The Flow of Lubricant as a Mist in the Piston Assembly and Crankcase of a Fired Gasoline Engine: The Effect of Viscosity Modifier and the Link to Lubricant Degradation
verfasst von:
Christopher J. Dyson, Martin Priest, Peter M. Lee
Der Artikel geht der komplizierten Dynamik des Schmierstoffflusses als Nebel innerhalb der Kolbenmontage und des Kurbelgehäuses eines befeuerten Benzinmotors nach. Sie unterstreicht die bedeutende Rolle von Viskositätsmodifikatoren bei der Beeinflussung des Schmiermittelabbaus und ihrer potenziellen Auswirkungen auf Motorleistung und Emissionen. Die Forschung zeigt, dass Nebeltröpfchen, die durch verschiedene Mechanismen gebildet werden, aufgrund ihres erhöhten Oberflächen-Volumen-Verhältnisses höhere Abbauraten aufweisen. Die Studie vergleicht auch die Abbauraten verschiedener polymerhaltiger Schmierstoffe und zeigt, dass Sternpolymere eine größere Scher- und Temperaturstabilität aufweisen als lineare Polymere. Die Ergebnisse haben wichtige Auswirkungen auf die Formulierung von Schmierstoffen und die Konstruktion von Motorkomponenten, um den Abbau und die Emissionen von Schmierstoffen zu minimieren.
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Abstract
Droplet flows, termed misting, are significant lubrication flow mechanisms to, in and around the piston assembly. Therefore, these are important in piston assembly tribology and engine performance. Crankcase lubricant degradation rate has been hypothesised to be influenced by lubricant droplet flows through the piston assembly and crankcase, but not previously confirmed. Lubricant was sampled from the sump, top ring zone (TRZ), and mist and aerosol from the crankcase during an extended run. The physical and chemical degradation of these samples was characterised. Droplet flows were intermediate in degradation and fuel dilution between TRZ and sump. Flows with smaller droplet sizes were more degraded that those with larger droplets. The degradation of polymers was dependent on their molecular architecture.
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1 Introduction
Lubricant in the piston and crankcase of an automotive internal combustion engine circulates through several mechanisms [1‐5], supplying critical tribological interfaces, e.g. the piston-liner interface, and passing through areas where high levels of lubricant degradation occur [6‐8]. Lubricant droplet flows within the crankcase and piston assembly, termed misting, have long been known but not greatly understood. As well as tribological performance (influencing fuel economy), misting has been hypothesised to influence lubricant flow to the combustion chamber [2, 4, 6]. Consequently, misting may also influence emissions and damage to after-treatment systems [9‐11], valve deposits [12], oil consumption through the piston assembly [11] and via the crankcase ventilation/recirculation system [12‐15], low-speed pre-ignition [1, 2, 4, 16‐22], and lubricant degradation [23, 24]. The purpose of the research reported in this paper was to test some of these hypotheses and identify links between lubricant flow as droplets in a fired gasoline engine and lubricant degradation.
Whilst known as misting, droplets in the crankcase and piston assembly include droplets that would be termed sprays and aerosols. Formation mechanisms have been previously described in detail by the authors [25] so are summarised here:
Blow-Through of oil pockets in the oil control ring or ring-liner interface [34, 35]. Mochizuki et al. [36] produced a detailed analysis of lubricant flow around the oil control ring.
5.
Inertial Throw-Off [1, 4]—Thought to form spray-sized droplets (102-103 μm) [1, 4, 32, 37, 38].
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Understanding droplet flows through the engine is significant for modern internal combustion engines, where fuel economy improvement and emissions reduction places new challenges on lubricants: Lubricant droplets entering the combustion chamber are an initiator of Low Speed Pre-Ignition (LSPI) in turbocharged gasoline engines [16‐22]; Cylinder deactivation causes vacuums that can draw more oil into the combustion chamber due to increased upward gas flow rates [39, 40]; Hybrid vehicles undergo high frequency stop-starts [41], often at speed rather than stationary i.e. increased frequency of transient phenomena that are associated with blow-through of oil pockets [35]; Exhaust Gas Recirculation (EGR) and turbochargers can draw oil droplets into hot areas of the engine, generating residues that can reduce turbocharger efficiency and emissions [42‐45].
A laboratory simulation of mechanisms 3 and 4 was performed by the authors [25, 46]. These mechanisms produced droplets with three characteristic size ranges and properties: Mist-sized droplets were found in two ranges 0.1–18 μm (Minor Mist region) and, more commonly, 18–135 μm (Major Mist region). Spray-sized droplets were 135–1000 μm. When entrainment from a component edges was the primary mechanism of droplet formation, mist-sized droplets predominated. When blow-through was the primary mechanism, spray-sized droplets predominated. Droplet formation tendency of the lubricant was most influenced by lubricant viscosity, and the presence and structure of polymeric viscosity modifiers. Higher viscosity lubricants had a lower tendency to form mist-sized droplets (entrainment from a component edge) but a greater tendency to produce spray-sized droplets (blow-through). Viscosity modifiers did not merely change the viscosity but influenced droplet formation through the viscoelasticity that they imparted. A greater viscoelastic response reduced the tendency to form mist-sized droplets by entrainment from component edges. However, greater viscoelastic response reduced the flow rate from the inlet area and, therefore, the formation of droplets by blow-through (spray-sized droplets) occurred at lower lubricant flow rates. Under the same flow conditions, a greater viscoelastic response increased the tendency to form spray-sized droplets by blow-through. These viscoelastic effects were greater with high molecular weight linear polymers than with high molecular weight star polymers [46].
