The study of fuel accumulation in the oil of a gasoline engine with direct fuel injection

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1. Relevance of the problem
For a long time, the problem of fuel accumulation in engine oil was mainly inherent in diesel engines due to their peculiarities in the organization of mixture formation. For gasoline engines in the "carburetor" era and with multipoint injection (MPI), this phenomenon also took place, but manifested itself to a lesser extent. Moreover, mid-20th century aircraft engine manuals recommended using the method of diluting the oil with gasoline to thin the oil and facilitate cold starting of the engine [9], and the presence of fuel in the oil was not considered to be something seriously affecting on the performance and life of the engine. However, at present, in connection with the massive introduction of gasoline direct injection (DI) systems on passenger cars with highly forced supercharged engines, the problem of diluting oil with fuel has acquired new emphasis and serious significance. It became obvious that the fuel that got into the engine oil has a negative impact both on the functional properties of the oil and on the working conditions of the parts and the flow of the working process of the engine itself.
We list the main negative aspects of this phenomenon:
• Fuel, washing off the oil film from the cylinder mirror, prevents the formation of a lubricating layer and thus increases wear in friction pairs: sleeve-piston, bushings of KV journal bearings, timing parts. With a fuel concentration in synthetic oil of 3%, 6% and 20%, the oil viscosity decreases by 19%, 35% and 73% and, as a result, the wear of cast iron sleeves increases by 7%, 59% and 212%, respectively [1] . According to [2], when the oil contains 4% and 8% of fuel, the wear of parts of the cylinder-piston group increases by 25 and 50%, respectively.
• The ingress of fuel into the oil leads to a decrease in its viscosity (Fig. 1) and, as a result, to a violation of the lubrication regime, loss of the bearing capacity of the oil wedge and destruction of the oil film in friction pairs.

Figure 1. Kinematic viscosity of 0W-40 oil at 40 and 100°C depending on the fuel content in the oil

• Oil dilution leads to a decrease in the operating pressure in the lubrication system, which further complicates the operation of plain bearings. In addition, oil diluted with fuel loses its design characteristics where the oil performs the function of a hydraulic fluid, for example, in valve timing control mechanisms.
• The fuel dissolves (liquefies) the oil on the cylinder walls and then, after ignition in the cylinder, burns out, entraining oil particles in this process. The liquefied oil film is scraped off by the piston rings and enters the gap between the sleeve and the piston in the area of ​​the piston top land. Given the common "T"-shaped piston skirt in modern engines, this is one of the reasons for increased oil consumption. When the oil is diluted with fuel, a significant decrease in the flash point of the oil (Flash point) occurs, fig. 2, and the oil, sandwiched between the piston and the sleeve in the zone of the top zone, begins to burn out, however, due to the lack of oxygen, the oxidation reaction is incomplete, with a large soot formation, fig. 3. In these zones, soot deposits accumulate, which, under certain conditions, can become provocateurs of a destructive phenomenon for the engine - pre-ignition or pre-ignition [6, 7].
• Fuel accelerates oxidative processes in the oil, which leads to oil degradation, reduces the concentration and effective life of additives. Accelerated oxidation shortens oil life and requires more frequent oil changes.

Figure 2. Dependence of oil flash point on the degree of fuel dilution according to [3] and [4]

Figure 3. Soot deposits in the heat zone

2. Causes of fuel getting into the oil

The following causes of fuel getting into the oil can be distinguished:
Structural*:
 shortcomings in the design of the nozzle spray, such as the number, location and diameter of the nozzle holes, the direction of the fuel jet;
 Loose fit of oil scraper rings to the walls of the cylinder liner.
Adjusting:
 incomplete combustion caused by over-depletion or over-enrichment of the fuel-air mixture, and leading to misfiring;
 incorrect organization of phased, "pilot" injection, start and duration of injection.
Operational:
 engine operation at low coolant and/or oil temperatures;
 frequent and short trips during which the engine does not have time to warm up;
 leakage of fuel equipment – ​​leakage of fuel from injectors in the closed position;
 a malfunction of the ignition system, in particular, a malfunction of the candles - misfiring;
 wear of the cylinder-piston group, as a result - a large consumption of crankcase gases, with which fuel vapors enter the crankcase, where they are further condensed and saturate the oil.
Some authors also name one of the reasons for the high fuel content in oil to be idling for a long time, “which is typical for urban traffic conditions” [8], however, experiments conducted at NAMI do not confirm this.

