Economics of Diesel Versus Gasoline in Small Wheel TractorsEngineering Features of Diesel Engines 4
Advantages of Gasoline Engine for Small Wheel Tractors 5
Advantages of Diesel Engine for Small Wheel Tractors . 6
Costs of Owning and Operating Diesel and Gasoline Tractors 6 Cost of Ownership . . . . . . . . . . . . . . . 6 Cost of Operation ............... . 7
Cost of Ownership and Operation Per Hour as Related to Hours of Use for Diesel and Gasoline Tractors. 9
Hourly Ownership Cost ... . 10 Hourly Operating Cost ... . 12
Economic Analysis of Diesel Versus Gasoline. 12 Significance of Cost Differences. 15
Sun:nnary. . . . . . . 15
TABLES Number
1. Yearly ownership (fixed) costs of diesel and gasoline tractors .... 7
2. Fuel consumption and fuel costs per hour of operation for diesel and gasoline tractors . . . . . • . . . • . . . . . . . ... 8
3. Cost of owning and operating diesel and gasoline tractors per hour as related to hours of use per year. . . . . . . . . . . . . . 10
4. Total annual costs of owning and operating diesel and gasoline tractors as related to hours of use per year ................. . 13
FIGURES
1. Cost of owning and operating 641 series diesel and gasoline tractors per hour as related to hours of use per year, ........... . 11
2. Cost of owning and operating 841 series diesel and gasoline tractors per hour as related to hours of operation per year ........ . 11
3. Total annual cost, annual fixed cost, and break-even point for 641 series diesel and gasoline tractors as related to hours of use per year. . . . . . . . . . . . . . . . . . . . . . . 14
4. Total annual cost, annual fixed cost, and break-even point for 841 series diesel and gasoline tractors as related to hours of use per year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
THE AUTHORS
JOSEPH T. KEELER is Assistant Agricultural Economist at the Hawaii Agricultural Experiment Station.
DONALD M. KINCH is Agricultural Engineer at the Hawaii Agricultural Experiment Station.
AUTHORS' ~OTE
The use of brand name in this report does not constitute, implied or otherwise, a recorrnnendation for or against this make of tractor.
This report is a contribution of the Hawaii Agricultural Experiment Station to Western Regional Project W-62 entitled, "The Cost of Owning and Operating Farm Power and Machinery Used for Seedbed Preparation and Tillage." Cooperating with Hawaii in this study are the Agricultural Research Service of the U.S. Department of Agriculture and the Agricultural Experiment Stations of Arizona, California, Idaho, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming.
ECONOMICS OF DIESEL VERSUS GASOLINE IN SMALL WHEEL TRACTORS Joseph T. Keeler and Donald M. Kinch
INTRODUCTION
While diesel powered track layers and large wheel tractors have been in operation for a number of years, the application of the diesel engine to small wheel tractors in the United States is a relatively new development . In European countries where fuel costs are high, diesel engines have often been used in small wheel tractors. The Germans in particular used diesel engines in a number of small vehicles including small tractors and automobiles. In the United States, diesel engines predominate as a source of power for heavy equipment such as earthmovers, highway trucks, industrial locomotives, fishing boats, and field electrical generators.
The major reason for not using diesel engines a s a source of power in small wheel tractors in the United States is that they have tended to be considerably more expensive than gasoline engines of comparable horsepower. This disadvantage, however, can be partially or completely offset by fuel economy provided the tractor is used a sufficient number of hours.
The purpose of this study is to compare diesel and small gas tractors with respect to engineering features and economy of operation.
HOW STUDY WAS MADE
The major problem was to select two tractors, one diesel and one gasoline, which were comparable in all respects except power source. Thus, the two tractors selected for each test bore the same brand and series identification and were similar in all respects; except one was powered by a diesel and the other by a gasoline engine.
The second requirement was that the tractors selected for the study be small wheel tractors, since the attributes of diesel versus gasoline are reasonably well known in larger tractors.
