Tractor fuels throughout history

Tractor fuels have changed substantially throughout history. Often following technological and economic trends. Some have come and gone while others have remained a popular choice since the early days.

The story starts in the mid 1850 with the invention of the steam engine. Being an external combustion engine, these typically burned a variety of solid fuels. Generally coal was the fuel of choice for its hot burning temperatures, energy density, and ease of use. Coal was easy to get and easy to shovel into a firebox. Wood was also an option although this involved splitting and cutting large amounts of wood to keep a hungry boiler fed, especially while working hard. And with a lower energy density than coal it took a LOT of wood to keep it burning. Another popular alternative to coal that could be used while threshing though was straw. Usually it was a by-product of threshing grain and available in large quantities. Some boilers were specifically built to burn straw although I hear that fueling with it was a non stop job. All three of these fuels had a few things in common though. They created ash. Which meant that cleaning boiler flues was a daily routine. And it took a long time to heat up the hundreds of gallons of water used in a steam engine. And many times just making sure the steam engine was running correctly was a full time job for one person. By the late 1890s there was a new, cheaper, easier, and more convenient option than steam engines emerging. It was the rise of the internal combustion tractor.

Internal combustion engines in tractors dramatically reduced the initial cost of a tractor and made operation much easier. The early ones typically ran on kerosene. This was a cheap and available fuel that was great for powering tractors. Now days it is considered to be a low grade fuel. At the time though it was the most common and affordable fuel. Kerosene was used for heating and lighting on a regular basis. Despite being built to run on kerosene though, almost all engines had to be started and warmed up on gasoline first. Kerosene has a high flash point between 100-150f. This means that the fuel would not vaporize correctly at normal ambient temperatures. It would remain as a hard to burn liquid unless something was done to get it hot enough to want to turn into a vapor. Vaporizing the fuel was critical since these engines were spark ignition engines. This was achieved in the engine’s intake manifold. The intake and exhaust were typically built into one manifold often with the hot exhaust surrounding the pipe that fresh fuel and air were coming through. This got the kerosene hot enough to be over its flash point where it could then burn. It was also common to have a heat shield over the carburetor to help the fuel start warming up and help the carburetor work correctly. Once the fuel was in the cylinder more issues arose though. The engine has to have cooling, but without a thermostat the operator had to manually control the cooling. Usually this was done by limiting the air through the radiator with a curtain or shutter. The tractor would be started with it completely closed and once warmed up the operator would adjust the shutters to keep the tractors coolant in the operating range of typically 180 to 200f. This worked well enough but with such a high flash point it was still inevitable that some fuel would condense on the cylinder walls and get past the piston rings leading to oil dilution. And if the tractor stalled it sometimes meant draining the carburetor so you could fill it with gas again to get the tractor restarted. Kerosene also didn’t burn cleanly like gasoline and contributed to oil contamination. Typically at the beginning of the day the oil was supposed to be drained to the lower petcock level of the oil pan and a fresh quart was to be added. With a full oil change often being every 40 hours. This operation and maintenance routine was common among many fuels though since most cheap fuels fall under the low grade and heavy oil categories.

Heading through the 1920s and 30s the automobile industry was picking up. And so was the supply of gasoline. As gas prices became more competitive with the cost of operating on kerosene many manufacturers began building more gasoline tractors. This allowed for less maintenance since gasoline burned cleaner than kerosene. Another major benefit was the increased octane. Octane is a fuels ability to resist detonating. Detonations in an engine could prove to be catastrophic. The trick to maintaining a controlled burn was to keep the cylinder temperatures lower which was controlled with the compression ratio. Higher compression ratios make for a more efficient engine but also higher cylinder temperatures. The fuel has to be up to the task. Kerosene has an octane rating of about 0. Compression ratios were often limited to below 4:1. But with gasoline in the 1930s having an octane rating closer to 70 compression ratios could be pushed closer to 6:1. This substantially increased the horsepower and efficiency. And with the flash point of gasoline being -9f it meant you could start and run the tractor on the same fuel regardless of ambient temperatures. It also meant manufacturers could build a smaller, cheaper tractor without all the additional heat shields and complicated parts that were needed to make kerosene burn well. Operating the tractor was also simplified slightly since an engine running on gas wasn't as picky about the coolant temp. Radiator shutters weren’t needed anymore and there wasn't a possibility of the engine cooling off and stalling if left at idle too long. Gasoline was here to stay. Cheaper options were coming back around though.