Extending this laboratory simulation to measure droplet flows in a fired engine (the same engine that is used in this study) was achieved by extracting gas and droplet flows from the crankcase [25, 46]. Like the laboratory simulation, a trimodal droplet size distribution was observed. However, in the engine, the characteristic ranges had generally larger diameters whereby Minor Mist = 0.1–30 μm, Major Mist = 30–250 μm and Spray = 250–1000 μm.
Other studies measuring droplets in and from the engine have been discussed by the authors previously [25]. In addition to these, Paoloni et al. [47] considered low viscosity lubricants (SAE 0W-12) and found that volatility only partially explained differences in oil emissions between lubricants. Viscosity modifier additives influenced lubricant droplet sizes, which were associated with variations in oil emissions from the combustion chamber arising from upward flow of droplets. Adlercreutz et al. [48] observed that intermittent spikes in oil-derived emissions in a natural gas engine were linked to the relative positions of the piston rings gaps, and that upward flows of gas contain lubricant droplets. Droplet formation depended on the accumulation of oil pockets during low flow conditions, followed by rapid droplet generation during transients when ring gaps were optimally aligned. Higher viscosity lubricants and lower volatility lubricants generated fewer particulate emissions. Boronat Colomer et al. [49] positively correlated the quantity of fuel retained in the Top Ring Zone (TRZ) of the piston assembly with higher frequency of LSPI events and particulate emissions i.e. linked to the mixture of fuel and oil in this region.
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Previous work [25, 46] showed, in many cases, greater droplet formation tendency generated a greater quantity of minor mist-sized droplets. This increased specific surface area of the droplet flow and decreased in the Sauter Mean Diameter (SMD). SMD relates the average droplet volume and surface area, so correlates well with chemical and physical reaction rates affecting the droplet [50]. SMD is used to evaluate potential combustion rates of droplet flows [38, 51]:
where dv is the volume-weighted average droplet diameter and ds is the surface area-weighted average droplet diameter [52].
Engine lubricants degrade in several ways in service: By thermal and oxidative reaction, leading to increased viscosity and Total Acid Number (TAN) amongst other parameters [23, 24, 53‐55]; This can form higher molecular weight species [56],oil-insoluble species and, ultimately, sludge, deposit and varnish [23, 54, 57‐60]; By fuel dilution, which can reduce lubricant viscosity [23, 61, 62]; By scission of viscosity modifier additives [28, 53]; By contamination from blow-by gases and condensates, which can increase acidity [63]; By additive depletion, which can reduce performance over time [64‐66]; and by evaporation of the base oil at high temperature [8, 27, 28]. The upper piston assembly is the most aggressive engine environment. Lubricant samples from this region show high levels of degradation [8, 23, 24], including enrichment of additives as base oil fractions evaporate [8]. However, as the oil volume in the upper piston assembly is low and residence time is relatively short [7, 8], the oil in the sump will experience lower rates of degradation overall as lubricant returns from piston assembly, valve train etc [23, 24].
It has been suggested that lubricant droplets in the piston assembly and crankcase of an engine will degrade at a higher rate than oil elsewhere in the engine [6]. Uy [67] found significant differences between bulk oil in the engine and samples of aerosols and particulates from crankcase filtration system; soot and wear particles were present in these particulates too. Behn [68] showed that fuel concentration was significantly greater than oil concentration in droplet and gas flows sampled through the cylinder wall of a diesel engine. It was hypothesised flows of smaller diameter droplets in the engine could be more highly degraded than those from higher droplet diameter flows and bulk lubricant in the sump.
Therefore, in this study, samples were taken from various lubricant flows in a fired gasoline engine. Their physical, rheological and chemical properties were analysed to determine their condition. Previous studies have sampled lubricant from different positions in the engine [7, 8, 23, 68, 69]: Others have sampled droplets and particulates from the crankcase and analysed their composition [14, 67, 70]. However, the different types of droplet have not been sampled simultaneously to determine their relative degradation.
2 Test Engine
A Ricardo Hydra engine was used: A laboratory-based single cylinder, gasoline engine with properties described in Table 1. The cylinder head of this version is based on a General Motors 2.0 L, 4-cylinder automotive engine. The engine is connected to a dynamometer and can be either motored or fired. To better control and measure the engine’s tribological parameters, several modifications had previously been made: A separate sump and lubrication circuit was used for the valvetrain to remove the influences of this system. A fully formulated, commercial lubricant was used in the valvetrain. Model lubricants were used in the crankcase circuit. Crankcase and valvetrain sumps were external to the engine, meaning the crankcase was nominally dry and the effects of crankcase lubricant churning were removed. The cylinder head has a replaceable wet liner. A fresh liner and set of piston rings were used for this work. These were run in using the standard procedure for this engine: 40 h at 1500 rpm, 75% load and 50% throttle.