For the purpose of the study, four tractors were selected. The first comparison was made between the Ford 641 gasoline and the Ford 641 diesel. The second comparison was made between the Ford 841 gasoline and Ford 841 diesel ..!/
The performance data presented in this report are based on the University of Nebraska's official tractor tests as modified by field experience tests conducted by the University of Illinois.1/
The initial prices of the tractors used in this report were obtained from the Honolulu retail dealer of this make of tractor, and include taxes and other fees which farmers would normally pay. In the same manner, fuel prices were obtained from local petroleum suppliers based on the prices farmers pay including any allowances for tax deductions or quantity purchases.
_!/ The selection of this brand of tractor was made for the convenience of the study and in no way implied or otherwise reflects a superiority or infer\ority of this brand of tractor over any other.
2/ University of Illinois findings as reported in County Agent and Vo-Ag Teacher, August, 1957, page 7.
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ENGINEERING FEATURES OF DIESEL ENGINES
The basic engine components of the diesel and gasoline engines used in this study are quite similar. Both engines, for example, use the same type of cooling system, and have pistons, crankshafts, camshafts, and valves. Both engines use electric starting motors and both use a four-stroke cycle which consists of intake, compression, power, and exhaust strokes.
The principal difference between diesel and gasoline is in the method of igniting the fuel mixtures. Diesel engines depend upon the heat of compression for igniting the fuel mixture whereas in gasoline engines a spark is induced
j !
into the combustion chamber via an ignition system. In the diesel engine the fuel is pumped through a metering jet which sprays it into the combustion \ chamber where it is ignited by the heat of the compressed air in the chamber. I
Because diesel engines use compression for ignition, they have a much higher compression ratio than gasoline engines. Compression ratio compares the volume of air in a cylinder before compression with the volume after compression. In the, Ford 841 diesel the compression ratio is 16 .8 to 1, which means that at the top of the stroke the air volume in the cylinder is approximately 1/17 of what it was at the bottom of the stroke. The 841 gasoline engine has a compression ratio of 7.5 to 1.
Because diesel engines have their fuel injected into the cylinder, they do not have carburetors or electrical ignition systems (coils, distributors, spark plugs, wiring harness), found on gasoline tractor engines. Diesel fuel is pumped through high pressure fuel lines to the injectors. However, the speed of diesel engines used in tractors is controlled by the throttle and governor, the same a.s in gasoline engines.
Another major difference between diesel and gasoline engines is in the type of fuel used. Diesel is made to burn rapidly while gasoline is made to burn slowly, otherwise a gasoline engine will knock or "ping.'' The octane rating of gasoline is a measure of its controlled rate of burning. An entirely different rating is applied to measure the quality of a diesel fuel; this is called the cetane rating and is a measure of how rapidly it burns. The fuels differ also in their density. For example, diesel fuel weighs about 7 pounds per gallon while gasoline weighs around 6 pounds per gallon. Due to its heavier weight, diesel. fuel contains more energy per gallon as measured in standard heat units (BTU's). Diesel fuel has approximately 136,000 BTU's per gallon while gasoline has about 123,000 BTU's.
Since diesel fuel has more potential energy per gallon, the efficiency of a diesel engined tractor should be greater than that of a gasoline engined tractor when efficiency is measured in terms of horsepower--hours of work accomplished per gallon of fuel used. If, in addition, diesel fuel costs less per gallon, the increased efficiency of a diesel engined tractor can become outstanding. Furthermore, more nearly complete combustion generally occurs in diesel engines due to the presence of more air. This is true because in a gasoline engine the air-fuel ratio remains about the same regardless of the power output. In the diesel engine a full charge of air is inducted with each suction stroke regardless of the power output.
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ADVANTAGES OF GASOLINE ENGINE FOR SMALL WHEEL TRACTORS
The most obvious advantage which gasoline powered tractors have is a lower initial cost. In both series of tractors used in this study, diesel powered models cost approximately $480 more than the comparable gasoline tractor.
