**1. Introduction**

Human activities have an impact on the environment by producing large quantities of greenhouse gases and polluting emissions. In the field of transport, the main pollutants linked to internal combustion engines are CO, NOx, unburnt hydrocarbons, and particles. The size of these particles can be less than a few tens of nanometers and therefore they can penetrate the bronchial tubes, reach the pulmonary alveoli, and have adverse effects on human health. It is therefore necessary to reduce particulate emissions from an internal combustion engine. In Europe, the Euro 6d regulation limits these emissions to 6.1011 particles by km and 4.5 mg of particles by km with the Worldwide Harmonized Light Vehicle Test Procedure cycle (WLTP) [1], for particles larger than 23 nm. The particle formation is a complex mechanism. It consists of a solid part composed of carbon and hydrogen containing a soluble organic fraction such as unburnt hydrocarbons, oxygenated derivatives or polyclinic aromatic hydrocarbons (PAH), or containing non-organic elements, such as

**Citation:** Berthome, V.; Chalet, D.; Hetet, J.-F. Consequence of Blowby Flow and Idling Time on Oil Consumption and Particulate Emissions in Gasoline Engine. *Energies* **2022**, *15*, 8772. https:// doi.org/10.3390/en15228772

Academic Editors: Tomasz Czakiert and Monika Kosowska-Golachowska

Received: 13 October 2022 Accepted: 18 November 2022 Published: 21 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mineral derivatives or metallic residues [2]. A particle arises from the collision between two PAH of pyrene (C16H10). This is called nucleation [3–8]. For internal combustion engines, the main factor generating particulate matter is the high-temperature combustion or pyrolysis of unburnt fuel [9]. Particle fluctuation at steady-state regimes was observed by Thawko, et al., [10] on a gasoline engine and by Swanson, et al., [11] on a diesel engine. Berthome, et al., [12,13] found variations in particulate emissions from a TGDI engine up to 80% on strictly identical transients. Amirante, et al., [14] found that the predicted soot particles mass was lower than experimental values and they suggested that other sources than fuel combustion should be considered, such as oil consumption. The main source of particulate emissions after unburnt fuel is oil consumption [15–18]. This consumption has several origins, such as the expulsion of oil through the piston rings into the combustion chamber, the evaporation of the oil film, and the transport of oil through the valve guides or via the blowby gases [19–22]. During a transient, Berthome, et al., [12,20] determined that 57% of the particulate emissions from an ICE were related to the variation of richness, while 31% were related to blowby, and 12% to backflow.

Blowby gases formation:

Blowby and backflow gases therefore have a very significant impact on particulate emissions. For example, an TGDI without blowby gases reintroduced at the intake emits 1.5 times less particle emissions than the same engine in normal configuration [20]. These gases, resulting from combustion, pass through the labyrinths of the cylinder/piston, ring, and grooves to the oil pan. These hot gases absorb oil droplets as they pass through the cylinder. Most engines use an oil sump. This means that the oil falls back into the crankcase by gravity after lubricating the components such as the crankshaft bearings, camshafts, valves, turbocharger, etc. A pump immersed in this housing sucks the oil and delivers it to the components to be lubricated after passing through the oil filter. With this oil falling by gravity, as well as the spray lubrication of the cylinder walls and moving parts, an oil mist is created which is partially absorbed by the hot gases of the blowby. If nothing is done, the pressure in the lower crankcase increases and can lead to problems with sealing, lubrication, etc. It is therefore necessary to evacuate these gases for the proper functioning of the engine. Since the introduction of anti-pollution standards, these gases are no longer released into the atmosphere but are reinjected at the intake. The oil in these gases must be removed before re-introducing them into the intake manifold [23].

The blowby oil sweep phenomenon:

Min, et al., [24] found that the blowby gases had the particularity of sweeping the oil stored between the compression and sealing rings of a piston. The position of the first two rings, characterized by the position of the "endgap", influences the flow rate and oil concentration of the blowby gases [24–27]. Thirouard, et al., [28] deduce that the amount of oil returned to the oil pan (*Qoil*) depends on the blowby flow rate (*Qblowby*) and the position of the "endgap" of these rings (*θrings*), cf., Equations (1) and (2).

$$\mathbf{Q}\_{oil} = \mathbf{Q}\_{blauby} \left(\frac{\mathbf{1}}{2\pi}\right) \frac{\mu\_{air}}{\mu\_{oil}} \cdot \frac{\mathbf{3}}{K\_2} \left( (2\pi - \theta\_{rings}) \left( 1 - e^{\frac{-K\_2\theta\_{rings}}{C\_1(2\pi - \theta\_{rings})}} \right) + \theta\_{rings} \left( 1 - e^{\frac{-K\_2(2\pi - \theta\_{rings})}{C\_1\theta\_{rings}}} \right) \right), \tag{1}$$

with

$$\mathcal{C}\_1 = \mathcal{Q}\_{blowby} \frac{\mu\_{air}}{\mu\_{oil}} \cdot \frac{\mathbf{3}}{2\pi (h\_{air})^2 \mathcal{R}\_{land}} \tag{2}$$

where *Rland* is the clearance between piston radius and cylinder radius, *K***<sup>1</sup>** and *K***<sup>2</sup>** constants are mainly related to the engine speed, and *hair* is the clearance between the piston and the cylinder.