Table 1
Performance Parameters of the Ricardo Hydra Engine
Parameter
Condition
Maximum Speed
6000 rpm
Test Speed
2500 rpm
Maximum Torque (Load)
36 Nm
Test Torque (Load)
27 Nm (75%)
Cylinder Bore
86 mm
Piston Stroke Length
86 mm
Fuel
Reference ULG95 Gasoline
Ignition Timing
12º Before TDC
Test Throttle Setting
50%
Sump Volume (external)
1.5 Litres
Piston Ring Pack
Rings
Profile
Fitted Gap (mm)
Tangential Tension (N)
Top
0.603
28
Second
0.577
25
Oil Control:
Upper
Lower
0.812
0.784
17
18
3 Test and Measurement Procedure
In this study, the focus was on determining the lubricant degradation in the various lubricant flows in the piston assembly and crankcase. Lubricant was sampled from the engine during an extended run. Lubricant was sampled from four positions in the engine, shown schematically in Fig. 1:
Top Ring Zone Lubricant in the piston top ring zone (TRZ) was sampled from a 0.5 mm diameter hole behind the top ring on the anti-thrust side. Gas pressure drives lubricant down a PTFE tube on the connecting rod and on a grasshopper linkage between the bearing shell and the crankcase wall, Fig. 2a. The sample was collected outside the engine in a glass jar with a condenser plate, Fig. 2b. Samples were collected at 5-h intervals. This was a development of the technique used by Saville [71], Gamble [6] and Lee [23], and comparable to Watson [8]. Previous variants were limited to 2000 rpm, but this linkage design enabled higher speeds to be achieved. 2500 rpm was used here but higher is achievable in shorter runs. Verification work by Stark [7], Notay [72], Lee [23] and Gamble [6] determined that the TRZ lubricant sample rate represents around 15% of the flow rate of lubricant through this part of the engine and that, by monitoring wear and component temperatures, lubrication performance was not significantly affected by the sampling system extracting lubricant.
Crankcase Mist Lubricant carried as droplets in the crankcase mist was extracted from the crankcase using a 20 mm diameter tube, Fig. 2c. This was collected in a 0.1 μm coalescing filter. So that only mist-sized droplets were collected, i.e. not spray-sized droplets which would otherwise quickly be deposited, a 90° fitting was attached to the pipe in the crankcase so that the inlet was parallel and opposite to the major direction of flow i.e. only droplets small enough to be entrained in the gas flow could enter, Fig. 2d. Preliminary studies showed much higher flow rate through the sampling system when the fitting was not used, indicating that spray-sized droplets thrown from the crankshaft bearing assembly could enter the tube directly. This system prevented sampling of spray droplets. Mist samples were collected for 20 min every 5 h in parallel with TRZ sampling. Sample sizes were 8–10 ml—Sufficient for oil analysis but not too great to significantly affect oil availability in the engine. Similar techniques have been used by Krause, [14].
Crankcase Aerosol: Lubricant in fine droplets, i.e. aerosols, was collected from a 6 mm diameter pressure tapping on the upper surface of the crankcase, Fig. 2e. Samples were collected outside the engine in a glass jar fed by a tube, Fig. 2f. In this part of the crankcase, the gas flow rate was low and, as it was on an upper surface, only droplets sufficiently small to be entrained in the gas flow by Brownian motion, i.e. aerosols, would be present. The pressure difference between the crankcase and the sample jar that produced the flow was in the order of 0.01 MPa (whereas it was 0.5 MPa for the TRZ sampling system). Because the volumetric flow rate of lubricant was very small, 0.15–0.20 ml/hr, samples were collected after 20 h. Aerosol-sized droplets were within the range (and often significant volumetric constituents) of ‘minor mist’ flows [25, 46]. However, the minor mist diameter range (0.1–30 μm) also contained droplets that were larger than aerosols that would not be sampled by this system. Thus, aerosol flow should not be equated with minor mist flow measured previously. This was a similar sampling technique to Watson [8], who explained that blow-by gas flow over a small sample would not accelerate chemical degradation. Crankcase aerosols were reported to be approximately 30–80 °C by Krause [14] and 50–90 °C by Arnault [29].
Sump 5 ml samples of sump lubricant were taken every 5 h in parallel with the TRZ and mist samples.
Fig. 1
Schematic of Engine Sampling Locations
Fig. 2
Sampling Apparatus: a Top Ring Zone Sampling System; b Glass Sample Jar and Condenser Plate for TRZ Sampling; c, d Mist Sampling System; e, f Aerosol Sampling System
×
×
Because sampling systems could not accurately measure the flow rate of these oil paths, estimating residence times, return rates and sump turnover rates was not possible. Emphasis was on chemical and physical properties of the lubricant flows.
Each lubricant was tested for 20 h of engine running at 2500 rpm, 75% load and 50% throttle. The engine was run for an hour to reach thermal equilibrium before the test time was started. During this period, different sample jars were used ensuring that final lubricant samples were only from the desired engine conditions. The test procedure is shown schematically in Fig. 3. Additional samples of TRZ and mist were taken after half an hour of engine warm-up. Tests were repeated so that the statistical significance of results could be considered. Error bars are 95% confidence intervals generated from these repeated tests. This test duration was not expected to extensively degrade the entire quantity of lubricant in the engine. Lubricant degradation is dominated by rapid degradation of small quantities in particular engine systems, especially the piston assembly [23, 24]. This study aimed to identify whether similar degradation occurs in other lubricant flows, particularly droplet flows, and identify any links between these droplet flows and lubricant flow mechanisms in the piston assembly.
Fig. 3
Sampling Schedule for Engine Tests
×
Lubricant samples were analysed as follows:
Fourier Transform Infrared Spectroscopy (FTIR) using the ATR (attenuated total reflectance) method, to determine chemical degradation. Peak areas of the carbonyl (1690–1750 cm−1) and at another key point (1550–1650 cm−1) were used to determine the oxidation [23, 28, 73, 74] and nitration/thermal degradation of the lubricant, respectively [23, 27, 49, 53, 55, 68]. These are indicators of engine oil degradation, [23, 28, 54, 58, 60, 75]. Peak areas were calculated on spectra that were baseline corrected and normalised to the height of the tallest peak (mitigating error arising from the optical darkening of used samples). The peak area was calculated using a baseline value from the peak limits, Fig. 4.
