Why Do Diesel Fuel Injectors Fail?
There are two major causes of diesel fuel injector failure associated with the properties of the fuel itself: excess wear and deposits.
One mode of injector failure is excess wear. Prior to 2006, diesel fuels in the United States contained relatively high amounts of sulfur. This sulfur comes from the crude oil refined into the fuel. This sulfur in the fuel acted as a natural lubricant for the fuel system. Ultra Low Sulfur Diesel (ULSD) was gradually introduced into the market and is now mandated in all diesel fuel segments including on-highway, off-highway, and railroad. ULSD has a maximum allowable sulfur content of 15 ppm. As refiners removed this sulfur the lubrication benefits went away as well. As a result additives are now used to restore lubricity. The standard for measuring this lubricity is the High Frequency Reciprocating Rig (HFRR) Test, ASTM D-6079, which measures the size of a wear scar between two metal surfaces lubricated with the fuel. The less lubrication the fuel provides, the larger the wear scar. The maximum allowable wear scar in the United States is 520 microns (460 microns in Canada). Many fuel distributors add additional lubricity improvers to the fuel to limit premature wear.
While fuel lubricity is an important factor in determining the wear characteristics of the fuel injection system, it is not the only fuel-related cause of excess wear. The other potential cause of premature injector failure due to wear is caused by abrasion. All fuels contain small amounts of impurities, even the highest quality diesel fuels. Some of these impurities include very small (a few microns in size) particles that can pass through even the tightest on-board vehicle filters. If the fuel contains a large amount of these small insoluble particles, over time they can abrade the injectors as they pass through them during normal engine operation. In extreme cases, this abrasion can significantly alter the fuel spray pattern, causing reduced engine performance, and even increased downtime and maintenance due to severe abrasion. Good housekeeping practices by the fuel supplier, and good fuel filtration can reduce the damage caused by this abrasion.
While excess wear, whether caused by poor fuel lubricity or abrasion, is important to consider when discussing the cause of injector failure, the major reason for injector failure today is due to excessive buildup of deposits. There are two major types of deposits: external injector deposits and internal injector deposits. External injector deposits generally are caused by incompletely burned fuel that builds up around the injector holes. These deposits are referred to as coking deposits. While in most cases these deposits may not lead to injector failure, they can build up enough to disrupt the fuel spray, which leads to less efficient fuel combustion. This is often observed by the vehicle operator as a noticeable loss in power or lost fuel economy. Detergent additives have been used quite successfully to help control these external deposits, and restore the injector to its most efficient performance; restoring both the lost power and lost fuel economy caused by the buildup of these external deposits.
Internal Diesel Injector Deposits (IDID)
In the last few years, a new type of injector deposit has begun to appear. This deposit does not form on the external tips of the injectors, but rather on the internal parts like the injector needles and pilot valves. These deposits often look similar to the coking deposits (dark brown in color), but also can be very light, almost grayish to off-white in appearance. While they can form in virtually any type of diesel engine, they typically only cause operational issues in the newer engines with highly engineered injection systems.
Engine manufacturers are now offering injection systems that operate at very high injection pressures (greater than 30,000 psi in some cases) that supply fuel to all injectors through a common fuel rail. These engines are often referred to as High Pressure Common Rail (HPCR) engines. They were designed to help meet the ever-tightening emission regulations engine manufacturers must meet. The extremely high injection pressures create a very fine fuel mist spray in the combustion chamber, resulting in more complete burning of the fuel. This more complete fuel burning yields lower emissions and can also improve fuel economy.
In order to maintain these high injection pressures, the injector assemblies have been highly engineered, and have very tight clearance tolerances, sometimes as small as 1 – 3 microns (a human hair is typically 70 – 100 microns thick). So, you can imagine that it would not take much material depositing on these parts to cause poor injector needle actuation, leading to poor engine performance. In extreme cases, these deposits can lead to complete sticking or seizing of the injector needles, particularly after the vehicle has been shut down and the engine has been allowed to cool.
As these internal deposits build up, they can cause the same symptoms as the more traditional external coking deposits, namely lost power and reduced fuel economy. In extreme cases where the injectors begin to completely stick, they can lead to excessive vehicle downtime and high maintenance costs.