All (injection) pumped up! Understanding the diesel difference.

All (injection) pumped up! Understanding the diesel difference.

January 7, 2021 0 By Ray Bohacz

A wonderful thing about the collector tractor hobby is the bond that these machines create between an eclectic group of people.

If you have never experienced that, just go to a gathering. You will find individuals from all walks of life and demographics.

A lifelong farmer will be talking to a guy that grew up and still lives in the city and never hoed a row, but the passion they both share for old iron is just as real and powerful.

In like fashion, some only collect a particular color or era. Yet, there are those that share their heart and are open to making a home for any machine.

Being close to the community, I have seen that many people have avoided diesel-powered tractors since they feel much more comfortable with a carburetor. This obstacle removes them from getting involved with some incredible machines, especially the growing and exciting Muscle Tractor era. 

I never want anyone to pass by owning a tractor due to a lack of familiarity. Since you have read this far, consider yourself already enrolled in Diesel 101!

The diesel differences

In the vernacular of engines, diesel and gasoline are not qualifiers; they represent the composition and combustion characteristics of the fuel. A spark ignition (SI) engine uses gasoline, while a compression ignition (CI) design consumes diesel.

With an SI design, the fuel and air are mixed, and the combustion even is initiated by the arcing of the spark plug.

A CI engine has no spark plugs but instead compresses the air to such an extent that the temperature from friction raises it to around 350 F degrees, the fuel’s auto-ignition point.

When the piston is nearing the end of the compression stroke, the fuel is introduced in an atomized form and combusts.

Standard terms when speaking about both fuels are octane (gasoline) and cetane (diesel). Octane is the fuel’s ability to resist combustion from pressure or heat and wait for the spark plug’s arcing. Cetane identifies the exact opposite. It is the fuel’s eagerness to combust.

Suppose an SI engine is exposed to exceptionally low octane fuel. In that case, it will knock or ping under load, but starting (both cold and hot) along with idle quality will not be affected.

Conversely, too low a cetane rating with diesel fuel will show itself in difficult cold starting, possible exhaust smoke, excessive combustion noise (diesel knock), idle quality along with a lack of power.

Since a CI engine relies on the fuel’s auto-ignition properties, it has a broader effect on the engine than gasoline would in an SI application.

Whereas an SI engine employs a carburetor to prepare to convert the fuel to a combustible mixture and throttle the engine (control engine speed), a CI engine uses an injection pump and nozzles (injectors) at each cylinder.

Liquid gasoline does not burn, so the task of the carburetor is to first break the fuel into small particles (atomize) and then mix it with air (emulsify). Then through the law of Latent Heat of Vaporization, the atomized and emulsified fuel experiences a phase change to a rarefied form (gaseous), which will now easily combust.

The rate of vaporization is intrinsically linked to the ambient temperature. At approximately 60 F degrees, only 50% of the fuel will phase change. For this reason, a choke is used on the carburetor to richen the mixture so that a suitable volume will vaporize, and the engine can start and run. As the engine warms, heat is transferred to the intake tract through thermal inertia, and the choke is no longer required.

A CI engine does not use a choke since heat is required for the fuel to auto-combust. Early applications had an integral can of ether (starting fluid) that would be sprayed into the intake manifold via a button on the tractor’s dashboard. Its chemical composition made it very eager to combust and would allow the engine to start. Then the injection pump and nozzles would supply fuel to the cylinders that gained heat from the ether’s combustion.

Either has a very explosive and uncontrolled combustion event that is very noisy and taxing to the engine. A better method was required that did not damage the engine and would not be linked to a can that, when empty, meant the engine would be almost impossible to fire.

The glow plug was introduced and eliminated the starting fluid. It resides in the cylinder head and uses tractor battery power to heat a thin metal tip much like a toaster works.

The glow plug can reach a tip temperature of nearly 1,400 F degrees. When the nozzle introduces the fuel to the cylinder while the piston is still traveling upward, the mixture interacts with the glow plug and ignites. Once the engine started, the glow plug was no longer required.

On older engines, the glow plug and the nozzle would be situated in a prechamber connected to the main combustion chamber. This design was identified as an indirect injection (IDI) system. A newer engine eliminated the prechamber and is direct injection (DI).

A CI engine uses the injection pump for timing adjustment, while a SI application employs an ignition distributor. The timing procedure on a SI engine is basically the same across all color tractors and engines. The injection pump timing process (CI) is more brand-specific and needs a shop manual for reference.

Just as there are different carburetors found on farm tractors, the same applies to injection pumps and nozzles on diesel engines.

Cutaway of a diesel injection pump with 4 inline cylinders and centrifugal governor

For the most part, both engine styles use a pump to either deliver gasoline to the carburetor (Some SI systems do not, and gravity feeds fuel from the tank.). It is identified as a fuel pump. On many CI engines, the pump looks similar but is recognized as a lift pump.

Once you get past the fuel’s differences and the way it is delivered to the engine, you will find that your gasoline engine experience will all apply.

The cooling system requires a unique chemical blend with a CI engine due to most having pressed-in cylinder liners. Still, service procedures and diagnostics are all in lockstep with a SI engine.

The charging and cranking circuits though more robust due to the compression ratio, are no different along with all other aspects of the engine and transmission.

There is no reason at all to shy away from adding too or even starting your collection with an “oil burner.”

Come on over to the other side and see what you were missing!

Something old… something new

Not many things in life are the same as when your collector tractors were new, and diesel fuel is one of them.

Several years back, due to emissions standards, diesel fuel was required to drastically reduce the level of sulfur it contained. The process of removing the sulfur dramatically reduced the natural lubricity of the fuel. In lay terms, it made the fuel “dry.”

Previously, this inherent lubricity kept the injection pump and nozzles (injectors) happy and minimized wear and sticking. That is no longer the case with modern diesel fuel.

The good news is that lubricity is simply a pour away. There are many excellent products that you can add to the fuel when filling the tractor’s tank.

A quart bottle of most products treats 275 gallons of diesel, so it will only cost a few pennies per gallon to keep things running correctly.

Choose a product that stabilizes the fuel, add an injector cleaner and a cetane booster while supplying lubricity. The engine will love you for it!