Gas Chromatography (GC) analysis to quantify diluted fuel, in line with previous studies [23] and using ASTM D3525 [76].
Gel Permeation Chromatography (GPC) to determine the concentration and molecular size distributions of polymeric viscosity modifiers in the lubricants that contained these. Two measurement techniques were used:
o
Light Scattering (LS)–Particularly sensitive to larger molecules. The distributions indicated the presence of particular molecule sizes, but not their relative concentration.
o
Refractive Index (RI)–A quantitative technique that allows comparison of the relative concentrations of particular molecules in different samples.
A parallel plate viscometer was used to measure the viscosity of lubricant samples. A shear rate sweep from 100 to 105 s−1 was performed at 20 °C. 20 °C was used to minimise evaporation of the fuel in used samples [77]. Viscosity on the first Newtonian plateau [78] is reported unless stated otherwise, i.e. avoiding shear-thinning effects at high shear rates.
Viscoelastic behaviour was measured using the same rheometer but where oscillation frequency was increased until the lubricant exhibited more elastic properties than fluid.
Fig. 4
FTIR Peak Area Baseline Example
×
The three lubricants tested used the same API Group III [79] SAE 20 base oil [80] with a nominal KV100 (kinematic viscosity at 100 ºC) of 6cSt. Standard procedure to protect the engine was to add 1% succinimide dispersant and 1% overbased calcium sulphonate detergent [6, 23]. Two of these blends contained polymeric viscosity modifiers, producing an SAE 15W30 multigrade [80]: Solid polymer was top-treated into the original blend to produce a KV100 of 10cSt. Two commercial viscosity modifiers were used, an olefin copolymer (polyethylene-co-propylene) with linear molecular structure, Polymer 1, and an polyisoprene-co-styrene with fixed star structure, Polymer 2, [81]. Properties are shown in Table 2. Coincidentally, both polymers had equivalent thickening efficiency, i.e. 1%wt to meet the viscometric requirements. Polymers were included as solids, rather than as viscosity modifier (VM) concentrates [82], to give greater precision in formulation. Typical commercial crankcase lubricants contain 7–10%wt VM concentrates [81]. As concentrates typically contain 6–15%wt polymer [81], so typical polymer concentrations are 0.4–1.5%wt. Thus, blends were representative of commercial crankcase lubricants. Based on calculations described in [46, 83], the following define the coiling, entanglement and inter-molecular interaction of these blends:
Number Average Molecular Weight, MN—Measured using GPC Analysis. Number average molecular weight for a single arm of star polymer is also reported based on 10 arms per molecule, representative of typical star polymers [81, 84].
Hydrodynamic Volume, HDV—Volume of lubricant that an average polymer molecule influences. Calculated thus:
$$HDV = \frac{{4\pi R_{G}^{3} }}{3}$$
(2)
Where RG is the radius of gyration of the polymer, which is measured in GPC analysis of the fresh polymer [83].
Table 2
Test Lubricant Formulations
Blend
Base oil
Additives
Polymer
%wt
Nominal Kinematic Viscosity @100℃ cSt
Dynamic Viscosity @20℃ mPa.s
Number Average Molecular Weight Da
Hydrodynamic Volume HDV m3 × 10–24
Self Concentration ϕmoleclue g/100 ml
Entanglement Density EBlend
Ratio of Concentrations ϕmoleclue/ϕ
Base Oil
6cSt
API Group III
1%wt Detergent
1%wt Dispersant
-
-
6
56.9
-
-
-
-
-
Polymer 1
Linear
1%
10
152.6
51,945
2.3
3.7
0.086
0.22
Polymer 2
Star
1%
10
157.5
146,499 (arm = 14,650)
5.9
4.1
0.018
0.20
Self Concentration, Фmolecule—Concentration of the average polymer molecule within its hydrodynamic volume, typically expressed in g/100 ml. This indicates the extent of coiling and, thus, capacity for extension. This is a strong indicator of viscoelastic effects in polymer blends [46]. Calculated thus:
Two methods estimated inter-molecular interactions within the polymer blends:
1—Entanglement Density, EBlend—Average number of inter-molecular overlaps per polymer molecule. Values greater than 0.1 indicate a degree of inter-molecular interaction will occur, a semi-dilute blend [83].
Where ME is the entanglement molecular weight of the solid polymer i.e. average molecular weight of a polymer molecule between ovelaps for each polymer chemistry, Figure 5. E is the entanglement density of the polymer as a solid [83], obtained from Ferry [85].
Fig. 5
Schematic of ME, Entanglement Molecular Weight
×
When a star polymer is used, due to the hindered structure, inter-molecular interaction correlates most closely with molecular weight of the arm rather than the entire molecule [83]. Thus, for star polymers:
where EABlend is the entanglement density of the arms in solution, EA is the entanglement density of the arms as a solid, and MA is the number average molecular weight of a single arm [83]: This is approximate because it does not consider interaction between arms on the same molecule, i.e. steric hindrance [83, 86].
2—Ratio of concentrations, Фmolecule/Ф—Ratio of the self concentration of the average polymer molecule to the concentration of polymer in the blend, Ф, [86, 87]. Around 1, the blend is, on average, occupied entirely by hydrodynamic volumes of polymer molecules, generating significant inter-molecular interaction—A semi-dilute solution, [87].
