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Why Oil Engines Reverse

3rd August 1956, Page 54
3rd August 1956
Page 54
Page 57
Page 54, 3rd August 1956 — Why Oil Engines Reverse
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Which of the following most accurately describes the problem?

By John Savage, A.I.R.T.E.

UNLESS suitable precautions are taken, there is nothing to stop a compression-ignition engine. from starting backwards and continue so to run. In certain circumstances, the results may be serious, because, in the case of a suction-operated governor, the .pump will be set to the full-delivery position.

Whilst the . problem is not new, .there is evidence that greater attention is being paid to its prevention. This is shown by the. number of patents which has been filed during the past 12 months in which the subject-matter has been based on suggested means for curing the tendency.

The propensity of an oil engine to run backwards is inherent in most conventional designs. The inlet and exhaust-valve operation is sufficiently symmetrical to Permit the engine to function in either direction of rotation. Injection timing is not normally affected, except perhaps for the slack in the timing-chain system, as . injection-pump cam lobes are usually symmetrical. Hence, injection occurs at the same number of degrees before top dead centre, and at the same rate, whichever way the camshaft is rotating.

II, for any reason, the engine is contra-rotated, air is drawn in through the normal exhaust system and the exhaust gases are exhaled through the inlet manifold and the air cleaner. The exhaust valves take over the duties of the inlet valves, and the inlet valves act as exhaust valves. The oil in the oil-bath-type air cleaner is thrown out, and the cab soon fills with exhaust smoke.

The most common cause of reverse running is if a driver stalls his engine when reversing up a gradient, declutches and then re-engages the clutch while rolling forward down the slope with the reverse gear still engaged. The heavier the vehicle and load in relation to the engine size, or the steeper the incline, the easier the engine will be forced over compression in the wrong direction.

Reverse Running on StoppIng

Some cases of backward running have been reported to have occurred at the instance of stopping, due to the engine rocking backward as it came to rest. For this to happen the driver must release the stopping control at the moment the engine turns over compression in the wrong direction. Attempts to produce this condition to order have failed, which would indicate that the chances of it

happening, are slender. However, certain engines may be more susceptible than others.

Unfortunately, the smaller types of oil engine which are the ones more easily forced into reverse running are usually fitted with pneumatic engine-speed governors. If a pneumatically governed engine runs backward, the governor diaphragm is pushed forward by the exhaust gases which pressurize the inlet (which has now become the exhaust) manifold. The distended diaphragm carries the control rod of the injection pump into the full-fuel position, causing an uncontrollable rise in speed.

With most designs of pneumatic governor it should be possible to stop the engine by pulling the stop control in the usual way. However, these devices are often operated by flexible cables which are difficult to move in normal eircumstaaces, and when the engine is running in reverse the exhaust-gas back pressure in the governor has to be overcome. It is, therefore, desirable that the stopping controls be kept in good working order. The pressure in the governor can be relieved to some extent by opening the throttle, thus making it easier to pull the control rod to the stop position.

A runaway engine can ultimately be stopped by cutting off the fuel supply but it usually takes too long to be of any practical use in an emergency. A favoured method of stopping the engine in cases of this type is to overload it by engaging direct drive and letting the clutch in with the brakes full on. As this method is liable to ruin the clutch, drivers working under conditions which may produce reverse running should prepare to stop the engine in the usual way.

A centrifugal flyweight-type of governor is not affected by the change of rotation. The weights still move out against the idling springs and the linkage moves the injectionpump control rod to the normal idling or slow-running position. The engine will thus idle in reverse hut will have no tendency to runaway,

Oil engines will not run as efficiently in reverse, but most of them will operate reasonably well. Few engines can be made to run backward when they are cold, hut, under favourable conditions, a warm engine can be forced into reverse running. Once in reverse the behaviour is usually dependent on the type Of governor fitted.

During the past two or three years. engine builders and the manufacturers of fuel-injection equipment have made several _useful contributions towards eliminating • the risk of reverse running. The most simple reverse-running deyice is an injectionpump camshaft with-asymmetrical. cam lobes. When operating normally, . they impart a quick lift to the pumping plunger in the usual way, but the return. to the bottom-of-stroke position is made slowly over the remainder of the periphery. • Inre-terse rotation, the injection stroke is consequently very slow, proper injection characteristics are not obtained and the engine fails to rim.

This method works best on-engines . having direct-injection combustion .chambers. These combustion systems require precise injection features. and if optimum conditions do no': . exist the engine is reluctant to run in the correct direction and, almost certainly, will not run at all in reverse.

.• Air-cell Engines

With pre-conbustion chamber, or air-cell engines, .injection charac teristics can be less rigid. Once warm, some air-cell engines will

tinue to run no matter how the fuel may be injected into the combustion chamber. With engines of this type the slow-lift earn is not' a complete safeguard and some other device is necessary.

When the Standard 2.1-litre engine was first fitted to London's taxicabs, the need for an anti-reverse-running device became eVident. Taxicabs turning in confined places, stopping on hills and bumping up kerbstones are liable to cause an oil engine to run backward especially if the unit be prone to this trouble. Early experience resulted in the production of a uni-directional drive for the injection pump, and the unit is now fitted to all road transport applications of this engine.

The drive consistsof a .sim,ple dog clutch which operates on the freewheel principle. If the engine crankshaft rotates in the wrong direction the pump camshaft remains stationary. The spacing of the three protrusions on the dog clutch is arranged so that the clutch will engage in only one position. In this way the correct timing is maintained when the normal rotation is resumed.

A non-reversing device is now used on the B.M.C. 2.2-litre oil engine which is also -fitted in taxicabs. It consists of a single spring-loaded -'linger that pushes against a pawl

which will drive the pump in only one direction. The Rootes twostroke oil engine is also fitted with a device to prevent back running.

Although these fittings are fully successful in preventing reverse running, some of them are inclined to rattle and they Complicate engine design. As simplification should be a primary objective, deviceswhich complicate design are to be deprecated. The best solution, therefore, lies in the use of an injection pump that is inherently anti-reverse running; one that does not inject fuel, when rotated in the wrong direction.

At least one type of pump embodying this feature exists, and is extensively used in America,

The type of distributor pump that is currently receiving a good deal of attention in America and in Europe has a . rotating distributor which carries two opposed . pumping plungers. The sequence of events that takes place in the process of pumping and distributing ensures that no fuel is injected if the pump be rotated in the wrong direction. Inherent antisreverse running is by no -means limited to this design. Many other distributor-pump arrangements, .both mooted and explored, are

directional in function.

Whether any distributor pump will eyentually he adopted in this country still remains to be decided. Nevertheless, whatever type of fuetinjection equipment is 'tised on the smaller range of oil engines. of the future, some method of preventing reverse running .is likely to be embodied in the design. This may' become an important feature, and selling point, and may ultimately be enforced by law.

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People: John Savage
Locations: London

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