Proper Maintenance Is Key To Dependability of Diesels
Diesel engines have steadily increased in fire service popularity during the last few years. At the present time, between 40 and 50 percent of all new custombuilt fire apparatus are being powered with diesel engines. Some manufacturers report the percentage of diesel engines being specified is about 75 percent.
A diesel engine powered fire truck was first offered to the fire service about 20 years ago, but little interest was shown. Within the last 10 years, however, there has been a growing acceptance of diesels.
There must be a reason for this trend, particularly when a diesel costs more than a gasoline engine of comparable power. (We are talking about net brake horsepower with a maximum power rating at full load governed speed.)
Diesel advances: With the more recent diesel engine weights comparable to those of gasoline engines of equivalent power, easier starting and improved fuel injection systems, the diesel engine has become well suited for emergency fire service. We recommend a review of the article, “Diesel Engines for Fire Departments,” by John Lehoczky of the Fire Apparatus Division of Mack Trucks, Inc., in the May 1964 issue of FIRE ENGINEERING. The two-cycle diesels made by the Detroit Diesel Engine Division, General Motors, also are to be included in the list of diesel engines being used in increasing numbers.
Sufficient time has elapsed since the delivery of a significant number of diesel engines to the fire service to provide an indication of relative performance. Nearly every fire department purchasing dieselpowered fire apparatus has gasolinepowered fire apparatus working under practically identical conditions. So, a valid comparison can now be made.
As with most new things, there was the initial enthusiasm which had to be tempered by time and experience. Singularly, most of this enthusiasm still persists. The reason, we find, is in the consistent and dependable service rendered by the diesel engine.
Maintenance vital: To achieve this record performance, in addition to the built-in features of the diesel engine, there is one important key—preventive maintenance!
Where the engine manufacturer’s instructions have been faithfully followed, the results have been excellent. Where trouble has developed, manufacturers have given fast service to correct any problem, and this has created considerable goodwill.
Where strict attention is paid to fuel specifications, engine oil specifications, engine oil and oil filter changes, fuel filter changes, and air cleaner maintenance, no problems have developed with diesel engines.
Heavier batteries supplied: Batteries must be kept charged. Even though they are not required for ignition, they are most essential for starting diesel engines. In some installations, the starting motor is 24-volt, supplied by two 12-volt batteries connected in series for starting use only. Also, heavier batteries with a higher ampere-hour rating are often supplied.
The economy of the diesel engine, while not considered too important, does make a difference in operations. One fire chief recalled that at a recent allnight fire, a diesel pumper operating with four 2 1/2-inch lines did not require added fuel and had better than a quarter of a tank when it returned to quarters. The gasoline-powered pumpers, not doing any more work, required filling the fuel tanks twice. And the tanks were all the same capacity!
It is necessary to pay very strict attention to the fuel used in a diesel engine to have continued reliable operation. There are three important properties that must be considered in the fuel selection: distillation range, sulfur content and cetane number.
Cetane rating: First we have a new word, cetane. The ignition quality of diesel fuel is measured by the time lag between fuel injection and ignition, and this measurement is in cetane units. The higher the cetane number, the shorter the ignition lag. In cold weather this cetane rating becomes more important. Normally, the engine manufacturers specify a minimum cetane rating of 40 for No. 2D fuel. At high altitudes and low temperatures, it is sometimes necessary to use fuel with a cetane rating of 45. A sample of the oil should be subjected to temperatures at least 10 °F below the lowest ambient temperature you will experience in your area. This is to check the cloud point, which is the point at which wax crystals begin to form. These crystals can, and will, clog fuel filters.
The fuel should meet the ASTM designation D-975-60T, 2D with a maximum sulfur content of 0.50 percent (1/2 of 1 percent).
The distillation range (625 to 675°F) controls the volatility of the fuel and is very important. Less volatile fuels leave a tarry residue and produce smoking.
Cleanliness of the engine, as well as crankcase dilution, are affected by the sulfur content of the fuel. Engine wear is directly related to the amount of sulfur in fuel oil.
Do not mix diesel fuel from different sources. Buy from a reliable supplier and do not change. Some mixtures of diesel fuel are not compatible and precipitate sludge which will clog fuel filters.
The lubricating oil for diesel engines is a DS oil of S-l type, which conforms to Mil-L-2104A. In fire service, the Series 3 type is not usually recommended. The S-3 type tends to form excessive ash deposits, resulting in valve-burning and top ring-sticking.
The multi-graded oils are not recommended, unless the apparatus is stored in an unheated building subject to very low temperatures.
Pump gear ratios: The one complaint we have heard is that fire apparatus manufacturers seem to ignore the difference in characteristics of diesel and gasoline engines, and they use the same pump drive gear ratios for both engines. In such an installation, a diesel engine will run near its full load governed speed when the pump is operating near its rated capacity. Even at lower pump discharges, the engine sounds like it is about to take off in flight, and the roar is very fatiguing to an operator.
One fire chief is partially solving the problem by operating the pump with the road transmission in overdrive. It’s not the best way to do it, but he gets some relief.
Diesel engines are governed in the range of 2100 to 2300 rpm at full load. The pump ratios should be selected to provide capacity rating of the pump at 1700 to 1800 rpm of the diasel engine.
There can be no doubt that with the fine service record that diesel engines have made, the future will see a continuous increase in the percentage of diesel engines in the fire service.