Gasoline tractors do not require special skill or tools for repairs or maintenance. Gasoline engines used in small tractors are quite comparable to those used in automobiles and thus a number of maintenance and repair garages would be able to work on them. Spare parts would also be more available than they would for diesel engines. Many farmers would be able to do a large amount of their own work such as timing and valve grinding if their tractor was gasoline powered.
Gasoline and diesel engines of equal displacement will not have equal horsepower ratings at maximum power. In the Nebraska tractor test ratingsll on the 841 series Ford tractors--one diesel, the other gasoline--both had a displacement of 172 cubic inches. The gasoline tractor had a maximum drawbar horsepower of 42.12 and the diesel a maximum of 37.75. However, out of 12 different pulling gears the diesel tractor developed more drawbar pull in 4 out of the 12 gears. The diesel tractor outpulled the gasoline in 1st, 2nd, 11th, and 12th gear. The tests were based on a given set of conditions as follows: (1) fuel setting or carburetor is set for economy; and (2) load is applied so that engine runs at rated speed.
Engine adjustment for maximum horsepower is entirely different on the two types of engines. On the gasoline engine there is a carburetor setting that gives maximum power. A richer or leaner setting will give less power. On the other hand, there is no specific injector setting on the diesel engine for maximum power. This setting is arbitrarily determined by the manufacturer; however, if it is increased significantly, the engine will be subject to over­ heating, valve warping, and burning.
In the Ford 641 series the piston displacement of the diesel powered tractor is slightly greater than its gasoline powered counterpart, making the horsepower of these two tractors quite comparable. While the horsepower rating of a tractor engine is a general indication of its pulling power, other factors have considerable influence on performance. Among these factors are the torque characteristics of engine, i.e., at what speed (rpm) does the engine develop its maximum horsepower, the gear ratio obtained in each gear, the weight of the tractor and its distribution between front and rear wheels.
In comparing the drawbar pulling power of tractors as closely matched as those in the Ford series, very little difference exists between diesel and gasoline powered tractors. However, what power differences do exist would tend to favor the gasoline powered machine.
11 University of Nebraska Agricultural Experiment Station test numbers 641 and 653.
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Because diesel engines use injectors it is essential that the fuel be kept absolutely free of all foreign matter. Much greater care must be taken in handling and storing diesel fuels for this reason. Related to this advantage for gasoline tractors is the fact that farmers having diesel tractors must main­ tain facilities for handling two kinds of fuels, for they will need gasoline for their trucks and other power requirements. These disadvantages of diesel may be partially offset by the fact that diesel fuel is less volatile than gasoline, and therefore constitutes much less of a safety hazard.
ADVANTAGES OF DIESEL ENGINE FOR SMALL WHEEL TRACTORS
The most important advantage of the diesel tractor is that it burns less fuel :i.n providing the same amount of power as the gasoline tractor. Further, the cost of diesel fuel is less than gasoline.
Diesel engines in general are more dependable in that they are free of ignition system difficulties. A large percentage of engine problems in gasoline engines center around the ignition system; this is particularly true in moist, cold areas. Diesel engines require little or no adjustment in that they do not have either breaker points, distributors or spark plugs which demand periodic inspection in gasoline engines. In short, diesel engines are simpler in design and therefore are reasonably free from the need for minor repairs and adjustment.
Because of their high compression, diesel engines are heavier built than gasoline engines of comparable horsepower. This heavier construction tends to give diesel engines a longer working life before it becomes necessary to under­ take a major overhaul. However, once it becomes necessary to overhaul a di esel engine or replace injectors the cost would be higher than similar work on a gasoline engine.
Since diesel engines depend on the heat of compression inside the cylinder for igniting the fuel charge, and in addition use a relatively low volatile fuel, they are a very safe engine to use from the standpoint of fire hazard. ~any serious farm accidents have been caused by gasoline explosions.
