Quantcast Causes of Unbalance

indicator cock on each individual cylinder and check the color of the exhaust. 2.   High   exhaust   temperatures.   If   the temperatures of exhaust gases from individual cylinders become higher than normal, it is an in- dication of an overload within the cylinder. If the temperature of the gases in the exhaust header becomes higher than usual, it is an indication that all cylinders are probably overloaded. Frequent checks on the pyrometer will indicate whether each cylinder is firing properly and carrying its share  of  the  load.  Any  sudden  change  in  the exhaust temperature of any cylinder should be investigated immediately. The difference in ex- haust  temperatures  between  any  two  cylinders should  not  exceed  the  limits  prescribed  in  the engine manufacturer’s technical manual. 3.  High  lubricating  oil  and  cooling  water temperatures. If the temperature gages for these systems show an abnormal rise in temperature, an overloaded condition may exist. The causes of the  abnormal  temperature  in  these  systems  should be determined and corrected immediately if engine efficiency is to be maintained. 4. Excessive heat. In general, excessive heat in   any   part   of   the   engine   may   indicate overloading. An overheated bearing may be the result of an overloaded cylinder; or an abnormally hot crankcase may be the result of overloading the engine as a whole. 5. Excessive vibration or unusual sound. If all cylinders are not developing an equal amount of power, the forces exerted by individual pistons will be unequal. When this occurs, the unequal forces cause an uneven turning movement to be exerted on the crankshaft, and vibrations are set up. Through experience, you will learn to tell by the vibrations and sound of an engine when a poor distribution  of  load  exists.  You  should  use  every opportunity  to  observe  and  listen  to  engines running  under  all  conditions  of  loading  and performance. Causes of Unbalance An   engine   must   be   kept   in   excellent mechanical  condition  to  prevent  unbalance.  A leaky valve or fuel injector, leaky compression rings, or any other mechanical difficulties will make it impossible for you to balance the load unless you secure the engine and dismantle at least a  part  of  it. To  obtain  equal  load  distribution  between individual cylinders, the clearances, tolerances, and the general condition of all parts that affect the cycle must be maintained so that very little, if  any,  variation  exists  between  individual cylinders. Unbalance will occur unless the follow- ing conditions are as nearly alike as possible for all  cylinders: 1. Compression pressures 2. Fuel injection timing 3. Quantity and quality of fuel injected 4. Firing pressures 5. Valve timing and lift When unbalance occurs, correction usually in- volves repair, replacement, or adjustment of the affected part or system. Before any adjustments are made to eliminate unbalance, it must be deter- mined beyond any doubt that the engine is in proper  mechanical  condition.  When  an  engine  is in good mechanical condition, few adjustments will be required. However, after an overhaul in which piston rings or cylinder liners have been renewed, considerable   adjustment   may   be necessary. Until the rings become properly seated, some  lubricating  oil  will  leak  past  the  rings  into the combustion space. This excess oil will burn in  the  cylinder,  giving  an  incorrect  indication  of fuel oil combustion. If the fuel pump is set for normal compression, and the rings have not seated properly, the engine will become overloaded. As the compression rises to normal pressures, there will be an increase in the power developed, as well as in the pressure and temperature under which the combustion takes place. Therefore, when an overhaul  has  been  completed,  the  engine instruments must be carefully watched until the rings are seated, and all necessary adjustments are made. Frequent compression tests will serve as a helpful aid in making the necessary adjustments. Unless an engine is so equipped that compression can  be  readily  varied,  the  engine  should  be operated  under  light  load  until  the  rings  are properly seated. Chapter  5—ENGINE  PERFORMANCE  AND  EFFICIENCY 5-5


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