Deterioration of the fuel injection parameters as a result of Common Rail injectors deposit formation

The article describes external and internal Common Rail injectors deposits formed in dynamometer engine simulation tests. It discussed not only the key reasons and factors influencing injector deposit formation but also the resulting way of fuel preparation and engine test approaches. The effects of external coking deposit as well as internal deposits two most common form types that is carboxylic soaps and organic amides on deterioration of the fuel injection parameters were assessed. The assessments covered both deposits impacts on quantitative and qualitative changes of the injectors diagnostic parameters and as a result on deterioration of the injector performance. Finally the comparisons between characteristic of dosage of one fuel injector before test and characteristics few injectors after engine tests of simulated deposit formation were made.


Introduction
Problems arising from build-up of different types of damaging deposits in diesel fuel injectors is well known since many years now and was extensively described in many scientific publications including [1][2][3].In the 1980s the build-up of external injector deposits on the pintle of a fuel injector has caused a significant concern of fuel systems producers and introduction of DCA (Deposit Control Additives) to reduce injector fouling occurred [4].Continuing since 1997 development of HPCR (High Pressure Common Rail) systems which design features are more conductive or susceptible to deposit formation, especially IDID (Internal Diesel Injector Deposits), such as severe high temperature along with high pressure operating conditions as well as precision engineering to operate highly controlled multiple injection strategies [5][6][7].This may rise problems in the field due to deposits that have formed on their critical moving parts, such as the needle and control valve and manifested themselves as injector sticking, rough idling, power loss, issues with drivability, high emissions and many more [8][9][10].The current industry focus is on internal injector needle deposits.These IDID have been categorized in four main types: higher carboxylate deposit, amide deposit, ester lower carboxylate -cold flow improver deposit and ester lower carboxylate -FAME deposit [10][11][12][13][14][15].The mechanism of IDID deposit formation is different from that of external deposits on the injection nozzles, because internal deposits are not exposed to exhaust gas generated during combustion and surface evaporation of fuel.IDID are formed as a result of deteriorated fuel which becomes deposit adheres to the material surface by physical binding or deteriorated fuel which adheres to the material surface by chemical binding [2,16].It was concluded that in case of diesel engines with indirect fuel injection the amount and rate of coke formation were widely influenced by the type of injector used, in case of direct fuel injection it is the fuel that plays the key role in formation of the subject coke deposits.Wider use of low sulfur diesel fuels and fuel additives gave way to fuels containing acid components.For example, to a different extent, unsaturated fatty acids are commonly used as lubricant additives.These acids easily react with metallic ions of the fuel contaminants forming carboxylic salts, that, similarly to low molecular mass polar compounds, much worse dissolve in low sulfur diesel fuels of low content of aromatic hydrocarbons compared to old high sulfur content fuels [1-4, 16, 17].A growing share of FAME (Fatty Acid Methyl Esters) characterized by low stability in the diesel fuels accelerates the process of fuel oxidation and degradation.FAME are a source of weak acids and lead to an increase in the fuel contamination (sodium compounds) -a component of typical catalysts used in the reaction of transestrification [1][2][3][4][16][17][18][19].Hence, due to their low stability, the resistance to oxidation is one of the most important properties of diesel fuels containing FAME.Consequently, in the market today there are a variety of fuel qualities.At the same time DCA additives used in fuels are generally considered to be the most effective way to counteract injector deposit formation.However, currently, widely applied detergents preventing external coke deposits on the injectors are based on PIBSI (Polyisobutylene Succinimide) and in combination with the previously described factors may facilitate the formation of IDID [1][2][3][4].Therefore, the development and manufacture of these type additives is critical to their performance in the field in terms of functioning to control both external and internal deposits but also from a nocharm standpoint [4,20,21].This paper reports results generated to develop further knowledge relating to the engine test investigations into external and internal common rail injector deposit formation.The main research problems concerned the possibilities of reproducing different type of injector deposits in engine bench tests that simulated conditions of real operation.The main target of the research was to investigate the problem of injector deposit formation and to try to identify the source of the problem.It was important to study the effects of diesel fuel properties on the common rail injector functioning because the fuel could deteriorate inside an injector at such severe conditions and build-up of damaging deposits.As a criterion for assessing the effects of external and internal deposits reproduced during engine simulation tests were used diagnostic parameters of injector functioning and their characteristics of fuel injection.In addition, visual assessments of the deposits were performed.

Reproduction engine test of injector deposit
The engine simulation tests of injector deposits reproduction were performed on a multi-purpose test stand equipped with a modern, widely used HSDI (High Speed Direct Injection) FORD 2.0i 16V Duratorq TDCi diesel engine coupled with an Alpha 160 AF eddy current brake from AVL, with a control module allowing the programming of the engine operation parameters (rpm, load, phase time and ramp time between phases).The basic technical parameters of the FORD 2.0i 16V Duratorq TDCi are given in Table 1.The engine tests were performed in a 4-phase, repeatable cycle which reflected the average engine operating conditions in a low-intensity city traffic.The parameters of the 4-phase cycle are given in Table 2.The test duration was 100 hours of actual engine operation in 8-hour periods which comprised repetitions of the 4-phase cycle (Table 2) and were separated by 16hour soak period during which the formed deposits stabilized.
When the test was complete, the injectors were removed from the engine and two of them were sent for full, professional assessment of diagnostics parameters and the other two were left to take photographs of the internal deposits.The necessity for such procedure resulted from the fact that the deposits may be subject to mechanical and chemical damage/changes during the diagnostics and in the final stage they are removed in order to evaluate the parameters and the adjustment possibilities of the "cleaned" injectors.On the other hand, before the diagnostics the injectors must not be dismantled (e.g. for photographing and evaluation of deposits) as this would affect (distort) the diagnostic parameters after the test.
The injector evaluation after the test comprised both the external coke deposits injectors and the IDID.In case of the IDID, the obligatory evaluation was performed on the injector needle and solenoid valve control piston as components with critical impact on the correct injector operation, and in addition most often subject to damage.
The measurements and evaluations of selected diagnostic parameters were performed on the Zapp CRU.2i testing device.The evaluations of each diagnostic parameters are an average value from three measurements.Characteristics injectors fuel delivery were taken -off on the Hartridge CRi-PC test bench.

