing instruments which will enable a mechanic to make a complete and accurate diagnosis without the need for dismantling, and to work in a systematic way which eliminates the B6 speculative element and converts the operation into sequence tests.
Many types of instrument have been developed, ranging from small portable meters, as well as vacuum and compression gauges, ammeters and voltmeters, up to elaborate engine-analysing bays. In general, these have fulfilled their purpose and have simplified the problem, but their use has riot been accepted by the service industry as a whole, and many engineers are doubtful as to their value and practicability. This is partly because of a misconception of their purpose. They should be considered as gauges, for they perform functions similar to those of a micrometer in lathe work, or a Cylinder gauge in reboring.
Recently, however, simplified forms of instrument have been developed and combined with new and practical test procedures. The outstanding change has been the introduction of individual testers which can be used either collectively for the systematic diagnosis of a complete engine, or, apart, for testing separate components.
Attention is now given only to the three "divisions" responsible for performance, viz., compression, ignition and carburation, and if the proper instruments be available, these three divisions can be tested accurately and completely in a surprisingly short time.
The starting point of engine diagnosis should always be the battery, followed by a systematic test of the ignition system (first on the low-tension side and then on the high-tension), including a check of the dynamo and regulator. The diagnosis should then be completed on the fuel pump and the carburetter.
A typical engine analyser of the new unit design comprises a test panel', equipped with six individual test sets. These instruments include vacuum and pressure gauges, a combustion tester for indicating mixture strength, ammeter and volt meter with loading resistances and switches, condenser and coil testers, and what is known as a Tach-Dwell tester which gives engine r.p.m, cam angle or period of dwell in degrees of angular rotation of the distributor, and speed changes due to adjustments or the fitting of replacement parts; it also permits the setting of breaker, points without using feeler gauges.
Practical Application
Three further portable instruments are used in the analyser. These are for h.t. search test, ascertaining cylinder compression and checking ignition timing and the operation of automatic advance and retard mechanisms by the use of a timing light.
The author then proceeded to explain the application of all these instruments, this being helped by lantern slides. He folkiwed this by pointing out how such equipment could assist road transport undertakings and service stations.
For shop service, he advised, as essential equipment, a complete electrical test bench, an armature-test growler, puller press for dismantling and removing bearings, etc., pole screwdriver for removing and fitting field coils, ball-race extractor, mica undercutter and remagnetizer. An appendix included a complete procedure for diagnosis and tuning.
Answering questions during the discussion, Mr. Mundy said that the cost of a complete analyser should be under £250, but units of it could be bought separately. Sparking plugs need not necessarily be of the same make, but should have the same characteristics, particularly in respect of temperature.
Localized heat in an engine may cause' hot plugs, for which the plugs are not responsible.
The drop in potential across breaker points should not exceed 0.1 volt.
Uniformity of compression was more important than anything else. He added that practical proof by road tests is always advisable.