High velocity and high-pressure fuel flow, with even the smallest amount of contamination, will gradually erode the sealing surfaces of the injector valve seat. Once valve wear has initiated, a chain reaction gradually occurs, resulting in a partial functional failure evolving into the full functional failure of the valve and the need for replacement.
The Failure Chain Reaction
- Valve erosion initiates
- Fuel leakage through the valve mating surfaces initiates
- Localized hot spot generation through the leakage zone causes fuel oxidation
- Reduced fuel pressure at nozzle
- Reduced volume of fuel delivered. Engine management system compensates by increasing injection event time (more fuel).
- Reduced fuel atomization
- Soot generation within the cylinder
- Increased emissions
- Loss of power
- Partial Functional Failure Point
- Leakage rates continue to increase as wear continues
- Fuel consumption increases as the engine controller unit tries to compensate for leakage
- Visible and audible signs
- Full Functional Failure Point
There are three principal locations inside a high pressure common rail fuel injector that suffer from the erosive and abrasive effects of contamination, resulting in a loss of functional efficiency. These are:
- Fuel injector nozzle holes
- Needle valve and seat
- Electronic Peizo or solenoid controlled valve
HPCR Fuel Injector Critical Zones
There are two predominant diesel fuel injector nozzle designs that are in circulation today: the area around pintel tip (SAC) and valve covered orifice (VCO) type nozzles. Modern HPCR fuel injectors typically utilize the VCO type as it completely covers the nozzle holes. This design enables the injector to abruptly and completely shut off the fuel at the end of an injection event, thus providing a more stringent control of the fuel injection event. The two designs can be seen below.
SAC-Type (left) – VCO Type (right) fuel injector nozzels
Due to its design, the VCO type needle valve has extremely fine tolerances and is highly susceptible to valve misalignment during rise and fall. Remembering that the rise and fall can occur 29 times per second at 1400RPM in a large high-speed diesel engine, any misalignment, or changes in the tolerances within the valve, will give rise to variations in the flow area, the volumetric fuel passing through the various holes, and the atomization of the fuel – all of which effect the combustion of the fuel and fuel efficiency.
Fuel injector nozzle holes generally have two failure conditions which result in a partial functional failure of a fuel injector. These two conditions are Blockage and Erosion. HPCR fuel injectors are finely tuned and balanced precision devices that are designed to inject a very fine fuel mist consisting of micro-fine droplets into the combustion chamber with millisecond precision timing. Fuel droplets burn from the outside in, and as such, it is important for the fuel injection system to maintain the consistency of the mist for correct and efficient combustion to take place. Modern HPCR fuel injection systems are specifically designed to reduce the droplet sizes of the fuel while increasing the number of injection events per engine cycle. When correct combustion takes place as designed by the injector manufacturer, the fuel droplets burn out completely before they reach the engine cylinder liner. Failure to complete combustion in this way results in the build up of soot within the engine and increases in nitrogen oxide (NOx), carbon monoxide (CO), and diesel particulate matter (DPM).
To maximize combustion efficiency and reduce emissions, modern HPCR fuel injectors typically have 5-8 very fine holes which are machined into the injector tip using a process called Electro Discharge Machining (EDM). These holes allow the fuel to exit the fuel injector and immediately atomize within the combustion chamber. The size of these holes will vary from manufacturer to manufacturer and depends on the size and application for each fuel injector. Hole sizes are typically 20μm-250μm.
As a fuel injection event takes place, diesel fuel is sprayed into the combustion chamber, which unlike gasoline engines, is typically housed inside the piston crown. As the piston moves downwards in its power stroke, sprays from the injector protrude further into the volume of the combustion chamber. Fuel should be burnt out before any droplet reaches the cylinder walls. When, however, the spray pattern generated by the injector is not as designed, the fuel droplets become larger and therefore take longer to burn out. Soot generation, high DPM, and smoke are by-products of this problem. The soot generated within the combustion chamber gradually builds up on the injector tips, causing blockages to occur, as well as, accumulate within the exhaust system, the valves, and on cylinder walls where it is typically removed by the piston moving up and down and thus washed into the engine sump where it contaminates the engine oil. Excessive soot build up within engine lubrication oil is directly correlated to poor fuel injection or combustion.
Fuel injectors that have one or more of the nozzle holes blocked due to contamination or soot build up will cause an increased fuel velocity through the open nozzle holes, thus reducing atomization. Micron sized contaminants can also gradually block some or all of the individual nozzle holes as a result of tight injector clearances and the electromagnetic conditions present inside. Nozzle holes that remain unblocked will see an increased flow rate, causing fuel to be ejected faster and increase the potential for wear.
The jetting of fuel (a continuous stream of fuel in a concentrated direction) within the cylinder can eventually result in the engine lubricating oil to be washed from the side of a piston and cylinder if left unchecked. This loss in lubrication film can result in the development of a hot spot and uneven thermal expansion of the piston, potentially causing the eventual piston seizure to the cylinder sleeve, resulting in a catastrophic failure. Interestingly, such failures are commonly categorized as lubrication failures and not the result of poor fuel injection. In March 2004, staff members of the Department of Mechanical, Aeronautical & Chemical Engineering at the University of Pretoria South Africa delivered a paper at the International Conference of the South African Institute of Tribology, which provided hard evidence of such failures.
The common approach to rectify the build up of soot or contamination within the injector is to seek out and use diesel fuel additives that are designed to clean soot from the injector tip and internal deposits. While this can be an effective means of cleaning the injector, in most cases the problem continues, as the root cause of the problem (contaminated fuel and worn injectors) has not been corrected. Again, it must be stressed that the build up of contamination and soot is a symptom of a far greater problem that should be corrected as a first step.