Tests Fuels Preparation
In order to accelerate the formation of injector deposits (especially the IDID), the engine was running on the naturally aged, commercially available diesel fuel containing 4.78% (v/v) of FAME -Table 3. Furthermore, an additional admixture of naturally aged FAME -Table 3, was used in some tests fuels.
The results of the to date analysis of sodium soaps (or sodium carboxylate) type IDIDs identified alkenyl succinic acids, like dodecenyl succinic acid (DDSA) and hexadecenyl succinic acid (HDSA), which are commonly used as pipeline corrosion inhibitors in the petroleum industry [22][23][24].It was also considered that if higher molecular weight carboxylates and sodium are contained in diesel fuel simultaneously, carboxylate salt deposits are build-up on metal surfaces at temperatures of 130 degress C or higher [8].Moreover, sodium ions are captured by fatty acids, which are a by-product of FAME blended into diesel fuel.When sodium ions and fatty acids meet, sodium soaps form [25].
Sources of amide deposits as well mechanism for amide lacquer formation is less recognized but it is most likely derived from polyisobutylene succinimides (PIBSI) which are added to diesel fuel as a major component of DCA.Two hypothesis are widely known for the formation of amide type IDID.One hypothesis assumes that deposits are formed through the reaction of PIBSI with fatty carboxylic acid to form an amide [8,17,22].The other assumes that it is the low molecular weight portion of the PIBSI that is responsible for the deposit formation as it is only sparingly soluble in the diesel fuel and thus deposits on the surfaces of the inner injector parts [20,22].
For this reason, added to the test fuel as 1-litre premixes were mixtures of chemical compounds which according to the available results of tests performed at various laboratories have the greatest impact on the formation of various IDID [9,18,20,[26][27][28][29][30][31].The said chemical compounds comprised the ingredients present in detergent and lubrication additives, corrosion inhibitors, additives increasing the cetane number and the contaminants which make their way into to the fuel.As a result, three fuels described below were prepared for the simulation engine tests.
In test No. 1, the propensity for the formation of external and internal deposits was tested on the aged, commercially available diesel fuel (physical and chemical properties are given in Table 3) with addition of 0.5 ppm Na + + 70 ppm dodecenyl succinic acid (DDSA).This aimed at accelerating the formation of carboxylate deposits, in this case sodium soaps by adding "Na" as metallic contaminant often appears in diesel fuels and dodecenyl succinic acid (DDSA) which is ingredient of corrosion inhibitors also occurs in diesel fuels.In test No. 2, the propensity for the formation of external and internal deposits was tested on the aged, commercially available diesel fuel (physical and chemical properties are given in Table 3) with addition of 120 ppm of PIBSI (Polyisobutylene Succinimide) + 100 ppm of hydrogenated dimer of oleic acid + 1ppm of formic acid.This aimed at accelerating the formation of amide deposits by adding PIBSI on which the detergent additives to diesel fuel are based and by adding acidic substances which are ingredients of lubrication additives.
In test No. 3, similarly to test No. 1, the propensity for the formation of external and internal deposits was tested on the aged, commercially available diesel fuel (Table 3) in which the content of biocomponents (4.78% (v/v) FAME) was increased to 10% (v/v) by adding aged B100 (properties given in Table 2).Moreover, 500 ppm of 2 ethyl-hexyl nitrate (2-EHN) + 120 ppm of PIBSI (small molecular mass) were added to the fuel.This aimed at accelerating the formation of amide (polymertype) deposits by adding PIBSI on which the detergent additives to diesel fuel are based and the formation of ester deposits from aged FAME in tandem with 2 ethylhexyl nitrate (2-EHN) added to increase the cetane number.

Results
Figure 1 pre after test No.

Fig. 1
Fig. 1.Photog On the in unevenly dis deposits are few outlet ho the deposits present in so of deposits o similar.Figu injectors afte

Fig. 4 .Fig. 5 .
Fig. 4. Compa after engine te Figure 6 injectors afte The injec mat, dry, da much thicke and peeling o The size surfaces of b Fig. fuel ed out using Z h injectors did ze at the injec opening time om the point injectors wer enance-repair ment and poss internal par ctor needle afte osits, covering found on the cylindrical pa rt -Fig.

Table 1 .
Selected technical parameters of the FORD 2.0i 16V Duratorq TDCi engine.

Table 2 .
Parameters of the 4-phase engine cycle.

Table 3 .
Selected physical and chemical properties of fuels used during engine tests.

Table 4 .
The ected diagnos pp CRU.2i tes it formed at the rol valve after t xt part of t of selected er test No. 1.

Table 4 .
IDID that collected on the nozzle needle guide and inside the nozzle body, metering valves and other parts of the armature group of the fuel injectors could slow the response of the fuel injector or cause sticking of the moving internal parts, impairing the injection timing and quantity and the quality of fuel delivered per injection.•The change in injection rate due to IDID adhesion manifests itself in injection start and end timing changes which results different fuel injection volume