Failure Analysis of a Damaged Direct Injection Diesel Engine Piston
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This work deals with the piston failures found in diesel engines functioning in light trucks. The failure was a hole inside the bowl located at the top of the piston, which is the combustion chamber of the engine. This hole progressed and finds its way out between the second and third rings. The operational characteristics of the engine were analyzed in order to elucidate the probable acting failure mechanisms. For the evaluation, obviously were assessed the damaged pistons but also pistons of the same engine without the mentioned failure and a new piston. Different studies and measurements were carried out such as microstructural morphologies and transformations, hardness, fracture surface analysis and chemical composition. The piston design, the material used and the operational conditions (thermal and mechanical) were studied with the purpose of determining the failure origin. It was concluded that thermomechanical fatigue along with some degree of degradation suffered by the material is responsible of the reported failure.
KeywordsPiston Failure Material Fatigue Temperature
The requirements for low consumption and emissions combined with higher specific power led the current diesel engines to use high-pressure direct injection and turbochargers. Therefore, the design and operation of internal combustion engines of this kind are complex, and the mechanical components are exposed to higher requirements than engines of previous generations.
These kinds of engines produce specific powers of up to 80 kW/l which results in mean effective pressures over 20 bar and high peak pressures into the combustion chamber.
The piston is the first mechanical component in the line of power generation, sustaining the pressure generated at its crown when the operating fluid chemical energy is released by means of the combustion process. The head of this element representing a moving wall of the combustion chamber is exposed to severe thermal loads besides the extensively studied mechanical efforts produced by its alternative movement.
The purpose of this article is to explain the repeated catastrophic failure of direct injection diesel engine pistons placed in light trucks.
Design and Operation of Internal Combustion Engine Pistons
Some aspects are examined in this section with the objective of establishing the basis for the analysis developed in this work.
The shape of the piston bowl contributes to a vigorous air movement, during the intake (swirl) and during the compression stroke (squish). In this manner, the air moves into a vortex inside the piston bowl before combustion with the purpose of creating better burning conditions once the fuel is injected.
The bowls have a variety of different shapes affecting the air/fuel mixture achieving better and more efficient combustion, which leads to more power together with fuel economy.
The energy conversion produced by the combustion process causes a very fast pressure and temperature development inside the combustion chamber. Peak temperatures could range 2000–2500 °C or even more, falling down very fast, although exhaust gases could still have temperatures above 700–1000 °C [1, 2].
The temperature changes in the piston crown introduce thermal gradients, which in turn produce the generation of thermal stress–strain fields in the component.
The heat is mainly transmitted by convection through the combustion chamber walls and the piston crown, with cyclic fluctuations of temperature . The heat on the crown of the piston is principally released to the cooling fluid following in the path piston ring–cylinder wall, and a small part is transferred to the fresh charge of gas of the new operative cycle.
The lubricant oil, which wets the inner walls of the piston and in some designs flows through internal galleries behind the rings, as detailed (A) in Fig. 1, absorbs another part in this particular zone.
The combustion strategy of each engine including pre-, main and post-fuel injection obviously influences the environment of the heat transfer to the piston crown.
The mechanical loading conditions of the piston include the gas pressure and the inertial forces transmitted by the pin following the power transmission line, generating a complex stress map in this type of piston. The pressure forces acting on the piston crown during the combustion reach about 180–200 bar with piston speed about 25 m/s .
The mechanical load on the crown over the entire operative cycle (720° of crank angle) consists of a periodic function, having its maximum soon after the top dead center position entering the expansion stroke, corresponding with the maximum pressure reached by the combustion. In the bowl lip, the load presents a sinusoidal and asymmetric variation.
The temperature field generated at the crown produces deformations, which add to increase the mechanically generated distortions. Both contributions work together, producing alteration of the sizes from the skirt to the top of the piston. Also considered must be the supplementary inhibition of deformation in the surrounding colder zones; all these contributions put the piston material through an exigent test.
Analyzing the acting force on the piston from the pin to the open area, it is possible to observe that the result is an oval deformation of the component.
These considerations require a design with the proper wall thicknesses able to prevent fracture or excessive deformation. Considering that the piston is a moving part contributing to inertial forces of the engine, also the weight must be kept at a minimum.