A ring is a split elastic ring made mainly of steel. This endgap is necessary to counteract thermal deformations that cause a ring dilatation. Depending on the chemical properties of the ring, this cutting clearance is more or less important. When the endgaps of the compression and sealing rings are opposite each other at 180◦, the blowby gases sweep over the entire inter-ring area, see Figure 1a. This means that a maximum amount of oil stored in this zone is redirected to the oil pan. This phenomenon is called "blow-down effect".

**Figure 1.** Oil removed by the blowby gas vs. endgap position of rings.

The gases, reintroduced into the cylinder during the next cycle, and which will pass through this zone again in the other way via the backflow phenomenon, will be lightly charged with oil and, consequently, the associated level of particles will be limited. On the other hand, if the endgap positions are close, for example, 0◦, the area swept by the gases will be very limited and there will be a lot of oil left in this area, see Figure 1b. The backflow gases will then recharge with oil and redirect it to the cylinder, resulting in a higher particle level in the next cycle. Nakashima, et al., [29] and Agarwal, et al., [30] measured very significant variations in oil flow as a function of endgap position, up to 400% depending on the piston design.

Rings dynamic behaviour:

The ring dynamic behaviour is a complex mechanism. A piston ring rises and falls in its groove several times per cycle. This is due to the different forces it undergoes. Tian, et al., [31] determined that the position of the ring depended on the gas pressure upstream and downstream of the ring, inertial forces, frictional forces, the pressure distribution on the side of the ring, and the contact surface of the ring on the cylinder. However, under special engine conditions, that is to say under heavy load and for high rotational speed, the first two rings can sometimes move radially inwards of the piston grooves and allow unburned gases to pass directly into the oil pan via the scraper ring [32]. Tian [33] and Chen [34] showed that this dynamic behaviour, called radial collapse, is very sensitive to the pressure above the ring and to the tilt angle of the ring. When the ring collapses, it generates more blow-by. Tian [31] demonstrated that it can be limited via the shape of the rings. Rabuté and Tian [35] found out that blow-by is sensitive to the choice of ring materials. Wroblewski and Koszalka [36] measured the influence of various anti-wear coatings on frictional losses on the rings. Wroblewski and Iskra [37] demonstrated that the asymmetrical shape of the rings impacts the amount of oil scraped into the combustion chamber during the compression and exhaust stroke. Zarenbin, et al., [38] studied the impact of piston ring mobility on the blow-by gas and determined that the movements of the rings in the grooves noticeably affect the gas escape into the crankcase. Turnbull, et al., [39] showed that the power losses due to gas leakage can be more important than frictional losses.

There are various singularities in the dynamics of the rings, such as axial flutter and radial flutter, but the one that most impacts the blowby phenomenon is rotational movement. The rotation of the rings is due to two phenomena: micro-scratches in the cylinder, and piston oscillations. In fact, the rings do not move back and forth in a radial direction, but by reversing them in the cylinder bore, the piston changes its support from one cylinder wall to the other if it is on the thrust or anti-thrust side. This occurs both at Top Dead Center (TDC) of the piston and at Bottom Dead Center (BDC). This results in a radial displacement of the ring in its groove. This leads to the rotation of the ring in relation to the cross pass and the honing structure. Schneider, et al., [40] and Min, et al., [24] measured ring rotation of up to 10 rpm. This depends on the load and engine speeds [41]. Thirouard, et al., [26] studied the impact of the position of the rings and the amount of oil

in the space between the first two rings (Land 2)**.** It appears that when the rings are free to rotate, the amount of oil is very unstable, whereas it remains constant when they are locked in rotation.

Blowby gas simulation:

In order to better characterise the quantity of oil swept by the blowby gases and its impact on particle emissions, it appears necessary to create a simulation model of these gases. The novelty of this article is that it is possible to quantify the amount of oil swept by these gases from a simulation model and as a function of the position of the piston rings and the engine operating points. The first part of this paper is devoted to the calibration and analysis of the simulation model based on experimental tests, while the second part refers to the analysis of a particular engine operating point that is idling.
