Powered Up: Turbocharged Antique Tractors
Adding aftermarket turbo/superchargers to antique and classic tractors has gained popularity in recent years for both work tractors and “pullers.” Since most of us live at some distance above sea level, this can restore lost power plus add a little more.
A supercharger is a device that increases the pressure of the fuel-air mixture in an internal combustion engine. It is used to achieve greater volumetric efficiency, to make up for pressure loss due to altitude, or to garner more horsepower from a given engine displacement. The two main types are the exhaust-driven turbocharger and the mechanically driven supercharger.
Turbochargers have become commonplace on diesel engines, and, lately, on gasoline-powered autos and light trucks where the purpose in both cases is to achieve more horsepower from a given displacement and to normalize power loss due to altitude.
The exhaust-driven turbocharger generally uses a scroll-type compressor and a scroll-type turbine, or turbo, on a common shaft with foil, or air, bearings, and turning at speeds of 30,000-60,000rpm. Pressure ratios of 4:1 are achievable with speed and pressure limited by an exhaust flow bypass, or “waste gate.” Other types of compressors include Roots-type, screw-type, sliding-vane, Wankel and multi-stage axial-flow. These, as well as the scroll-type, can be used in mechanically driven (non-turbo) applications.
A matter of efficiency
Some engines, such as aircraft radials, use low-pressure “blowers” to ensure distribution of the fuel-air mixture to all cylinders. Also, some 2-cycle engines use blowers to purge exhaust gasses, but there is little, or no, supercharging.
The 4-cycle gasoline engine ingests a mixture of fuel and air, compresses it and ignites it by a spark, thus converting chemical energy into thermal energy, expanding these combustion products to produce mechanical energy, and then ejects the combustion products, replacing them with a new charge of fuel and air.
The four strokes, according to engine expert Herschel Smith, can be summarized as: “suck, squish, pop and ptui!” In an un-supercharged (naturally aspirated) engine, when the piston goes down on the intake stroke, atmospheric pressure tries to fill the void now present in the cylinder. If the cylinder filled completely with air at ambient pressure, the engine would have a volumetric efficiency of 100 percent.
Due to the restrictions in any engine created by the air cleaner, cylinder head and valve timing, all of the air that should get into the cylinder can’t, so the typical engine’s volumetric efficiency is less than 100 percent. Further, the supercharger on a gasoline engine only goes into boost when the throttle is opened. In order to function, the compressor must get air, and during some operations, the throttle/governor will only be open a small amount.
Supercharging cause and effect
Compressing the air to be ingested by the engine raises the temperature of the air, due both to the inefficiencies in the compressor and to the consequences of Boyle’s law of physics, which states that the temperature of the un-compressed volume of air is proportionally higher in the compressed volume. Scroll-type compressors can be as much as 85 percent efficient, while Roots-type positive displacement types run about 50 percent efficient.
High inlet air temperatures can lead to destructive detonation. After-coolers can negate these effects, but at the expense of additional system complication. In any case, the effect of supercharging is much the same as raising the compression ratio, and increased octane fuel may be required.
Mechanically driven compressors have the advantage of instantaneous response, while exhaust driven units take time to “spool-up.” Instantaneous response may be a requirement for tractor-pulling or drag racing, but is not a factor for increasing the power of a mowing tractor. The disadvantage of the mechanically driven unit is that it takes about a third of increased power just to drive the compressor. The following discussion applies to both mechanically driven and exhaust-driven superchargers.
Supercharging the antique tractor
For antique and classic tractors, the addition of aftermarket turbo/superchargers offers an intriguing prospect. With a super/turbocharger, the amount of air and fuel that can be packed into the cylinders greatly exceeds the volumetric efficiency of a naturally aspirated engine. Since the air is now being forced into the engine, a substantially denser fuel/air charge can be put into the cylinders.
For a typical classic work tractor, such as a Farmall or Ford 8N with a 6.5:1 compression ratio, running 6 to 7 pounds of boost, approximately 40 to 50 percent more fuel/air mixture can be induced into the cylinders for a 40- to 50-percent increase in horsepower, and you could expect your 8N to have the power of a Ford Jubilee.
As a result of supercharging, a small displacement engine can produce similar horsepower and torque to a naturally aspirated larger displacement engine. This is especially good if the added power is taken out via the PTO, rather than through the drive axle(s) as they may not be able to handle the increase.
Adding a supercharger to a tractor will increase its fuel consumption, the radiator will be required to expel more heat and it is likely that a larger-diameter carburetor and improved ignition will be required. Further, don’t count on adding a supercharger to a worn-out engine to revive its power.
Also note that too much boost can cause harmful detonation unless expensive high-octane gas is used (aviation gas can cost twice as much per gallon as regular). Further, too much boost can cause internal engine problems with connecting rods, caps and bearings and blown head gaskets.
Supercharged since the 1800s
Francis Roots obtained the first patent of record for his namesake compressor in 1860. Nothing automotive came of it until 1900, when Daimler-Benz patented its “Kompressor” for automobile supercharging. The name, by the way, is still used on their turbocharged cars today.
Rudolf Diesel patented the supercharged diesel engine in 1896. Louis Renault patented the centrifugal-compressor supercharger in 1902. Italian Alfred Buchi obtained a patent on the turbocharger in 1905. Turbochargers were used on diesel engines beginning in the 1920s, but manufacturing reliable turbines capable of enduring the higher exhaust temperatures of gasoline engines prevented their routine use into the 1930s.
Gasoline-powered production automobiles with supercharging came into vogue in the 1930s on the Auburn-Cord-Duesenberg line. In 1937, Robert Paxton McCulloch began making mechanically driven scroll-compressor superchargers as add-ons for the Ford V-8 engine widely used in racecars and so-called hot rods. McCulloch later went on to fame with chainsaws and other 2-cycle engines.
Turbochargers became common on aircraft engines shortly before and during World War II, although shortages of critical material for the turbines limited supply. Many aircraft models, including those powered by the famous Rolls-Royce Merlin V-12 engine, achieved notable success using multi-stage and intercooled engine-driven superchargers.
1962 Oldsmobile Jetfire leads the way
Supercharged production cars again appeared in 1954 in the form of the Kaiser Manhattan (with a McCulloch unit driven by a variable-speed belt-squeezer drive). These were followed in subsequent years with similar superchargers on Packard and Studebaker cars.
Turbocharged racecars began to appear in the early 1950s, but the world’s first production car with a turbocharged gasoline engine was the 1962 Oldsmobile F-85 Jetfire, followed a few weeks later by the Chevrolet Corvair Monza Spyder. The Jetfire turbo lasted only two years, the Corvair turbo for four.
Supercharged tractor engines became popular with the growth of the “unlimited tractor-pulling” sport. These specialized giant machines use every trick in the book to get more and more torque to the rear wheels. Along with multiple stages of supercharging, special alcohol-based fuels are used. Of course, extremely heavy-duty drive trains are employed. Observing the air inlet butterflies as the pull starts, shows how power is regulated to provide just the right amount of wheel spin. 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.
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