Diesel nozzles: Maintaining mechanical injectors

By Hot Rod Farmer
6th December 2019

For more than fifty years most if not all engines that power farm equipment have been of the compression ignition (CI) design, or more commonly known as a diesel.

When compared to a gasoline (spark ignition, SI) engine a diesel is more robust, has a longer service life, requires less maintenance, and is more economical to operate for a given specific output.

Many farmers and others do not realize the reason for the fuel consumption advantage. It is due to the increased thermal efficiency that is inherent to the design’s higher compression ratio and that the fuel offers more Btu (energy) per gallon than gasoline.

The industry states that a gallon of gasoline without ethanol has around 117,000 Btu while the same amount of #2 diesel fuel posses approximately 132,000 to 152,000 Btu, depending on the density of the blend. As with gasoline, all diesel fuel does not have the same density and this is due to the refining process that was employed along with the source of the crude oil.

The US government mandates a minimum energy content defined by the fuel’s specific gravity. As long as that is met it is a viable consumer product. As an aside, this is why with both SI and CI engines fuel consumption for the same weather and load conditions can and often does vary from tank to tank, if different sourced fuels are used.

Though the diesel has earned a reputation to do much work with little care it does need to be understood and maintained for efficient operation and long life. Its requirements are different than a SI engine.

If you were to talk to any diesel power plant manufacturer they would tell you that the majority of the cost in the engine is found in the fuel system. On a mechanical injection diesel this consists of the injection pump and nozzles.

These components are the heart of the diesel and are not only critical to its operation but are extremely costly to replace if a failure occurs. They also have the ability to degrade engine performance, fuel consumption and longevity if not operating properly.

Getting to know nozzles

The nozzles on a diesel engine are responsible for taking the liquid fuel and atomizing it (breaking into small particles) so that it can burn. They need to deliver the proper amount of fuel to each cylinder for the load and horsepower demand.

They perform this job a countless number of times. Over the course of the life of the engine the inject cycles can be in the billions and possibly trillions. In addition, the nozzles are subjected to an extremely abusive environment — more than any other part of the engine.

The nozzles are exposed to temperature peaks of more than 2,550 F degrees on the outside while the internal pressure may exceed 30,000 psi. Though almost every manufacturer recommends servicing the nozzles to maintain proper atomization, these procedures are often neglected by the farmer and only addressed when a problem exists.

Preventive maintenance should be part of every diesel engine owners’ plan if long life and trouble-free performance is desired.

When discussing diesel engines many refer to the part that delivers fuel to the cylinder as an injector. To a diesel expert the injector is the nozzle holder assembly but over time it has been used to describe the actual nozzle.

This has become complicated by the different fuel system designs that are employed on diesel engines. There are now mechanical unit injectors (MUI), electronic unit injectors (EUI) and hydraulic actuated electronic unit injectors (HEUI) that became popular on light-duty designs with the Ford Power Stroke engine.

The common complaint that prompts the injectors to be removed is blue/black smoke at idle, missing under load, poor idle quality, a decrease in power and an increase in fuel consumption along with hard starting.

There are brand specific variations in the nozzles from different manufacturers but the basic function, service procedures and maintenance tips all apply.

To complicate matters, within the mechanical category there are many different designs of nozzles that in some instances share operating characteristics but not in every case.

Hydraulic injectors are usually classified by the nozzle design. There is poppet, pintle, multi-orifice and electro-hydraulic styles. With each design category there are often subsets of styles such as those used strictly with in-direct injection (IDI) or direct injection (DI) applications.

Regardless of the design, a mechanical injector that contains no electronic parts can and needs to be serviced. Electronic enhanced injectors in light-duty applications are traditionally not serviceable and need to be replaced as a unit.

There are three terms that should be understood that pertain to nozzle testing and service. They are nozzle opening pressure (NOP), back leakage, and forward leakage.

An injector nozzle can be considered a hydraulic switch. One of its design elements is the pressure at which it opens. This is usually set with either a spring tension adjustment or on some models with shims. The term pop-open pressure or popping pressure is also used instead of nozzle opening pressure.

Regardless of which term is employed it describes the amount of pressure that must be created by the injection pump before the nozzle will pass fuel into the cylinder. Each model of engine and nozzle design has its own NOP value that typically varies from 1,000 to 5,880 psi.

Some nozzles employ an internal opening valve that returns unused fuel to the tank. The internal leakage is a result of the nozzle-valve-to-nozzle-body clearance. It is measured during bench testing for ten seconds and recorded as back leakage.

Forward leakage is the nozzle’s ability to not drip or leak until the NOP is realized. It confirms the nozzle’s ability to seal. To test for forward leakage a pressure of approximately 150 psi below the NOP is created on the test bench. No visible dripping is allowed.

To service a nozzle properly it needs to be removed from the engine and brought to a facility that specializes in these procedures. These shops are traditionally listed as injection pump and nozzles services. There the technician will document the issues and concerns and using a test bench, confirm all the values while visually inspecting the fuel atomization pattern.

Then the nozzle will be disassembled, cleaned ultrasonically, wear items replaced and reassembled. The nozzle will then be put back on the test fixture and critical pressures will be set and the spray pattern revaluated.

Prevention is the best medicine

The most effective nozzle program to institute on your farm is to not allow the component to deteriorate to the level where major service is required. This is not to be understood that the nozzles will never need professional service but to extend the hours of operation before that is required. The good thing being this is very easy.

The key to maintaining nozzle performance begins with clean fuel and filters on both the equipment and your on-farm fuel storage tank. The second step is to never use untreated fuel in the engine, especially if low or ultra low sulfur diesel is employed on a mechanical injection system.

The process to remove the sulfur also undermines the natural lubricity of the fuel (the sulfur is NOT a lubricant) and the nozzle wears at an exponential rate. This creates pressure and internal leakage issues that will require a replacement of parts. A product that adds lubricity and promotes varnish and carbon removal will attack any deposits that will impede nozzle performance.

The good thing being there are many excellent and readily available brands of additives that can be used to treat your bulk fuel tank prior to a delivery from the vendor.

The better products usually include not only lubrication and detergent but also other key ingredients for proper performance such as a cetane improver, anti-gel, moisture reduction, and fungicide. Many farmers balk at the already high cost for diesel fuel and see the expense of a good additive as unnecessary or snake oil — but that logic is faulted if the math is done.

A quality additive will only increase the fuel cost by about five to seven cents per gallon and may be slightly lower if purchased in bulk. If your farm uses 5,000 gallons of diesel per year it equates to $250.00 to $350.00 in increased fuel costs.

Weigh that against the improved performance, lower fuel consumption, and the time and expense to service the nozzles; there is no better return on your equipment dollar than treating every gallon of fuel used and keeping the nozzles working as designed.