Water and particles are the two contaminants that cause the greatest damage within a diesel engine fuel system. Not only are they a primary contributor to failure, they are also the two sources for secondary failure points within an engine and degradation of the fuel. There are two classifications for the types of failure within a fuel injection system: Partial Functional Failure and Catastrophic Full Functional Failures. Partial Functional Failure can also be referred to as a performance failure. Catastrophic Failures are well understood and result in the fuel injector or engine ceasing to function. Often, partial functional failure of an engine will go unnoticed until it is too late. Engine inefficiencies are seldom felt by a user, but can result in real world losses in revenue. A major reason for engine inefficiency can stem from partial failure of the engine’s fuel injection system, which is less well understood. Injector partial functional failure is rarely well documented and poorly investigated for use in formulated strategies to mitigate similar future events.
A partial functional failure within a fuel injection system is generally one that reduces the performance or efficiency of the injector and thus the overall performance or efficiency of the asset. The symptoms of such performance failures within an injection system may include the following:
- Low power from the engine
- Reduced engine RPM
- Increased fuel consumption
- Poor cycle times or low speed
- Lower gear selection
- Poor starting
- Poor idle
Many of the symptoms mentioned here are difficult to monitor accurately. An excellent example showing engines with various levels of fuel injector partial functional failure can be seen in the proceeding images. The varying levels of smoke opacity, with engines running at the same RPM and load, is directly related to poor fuel combustion – with the fuel injectors being primarily at fault. The two engines closest to the right side of the photo show little or no smoke from the exhaust stack, while the engine closest to the middle of the photo shows a high degree of smoke (DPM – Diesel Particulate Matter) emanating from the engine.
A diesel fuel injector, be it an electronically controlled unit injector (EUI) or high pressure common rail (HPCR), reduces in its designed functionality the moment any of the designed tolerances within the injector alter, thus affecting the design of the fuel spray profile within the combustion chamber. The changes in these injector tolerances can occur through the erosive and abrasive effect that contamination plays on metal surfaces, the alteration in injector nozzle hole size, or the number of open holes. As further evidence of this, Robert Bosch, author of the book “Diesel Fuel Injection 1st Edition” wrote that “deviations of ≥2° from the optimum injection direction will lead to a measurable increase in black smoke emission & fuel consumption. Catastrophic full functional failures are those that simply cause the engine to cease functioning. They are often dramatic events, highly visible to operations and high in cost and resulting downtime. It is for these reasons engineering, CM, and RE businesses typically focus their attention on monitoring, predicting, managing, and reducing such events. Additionally, hourly-based planned maintenance strategies are specifically designed around preventing such failures through known MTBF, advised by both OEM and industry specialists alike. An excellent example of this are the mandatory fuel injector replacements at half engine life (based on operating hours) that many OEM’s advise as part of their warranty or maintenance program. An obvious question should be raised here. If an engine is being providing the OEM specified fuel cleanliness level, then why would the fuel injectors need to be replaced halfway through the life of the engine?
Clearly, OEM’s are aware that cleanliness levels such as ISO18/16/13 do not provide the fuel injection system with a cleanliness level that will ensure its ultimate reliability, and it is for this reason why fuel injector change outs are recommended at engine half-life.
There are many types of common engine failures and most are typically well understood and managed by engineering and maintenance practitioners. However, there are some engine failures which can never be properly diagnosed, making it difficult for professionals to develop a suitable solution to the problem. Some of these more common engine failures can be misdiagnosed, and if correctly analyzed, attributed to failures within the fuel injection system. 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
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.
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. Contaminants in diesel fuel also have an erosive effect on the needle valve within the injector. This valve is designed to seal off the fuel within the injector following an injection event. Poor sealing of the valve can result in the fuel injector dripping fuel into the cylinder and onto the piston crown. This problem is more predominant within HPCR fuel systems with the fuel injector being pressurized 100% of the time. Such problems were less evident with EUI systems where full fuel injection pressure only exists for 5% of the engine run time. The image below provides two examples of fuel injector needle valves from an EUI SAC type fuel injector tip. The valve on the left failed within 1000 hours of operation in an Allen diesel engine used for power generation. The valve on the right is shown as evidence of a near new condition (100hrs), although some scratches are shown as evidence of the contaminated fuel this facility was faced with.
Dripping fuel injectors can cause a multitude of problems and catastrophic failures. The predominant failures caused by dripping fuel injectors are excessive piston crown temperatures causing the crown to deform or melt, resulting in engine failure. All electronic controlled fuel injectors, either EUI or HPCR, incorporate a control valve that is used to control the timing of each fuel injection event. In EUI type fuel injectors, the valve is controlled via an electronic solenoid. Most new HPCR injectors are controlled via a Piezoelectric actuated valve, which enables far greater control of valve movement (distance) and a far greater control of speed. No matter how the valve is actuated, it is by far the most critical component and the most sensitive to contamination. In most applications where HPCR fuel injectors are employed, the control valve inside the injector only opens a distance of approximately 20-30μm. When open, high-pressure fuel up to 40,000 psi travels over the metal sealing surfaces at speeds faster than a jet airplane. Extremely fine contaminants suspended within the fuel gradually erode and damage the sealing surfaces and the fine-machined tolerances.
Additional problems occur within the control valves when ultra-fine contaminants below 4μm, enter the critical clearances between the valve pintle and the valve body. The trapped silt within the clearance zone restricts the movement of the pintle, which results in sluggish movement of the valve, poor injection timing and eventual seizure. Such seizures typically result in the solenoid coil or the Peizo failing or a fault signal within the ECM. These failures are almost always classified as electrical faults by either manual detection or the ECM, when in fact it is contamination that has initiated the problem. 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