In these types of modern pistons such as the part under study in this work, the first and in some cases the second, ring groove is strongly affected by the thermal and mechanical loads discussed previously. Aluminum diesel engine pistons need to be reinforced in these areas. Designers have adopted a solution consisting of reinforcing the aluminum diesel piston first ring groove with cast iron (specifically an austenitic Ni resist alloy) (B in Fig. 1).
From the previously discussed, surge pistons must cover a wide range of unusual and conflicting demands, and together with these situations, users expect pistons with high durability and low maintenance.
The identified operational conditions could induce damage during service localized in the piston pin, the crown, the ring grooves or the skirt .
Traditionally, piston damage is attributed to wear, injection problems and lubrication sources; fatigue is also responsible for a significant number of piston damages. Mechanical loads together with high temperature induce severe operating conditions, which could produce serious damage in the piston crown.
Finally, a brief review of the piston materials should be considered in order to cover most of the issues that could be involved in the failure process. The piston material needs to meet the required mechanical properties, and this is closely related to the appropriate microstructure to undertake the complex set of thermal and mechanic requirements. The microstructure is affected by the chemical composition, fabrication process parameters and the final heat treatment [9, 10, 11].
Commonly, the materials used for fabrication are dedicated alloys not used generally in other components. Pistons are produced from cast or forged, high-temperature resistant aluminum silicon alloys.
In fact, common piston materials constitute a compromise among partly contradictory demands. Aluminum silicon alloys (predominantly eutectic) are overwhelmingly employed as materials to manufacture pistons. High thermal conductivity, low density, good castability and workability as well as good machinability and high-temperature strength are only a few of these lightweight alloy’s properties.
There are basic types of aluminum piston alloys, such as the widely used eutectic Al–12%Si alloy, regularly with the addition of approximately 1% of each Cu, Ni and Mg. With the objective of improving strength at high temperatures, several eutectic alloys of this kind have been also developed. Hypereutectic alloys with 18 and 24% Si provide lower thermal expansion and wear, but have lower strength. In practice, the supplier of aluminum pistons uses a wide range of further optimized alloy compositions, but generally based on these basic alloy types [12, 13].
Almost all the pistons are produced by gravity die casting. Optimized alloy compositions and properly controlled solidification conditions allow the production of pistons with low weight and high strength.
For those pistons used in high-performance engines with even higher strength requirements, forged pistons from eutectic and hypereutectic alloys are used. These types of pistons develop a finer microstructure than cast pistons with the same alloy composition, resulting in greater strength in the lower temperature range. The greater strength offers the possibility to produce lower wall thicknesses, reducing the piston weight.
In some special cases, aluminum metal matrix composite materials are used in piston applications. Pistons tops reinforced with Al2O3 fibers are produced by squeeze casting and are mainly used in modern direct injection diesel engines with improvements in mechanical properties and thermal fatigue behavior.
Failure Under Study and Experimental Procedures
The pistons analyzed in this work correspond to a four-cylinder diesel engine used in a light truck with pistons of 96 mm diameter, developing a volume of 3000 cm3, giving a maximum power of 127 kW at 3600 r.p.m. and 350 Nm of torque during a good range of r.p.m.’s.
A turbocompressor with variable geometry and intercooler helps the charge renovation process. The combustion chamber consists of the bowl at the top of the piston and a central injector with eight radial nozzles supplying the fuel.
Once the engine was disassembled, it was possible to detect that the propagating crack had advanced producing almost the fragmentation of the piston (Fig. 5c arrow).
Several pistons with the same failure pattern were examined; all of them were found in one piece joined by a ligament that survived the fracture process. They were separated mechanically using press equipment with the objective of observing the fracture surfaces and in this manner identify initiation locations and propagation mode. In addition, pistons that worked in the same engine (with equal operation conditions and hours of use) but not presenting any operational problems were also studied. Finally, a new piston was examined with the purpose of evaluating a microstructure that was never exposed to temperature, and assessing the degradation suffered by the used pistons.
The tasks included in the failure analysis of the piston involved inspection of crack initiation, analysis of material chemical composition (optical spark emission spectrometer BAIRD DV6), hardness at different points (universal hardness machine IBERTEST DU-250), optical microscopy (OLYMPUS PGM3) in different zones and SEM (JEOL JSM-6460LV) of the fracture surfaces and different zones of the bowl lip surface.
