Diesel—Engine That’s Different
Lack of spark ignition system, easy starting, idling ability, less maintenance among advantages cited by California chief
The value of the diesel engine to the fire service lies in its dissimilarity to a gasoline engine in design and operation.
The primary difference between gasoline and diesel engines is the ignition method. The diesel is a compression ignition engine and it is free of all the trouble inherent in a gasoline engine’s spark ignition system components, including spark plugs, ignition wires (with their radio interference),
coils, condensers and distributors. The diesel engine achieves ignition by injecting atomized fuel directly into the air in the cylinder, which is heated to nearly 1000*F by compressing it at 500 to 600 psi.
Compression ignition provides instantaneous starting, instant power, no loading or fouling during idling, reduced maintenance and increased reliability.
The change that opened the fire apparatus field to dieselization was the improvement in cranking capability to
provide instantaneous starting, a factor vital to the fire service. Due to improved fuel pump and fuel injector design and increased fuel compression ratios, diesel engines now excel gasoline engines in starting. In my own department, we found that the diesels normally fire on the first revolution of the flywheel and never later than the second, while our gasoline engines normally require 3 to 5 revolutions of the flywheel to achieve ignition.
Because a diesel has compression ignition. not only are starting time and reliability improved, but also full power is available immediately. With gasoline engines, the choking required for cold starts reduces power, both instantaneous starting and full power art’ important pluses the diesel engine offers to the fire service. Dynamometer tests, field tests and fire department experience all substantiate the claim that with a diesel, full horsepower is available immediately after ignition.
One of the weaknesses of gasoline engines is their inability to idle for extended periods without developing carburetion problems from loading and engine overheating. The diesel engine does not have these drawbacks. A large percentage of fire apparatus operation is under idling conditions. The diesel engine, without a carburetor to load up, spark plugs to foul and distributor contact points to arc, can idle all day without any difficulty. The diesel is also more economical than its gasoline counterpart while idling.
Gasoline engines in the 300-hp class idle only 45 to 50 minutes on a gallon of fuel. Diesel engines of comparable horsepower will idle from 3 3/4 to 4 hours on a gallon of fuel.
No overheating at idle
The combustion principle of the diesel engine provides for the introduction of 25 percent more fresh air into the cylinder for intake and exhaust functions than the cylinder can hold. This assures complete exhaust heat removal and prevents overheating during long idling. Under any operation, a diesel receives 50 to 60 percent more fresh air in the combustion chamber than its counterpart gasoline engine.
In gasoline engines, the coolant load remains high at light loads and slow speeds when the fan and water pump have slowed to their least efficient output. The diesel coolant load requirement is automatically reduced when the load drops during idling. A diesel’s fan and radiator are designed for maximum heating, which occurs at maximum load, and they can easily cool the engine under a light load or low speed conditions even with the water pump and fan speeds reduced.
The next point of difference I listed was the lower maintenance factor of diesels because they have no spark ignition system. According to the Society of Automotive Engineers, 75 to 85 percent of the maintenance and service required by gasoline engines is attributable to ignition difficulties.
In the July 1967 issue of Diesel and Gas Turbine News, an article quoting Burton G. Clark, battalion chief in charge of the New York City Fire Department’s division of repairs and transportation, states: “More than three-fourths of our shop mechanic fire apparatus engine service calls are made because of carburetion and ignition failures. In 1965 and 1966, our 17 diesel pumpers amassed a total of over 2(X),000 engine miles without a single engine service call while responding to 50,000 calls and performing their inspection duties and drills.”
Cliief David Gratz of Silver Spring, Md., in the March 1967 FIRE ENGINEERING quoted one of his department mechanics as saying, “Nothing could be more difficult than properly adjusting a dual ignition, dual carburetor gasoline engine.”
Certainly the lack of a spark ignition system and the absence of overheating during long periods of idling add to the reliability of diesel engines, but there are other examples.
The Mack Company on July 5, 1964, took a standard production model Mack C-95 pumper with a 1,000-gpm pump and a standard production model Mack END TF 675 diesel engine rated at 237 hp at 2,300 rpm and started drafting from the Detroit River. The pumper, at an 11-foot draft, discharged 1,000 gpm at 150psi net pump pressure. The engine pumped continuously for 168 hours before it was voluntarily shut down. During the seven-day, 168-hour test, the pumper delivered over 10 million gallons of water at 150-psi pressure and used 1,108 gallons of fuel for a 6.5-gph average consumption.