COSTS OF OWNING AND OPERATING DIESEL AND GASOLINE TRACTORS
Cost of Ownership
The cost of owning any piece of farm machinery can be divided into four major components: (1) depreciation, (2) interest on investment, (3) taxes and insurance, and (4) housing. Such cost items make up a group of costs variously referred to as overhead cost, fixed cost, or cost of ownership. The important characteristic of these costs is that they remain relatively unchanged on an annual basis regardless of how many hours a tractor or other piece of farm ~achinery is used.
The cost of ownership for the tractors used in this study are shown in table 1.
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Table 1. Yearly ownership (fixed) costs of diesel and gasoline tractors
Make and model Honolulu price
Depreciation Interest Taxes
Ford 841 diesel 3,520.00 352.00 105 .60 33.44 12.00 503.04
Ford 841 gas 3,057.00 305.70 91. 71 29.06 12.00 438.47
Ford 641 diesel 3,217.00 321.70 96 .51 30.58 12.00 460. 79
Ford 641 gas 2,754 .00 275.40 82.62 26.20 12.00 396 .22
Calculations made in arriving at the fixed costs shown in table 1 were computed as follows:
Depreciation. Depreciation per year was computed by dividing the Honolulu price by the estimated years of life of the tractor--10 years for tractors used in this study.
Interest on investment. This was computed by calculating the average investment (Honolulu price divided by 2) and multiplying it by an interest rate of 6 percent.
Taxes and insurance. The only tax charge was the 3\ percent state sales tax which was depreciated over a 10-year period. Insurance was based on the average value of the tractor and the type of insurance was all risk, full replacement type. The insurance charge was 10 cents per month per $100 valuation as quoted by a Honolulu insurance firm.
Housing. A structure having dimensions of 10 by 12 feet would be necessary to house a small tractor. In Hawaii shelter needs would be at a minimum; therefore, the cost of such a structure would be approximately $1.00 per square foot and would have an estimated life of 10 years.
Cost of Operation
Operating costs can be divided into three major components: (1) fuel costs, (2) repairs, and (3) oil and other lubrication costs. Annual operating costs differ from ownership costs previously referred to in that they increase with the number of hours the tractor is used.
Fuel: Fuel costs for tractors used in this study are shown in table 2.
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Table 2. Fuel consumption and fuel costs per hour of operation for diesel and gasoline tractors
Make and
mode.l Fuel
Gallons Gallons Dollars Dollars
Ford 841 Gas 3.288 2. 795 .2 79 . 779
Ford 641 Diesel 1. 746 1.484 .166 .246
Ford 641 Gas 2.393 2.034 .2 79 .567
1/ University of Nebraska Tractor Test Hat rated drawbar horsepower. }/ Eighty-five percent of fuel consumption for rated drawbar load, University
of Illinois as reported in County Agent and Vo-Ag Teacher, August 1957, page 7. ]/ Source: Shell Oil Company and Standard Oil Company of California with
federal and state tax adjustments made for farm use on the island of Oahu.
As can be seen from table 2, there is a difference of 11.3 cents between the cost per gallon of gasoline and of diesel fuel. If a farmer on the island of Oahu buys either fuel in at least 400-gallon lots, his cost at time of purchase including all taxes would be: gasoline, 31.9 cents per gallon, and diesel, 16.6 cents per gallon. Thus, at the time of purchase there is 14.3 cents difference in cost between these two fuels. However, a federal tax of 4 cents on gasoline is reimbursed to the farmer, providing his tractor is used entirely on the farm. In the case of state and local taxes there is an 8\-cent tax on gasoline per gallon and a 1-cent tax on diesel fuel per gallon at the time of purchase, neither of which is reimbursable. Thus, if a tractor is used entirely on the farm, the farmer will pay 7\ cents more in state and local taxes for gasoline per gallon than he will for diesel fuel. If both diesel and gasoline were taxed at the same rate, diesel fuel would then cost 3.8 cents per gallon less than gasoline.