In the 1930s distillate fuel was gaining traction. Lighter than kerosene and heavier than gasoline, it was cheap and it offered some octane. Radiator shutters were back due to distillate once again being a heavier fuel with a flash point around 100f. It had many of the same problems as running kerosene. But unlike kerosene it had around 30 to 40 octane. Compression ratios could be kept close to 5:1 which didn’t make as much power as gasoline but it was cheaper to operate. On a Farmall B this accounted for about a 10 percent loss in horsepower. This era brought on the all-fuel tractors. These tractors were happy to run on most flammable liquids that could be vaporized well enough. John Deere in particular liked to advertise their tractors as being the most cost efficient tractors available due to their ability to happily run on cheap, low grade, heavy fuels including, but not limited to kerosene, distillate and gasoline. Distillate was a popular fuel through the 1950s. As it lost favor though, many engines were converted to run gasoline exclusively and most radiator shutters, distillate manifolds, and starter gasoline tanks were removed. Distillate fuel is no longer available because of refinements in the gas making process turning what was once distillate into more usable and desirable fuels such as gasoline and diesel.

In my research I also came across references to tractor fuel and TVO or tractor vaporizing oil. Both of which seem to be very similar to or the same as distillate. One step above distillate but not quite gasoline however was power fuel. This fuel was popular for a while because it offered most of the power of gasoline but without road taxes. Power fuel eventually gave way to diesel in the 1960s. Before discussing diesel though I’m going to touch on LP.

LP or liquid petroleum was common in the 1950s and 60s due to its extremely low cost. LP has an octane a little bit over 100 so compression was sometimes over 7:1. And it could produce slightly more power than gas engines sometimes. It does have a lower energy density than gas though which leads to increased fuel consumption. The massive price difference compensated for this increased consumption though. LP is stored under pressure and required a special carburetor and fuel tank to handle the pressure. Identifying a tractor that runs on LP is easy thanks to the large cylindrical fuel tank that usually sticks out of the hood. LP burns cleaner than gasoline which extended service intervals. It doesn’t provide as much lubrication as other fuels though which tended to be hard on valves. LP engines have also been noted to start very well even in the coldest of weather, likely thanks to its extremely low flash point often lower than -70f. LP eventually became more than just a waste byproduct of the oil industry though and became popular for heating homes which drove the price up. LP fell out of favor shortly after.

Now we get to the diesel engine. Originally starting in the 1930s in some large tractors, diesel became popular as a fuel after the 1950s. This is the first fuel on this list that doesn’t use spark ignition, diesel engines rely on high cylinder temperatures to ignite fuel. This is done with very high compression ratios, typically over 14:1 or even over 20:1. With compression ratios this high the fuel is injected under high pressure through a nozzle that atomizes it as it enters the combustion chamber. Diesel engines are known for their robust, long lasting designs and high fuel efficiency. Since the 1960s most large farm tractors have relied on diesel. Smaller tractors were commonly diesel as well but were also available with gas engines as well.

Some experimental fuels have been tried as well. Some examples include the 1983 Oliver XO-121. This was a tractor with a 12:1 compression ratio intended to be run on high octane fuel. Only one was ever produced for experimentation. Another experimental fuel was hydrogen. In 1959 Allis Chalmers experimented with a hydrogen fuel cell tractor. This idea was quickly abandoned.

And lastly I would like to cover some European technology. This is more of an engine technology than a fuel technology but I feel that its important enough to mention here. In the early 1920s the hot bulb engine hit the market. This engine didn’t rely on spark ignition or compression ignition. The term hot bulb referred to the top of the engine that intentionally ran very hot. In fact the bulb had to be preheated with a torch before the tractor would start. The idea was that when the fuel was injected it would hit the red hot bulb and instantly vaporize and start to burn. This method of ignition made the engine capable of burning a very wide varieties of fuel. Some examples include: crude oil, gasoline, diesel, kerosene, vegetable oil, fats, alcohol, waste oil, and even tar oil. Essentially if it was close enough to a liquid to be poured into the fuel tank and flammable the tractor could probably run on it.