Preventing and Correcting Diesel Engine Troubles

Preventing and Correcting Diesel Engine Troubles

Apparatus Maintenance

Tabulation of the possible causes of trouble for the diesel engine in fire service shows preventive maintenance must start with the proper selection and storage of diesel fuel oil. Poor quality fuel oil heads the list of possible causes of trouble. Under this head is fuel oil not in conformity with the recommendations of the diesel engine manufacturer, dirt and water contaminants, improper storage of fuel, and mixing fuel from different suppliers.

The high-performance, high-speed automotive diesel engines used in fire apparatus will not “just run on anything” and still give the long life with low maintenance cost of which they are capable. While low-speed diesel engines used in industrial and marine service will operate on inferior grades of fuel oil, the performance and reliability required in automotive service cannot be obtained by use of fuels that do not meet engine manufacturer’s specifications. Fuel oils are marketed in many classifications. Four of the main classifications are:

  1. Residual fuel oils
  2. Crude petroleums and weathered crude petroleum
  3. Distillate fuel oil
  4. Blended fuel, a mixture of two or more of the above

Basic specification: Of these, only a completely distilled diesel fuel oil meeting the engine manufacturer’s specification is acceptable. The basic specification for fuel oil for automotive diesel engines is ASTM D-975 (American Society for Testing and Materials). This specification is modified by specific limits on API (American Petroleum Institute) gravity, cetane number and total permissible sulfur. In some areas, distributors may be marketing only one fuel oil, which they sell as either diesel fuel oil (ASTM D-975) or as a domestic heating fuel oil (ASTM D-396). Such fuel oil will usually be low in API gravity, that is, less than 34 at 60°F, which will contribute to sludging in the crankcase as the combustion is incomplete, cause sticking valves, piston rings sticking and other troubles attendant with the use of a heavy oil.

The cetane rating will also be low, less than 40, to cause hard stalling, dilution of the lubricating oil and increased knocking. Sulfur content will usually exceed the 0.5 percent limit to cause excessive cylinder and piston ring wear.

To avoid these troubles, insist on the diesel fuel oil supplier submitting a report on the test of each lot of fuel oil delivered to be sure it meets the engine manufacturer’s specification.

The storage and handling of diesel fuel oil is most important. If stoppage of fuel screens, filters and lines is to be avoided, care in handling and storage must be exercised. Scale from suppliers tanks and pipelines and from the fire department storage tank is not an uncommon contaminate of fuel oil. The fuel oil should always be filtered as it is pumped from the storage tank into the fuel tank on the fire apparatus.

Galvanized steel danger: Never, but never, use fuel storage tanks made of galvanized steel, or use galvanized piping, because there is a chemical reaction between the zinc (galvanized coating) and fuel oil. Small flakes of zinc are formed to clog the fuel strainer and filter and possibly damage the fuel pump and injectors.

Condensation of the moisture in the air occurs in both the fuel storage tank and the fuel tank on the fire apparatus. This contamination of the fuel oil with water is in varying degree, depending on the volume of air in the tank and the breathing action that takes place through the breather or vent due to temperature changes. Dirt suspended in the air is also introduced into the tank by this breathing action. This condensation can be almost eliminated by keeping the tanks full, particularly the fuel tank on the fire apparatus. The screen used at the storage tank pump must have a fine mesh to filter out rust, dirt and water.

That fuel oil from different refiners may not be compatible and should not be mixed has been discussed previously, so sufficient to mention that sludging and varnish formation are two of the problems developing from mixing fuel oil from different suppliers. Sometimes sludging may be caused by a fungus, requiring treatment with a biocide. Such treatment should only be undertaken by or with the cooperation and advice of the fuel oil supplier.

High fuel use causes: Excessive fuel consumption can be caused by:

  1. Poor quality of fuel
  2. Restriction in air intake
  3. Exhaust back pressure too high
  4. Restriction in fuel system
  5. External leakage in fuel system
  6. Low compression pressure
  7. Leaking cylinder head gasket
  8. Spray nozzle leaking, incorrect opening pressure
  9. Plugged spray holes in injector
  10. Cracked injector body
  11. Valves sticking, not seating properly
  12. Injection pump plunger(s) worn or stuck
  13. Injection rack stuck
  14. Scored piston rings or cylinder sleeves
  15. Piston rings worn, broken or improperly fitted

Items 1 through 7 were discussed in some detail in this column in the December 1969 issue of FIRE ENGINEERING. Only after tliese items have been checked, and only then, should the balance of the items, 8 through 15, be checked if the trouble is not located and corrected.

In every maintenance problem, the cause should be determined. It is not enough to mechanically correct some trouble or defect, but determine what caused the condition for action that will assure the problem will not recur.

Power loss causes: Loss of power can be caused by the following:

  1. Poor quality of fuel
  2. Restriction in fuel system
  3. Restriction in air intake
  4. High back pressure in exhaust system
  5. High ambient temperature, or high altitude
  6. Water in fuel
  7. Fuel tank vent partially plugged
  8. Air leak in suction part of fuel system
  9. Pressure regulator set incorrectly
  10. Setting of high-speed governor too low
  11. Long idle periods
  12. Low compression pressure
  13. Leaking cylinder head gasket
  14. Valves sticking and not properly seating
  15. Worn or broken piston ring(s)
  16. Injectors need adjustment
  17. Plugged injector spray nozzles
  18. Throttle linkage needs adjustment
  19. Injection rack stuck
  20. Scored piston rings or cylinder sleeves
  21. Return fuel temperature too high

Continued on page 53

apparatus maintenance

Continued from page 49

A high ambient temperature (5 above—temperature in the engine compartment) sufficient to cause a noticeable loss of power would indicate overheating of the cooling system, with temperatures of tire coolant exceeding 185°F. The quickest relief is opening the engine hood, which will probably provide temporary relief by permitting cooler air to enter the engine air intake. This condition may be caused by overloading the engine, assuming the cooling system is working properly. There is a power loss of approximately 3 blip for each 10°F air temperature rise above 90°F at the engine air intake. If the condition persists with coolant temperatures in the 160°F—185°F operating range, it may be necessary to extend the engine air intake at the air cleaner to the exterior of the engine compartment for cooler air.

Long idling periods (11) should be avoided when possible. During such periods, the coolant temperature will drop below 160°F, resulting in cold operation of the engine, producing incomplete combustion of the fuel with consequent dilution of the lubrication oil and formation of sludge. Also, gums or lacquer deposits will form on the valves, pistons and rings.

When long idling is necessary, the engine should operate at not less than 800 rpm to prevent too much drop in coolant temperature.

The return fuel temperature should never exceed 150°F. If the temperature exceeds this limit, the return line may be too small or it may be too close to the exhaust pipe or muffler. In the latter condition, the return line should be moved to a cooler location.

Technical data was obtained through the courtesy of the Detroit Diesel Engine Division, General Motors Corporation, and the Cummins Engine Company, Inc.

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