The difference in the cost of fuel is only part of the reason for the lower operating cost per hour of the diesel tractors. As can be seen in table 2 the diesel machine burns considerably less fuel per hour when compared with the gasoline powered model.
Repairs: The cost of repairs in tractor opera tion vary considerably from one farm to another or even between one tractor and another on the same farm. Most differences in repair costs are attributable to conditions under which the tractors are operated and the type of work they perform. Tractors used for heavy work such as plowing in tight stony soils will normally have higher repair bills than those used for lighter work such as farmstead hauling.
••
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Tractor repair costs in general are much more difficult to predict than any other single cost item. Even when conditions of use are known it is seldom that any two machines will be identical with regard to the cost of repairs. Since the use of diesel engines in small wheel tractors in this country is relatively new, there is very little empirical evidence which could be used in determining their repair costs as versus those in the gasoline powered machines. Theoretically the repair costs for small diesel tractors should be comparable to those experienced by gasoline models. The repair costs for small wheel tractors in Hawaii should average about 18 cents per hour of operation.
Oil and lubrication: Oil changes and lubrication will vary to some extent with the conditions of operation. When tractors are operated under dry, dusty conditions, oil changes and lubrication may be required more often than would be called for in the manufacturer's recommendations for normal use. On large farms or when contractors are operating a number of vehicles, all machines may be oiled and greased according to a predetermined schedule.
Oil changes for small wheel tractors of the type used in this study should be made about every 120 hours of operation under normal use conditions. At this time the tractor would also be greased and the transmission oil checked. The hourly cost of oil and lubrication materials under normal use conditions will average 1.5 cents.
COST OF OWNERSHIP AND OPERATION PER HOUR AS RELATED TO HOURS OF USE
FOR DIESEL AND GASOLINE TRACTORS
The cost of owning and operating any piece of farm equipment per hour will depend upon how many total hours it is used. The longer a tractor or other piece of machinery is used, the less it will cost per hour . This declining cost per hour of use is due to the fact that fixed or ownership costs are spread over a
· greater numb.er of hours. Table 3 illustrates this relationship for the diesel and gasoline powered tractors used in this study.
Hourly Ownership Cost
The cost of ownership for a diesel powered tractor will be higher than a comparable gasoline powered tractor regardless of how many hours a farmer intends to use his tractor. However, the difference in hourly cost of ownership between diesel and gasoline is of much less significance as the hours of use increase. Comparing the cost of ownership between the Ford 841 diesel and gasoline in table 3, there is a difference of 64.5 cents per hour when both tractors are operated only 100 hours per year, but when both tractors are operated 1,000 hours per year the hourly difference in the cost of ownership is only 6.5 cents. For the 641 series tractor the hourly ownership cost difference at 100 hours of use is 64.6 cents, but at 1,000 hours of use per year the difference in ownership cost per hour of use is only 6.5 cents.