Although viscometrics of both blends were comparable, they significantly differed in polymer blend properties. Polymer 1 had lower molecular weight than Polymer 2 by around a factor of 3. However, one arm of Polymer 2 was around a factor of 3 lower in molecular weight than Polymer 1. Polymer 1 was a linear chain polymer, which coiled effectively at low temperature in this base oil, so had lower hydrodynamic volume than Polymer 2, differences in molecular weight notwithstanding. Star polymers often have dense cores [81] so self concentration of Polymer 2 was higher than Polymer 1—Though the hindered structure restricted coiling of the arms (producing larger hydrodynamic volume than Polymer 1) the dense core produced higher self concentration. The Entanglement Density method, by differentiating intermolecular behaviour of the different polymer structures, predicted a far greater probability of inter-molecular interaction for the Polymer 1 blend, which may indicate greater viscoelastic response. However, because both blends were not predicted to be semi-dilute (EBlend > 0.1, [83]), i.e. dilute solutions, insignificant viscoelasticity arising from inter-molecular interaction was predicted. Despite using different polymers with different blend properties, inter-molecular interactions predicted by Ratio of Concentrations was similar for both blends. Low inter-molecular interaction was predicted, so differences in viscoelasticity would arise from differences in the individual polymer molecules.
4 Results
Figure 6 shows carbonyl peak volumes from FTIR analysis, indicators of oxidative degradation. The only significantly oxidised sample was fine aerosol droplets from the crankcase, Fig. 7. This was expected as the higher surface to volume ratio of aerosol droplets, expressed as Sauter Mean Diameter (SMD), increases thermal and chemical reaction rates. No significant difference in oxidation levels of the two polymer-containing samples occurred. The base oil aerosol sample was significantly less oxidised than the polymer-containing lubricants. Although not at the same speed, previous work suggested that, under high load conditions, the volumetric quantity of minor mist droplets (which potentially correlate with behaviour of these aerosol-sized droplets) was reduced when VM was present [25]. However, little significant variation in characteristic diameters of these droplets was observed between different lubricants [25]. Therefore, differences in oxidation between base oil and VM-containing blends were not likely to be caused by differences in SMD. Therefore, either VM-containing droplets sampled here were from a more greatly oxidised source (e.g. lubricants with higher residence time in the piston assembly) or that the droplets themselves had greater residence time in the piston assembly and crankcase.
Fig. 6
Carbonyl Peak Area for Engine Test Samples
Fig. 7
FTIR Trace of Aerosol and Sump Samples of SAE 20 Base Lubricant After 20 Hours Running
×
×
There was little significant oxidation of the sump, mist and TRZ samples. The main reason was probably because the engine operated under lean combustion conditions [88], typically producing greater nitration and thermal degradation of the lubricant [59]. Also, the sampling system could also have sampled a greater quantity of lower molecular weight species because of the condenser plate [72], which may partially explain some differences compared to previous results [23].
Therefore, the FTIR peak at 1600 cm−1 was also measured; This can represent C=C or C=N bonds in used lubricants, indicating thermal degradation or nitration [58, 63, 89, 90]. Figure 8 shows peak volumes for this:
TRZ samples were significantly more degraded than sump oil. This was expected as the TRZ where most degradation occurs [23, 24].
There was gradual increase in sump degradation with time as more lubricant from the piston assembly was returned.
Mist samples were comparable in degradation to sump samples but were less degraded than TRZ samples. This varied with time: Up to 10 h, mist samples were significantly more degraded than sump samples. After 10 h, increased levels of sump degradation meant no significant difference between mist and sump samples. This could imply that mist droplets were sourced in the piston assembly and that the reduced level of degradation was because either (1) misted lubricant had lower residence time in the piston assembly than TRZ samples, (2) misted lubricant was sourced from the lower piston assembly where temperatures and reaction rates were lower, (3) mist samples were a mixture of highly degraded piston assembly lubricant and less degraded lubricant from the crankcase, or (4) a combination of these mechanisms. Longer running durations could determine this more definitively.
Under many conditions, TRZ samples of polymer-containing blends showed greater degradation than base oil at the same time stage. This indicates that polymer-containing lubricants had higher residence time in the upper piston assembly than the base oil. The higher viscosity and viscoelasticity of these lubricants were the likely cause. Further testing could identify if this would produce long term differences in lubricant degradation rate. At almost every condition, insignificant different degradation levels were observed between polymer-containing lubricants.
Conversely, degradation levels of mist and sump samples of the polymer-containing lubricants were lower than the base oil under almost all conditions, especially for mist samples. This may have had several causes:
o
Despite higher residence time of polymer-containing blends in the TRZ than the base oil, residence time in the lower regions of the piston assembly may have been lower, leading to lower levels of degradation.
o
If degradation of mist flow was influenced by droplet size (SMD), changes in droplet size distribution may produce lower degradation rates for polymer-containing blends. Previous work measured lower SMD for mist-sized (30–250 μm) droplets in the crankcase when VM was present, which could predict the opposite trend to that observed here [25]. However, previous work used VM concentrate with a significant quantity of a low viscosity base oil as a diluent, which influenced the viscometrics [82]. In the current work, solid polymers were used, i.e. direct correlation with previous behaviour was not possible. Other authors using bench-top apparatus and lubricants without diluted VM concentrates suggested greater droplet diameter, hence SMD, when polymer was present in blends [91, 92]. This latter observation would support the hypothesis of higher degradation rate with lower SMD.
o
If lubricant returning to the sump was less degraded, degradation rate of the sump would likely be lower [24]. Hence, sump degradation levels may be lower when mist degradation levels are lower.
o
The more highly degraded TRZ lubricant with polymer-containing blends may result from significantly different lubricant flow through the piston assembly: More upward lubricant flow into the combustion chamber may have occurred with polymer-containing blends, reducing the downward flow rate of lubricant through the piston assembly to the crankcase. If so, a lower quantity of degraded lubricant would be returned to the sump, which may reduce the overall sump degradation level. If this mechanism occurred, higher oil consumption would be expected, especially emissions of lubricant-derived species. Specific emissions sampling would be required to test this hypothesis.
