Tractor Design, Power and Transmission.

Tractor configurations:

Tractors can be generally classified as two-wheel drive, two-wheel drive with front wheel assist, four-wheel drive (often with articulated steering), or track tractors (with either two or four powered rubber tracks).

The classic farm tractor is a simple open vehicle, with two very large driving wheels on an axle below and slightly behind a single seat (the seat and steering wheel consequently are in the center), and the engine in front of the driver, with two steerable wheels below the engine compartment. This basic design has remained unchanged for a number of years, but enclosed cabs are fitted on almost all modern models, for reasons of operator safety and comfort.

In some localities with heavy or wet soils, notably in the Central Valley of California, the “Caterpillar” or “crawler” type of tracked tractor became popular in the 1930s, due to superior traction and floatation. These were usually maneuvered through the use of turning brake pedals and separate track clutches operated by levers rather than a steering wheel.

Four-wheel drive tractors began to appear in the 1960s. Some four-wheel drive tractors have the standard “two large, two small” configuration typical of smaller tractors, while some have four large powered wheels. The larger tractors are typically an articulated center-hinged design steered by hydraulic cylinders that move the forward power unit while the trailing unit is not steered separately.

In the early 21st century, articulated or non-articulated, steerable multi-track “tractors” have largely supplanted the “Caterpillar” type for farm use. Larger types of modern farm tractors include articulated four wheel or eight wheel drive units with one or two power units which are hinged in the middle and steered by hydraulic clutches or pumps. A relatively recent development is the replacement of wheels or steel crawler-type tracks with flexible steel-reinforced rubber tracks, usually powered by hydrostatic or completely hydraulic driving mechanisms. The configuration of these tractors bears little resemblance to the classic farm tractor design.

Engine and fuels:

The predecessors of modern tractors, traction engines, used steam engines for power. Since the turn of the 20th century, internal combustion engines have been the power source of choice. Between1900 and 1960, gasoline was the predominant fuel, with kerosene and ethanol being common alternatives. Generally one engine could burn any of those, although cold starting was easiest on gasoline. Often a small auxiliary fuel tank was available to hold gasoline for cold starting and warm-up, while the main fuel tank held whatever fuel was most convenient or least expensive for the particular farmer. Dieselisation gained momentum starting in the 1960s, and modern farm tractors usually employ diesel engines, which range inpower output from 18 to 575 horsepower (15 to 480 kW). Size and output are dependent on application, with smaller tractors for lawn mowing, landscaping, orchard work, and truck farming, and larger tractors for vast fields of wheat, maize, soy, and other bulk crops. Liquified petroleum gas (LPG) or propane also have been used as tractor fuels, but require special pressurized fuel tanks and filling equipment so are less prevalent in most markets.

Tractor Fuel Types:

Diesel
Diesel fuel first started appearing in large agricultural crawlers in the 1930s, but it was not until the 1950s that diesel became a major fuel source for farm tractors. Difficult starting limited the use of early diesel engines. Some manufacturers built spark-ignition diesel engines, or engines that started on gasoline and were switched over to diesel. Others used small gasoline “pony motors” to warm and start the diesel main engine. By 1960, diesel engines had greatly improved and were becoming very popular for large farm tractors. By the 1970s, nearly all farm tractors used diesel engines.

Kerosene
Kerosene was commonly used as a tractor fuel in the early part of the 20th century. Like tractor-fuel, it was used in “all fuel” engines after the engine had warmed enough to allow efficient combustion of the kerosene. Cheaper gasoline after World War II, plus the onset of diesel engines, caused kerosene to disappear as a tractor fuel.

Gasoline
Starting with John Froelich’s 1892 tractor, gasoline had always been a fuel forfarm tractors. Most tractors built through World War II were either used gasoline, or could use gasoline (in an all-fuel engine). By the 1960s, diesel was replacing gasoline as the primary fuel, although gasoline was often an option into the 1970s. Today, gasoline is only used in lawn tractors or other small equipment.

LPG
Liquified propane, or LPG was commonly used in the 1950s and 1960s as fuel for farm tractors. Farmers began converting their gasoline engines to LPG in the 1950s to take advantage of the low cost. Manufacturers soon began offering these engines as an option. LPG engines were discontinued as diesel engines began the primary fuel for farm tractors.

Tractor-fuel/distillate/TVO
Known as tractor vapourising oil or distillate, this once-cheap fuel was commonly used in farm tractors until World War II. Many manufacturers built low-compression “all fuel” engines designed to burn tractor-fuel, gasoline, or kerosene. The engine was started on gasoline from a small tank, and switch to tractor-fuel once it was warm.Tractor-fuel was a low grade fuel produced between gasoline and diesel in the traditional distillation of crude oil. The refining techniques developed during World War II made it possible to convert this into more useful fuels, and it began to disappear, A tractor-fuel engine can be run on modern gasoline. The lowest grade of gasoline available today is often better than the highest grade available when these engines were built.

Transmission:

Most older farm tractors use a manual transmission. They have several gear ratios, typically 3 to 6, sometimes multiplied into 2 or 3 ranges. This arrangement provides a set of discrete ratios that, combined with the varying of the throttle, allow final-drive speeds from less than one mile per hour up to about 25 miles per hour (40 km/h), with the lower speeds used for working the land and the highest speeds used on the road.

Slow, controllable speeds are necessary for most operations that are performed with a tractor. They help give the farmer a larger degree of control in certain situations, such as field work. However, when travelling on public roads, the slow operating speeds can cause problems, such as long queues or tailbacks, which can delay or annoy motorists in cars and trucks. These motorists are responsible for being duly careful around farm tractors and sharing the road with them, but many shirk this responsibility, so various ways to minimize the interaction or minimize the speed differential are employed where feasible. Some countries (for example the Netherlands) employ a road sign on some roads that means “no farm tractors”. Some modern tractors, such as the JCB Fastrac, are now capable of much higher road speeds of around 50 mph (80 km/h).

Older tractors usually have unsynchronized transmission design, which often requires that the operator stop the tractor in order to shift between gears. This mode of use is inherently unsuited to some of the work that tractors do, and has been circumvented in various ways over the years. For existing unsynchronized tractors, the methods of circumvention are double clutching or power-shifting, both of which require the operator to rely on skill to speed-match the gears while shifting. Both of these solutions are undesirable from a risk-mitigation standpoint because of what can go wrong if the operator makes a mistake – transmission damage is possible, and loss of vehicle control can occur if the tractor is towing a heavy load either uphill or downhill – something that tractors often do. Therefore, operator’s manuals for most of these tractors state that one must always stop the tractor before shifting, and they do not even mention the alternatives. As already said, that mode of use is inherently unsuited to some of the work that tractors do, so better options were pursued for newer tractor designs.

In these, unsynchronized transmission designs were replaced with synchronization or with a continuously variable transmission (CVT). Either a synchronized manual transmission with enough available gear ratios (often achieved with dual ranges, high and low) or a CVT allow the engine speed to be matched to the desired final-drive speed while keeping engine speed within the appropriate rpm range for power generation (the working range) (whereas throttling back to achieve the desired final-drive speed is a trade-off that leaves the working range). The problems, solutions, and developments described here also describe the history of transmission evolution in semi-trailer trucks. The biggest difference is fleet turnover; whereas most of the old road tractors have long since been scrapped, many of the old farm tractors are still in use. Therefore, old transmission design and operation is primarily just of historical interest in trucking, whereas in farming it still often affects daily life.

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