FAULTS IN THE ENGINE

Press, Volume LXXIV, Issue 22597, 30 December 1938, Page 6

FAULTS IN THE ENGINE

DEALING WITH MISFIRING AND DETONATION

HINTS FOR THE AMATEUR MECHANIC

There is no need to-day for a motorist to be a skilful mechanic, but there is an advantage in kneeing one or two elementary things about a car engine, particularly if the owner frequently undertakes long journeys in the country.

For example, misfiring and detonation are nearly always due to very elementary and simple causes, but how many drivers who have turned to motoring within the last few years can attempt to cure one or the other with a reasonable hope of success? The most common cause of misfiring is a faulty sparking plug. When a car j has exceeded 10,000 miles, one or more of the plugs is likely to break down in service. Even if they do not fail completely, the chances are that they will fail partially or intermittently, and many a sluggish and irregular engine has been restored to something like its original condition by the simple expedient of replacing the whole set. But we are dealing at the moment with a regular and complete misfire on one of the cylinders. and the chances are about 10 to one in favour of the completely broken down sparking plug. Hence,, at least two spares should always be carried in the car. Discovering the faulty plug is a simple matter, but possibly unknown to some of the more recent motorists It consists in short-circuiting the central electrode of each plug m turn against the body of the engine with a screwdriver, and the faulty plug is found when this treatment docs not make any further difference to the running of the engine. The screwdriver must have a wooden or bakelite handle, and care mCist be observed that the metal part of the screwdriver is not touched with the fingers. Otherwise, the amateur mechanic will receive the full benefit of the high tension current. Replacement will effect a more or less permanent cure if the engine is in good order, but if that same cylinder soon starts mis-flring again more than likely the plug is “oiling up,” due to wear in the cylinder bore or a broken piston ring, and rectification of that is beyond the scope of the amateur. On the other hand, it is barely possible that the unevenness is caused by a tappet adjusted so tightly that it rides against the base of the valve stem, and it will be worth while examining both tappets of that cylinder to make sure that there is a certain amount of play between tappets and valve stems when both valves are in the closed position. About the only other thing which can cause the trouble is a fault in the high-tension lead running to the plug—a break in the insulation which allows the current to earth itself against the cylinder block. Detonation Detonation is the actual explosion of the mixture in the combustion chamber instead of the extremely rapid burning which should take place. Its occurrence in an engine which formerly has behaved well nearly always indicates an excess of carbon deposits. These have the effect of restricting the available space for combustion, and thus increasing the compression pressures beyond the point's© carefully worked out by the designer so that the mixture explodes on ignition. A top overhaul at the local service station is indicated. It may, however, be caused by using a low-grade fuel with the ignition too far' advanced, and experiments may be made with the little adjustment fitted at the side of the ignition distributor head on most cars of to-day. This is expressly provided for fitting a high compression modern engine to the use of bath low and high octane value fuels. On the contrary, this adjustment sometimes slips back to the fully retarded position, so that there is a very mysterious loss of power. In such cases it is the first thing that should be examined.

A phenomenon sometimes encountered and having a great deal to do with detonation is preignition, in which an engine continues to run for a few revolutions after the ignition has been switched off. This is caused by some small projection within a cylinder becoming incandescent under the heat of combustion, and usurping the function of the electric spark by firing the mixture before the proper moment and continuing to do so after the engine has been switched off. Sometimes a small projecting piece of carbon becomes red hot, and is responsible. In other cases the wrong type of sparking plug for that particular engine is fitted, the electrodes becoming incandescent. The plugs may rightly be suspected if the electrodes burn away in service far more rapidly than seems reasonable. A “colder” type of plug, that is, one with a greater resistance to heat, should be fitted.

Finally, intermittent misfiring may be caused by blackened and pitted contact breaker points, exposed when the bakelite cover of the distributor is lifted. They may be cleaned with a fine magneto file, available from any accessory store for a few pence. If the owner has occasion to replace a set of sparking plugs he must be careful not to mix the high tension leads running to them from the distributor. If he does, fireworks will result. DIESEL PROGRESS Most of the developmental work on the compression ignition or Diesel engine has taken place in England and Europe, with England undoubtedly leading the field at the moment with some of her high-speed, smoothrunning units. , , America entered the field fairly late, but her engineers have been working away quietly to such good effect that on of the largest manufacturers has announced a complete range of Dieselengined commercial vehicles for 1939. The engine used has many original points. It is a two-stroke design, but also has a simplified injection apparatus which, if it is as good as it appears to be on speefleations, will give a great impetus to the compression ignition principle. Practically all Diesel engines of today use a fuel injecting pump which, b-cause it has to calibrate the fuel so exactly for the different speeds, must be made to the fine limits of a wrist watch, and, by the same token, has much of the watch’s delicacy. It is an expensive unit which to some extent has held back the production of Diesels.