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Blown Engines Need Strict Attention

4th September 1959
Page 56
Page 56, 4th September 1959 — Blown Engines Need Strict Attention
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GIVEN a continuous supply of clean oil at a high enough pressure, the bearings of a turbocharger should last indefinitely, but even a few seconds of operation at speeds of up to 100,000 r.p.m. without oil pressure can damage them beyond repair. This was one of the points raised by Mr. James A. Hardy of the Schwit2er Corporation, at a meeting of the Society of Automotive Engineers in America. He also emphasized the need for large air cleaners which would supply cold air to the turbocharger in sufficient quantities.

Mr. Hardy summarized the servicing and maintenance of turbochargers by listing six "do's" and five "don'ts" These are the things which must be done:

(1) Maintain air cleaners so that highpressure drops do not exist. (2) Keep all intake and exhaust manifold connections tight and leakproof. (3) Maintain the engine so that oil pressure to the turbocharger remains high and constant. (4) Clean and inspect the turbocharger often enough to prevent excessive dirt accumulation, and catch bearing wear and minor damage before it becomes serious. (5) Keep engine-oil filters clean. (6) Be particularly careful not to let any foreign material get into the intake or exhaust systems.

"Don'ts" are: (I) Overfuel the engine.

(2) Operate beyond the safe altitude limit without derating the fuel pump. (3) Apply exhaust pipes or silencers that would cause excessive back pressure in the exhaust system. (4) Let heated air go into the turbocharger. (5) Use undersized and restricted inlet piping and air cleaners.

Mr. Hardy emphasized that particular care must be taken to avoid overfuelling the engine. for Which reason the fuel pump had to be accurately calibrated. No fuel-pump adjustment must be made without comprehensive instruments to check the engine load, speed, fuel rate, exhaust temperature and the turbocharger speed.

This problem stemmed from the fact that a turbocharged engine normally ran with relatively large amounts of excess air. Thus, it could burn more than the rated amount of fuel, but as more fuel was burned, so the turbocharger would run faster and supply stilt more air. Therefore, long before enough fuel could be supplied to use all the air, the engine would be severely overloaded, resulting in either an engine or a turbocharger failure.

Increases in altitude had been shown to produce effects similar to overfuelling, in that they caused both exhaust temperature and turbocharger speed to rise. Mr. Hardy thought that a reliable automatic altitude control in the fuel pump would be highly desirable on engines likely to be operated in a wide range of altitudes, but that the height at which an engine needed to be derateel depended on the characteristics of the particular engine and turbocharger.

Intake Restrictions

It was essential to use large air cleaners, because a turbocharged engine consumed almost double the amount of air required by a normally aspirated unit of the same cubic capacity. Air-cleaner and intake restrictions would reduce air supply to the engine and cause high exhaust temperatures.

If dry-type air cleaners were used. special care had to he taken to ensure that they were large enough and serviced often enough to prevent excessive pressure drops. An indicator which provided warning that the pressure drop through the cleaner had become excessive had been found to prevent trouble from this source. [The British Cooper dry air cleaner has a whistle indicator.] High oil pressure was essential to lead heat away from the turbocharger bearings, and it was easy to tell whether a turbocharger had been operated without sufficient oil pressure because the compressor shaft would be blue from the heat generated.

Factors likely to cause a pressure drop at the blower included excessive bearing wear in the engine, oil-pump wear. defective operation of the pressure relief valve and faulty oil lines.


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