Nitration and thermal degradation of aerosol droplets was comparable to TRZ degradation levels. Though droplets may be partially sourced in the piston assembly, this was not necessarily the reason for the comparable degradation to the TRZ: Mist degradation levels were significantly lower than aerosol flows—Mist flows being also partially sourced in the piston assembly and partially from crankcase-generated droplets, though perhaps in different proportions. It is probable that the more extensive aerosol degradation occurred after their formation. A longer test cycle could verify this if TRZ and aerosol degradation mechanisms were, to some extent, decoupled. It generally appears that, when aerosol-sized droplets form in the piston assembly and crankcase, they degrade at a higher rate than most other lubricant flows.
For samples taken 30 min after start-up, when the engine was still warming up, TRZ and mist degradation levels were largely similar to the equivalent samples taken at thermal and tribological equilibrium: Probably because a significant proportion of samples were resident in the TRZ when the engine was previously stopped, so had experienced normal engine operation. TRZ samples were significantly more degraded than mist and sump. Mist samples were more degraded than sump samples around the same time stage. No significant differences in degradation were observed between different lubricants under warm-up conditions.
Fig. 8
Nitration and Unsaturation Peak for Engine Test Samples
×
Figure 9 shows GC fuel dilution levels. These bear similarity to degradation levels for each region. TRZ samples contained 15–22% fuel, significantly more than the sump (3–4%), with mist samples intermediate (6–10%). This suggests that mist samples were sourced in the piston assembly, few other mechanisms exist whereby mist droplets could contain more fuel than the sump. Fuel dilution was consistent with time, reflecting the thermal equilibrium of when sampling took place. Using the same engine, Smith [69] found that fuel dilution sampled through the liner decreased through the piston assembly: The dilution at TDC is around 4%, around 1% at mid-stroke and 1% at BDC. This suggests a large proportion of the mist samples may have originated from various positions in the piston assembly and that the fuel in the droplets may have increased by the condensation of fuel from surrounding gases. The upper piston assembly of this engine equilibrates around 200 °C depending on operating condition [93] and the crankcase around 50–60 °C [25]: This temperature difference would likely cause condensation of fuel onto droplets in the crankcase [25]. This effect appeared pronounced in aerosol samples, where fuel dilution varied widely between 15 and 28%. Due to the low SMD, the rate of fuel condensation per unit volume of lubricant would be greater than droplets in mist flows under the same conditions.
Fig. 9
GC Fuel Dilution of Engine Test Samples
×
When thermal equilibrium did not occur, i.e. during warm-up where fuel evaporation rate was significantly lower than the supply rate, there were variations in the flow of fuel through the engine: The fuel quantity in the TRZ was very large. However, because evaporation rates and component temperatures were lower, the fuel quantity reaching the lower piston assembly and crankcase (indicated by mist sample dilution) was significantly lower. If more fuel were present in the TRZ during warm-up, then a corresponding increase in mist fuel dilution may have been expected. However, mist fuel dilution during warm-up had similar or, with polymer-containing lubricants, lower fuel dilution than samples at thermal equilibrium: This confirms the effect of fuel vapour condensation on droplet flows.
GPC traces for the polymer containing blends are shown in Fig. 10, Fig. 11 and Fig. 12.
Fig. 10
GPC Traces for Fresh Polymer Blends: a Light Scattering b Refractive Index
Fig. 11
GPC Traces for Polymer 1 Engine Samples from a TRZ LS, b Mist LS, c Sump LS and d RI
Fig. 12
GPC Traces for Polymer 2 Engine Samples from a TRZ LS, b Mist LS, c Sump LS, d RI and e RI in Closer Detail
×
×
×
Figure 10 shows light scattering (LS) GPC traces for both fresh polymers. Traces are reported with respect to retention time rather than molecular weight. The dominant retention peaks defining Polymer 1 and Polymer 2, respectively, were 22 min and 20.5 min. Polymer 2, the fixed star polymer, had lower variation in molecular weight, indicated by the narrower distribution around the peak and by the polydispersity, PD, values for the peaks which were 1.88 and 2.94 for Polymer 2 and Polymer 1, respectively. Polydispersity indicates variation in molecular weight:
where MW is the mass-average molecular weight and MN is the number average molecular weight [83]. These values were measured using the neat polymers in a previous analysis [46].
Fresh base oil had two characteristic peaks, primary at 34.5 min and secondary at 39 min. The fuel had a peak at 41 min. Dispersant and detergent molecules, which have MN ≈ 3000 Da, produced a peak at around 27–29 min [94]. In used samples, new species were observed with retention times of 25.5–28 min, with broad peaks that do not correlate with the dispersant/detergent peaks. RI traces show these have low concentration, though higher in the TRZ than mist and sump samples. These appear to be degradation products, of polymer scission [28, 53] or of base oil degradation and polymerisation [56]: However, similarity of these broad peaks in different polymer blends suggested these are predominantly base oil-derived. In the LS traces from TRZ samples at start-up, these degradation products had a more distinct peak: This suggests differences in lubricant flow or sampling occurs immediately after start-up, or that different species are present and available to the sampling system on start-up.
