Farm Machinery Digest Encyclopedia: Part 1, Engine Terms for Tractor Collectors.

By: Ray Bohacz

Once the lore and romance of old iron gets into your blood, you cannot help but desire to learn more about your new love… like two teenagers on a second date.

The readers of the Farm Machinery Digest website and listeners to my podcasts are 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 you are familiar with, will further cement your bond with that exceptional tractor!

Flywheel (brake) versus PTO horsepower

It needs to be understood that an engine does not produce horsepower but instead, torque; a 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.

Calculating The Horsepower Of Your Tractor

  1. Find your tractor’s torque numbers.
  2. Find your tractor’s engine speed.
  3. Multiply the torque by the engine speed.
  4. Divide the number by 5,252.

Flywheel horsepower is derived from a dynamometer that is connected to the flywheel of the engine. The term brake refers to the unit that attaches to the flywheel to creates 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).

Years back, it was common to identify an engine’s output as brake horsepower. Today, the qualifier is just horsepower alone.

PTO horsepower is measured by connecting a modified dynamometer to the tractor’s PTO. The theory of work divided by 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 will see around 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 also limits the rpm. Since 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 weight transfer to the rear wheels will reveal a higher drawbar power rating. 

With these facts established, tractors (and other engines) should have the torque advertised instead of horsepower. Old habits do not die quickly! 

Compression ratio

It is the difference in volume in the cylinder at bottom dead center (BDC) and then again at top dead center (TDC). It is volume and not area.

If an engine has ten times more cylinder volume at BDC than TDC, it has a 10:1 compression ratio.

Many think that the compression ratio of an engine is its key to produce power… it is not. Though the compression ratio has an impact on engine power, its dominant effect is on thermal efficiency. This is a qualifier of how efficiently the energy exchange from chemical to mechanical energy occurs.

A higher compression ratio increases thermal efficiency. To the farmer, that meant less fuel-per-horsepower used.

Gasoline tractor engines were traditionally low 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 knock or ping with low octane fuel.

A fuel’s octane represents its ability to resist auto-combustion through pressure or heat. A low octane fuel has a propensity to auto-combust.

The balancing act for the tractor manufacturer would be to enjoy as high a compression ratio as possible, while not evoking knock or ping.

The post-WW II era that brought higher octane fuels to market allowed the tractor makers to add compression ratio and often bragged about that in their advertising.

Many factors impact the engine’s propensity to knock (ping) that work together with the compression ratio. Some are the combustion chamber design (shape), spark plug location concerning the center of the bore, piston crown shape, and the intake port.

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 is an exact opposite.

Atmospheric pressure is the same in both locations, so the carburetor can function.

The benefit of an updraft carburetor is greater freedom in placement with a lower hood line, for better operator visibility.

Most but not all gasoline-powered farm tractors used an updraft carburetor. A side benefit being that the engine was harder to flood since any excess fuel would run out of the carburetor venturi.

The design also allowed for remote placement of the air filter and assembly and during that era, usually one that employed a wire mesh in an oil bath.

Crossflow versus reverse flow (non-cross flow) cylinder head

It was quite common for early farm tractor engines to employ a reverse or non-cross flow cylinder. This describes a cylinder head casting that has 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. The intake 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 across from it. Additionally, the exhaust heats the incoming air charge and reduces its oxygen content and makes the engine more prone to detonation.

As tractor manufacturers looked to improve both the power and efficiency of their engines without adding weight by increasing the displacement, the crossflow cylinder head was employed. Many of the first applications were found as diesel engines were being introduced 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-volt systems/generator and alternators

Other than having one half 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 negative ground that is used with 12-volts. The main reason for this being the electron flow theory.

It was accepted that on a D.C. circuit, the most efficient flow path for the electrons is to have the load connected to the negative battery terminal and the positive to ground. The logic being since a 6-volt battery is limited by design, every advantage was given to it.

Due to the unique electron flow path, the ignition coil is wound oppositely than for a 12-volt system. Also, the voltage from the ignition switch goes to the coil negative, and the positive connects to the breaker points.

There is no ballast resistor or resistance wire as would be employed with a 12-volt system, to support longer breaker point life.

When it comes to charging 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 an alternator A.C. An alternator internally converts the A.C. to D.C. using diodes. Thus, it internally is creating A.C. but exports to the battery D.C.

Though there were 12-volt generators, they were quickly phased out. The widespread acknowledgment 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 that would sense the battery 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 off.

A 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 level of linearity.