Analysis and Discussion
Chemical composition of piston material
The structure of the piston material is characterized by dendrites of Al-α-phase (solid solution Al(Si)), intermetallic eutectic compound (Al-α + eutectic Si) and primary Si particles.
The intermetallic phases act as the highest elevated-temperature strengthening phases in pistons with Al–Si alloys. These phases are thermally stable and affect mechanically the boundaries blocking the slide of Al-α grains. Frequently, some of the intermetallic phases found in these alloys include Mg2Si, Al7Cu4Ni, Al3CuNi, AL3Ni, AL2Cu and others.
It was reported that the addition of alloying elements such as Cu, Mg, Fe and Ni helps to improve the properties of the alloys used to manufacture pistons. For example it is known that adding copper and magnesium upgrades the strength because of the precipitation of fine secondary phases. On the other hand, Ni enhances the tolerance to high temperature, while Cu boosts the corrosion resistance .
The drop in hardness values measured in the zone of the bowl confirms the microstructural degradation process, which could contribute to the failure analyzed here.
Commonly the most affected zones during the operation are the piston pin and the ring grooves in the type of pistons studied in this work . In view of the failure type under study, the crown and specifically the lip of the bowl should be included in the list.
The biodiesel percentage present in the fuel used by the engine (operating in Argentina, where fuel is a blend) seems to have no evident influence on the failure. Moreover, it is possible to argue that the addition of biodiesel lowers the operational temperature, as reported, which is less detrimental to piston crown thermal fatigue [14, 15].
From the analysis of the piston crown in the damaged pistons, it was not possible to detect any problem in the injection system since the pattern was symmetric in the eight jets with no signs of an obstructed jet. Nor were there detrimental carbon deposits in any case as can be seen in Fig. 11. As observed, the failure was not located on any of these marks but in between them.
Another point worthy of note is that the higher temperatures should be located in the bowl lip and they could be a little higher in the exhaust valves zone.
The fracture surface was flat in the zone near the initiation (encircled black in Fig. 15), showing striations along with the crack travel direction. The zone encircled in white could be the point of meeting between the advancing crack and the cooling gallery at this particular surface, although it could also be intersected by a branch propagated from the inside of the bowl.
Once the crack advanced going down to the first compression ring, the fracture showed a sort of chevron marks probably influenced by the cast iron insert present in the groove.
Final Remarks and Conclusions
The present study explains the repeated failures observed in pistons of modern direct injection diesel engines.
Commonly, the failures of this component are attributed to combustion, variability of operative conditions or the inclusion of microstructural defects in the material because of the casting process.
From the tasks conducted in this work, it is possible to conclude that the action time and the operating conditions of the piston generated degradation of the material such as microstructural modifications that could affect the material properties. The lowering of hardness is evidence for this adverse effect, which combined with the severe thermomechanical fatigue load, led to this kind of failure.
Artificial aging of these alloys leads to precipitation hardening, breaking the as-cast dendritic structure, lowering alloying element segregation and spheroidizing the primary silicon crystals, which in turn improve bonding between the second-phase particles and the aluminum matrix. Overaging above the peak aging time produces the start of changes in the precipitate morphology, causing a clear reduction in the alloy hardness.
For especially high thermal and mechanical loads at the bowl edge, an improvement would be the achievement of a finer and homogeneous microstructure in the lip. This is a very common rule in materials selection for components working under severe thermomechanical fatigue conditions such as those borne by the piston of modern diesel engines, since fine grain structures can accumulate larger quantities of energy.
Influence of the biodiesel content in the fuel used was discarded since no adverse effects have been encountered.
Regarding the mechanical stress state of the piston, the tip of the bowl and the bowl itself at the pin axis direction, together with thermal loads, combine to generate a complex stressed location.
As a result of the material analysis carried out and the levels of stress and temperature sustained by the piston, it is possible to conclude that thermomechanical fatigue as a result of degradation in material properties is the main cause of failure. In addition, the generation of pits due to surface erosion contributed in the fracture initiation process. Once the fracture was present, brittle silicon particles acted as a preferential path of the fracture.
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