Peak retention time of degradation species vary between TRZ, and the mist and sump: This indicates different degradation processes in the TRZ, reflecting the severe TRZ environment, confirming previous observations [23, 24]. This may also reflect different flow paths for different degradation species—Some may have accumulated in the piston assembly and others may more readily return to the sump. The mist degradation product species had a broader and more Gaussian distribution with peak retention time of 28.5 min. The sump had a narrower and more skewed peak with peak retention time of 28 min. The broader peak with the slightly higher retention time for mist implies that mist was more degraded than the sump i.e. greater variation in molecular weight. This suggests lubricant flowing as mist either originated from a body of lubricant more degraded than the sump, i.e. the piston assembly, or suggests degradation in the mist formation process and mist flow itself.
TRZ samples for linear chain Polymer 1 blends showed little trace of the original polymer peak: Polymer molecules had been highly degraded. Almost all Polymer 1 TRZ samples contained little polymer or associated degradation products, mostly base oil and fuel. This may reflect either lower polymer quantities reached the TRZ or that base oil and fuel were preferentially sampled. Further work is required to determine this. When polymer-containing blends (both 1 and 2) were tested, the TRZ sample hole in the piston blocked after around 25 h, not previously observed for non-polymer-containing blends: Watson reported similar [8]. Degraded polymer was the likely root cause. If so, degradation of the polymer caused both changes in the TRZ lubricant composition and preferential sampling of base oil and fuel, as polymer precipitated in the piston assembly. Mist samples for Polymer 1 contained a quantity of Polymer 2, seemingly contamination of the mist sampling pipework. LS sampling is sensitive to the presence of species rather than concentration—RI traces confirmed contamination was extremely low.
For Polymer 2, LS traces of TRZ samples indicated little change in the main polymer peak. RI traces indicated reduced concentration, by loss or degradation. This differed with TRZ samples for Polymer 1, where little original polymer remained. This suggests star polymers degraded at slower rates than high molecular weight linear polymers. Mist and sump samples for Polymer 2 showed little change in nature or concentration of the original polymer peak, i.e. low levels of polymer degradation.
Though Polymer 2 experienced degradation, particularly in the TRZ, it was lower than Polymer 1, indicating greater shear stability and thermal/oxidative stability. For Polymer 2, there was greater similarity between TRZ and mist samples than for Polymer 1. This further confirmed lower degradation of Polymer 2 in the upper piston assembly and TRZ than Polymer 1. This broadly agrees with Devlin et al. [95] who observed high molecular weight polymers generally reduced in molecular weight by chain scission, and that polymerisation of lower molecular weight materials occurs but does not generate such high molecular weight species as the sheared polymers.
Table 3 shows sump oil after 20 h had significantly lower viscosity than fresh, primarily from fuel dilution, around 3–4%, Fig. 9, typical for this engine [23]. Degradation of polymer molecules may have contributed to viscosity loss but was less significant than fuel dilution because of the comparatively low concentration of degraded polymer molecules in the sump, [23, 24].
Table 3
Viscosity and Viscoelastic Crossover Parameters of Fresh and 20 Hour Sump Samples
Blend
Dynamic Viscosity at 20 ºC mPa.s
Crossover Parameters
Frequency
Hz
Maximum Strain γ* %
Maximum Shear Stress σ* Pa
Viscous, Elastic Moduli G‘, G“ Pa
Polymer 1 Fresh
157.5
38.3
3.0
1.1
26.5
Polymer 1 Used
82.6
33.8
3.4
0.8
13.7
Polymer 2 Fresh
152.6
33.7
3.4
1.5
27.2
Polymer 2 Used
79.4
33.6
3.4
0.8
14.0
Linear viscoelastic response is represented by a complex shear modulus G* which resolves into two components, [83, 96]:
Elastic modulus, G’, or storage modulus, describes energy stored elastically, i.e. recoverable: In an oscillating rheometer, in-phase response to input oscillation.
Viscous modulus, G”, or loss modulus, describes energy dissipated by viscous response: In an oscillating rheometer, 90° out-of-phase to input oscillation.
Phase angle, δ, described the relative contribution of these components [83, 96]. When δ = 45°, elastic and viscous components are equal, termed crossover point, which is the threshold between responses dominated by solid or fluid behaviour.
Linear viscoelasticity analysis was performed in two stages. Firstly determining a maximum strain value within the Linear Viscoelastic Limit, LVEL [83, 96]. The frequency evaluated was 43 Hz, i.e. 2500 cycles per minute, mirroring engine conditions. Oscillation amplitude progressively increased, increasing the maximum strain. The test halted when storage modulus reduced by 5% from the previous measurement, indicating a highly non-linear viscoelastic response and increased out-of-phase viscoelastic response i.e. non-viscous-related increase in viscous/loss modulus. Maximum strain of 5% was within the linear viscoelastic limit of all samples, so was selected as the test value.
Secondly, crossover parameters were determined. Figures 13, 14 show linear viscoelastic responses of fresh sump oil and after 20 h. These represent response to oscillating shear stress at a maximum strain of 5%. Shear stress was increased by increasing oscillation frequency up to 43 Hz: As frequency increased, maximum strain begins to reduce. Several key observations were made:
Viscous moduli of fresh lubricants were comparable, Table 2 and Table 3. Similarly, viscous moduli of degraded samples were comparable, significantly lower than fresh: Degradation had not reduced the thickening effect of one polymer more than the other. However, the test time here was not expected to produce severely chemically and physically degraded lubricant in the sump. Fuel dilution was the root cause of lower viscous moduli.
