♦ by RAY BOHACZ | Visit Ray’s website, www.farmmachinerydigest.com, for easy-to-understand technical articles and the “Idle Chatter” podcast, education delivered in an entertaining format. The “Idle Chatter” podcast is listened to in over 67 countries. Tune in to Farm Machinery Digest Radio on channel 147 on Sirius/XM.
Once theand romance of old iron get into your blood, you cannot help but desire to learn more about your new love… like two teenagers on a second date.
just as passionate about increasing their knowledge of their beloved machines, so this is Part 1 in a two-part series.
Consider it an encyclopedia of terms that, once youwill further cement your bond with that exceptional tractor!
Flywheel (brake) versus PTO horsepower
It needs to be understood that an engine does nota measure of work performed. Horsepower is a mathematical equation derived from torque and engine speed (rpm), created by James Watt hundreds of years ago. Horsepower is work divided by time.
The equation is: HP = torque X rpm/5252.
Flywheel horsepower is derived from ato the flywheel of the engine. The term brake refers to the unit that attaches to the flywheel to resistance. Traditionally it employs water and resembles a torque converter on an automatic transmission. In modern times the brake can instead create resistance via electricity (eddy current).
it was common to horsepower. Today, the qualifier alone. PTO horsepower is measured by connecting a modified dynamometer to the tractor’s PTO. The theory of work divided time is still relevant.
PTO horsepower is always less than flywheel or engine horsepower due to the energy consumed to operate the gear train. This is considered a parasitic loss.
It is widely accepted that a 20% loss from the flywheel is the norm when measured at the PTO. For example, an engine that produces 100 horsepower at the flywheel willaround 80 horsepower at the PTO. (This too holds true for road vehicles when power is measured at the tire.)
The gear reduction that runs the PTO alsoSince James Watt’s equation is work divided by time, the results represent that.
Another metric is drawbar horsepower. It is a function of the tractor’s engine output, along with its ability to transfer that power to the ground. The same engine in a different tractor that has more weightto the rear wheels will reveal a higher drawbar power rating.
With these facts established, tractors (and other engines) shouldof horsepower. Old habits die quickly!
It is the difference in volume in the cylinder at bottom dead center (BDC)dead center (TDC). It is volume and not area. If an engine has ten times more cylinder volume at BDC it has a 10:1 compression ratio.
Many think thatits key to power… it is not. Though the compression ratio engine power, its dominant effect is on thermal efficiency. This is of how A higher compression ratio increases thermal efficiency. To the less fuel per horsepower used.
Gasoline tractor enginestraditionally compression compared to their on-road counterparts. This was due to the constant load the engine typically was used under and the need to not experience or ping with low octane fuel. A fuel’s octane represents its ability to resist auto-combustion through pressure or heat. A fuel
to enjoy as high a compression ratio as possible, while not evoking knock or ping.
Many factors impact the engine’s propensity to knock (ping) that work together with the compression ratio.
Updraft versus downdraft carburetors
Since a carburetor works on a pressure differential, it is almost an anti-gravity device. An updraft carburetor has its inlet facing downward (the ground) while a downdraft isopposite. Atmospheric pressure is the same both locations, so the carburetor can function.
greater freedom placement with a lower hood line, for better operator visibility. Most but not all gasoline-powered farm tractors A side benefit that the engine was to flood fuel would run out of the carburetor venturi.
The design also allowed for remote placement of the air filter andthat era, mesh in an oil bath.
Crossflow versus reverse flowcylinder head
a reverse or cylinder. This describes a cylinder head casting both the intake and exhaust ports on the same side. It would leave the other side of the engine to house either the spark plugs or injectors (nozzles) on a diesel.
A reverse flow cylinder head offers packaging advantages but is also very inefficient.and exhaust port designs are extremely compromised. They cannot enjoy the more efficient flow path of having the charge enter on one side of the bore and exit heats the incoming air charge and reduces its oxygen the engine more As tractor manufacturers looked to improve both the power and efficiency of their engines without adding weight by the crossflow cylinder head began to be adapted. Many of the first applications were found as diesel engines and were often a clean slate design and not a retrofit of an older gasoline powerplant.
Next time you are at a tractor show, see if you can spot a crossflow cylinder head!
6-volt and 12-voltand alternators
Other thanhalf the voltage of a modern electrical system, there are some differences that tractor collectors need to understand.
Since voltage is electrical pressure, traditionally, a 6-volt electrical system employed a positive ground instead of the negativewith 12-volts.
It was accepted that on a D.C. circuit, the most efficient flow pathis to have the load connected to the negative battery terminal and the positive to ground. The logic a 6-volt battery is limited by design, every advantage was given.
Due to the unique electron flow path, the ignition coil is wound oppositely thana 12-volt system. Also, the voltage from the ignition switch to the coil negative, and the positive connects to the breaker points. There is no ballast resistor or resistance with a 12-volt system, to support longer breaker point life. the battery, a 6-volt system has a generator instead of an alternator.
For this discussion, the significant difference is that a generator produces D.C. and anAn alternator internally to D.C. using diodes. Thus, it internally A.C. but exports to the battery D.C.
out. The widespread that an alternator has a higher output at lower engine speeds and a more significant total output potential was the death knell for the simple generator.
Many early generators used a cut-out thatbattery voltage and either supply current to the generator’s field circuit (to create output) or not. It was a simple magnetic switch. It was either on or voltage regulator does what its name implies and varies (regulates) the input to the field circuit in either a generator or alternator to produce smoother output with a higher