Table 3. Cost of owning and operating diesel and gasoline tractors per hour as related to hours of use per year
Hours of Ford 841 diesel Ford 841 gasoline Ford 641 diesel Ford 641 gasoline use
per year Total Owning Operating Total Owning Operating TotalOwning Operating Owning Operating Total
DollarsDollars Dollars DollarsDollars Dollars Dollars Dollars
5,359 4.608 0.441100 5.527 4.385 0.974 5.049 3 .962 0. 7625.030 0 .497 4. 724 200 3.166 2.304 .4413.012 2 .192 .974 2.745 1.981 .7622.515 .49 7 2.743 300 2.436 1.536 .441 1.977 1.321 .7622 .174 1.462 .974 2.083 400
1.677 .497 2.070 1.152 .4411.096 .974 1.593 .991 .7621. 755 1. 753
500 1.258 .497
1.851 .922 .441 1.363 . 792 .7621.503 .877 .974 1.554 600
1.006 .497 1.705 .768 .441 1.209 .660 .7621.335 .731 .974 1.422
700 .838 .497
1.216 .626 .974•719 .497 1.522 .576 .441 1.017 1.257
900 1.126 .548 .974 .495 .762.629 .497
1.461 .512 .441 1.202 1000
1.056 .487 .974 .953 .440 .762.559 .497 1.412 .461 .441 .902 .396 .762 1.158
1100 1.000 .438 .974.503 .497
.419 .441.399 .9 74 1.373 .860 .360 .762 1.122 1200
.954.457 .497 1.339 .384 .441 .825 .330 .762 1.092
1300 .916 .365 .974.419 .497
1.067 1400
1.311 .354 .441 . 795 .305 .762.884 .33 7 .974.387 .497 1.287 .329 .441 . 770 .283 .762 1.045
1500 .856 .313 .974.359 .497
1.266 .307 .441 .264 .762 1.026 1600
.748.832 .292 .974.335 .497 1.010
1700 1.248 .288 .441 . 729 .248 .762.274 .974.811.314 .497
•712 .995 1800
1,232 .271 .441 .233 .762•793 .258 .974.296 .497 .220 .762 .982
1900 1.218 .256 .441 .697•776 .244 .974.279 .497 1.205 .684 .209 .762 .971
2000 .243 .441.231 .974.762.265 .497
.198 .762 .9601.193 .230 .441 .671.219 .974.749.252 .497
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D Operating cost
100 300 500 700 900 LlOO 1300 1500 1700 1900
Hours Per Year
Figure 1. Cost of owning and operating 641 series diesel and gasoline tractors per hour as related to hours of use per year.
Cost Per Hour Dollars
Q/ .; c.:,• c.:,• .... Q/ Q/0 ., -.... .
Q/0 .... 0
100 300 500 700 900 1100 1300 1500 1700 1900
Hours Per Year
Figure 2. Cost of owning and operating 841 series diesel and gasoline tractors per hour as related to hours of operation per year.
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Hourly Operating Cost
Unlike ownership costs, operating costs per hour do not vary with hours of use. However, the cost of operation per hour for the diesel tractor is less than for the gasoline powered model; hence, the more the tractor is used the greater the total savings in operating costs will be by using diesel power. The magnitude of the saving in operating cost by using diesel can be calculated by multiplying the hourly difference in cost between diesel and gasoline tractors by the hours of intended use . In the 841 series tractors the cost of operating a diesel tractor for 1 hour is 47.7 cents less than for the gasoline, and in the 641 series the difference is 32.1 cents in favor of diesel.
ECONOMIC ANALYSIS OF DIESEL VERSUS GASOLINE
For farmers who use their tractors over 200 hours a year, there are some important economies to be gained from using diesel power. In the 641 series tractors studied in this report, a diesel tractor required 202 hours of operation per year for the economy of operation to compensate for its additional cost of annual ownership. In the 841 series of tractor, which is a larger machine of greater horsepower, the cost of owning and operating a diesel powered model became equal to the gasoline after 135 hours of annual use.
Figures 1 and 2 illustrate how the total cost per hour declines as the hours of annual use increase. Figure 1 shows this relationship for the 641 series tractors, both gasoline and diesel, and figure 2 for the 841 series. It is noticeable from these figures that as more and more hours of use are incurred, the total cost per hour declines rapidly. Along with this reduction in cost per hour is the increasing cost advantage which the diesel tractor has over the gasoline. This condition is brought about by the fact that as the fixed costs are spread over a larger number of hours, the fixed cost advantage of the gasoline tractor is reduced to a small amount while the operating cost advantage of the diesel remains unchanged. The bar graphs presented in these figures are based on the data shown in table 3.
Table 4 of this report shows the total annual cost including both ownership and operation as it varies with hours of use for both the 641 and the 841 series of diesel and gasoline tractors. When costs are computed on an annual basis, the more hours a tractor is used, the greater will be the total annual cost. Even though the total cost per hour declines as a tractor is used a greater number of hours, each additional hour of use adds to total annual cost.