3. Permissible values ​​for the amount of fuel in oil
What is the maximum fuel content in oil? This question does not have an unequivocal answer for either diesel or gasoline engines. Too many factors to take into account - operating conditions, climatic conditions, fuel quality, type and chemical composition of the oil. For example, one of the largest manufacturers of synthetic motor oils, AMSOIL, declares the following restrictions:
 up to 2.4% for gasoline engines;
 up to 3.4% for diesel engines [10].
The American laboratory POLARIS Laboratories [11] indicates the permissible limits for the content of gasoline in oil of 3-7%. The source [12] speaks of 1.5% as the permissible concentration of fuel in oil, and 5% as the limit. Diamas laboratory [13] allows the presence of 7% fuel.
On the other hand, various sources state that during operation under average conditions, the concentration of fuel in the oil can reach 9%, which is a natural and normal state. According to the source [14], the limiting content of fuel in oil for gasoline and diesel engines is 7%, although it is also indicated there that if the fuel concentration is at the level of 20%, then short-term operation of engines in winter is possible ( more detailed comments are missing). At the same time, statistical processing of the results of the analysis of engine oil of cars that are in normal operation in Russia and undergoing service maintenance (regular oil change after 6-8 thousand kilometers) shows that the actual fuel content in the oil of passenger cars is in the range of 0.7-25.6% [14].
In this regard, of interest are the NAMI data obtained during a long-term survey of 30 cars of different classes and weights, but equipped with engines of the same model. The cars passed laboratory road tests of various kinds (high-speed, mountain, "urban cycle", etc.) for several years in different climatic conditions. The statistical picture of the fuel content in oil for all vehicles during these tests is shown in Figure 4. The results correlate well with the above source data [14], but also allow us to estimate the degree of deviation of these measurements from the conditionally limit value of 10%.
It is important to note that values ​​above 15% were recorded both in winter and in relatively warm climatic conditions.

Figure 4. Statistics of changes in the fuel content in oil for 30 vehicles during trial operation

Figure 5 shows the variation in the concentration of fuel in oil for one of the experimental engines for 9 months (within the service interval). Thus, based on the distribution density of the measurement results (Fig. 4) and the results of observations of individual vehicles (Fig. 5), we can state that for vehicles equipped with this engine, the fuel content in the oil, as a rule, is within 1 ... 15%.

Figure 5. Change in the fuel content in engine oil in one of the cars during the service interval

Thus, there is a rather significant spread of opinions and recommendations on the permissible concentration of fuel in oil. The most important are two questions: in what modes and how does the fuel get into the oil, and are there ways to return the oil to its original state by removing fuel from it. To answer these questions, a number of studies are needed.

 

About the authors

Vladimir M. Krasnov

State Scientific Center of the Russian Federation “NAMI”

Author for correspondence.
Email: vladimir.krasnov@nami.ru
ORCID iD: 0000-0003-4855-9105
SPIN-code: 3726-5821

Cand. Sci. (Tech.), Head of the Research Department of Fuels and Lubricants and Special Fliuds of the Power Units Center Powerunits

Russian Federation, 2 Avtomotornaya street, 125438 Moscow

Alexey S. Terenchenko

State Scientific Center of the Russian Federation “NAMI”

Email: terenchenko@nami.ru
ORCID iD: 0000-0002-1371-3179
SPIN-code: 8166-4320

Cand. Sci. (Tech.), Head of the Power Units Center

Russian Federation, 2 Avtomotornaya street, 125438 Moscow

Dmitriy S. Timofeev

State Scientific Center of the Russian Federation “NAMI”

Email: dmitry.timofeev@nami.ru
ORCID iD: 0000-0002-9946-4440
SPIN-code: 3083-1135

Head of the Sector of Units, Parts and Systems of Diesel Engines of the Power Units Center

Russian Federation, 2 Avtomotornaya street, 125438 Moscow

Ivan S. Shibaev

State Scientific Center of the Russian Federation “NAMI”

Email: ivan.shibaev@nami.ru
ORCID iD: 0000-0001-7741-9860

Head of the Internal Combustion Engines Department of the Power Units Center

Russian Federation, 2 Avtomotornaya street, 125438 Moscow

Nikita D. Stepin

State Scientific Center of the Russian Federation “NAMI”

Email: nikita.stepin@nami.ru
ORCID iD: 0000-0002-2826-7677

Chief Planning Specialist of the Project Management Center

Russian Federation, 2 Avtomotornaya street, 125438 Moscow

References

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