When Caterpillar delivered its new 60 diesel crawler for testing at the University of Nebraska tractor test facility on June 14, 1932, diesel fuel was so hard to find that No. 2 furnace oil was used instead.
As Test No. 208, the Cat 60 was the first diesel tractor tested under the Nebraska Tractor Test Law. An International Model 284 was the last tractor to be tested at the University of Nebraska using other than diesel fuel (gasoline) in Test No. 1277 on May 24, 1977. During the intervening 45 years, evolving technology advanced use of the more fuel-efficient diesel engine for agricultural tractors, as well as for equipment in most other industries.
The diesel internal combustion engine differs from the gasoline engine in that, in the diesel, compressed hot air ignites the fuel rather than a spark plug (compression ignition rather than spark ignition).
Dr. Rudolf Christian Karl Diesel invented the compression ignition engine that bears his name, obtaining a patent and running his first engine in 1892. The engine was fueled by peanut oil. The first commercially viable engine was available in 1897. Early diesel engines were large and heavy and operated at low speeds due to both the then-current state of metallurgy and the limitations of existing compressed air-assisted fuel injection systems.
Early applications included stationary power plants and propulsion of ships. Higher-speed diesel engines were developed gradually and finally introduced in the 1920s for railroad engines and, in the 1930s, for trucks, tractors and passenger cars. The main advantage offered by the diesel is its frugal fuel consumption due to thermal and volumetric efficiencies that exceed those of spark-ignition engines.
How the diesel engine works
Thermal efficiency is the difference between the theoretical heat value of the fuel and the amount of heat that actually produces work. The diesel has the advantage to begin with, in that there is more heat value in diesel fuel than in gasoline. Further, spark engines run fuel-rich for cooling purposes, while diesels run on the lean side of stochiometric (the ideal combustion process where fuel is burned completely).
Volumetric efficiency is the difference between the swept volume (or displacement) of the cylinder and the actual size of the charge as the piston moves up and down in the cylinder. The volume above the piston at the top of the stroke represents a waste since it is not swept by the piston on the intake stroke.
Also, on the exhaust stroke, that volume is not expelled. Thus, the higher compression ratio of the diesel relates directly to higher swept volume and lower fuel consumption. And, since diesel speed and power are controlled by fuel injectors rather than by a throttle, there are fewer restrictions in the air intake passages.
In the diesel engine intake stroke, only air is initially introduced into the combustion chamber. The air is compressed with a compression ratio typically between 15:1 and 20:1. This high compression causes the temperature of the air to rise. At about the top of the compression stroke, fuel is injected into the compressed air in the combustion chamber through a high-pressure fuel injector timed to coincide with the engine’s rotation.
The situation in the combustion chamber is represented by the Combined gas law (or the General Gas Equation), which is a combination of Boyle’s Law, Charles’s Law and Gay-Lussac’s Law, showing the relationship between pressure, volume and temperature with the equation:
Thus, on a standard sea level day, a 15:1 compression ratio would yield about 900 degrees Fahrenheit combustion chamber temperature to ignite the fuel.
Introduction of the hot-bulb engine
In its early years, the diesel engine competed with other oil fuel engine concepts (those known as semi-diesels) such as the Akroyd-Stuart hot-bulb engine, which was the first internal combustion engine to use pressurized fuel injection.
In the hot-bulb engine, fuel is injected into a bulb attached to the main combustion chamber and is ignited by coming in contact with a red-hot metal surface inside the bulb, followed by the introduction of air compressed into the hot-bulb by the rising piston. The bulb is connected to the cylinder by a narrow passage. Burning fuel shoots through the passage to the main combustion chamber to ignite the main charge.
An external heat source, such as a blow torch or wick, is used to heat the bulb for starting. Compression ratios of 5 or 6:1 are used and generally these engines are of the valveless two-stroke design. The advantages of the hot-bulb approach are that no electrical components are required, the engine can be started and run in either direction of rotation, and operation is possible on almost any combustible fluid.