Figures 3 and 4 show the competitive relationship between the diesel and gasoline powered tractors in series 641 and series 841 as defined by annual costs. The horizontal lines in the lower portion of the graphs show the fixed or annual ownership costs of the tractors. Notice that these costs remain the same regardless of the number of hours the tractor is used and that the fixed costs of the diesel tractor exceed the gasoline. The sloping lines originating at the fixed cost line on the left side of the graph represent the total annual cost and are shown numerically in table 4. The area on the graph falling between the total cost line and the fixed cost line for a given tractor defines the operating costs for this machine.
Table 4. Total annual cost of owning and operating diesel and gasoline tractors as related to hours of use per year
O fHours Ford 841 dieselI Ford 641 gasolineFord 841 gasoline I
' Ford 641 diesel use
Dollars Dollars Dollars Dollars DollarsI Dollars i Dollars
472 .42 200 Ij503.04 99.40 100 i503.04 49.70 552.74 438 .47 97 .40 535.87 504.89 : 396.22 76.20460.79 44.10
548 . 62 300 i503.04 149 .10
602.44 438 .47 194.80 633.27 1 460.79 88.20 548.99 :396.22 152.40
730.67 593.09 · 396.22 228.60 624.82 400 503.04 198.80
652.14 438.47 292.20 460.79 132.30 701.02
500 503.04 248.50 438.47 389.60 828.07 637.19 396.22 304.80701.84 460.79 176.40
777.22 600 503.04 298.20
751.54 438 .47 487 .00 925.47 460. 79 220.50 681.29 396.22 381.00 853.42
700 503.04 347 .90 801.24 438 .47 584.40 · 1,022.87 460.79 264.60 725 .39 396 .22 457.20
929.62850.94 438 .47 681.80 1,120.27 460.79 308.70 769 .49 396. 22 533.40 ' 1,005.82
900 800 503.04 397 .60 1,217.67 813.59 396.22 609.60900.64 438 .47 779.20 460.79 352.80
1,082.02 1000
503.04 447 .30 438.47 876.60 1,315.07 460.79 396.90 857.69 396.22 685.80950.34 1,158.22
1100 503.04 497 .00 1,000.04 438.47 974.00 1,412.47 460.79 441.00 901.79 396.22 762.00
1,234.42 1200
438 .47 1,071.40503.04 546.70 1,049.74 1,509.87 460. 79 485.10 945.89 396 . 22 838.20 1,310.62
1300 503.04 596.40 1,099.44 '438.47 1,168.80 1,607.27 460.79 529.20 989.99 396 . 22 914.40
1,149.14 1,386.82 1400
503.04 646.10 438.47 1,266.20 1,704.67 460.79 573.30 1,034.09 396 .22 990.60 396. 22 1,066.80 1,463.02
1500 1503 .04 745.50 503.04 695.80 1,198.84 438.47 1,363.60 1,802.07 460.79 617 .40 1,078.19
396 .22 1,143.00 1,539.22 1600
1,248.54 438.47 1,461.00 1,899.47 460.79 661.50 1,122.29 1,615.42
1700 503.04 795.20 1,298.24 438.47 1,558.40 1,996.87 460.79 705.60 1,166.39 396. 22 1,219.20
1,691.62 1800
438.47 1,655.80 2,094.27 460. 79 749.70 1,210.49 396 .22 1,295.40503.04 844.90 1,347.94 396 .22 1,371.60 1,767.82
1900 1,397.64 438.47 1,753 . 20 2,191.67 460.79 793 .80 1,254.59503.04 894.60
1,844.02 2000
1,298.69 396.22 1,447.802,289.07 1460.79 83 7. 90503.04 944.30 11,447.34 ,438 .47 1,850.60 503.04 994.00 j 1,497 .04 1438 .47 1,948.00 i 2,386.47 1460.79 882.00 I 1,342 •79 396 • 22 1,524.00 11,920.22
2000
1600
1200
800
O'-------'----...,______._____..._____.______,_____......._____._____.,______
200 400 600 800 1000 1200 1400 1600 1800 2000 Hours Per Year
Figure 3. Total annual cost, annual fixed cost, and break-even point for 641 series diesel and gasoline tractors as related to hours of use per year.