For fresh lubricants, the crossover point occurred at a higher frequency for Polymer 1 than Polymer 2 i.e. Polymer 1 blend retained predominantly liquid properties to a higher oscillation frequency than the Polymer 2 blend. This was probably because the hindered star polymer structure prevented it aligning to the shear axis as effectively as linear polymer. This suggests that elastic deformation of Polymer 2 molecules had greater influence on fluid flow and deformation than Polymer 1. Star polymers have been found to have a greater shear-thinning response in unidirectional shear when compared to linear OCPs [86, 97]: Oscillatory shear perhaps produced responses which may not correlate with other rheological techniques.
Degraded Polymer 1 sample had significantly lower crossover frequency than fresh. Elastic modulus changed little, as expected with low polymer degradation. Because fuel dilution reduced viscous modulus, elastic response became dominant at lower frequency.
For Polymer 2, crossover frequency was comparable for degraded and fresh samples. This was apparently coincidental – Both viscous and elastic moduli were reduced. Polymer degradation appeared low, Fig. 12, suggesting a changes in blend or interaction properties of the polymer. Mary and Cusseau noted that in fresh Group III base stocks OCPs show ‘good solvent’ behaviours and isoprene-co-styrene star polymers shown ‘theta solvent’ behaviours, [86, 97]. Fuel dilution may have affected the solvency of Polymer 2 in the lubricant, perhaps changing hydrodynamic volume: Shifting theta solvent behaviour towards good solvent behaviour could enlarge hydrodynamic volume, leaving lower potential to dissipate energy under extension: Or shifting theta solvent behaviour towards poor solvent behaviour could reduce hydrodynamic volume, reducing the shear energy transmitted through the polymer molecules. Further study would be required to investigate. Research has highlighted sensitivities in fuel, base oil and polymer interactions [98, 99].
Whilst linear oscillating viscoelastic response identified differences, extensional rheometry may correlate more strongly with degradation mechanisms affecting droplet formation, [94, 100].
Fig. 13
Viscoelastic Response of Fresh and Degraded Polymer 1 Blends
Fig. 14
Viscoelastic Response of Fresh and Degraded Polymer 2 Blends
×
×
Various techniques were applied to lubricant samples from various flows in a fired gasoline engine. Differences in polymer degradation were clearest in GPC analysis. More extensively degraded samples may enable resolution of degradation differences using other techniques.
5 Implications for Industry
Implications for industrial practice and development:
High molecular weight linear polymers degraded rapidly in the piston assembly, especially TRZ. Viscometry and rheology of sump samples cannot be assumed to correlate with piston assembly lubricant, especially when polymers are used.
Star polymers degraded less extensively in the TRZ, confirming higher shear stability and thermal stability than the linear polymer used. One polymer architecture is not necessarily better than the other, as commercial linear and star polymers vary broadly. However, differences in shear stability (and thermal and chemical stability) significantly affected the extent of polymer degradation in the piston assembly and TRZ. Thus, considering the Yasutomi effect [23, 24], wise formulation may reduce rheological degradation rates.
Lubricant-fuel mix composition around the TRZ influences LSPI and particulate emissions [47‐49]. Varying polymeric additive degradation may influence these in ways not readily predicted from fresh lubricant properties in isolation.
In hybrid engines, where frequent start-stop cycles occur [41], fuel accumulation and longer lubricant residence time in the piston assembly is likely to affect lubricant degradation rate and emissions linked to oil-fuel in the TRZ.
Lubricant droplet formation and their degradation are indeed linked. Aerosol-sized droplets, i.e. lower SMD [38, 51] and higher crankcase residence time, were significantly more thermally and oxidatively degraded than lubricant in other flows. The effect on mist-sized droplets may require larger timescales to resolve due to their higher SMD and shorter residence time in the crankcase. Using a Yasutomi-type analogy, whilst the droplet lubricant volumes are small, engine designs and lubricant formulations that reduce formation and residence time of aerosol-sized and small mist-sized droplets may reduce the lubricant degradation rate overall.
The degradation and properties of mist-sized droplet flows strongly indicated their formation is linked to the piston assembly. The role of droplets in maintaining lubricant flow through the engine is significant, [1, 2, 4, 6].
6 Conclusions
Lubricant was sampled from the sump, top ring zone, mist and aerosol flows of a fired gasoline engine.
Mist lubricant degradation was intermediate between sump and TRZ samples. This partially confirms that lubricant droplet flows degrade at a faster rate than the bulk lubricant in the sump.
Under some conditions, lubricant degradation in mist and sump flows was lower when polymer additives were used than base oil only.
Aerosol oil samples were significantly more degraded than mist samples. This confirmed that higher surface area to volume ratio of droplets have higher degradation rates than other flows.
GC analysis of fuel dilution showed TRZ oil contained significantly more fuel than the sump. Mist oil was intermediate between these values, suggesting some crankcase mist originated in the piston assembly: Misting was a significant oil transport mechanism.
GPC analysis showed no significant change in viscosity modifier concentration during testing. A star polymer showed low degradation. However, a linear polymer was extensively degraded in the TRZ.
Fresh and degraded sump samples had viscoelastic properties: Linear viscoelasticity changed when degraded, showing some differences between polymer types.
Acknowledgements
Many thanks are owed to our collaborators who produced the GC and GPC data, and generated the FTIR spectra. Many thanks are also owed to Professor R C Coy and Professor M F Fox for their guidance and advice in this work.
Declarations
Competing Interests
The authors declare no competing interests.
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The Flow of Lubricant as a Mist in the Piston Assembly and Crankcase of a Fired Gasoline Engine: The Effect of Viscosity Modifier and the Link to Lubricant Degradation
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