The significant comparison is that this sevefi-day test was the equivalent of 10 years of normal fire department operation.
In the field of commercial truck service the diesel engine has established an unexcelled record of trouble-free service. It is not uncommon for heavyduty over-the-ioad diesel trucks to run 15,000 hours or up to .500,000 miles before major overhauls.
There are some diesel engine disadvantages. I think they are minor, but they involve odor and dirt, noise, vibration, heating, driving difficulty and maintenance complications.
One of the most common complaints by the non-diesel user is that diesel fuel is dirty and smelly and the exhaust gases are objectionable. My reply is a qualified “yes.” I am sure a closed fire station will be rapidly polluted with diesel odors if an engine is run for an extended time indoors, but a gasoline engine is considerably more hazardous to personnel because of carbon monoxide coming from a gasoline engine exhaust. Diesel exhaust fumes contain 1/10 of 1 percent carbon monoxide, and gasoline exhaust contains 6 to 7 percent carbon monoxide.
Responding to this objection in the March 1967 FIRE ENGINEERING, Gratz stated, “Our experience does not indicate any excessive odor or smoke. Most of these problems can be traced to a poor grade of fuel and improper adjustments in the fuel systems. Commercial deodorant additives are available for diesel fuel.”
There is no denying diesels are noisy. However, with one single exception, this has not presented a problem in our department. That exception is the reading of voice radio transmissions en route to a call, particularly during gear shifting. Our answer has been to mount (as many fire departments do) an all-weather speaker behind the captain’s seat and place a second speaker at the pump control panel. The noise level is actually not much higher than from our 300-hp class gasoline engines, and it is not impossible to live with.
Vibration is a complaint that has not been substantiated by our diesel engine experience. When our diesel pumper was undergoing its acceptance test in Los Angeles, an assistant chief from the Vernon Fire Department balanced a quarter on edge on the front fender of the pumper while it was pumping 1,560 gpm at 150 psi. The quarter remained in the balanced position for 10 minutes until he removed it.
I have refuted the heating objection in discussing the heating factors under idling conditions. Diesels can operate at maximum power output or at minimum load for unlimited periods of time and never overheat.
Diesel easier to drive
The plaint that diesels require special driving skills is not true in Modesto. The reverse is actually the case. The large gasoline engine requires greater driver skill, and it is less tolerant of lugging or peaking than the diesel. In most cases, the higher net horsepower available in the diesel engine and the higher torque at lower rpm means less shifting and, consequently, a reduced demand on the driver’s shifting ability.
That diesels are complicated to maintain may have been true in the days of the old-style massive fuel pump that required two men and a boy to time and synchronize. It is no longer the case. Your own mechanic can be trained in a very short time in diesel service and repair. Also, independent shop service is available in nearly every community.
Most diesel manufacturers today make only two series of truck engines, so the variations in mechanical problems are a fraction of those experienced in gasoline engine maintenance. Most fire chiefs are concerned with tliree major factors in selecting an engine: cost, efficiency and obsolescence. The cost includes the purchase price and operation and maintenance expenses. Efficiency is concerned with the net horsepower, adaptability to local conditions and fuel economy. Obsolescence involves the availability of parts and the stocking requirements for parts and tools for the life of the engine.
Gasoline and diesel engines of comparable quality and capacity are competitively priced. Due to the net horsepower differential, smaller diesel engines can provide the same power available from higher horsepower gasoline engines, so savings may actually occur when diesel is specified. When the Modesto Fire Department obtained a cost estimate for replacing a gasoline engine, the prices for a 324hp gasoline engine and a 335-hp diesel were identical.
On a net horsepower basis, a diesel is your best buy.
While the purchase price of an engine is important, the operating cost is probably of greater concern. An engine may cost $5,000, but the fuel bill for the life of the engine may amount to twice that much. Diesel engines for fire apparatus application operate efficiently and economically on No. 2 diesel fuel, which is equivalent in price to regular gasoline in most areas and is readily available.
Various manufacturers and users of diesel engines quote figures reflecting 33.3 to 87.5 percent reductions in fuel consumption, compared to gasoline engines.
In the article previously mentioned, Gratz stated, “Our aerial ladder was repowered with a diesel engine, and after a year of service, we recorded the following cost data:
“The large gasoline engine removed from the aerial cost 12 1/2 cents per mile for fuel. The new diesel engine, having greater horsepower, has recorded a decrease in fuel costs to 3 1/2 cents per mile, or a savings of 9 cents per mile. With the apparatus travel for one year being approximately 2,500 miles, the annual savings in fuel cost alone amounted to $235. The extended life of the aerial is expected to be 20 years . . . This will produce a net savings of $4,700 over the fuel cost for operating a comparable gasoline engine.”