~ / Cost Dol lar s
Hour s Per Year
Figure 4. Total annual cost, annual fixed cost, and break-even point for' 841 series diesel and gasoline tractors as related to hours of use per year.
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Since the operating costs of the diesel tractors are less in both series, when enough hours of use are put on a diesel to compensate for the difference in fixed costs favoring gasoline, the total annual cost lines converge. It is at this point, or hours of use, that total annual cost of the diesel and gasoline tractors are identical. This point on the graph is marked by a dotted vertical line extending from the point of intersection of the total cost lines to the horizontal axis of the graph. The point of intersection of the total cost lines of the diesel and gasoline tractors could be referred to as the break-even point for diesel versus gasoline operation. To the left of the break-even point the total annual cost of the diesel tractor is higher than the gasoline; to the right of this point the total annual cost of the diesel becomes progressively less than the cost of the gasoline powered tractor.
The break-even point in annual cost between diesel and gasoline tractors can be calculated arithmetically by dividing the difference in annual fixed costs by the difference in hourly operating costs. The hours of operation necessary to equate annual cost of the two tractors in the 641 series, for example, would be: $64.57 difference in annual fixed costs divided by $0.32 difference in hourly operating cost equals 202 hours of operation per year.
Significance of Cost Differences
The cost differences between the diesel and gasoline wheel tractors as illustrated in this report show the diesel tractor to excellent advantage. In contrast to the 135 to 202 hours of operation for a diesel to overtake the gasoline machine costwise in Hawaii, the average rule-of-thumb figures for the Mainland would approach 1,200 hours of operation before the diesel would be competitive with gasoline. However, mainland rule-of-thumb figures refer to larger-sized tractors where the initial cost difference between the diesel and the gasoline tractors is much greater than for the smaller models compared in this report. The recent use of diesel engines in small wheel tractors where the initial cost between the two machines is small will make diesels more competitive with gasoline powered tractors, even on the Mainland where fuel costs are lower.
There are several other reasons why the diesel tractors discussed in this report were more competitive with gas·oline powered machines than would normally be expected under mainland conditions. As mentioned previously, a primary reason for the diesel's economy of operation in Hawaii is tax advantage which this fuel has over gasoline when used for farm power. As long as a tax differential of this magnitude exists, there will be some important economies in using diesel.
SUMMARY
Diesel engines have long been considered superior as a source of power for heavy equipment. Their efficiency, durability, and dependability have made them particularly valuable for use in fishing boats, large trucks, and crawler tractors. The use of diesel engines in small wheel tractors has been limited because of their high initial cost as compared to gasoline engines, and because small wheel tractors are seldom used enough hours for the fuel economy of the diesel to make up for its higher first cost.
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In recent years several major tractor manufacturers have offered diesel engines in their tractors for a nominal difference in cost. This has tended to make the diesel more competitive with the gasoline tractor and particularly so in areas where a wide margin exists between the cost of gasoline and diesel fuel.
This study sought to answer the question: Will the operating economy of the diesel powered tractor compensate the owner for its higher initi a l cost and, if so, at how many hours of operation per year? In conducting this study two small-sized tractors of the same make were compared--one a diesel, the other gasoline. This was also done for two larger tractors. In the smaller series tractor the diesel cost $64 more per year to own, but saved $0.32 in fuel over the gasoline machine for each hour of operation. In this size tractor, therefore, the di esel must be operated 202 hours a year before the saving in its fuel would compensate for its higher ownership cost. In the larger series tractor the diesel machine must be operated 135 hours per year before its fuel saving over gasoline would compensate for its added initial cost.
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LAURENCE H. SNYDER President of the University
MORTON M. ROSENBERG Dean of the College and
Director of the Experiment Station
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