The Hesselman-Waukesha oil engine
Another semi-diesel concept was the Hesselman-Waukesha oil engine, built by Allis-Chalmers in 1935 for about 50 of their crawler tractors. Invented by Swedish engineer Jonas Hesselman in 1925, the hybrid of a gasoline engine and a diesel engine represented the first use of direct fuel injection on a spark-ignition engine.
With the Hesselman engine, regular intake air compression is followed by fuel being direct-injected into the combustion chamber at the very end of the compression stroke. Because the compression is too low for auto-ignition, the air-fuel mixture is ignited by a spark plug. Under full load, engine torque is controlled by the injectors without throttling, while under medium load (and when idling), an air intake throttle valve ensures a stable engine speed.
A Hesselman engine can run on oil, kerosene, gasoline or diesel fuel. In the 1930s, heavier oils were considerably cheaper than gasoline and were therefore more economical. Tests also pointed to slightly lower fuel consumption in comparison to gasoline engines of similar power.
The engine had some inherent problems, though. Incomplete combustion led to spark plug fouling and very heavy exhaust smoke. The engines generated toxic exhausts on a scale that would now be considered completely unacceptable.
Tackling the starting problem
Starting of diesel engines, especially at colder temperatures, has always been problematic. Early (and very large) diesels were, and are, started by introducing high-pressure air into the engine’s intake manifold, and thus, into any open cylinder intake valves, causing the engine to rotate at somewhat above idle rpm. The fuel injection system does its job and the engine begins to run. In most cases, the engine also drives a compressor that will replenish the air-pressure tank.
Such pneumatic arrangements were not practical for tractors and trucks, so Caterpillar, Deere and others adopted the “pony motor” starter approach, wherein a small gasoline engine, either electrically or manually started, is clutched in to bring the diesel up to operating speed. Anyone familiar with the process knows that starting these small engines can also be problematic.
International Harvester solved the starting problem for their diesels in a unique but complicated way. Their system involved adding a gasoline intake manifold, carburetor and spark plugs to the diesel engine head. The system also had a device that closed off that portion of the head and raised the compression ratio. That allowed the engine to be started with an electric starter and warmed up on gasoline before switching over to diesel operation. Though far from ideal, this system worked well enough to be used on International tractors well into the late 1950s.
Development of the electric starter
Charles F. Kettering invented the automotive self-starter in 1915. At that time, and well into the 1960s, 6-volt automotive electric systems were considered to be the upper limit for voltage. That was because higher voltage caused switch contacts to weld themselves together when closing.
Electric diesel starting was used on lower-compression diesels by tying two 6-volt batteries together in series for 12 volts just for the starter, but leaving them in parallel for lights and charging. The 12-volt system required use of solenoid starter contacts, rather than the old “step-on” starter button. With 12-volt electrical systems, direct electric starting of diesels became routine. The problematic 12-volt generator was rather short-lived, but was used until the automotive alternator was perfected.
Until the 1960s, DC generators were used exclusively, but with the availability of affordable silicone diode rectifiers, alternators became practical. The 1960 Plymouth Valiant was the first to use an alternator in a production automotive application.
Some tractor makers resorted to using glow plugs to enable electric starting. Others tried adding heating elements in air intakes and oil pans. International Harvester and others added an ether injection system to their glow plug diesels when they discontinued the dual-head start system.
Important developments in diesel technology
The year 1963 saw two developments that spurred the acceptance of the diesel tractor: Use of the first tractor 12-volt alternator, in place of a generator on an Allis-Chalmers tractor, and development of the Roosa Master fuel injection system (with a pressure pump for each cylinder, replacing the Bosch-type in-line pumping system).
Besides the electric self-starter, Charles Kettering founded Dayton Engineering Lab Co. (Delco), later purchased by General Motors. Kettering became GM’s vice president of research and development. As such, he was the father of the General Motors 2-cycle diesel engine. FC
After 36 years in the aircraft industry, Bob Pripps returned to his first love and began writing about tractors. He has authored some 30 books on the subject and several magazine articles. Pripps has a maple syrup farm near Park Falls, Wisconsin. In harvesting the maple sap, he relies on a Ford Jubilee and a Massey Ferguson 85.