Gratz also said, “In another station we have two identical pumpers, one with a gasoline engine, the other with a diesel. The diesel unit is operating at 8 cents per mile cheaper, resulting in an annual fuel savings of $250.”
However, our experience with diesel in Modesto has not produced fuel economies of this magnitude. On pumping operations, we have recorded a high of 6.8 gallons per hour of fuel and a low of 4.9 gallons per hour. W e have operated the pump at 1,000 to 1.800 gpm for eight hours on a 45-gallon tank of diesel fuel, and the pumper still had enough fuel to return to the station.
Based upon our experience, you can expect a 50 percent reduction in fuel consumption with a diesel. Conservatively, I would say from our two years of experience you can figure on about $100 a year fuel savings. I recommend you specify an engine that performs satisfactorily on No. 2 diesel fuel if you are interested in fuel costs because No. 1 is a premium fuel that costs double the price.
I he third cost factor we must consider in the fire service is maintenance and repairs. Previously I cited the 75 percent reduction in engine service resulting from the elimination of the spark ignition system as a major savings. The rugged construction of the diesel engine necessary for high compensation ratios makes the diesel less susceptible to failure than lower compression gasoline engines.
Cheaper fo repair
The family of engines concept with its high interchangeability of components reduces parts costs. Conservatively, I can state that repairs for a diesel engine will average 50 percent less than similar repairs for gasoline engines. Diesel manufacturers offer 9 or 10 different engine models and capacities, all using the same component parts, pistons, rings, valves, injectors, etc. There is an actual 70 to 80 percent parts interchangeability with a single manufacturers complete series of automotive diesel engines. This permits greater volume production of replacement parts, resulting in lower production costs and smaller dealer inventories. All this is reflected in reduced maintenance costs.
Limited obsolescence of engines and parts due to a minimum of engine design changes eliminates the high cost of hand-producing repair parts for engines still in service but not in production.
Our concern with any engine is power production and basically with net horsepower. The mere fact that a gasoline engine has a listed dynamometer rating does not tell the whole story. Your interest is in the net working horsepower at the full load governed rpm.
Loss in rafed horsepower
Hubert Walker, in one of his monthly apparatus maintenance columns in FIRE ENGINEERING, “Calculation of Horsepower and Torque Curves of Engines” (June 1968), points out the tremendous inconsistency in establishing power factors for gasoline engines. Walker suggested that at best we should consider only 85 percent of the rated horsepower at governed rpm as available power. By comparing the 15 percent loss in gasoline engine horsepower with a maximum of 3.5 percent reduction of rated horsepower in diesel engines, you can readily see that the efficiency of the diesel engine is much greater than that of the gasoline engine.
The diesel engine has a compression ratio varying with manufacturers from a low of 16 to 1 to a high of 21 to 1, compared to a low of 6 to 1 to a high of 9 to 1 for gasoline engines. The diesel, because of its high compression, converts a greater percentage of fuel energy into power, and less fuel energy is lost in the exhaust than in gasoline engines.
With engines of equal horsepower rating at full load, the gasoline engine burns about 43 percent more fuel than the diesel engine. At one-fourth load, the gasoline engine uses twice as many gallons of fuel, and at idle speed, the gasoline engine uses four times more fuel than the diesel.
Diesel engines operate equally efficiently below sea level and at altitudes to 12,000 feet above sea level and in temperatures well below 0°F to above 100°F.
A constant concern and expense for the fire service is the rapid obsolescence of gasoline engines. It is common for a department to scrap a little used and far from womout fire truck when the cost of maintaining it becomes prohibitive because of the obsolescence of parts for repairs.
Diesel manufacturers say that the design changes in the basic engines have been so limited over a 30-year period that engines purchased in 1938 can still be serviced and can, for a fraction of the cost of replacement, be updated to take advantage of improvements in injectors, fuel systems, starting systems, etc. And parts to service engines 30 years old are still available at competitive prices. Diesel engines are the best answer to your problem of cost obsolescence.
It seems reasonable to conclude that the move to diesel-powered fire apparatus is a fact of life. The transition through new purchases has gone from 30 percent diesel in 1966 to an expected 90 percent this year, so it seems safe to predict that the diesel engine will replace gasoline-powered fire apparatus much faster than the gasoline engine replaced horse-drawn apparatus.