Use of Advanced Piston Ring Coatings on Agricultural Engines

Abstract

The use of combustion engines on agricultural vehicles will persist much longer than on-road vehicles. Introducing new technologies in agricultural engines is crucial to mitigating emissions while accounting for customer cost-sensitivity, harsh operation conditions, and typically sub-optimal maintenance. This work describes the use of CrN and tetrahedral amorphous carbon (ta-C) DLC-coated rings in small agricultural diesel engines. Compared with the gas nitride rings, the CrN and the ta-C DLC coatings exhibited, respectively, 74% and 86% lower wear in rig tests. The DLC also presented a very low coefficient of friction and high resistance to scuffing. A similar wear trend was observed on durability engine tests, where the CrN top ring showed an 80% lower wear rate than the GNS used in a similar engine. Wear on the DLC oil ring was below the measurement capability. Liner radial wear was measured on the piston ring reversal points in four angular positions, and except for one position, was lower than 3 µm. At the end of the test, engine performance and emissions are nearly identical to those at the test’s start, demonstrating that the use of advanced tribological solutions can significantly contribute to emissions mitigation in agricultural engines.

1. Introduction

The agricultural sector accounts for a substantial amount of diesel consumption, and hence CO₂ emissions, reaching 118 Mtoe diesel in 2022 [1]. This figure is expected to rise, particularly in emerging and large-population countries [2]. China, as the world’s largest emitter of CO₂, sees increasing emissions from its agricultural sector [3,4]. According to the China Mobile Source Environmental Management Annual Report 2021 [5] apud [6], non-road vehicles emitted 425,000 tons of hydrocarbon (HC), 4,782,000 tons of nitrogen oxides (NOx), and 237,000 tons of particle matter (PM) in 2020, accounting for 48%, 35%, and 39% of the total emissions, respectively.

Tractors are among the most fuel-consuming and polluting machines in agriculture [7]. They are used in various agricultural operations: plowing, soil tillage, pushing harvesting equipment, and others. Internal combustion engines will likely remain in use for a long time: however, maintenance and operation are often suboptimal, especially on small farms, where the engines are exposed to dust, fibers, etc. Robustness and low operation costs are essential for market acceptance and user satisfaction. While new engine technologies can reduce fuel consumption and emissions, they must be cost-effective. The introduction of new, improved, and efficient machinery can mitigate the environmental impact of agricultural mechanization, particularly in developing countries [8,9]. In some cases, the purchase of modern machinery increases fuel consumption more than the increase in productivity [10] showing that reducing fuel consumption is necessary. On the other hand, studies show that CO₂ can be mitigated with the introduction of new technologies and that increasing energy efficiency is a more effective means to reduce CO₂ emissions than changes in the fuel mix [11].

Until recently, CrN applied by PVD (Physical Vapor Deposition) and DLC (Diamond-Like Carbon) low-friction rings were applied only to more advanced, larger HDD engines. More rigorous emission regulations for non-road vehicles, the increasing use of alternative fuels, and market competitiveness are promoting the use of more advanced technologies on agricultural engines [12].

Friction losses account for one-third of automotive fuel consumption [13,14,15]. Reducing these losses can enhance energy efficiency. The piston rings and piston account for around 50% of the engine friction losses [13,16,17] and, being a reciprocating system with variable loads and temperatures, they experience both boundary/mixed and hydrodynamic lubrication regimes [17,18,19]. The main research paths on vehicle powertrain to friction reduction include [20]:

- Use of materials/coatings with a low friction coefficient.

- Surface finish and/or texturing, especially via optimization using computer models and detailed topographic characterizations.

- Use of low-viscosity oils and improved friction modifier additives.

The increased operational demands on piston rings caused by biofuel use [21], along with low-viscosity oils, increase the need for better tribological solutions such as coatings and improved surface finishes. On diesel engines, it is still common to use Chromium coatings. For agricultural diesel and two-stroke gasoline engines, it is also common to use gas-nitrided rings (GNS) in combination with chromed-coated liners. More modern HDD piston rings have CrN- or DLC-coated rings [22,23,24,25].

There are several variants of DLCs, and their tribological performances depend on their structure, surface finish, and lubricant additives [26,27,28]. For piston ring coatings, variants range from relatively soft hydrogenated DLCs, used for break-in [23,29], to very hard ta-C (tetrahedral amorphous Carbon) [24,25]. Recent optimizations for DLC coating processes have allowed the mass production of ta-C coatings thicker than 20 µm [30,31,32]. Some studies indicate that H-free coated rings against cast iron cylinder iron may benefit from biofuels [28]. Figure 1 and Figure 2 show the relative wear and friction of ring coatings in a reciprocating test.

The piston rings operate in a complex, transient tribological environment. Due to different transient gas pressures and temperatures, the rings face boundaries to hydrodynamic lubricant regimes. The top ring, closer to the combustion chamber, usually presents the highest wear rate. The use of lower-viscosity oils, one of the main paths to increase engine mechanical efficiency [17,18,20,34,35,36,37,38,39], tends to cause more boundary contact [18,36,37] and increase ring wear. Figure 3 shows the wear rate after 500 h tests. The use of a lower-viscosity oil (10W-30) caused a significant increase in wear; for example, 62% for the top ring. The use of ta-C DLC on the 10W-30 test returned the wear rate to a lower value.

From the cylinder-liner perspective, improving the honing surface finish is the primary method for reducing friction and wear [20,39,40,41,42,43,44,45]. Cylinder honing is typically characterized by the Abbot curve, which defines the reduced peak height (Rpk), core height (Rk), and valley depth (Rvk). Figure 4 shows the typical values of the three most common cylinder honing approaches: standard (almost obsolete nowadays), plateau, and side honing. Slide honing, also called “glide honing”, has lower peaks (Spk + Sk/2) to reduce boundary contact but also has shallower valleys to reduce oil consumption. Figure 5 shows the oil film thickness along the four strokes of an HDD at part load and the resultant contact area and oil volume for plateau and slide honing. The smoother slide honing has a much lower contact area, especially at the TDC (top dead center). The shallow valleys of slide honing also contain less oil, helping to reduce oil consumption and emissions.

Soderfjall (2017) [40] studied the effects of different honing techniques, DLC-coated rings, and low-tangential-load OCRs on engine friction. Smoother plateau surface cylinder liner surfaces presented lower friction compared to the standard honed surface. The use of DLC-coated rings and TLOCR with a low tangential load (one of the combinations) showed an almost 50% OCR friction reduction while still indicating a small difference in oil supplied to the top compression rings. Tas [28] investigated the use of DLC rings against mirror-like cylinders and concluded that the low wear and high scuffing resistance were improved by the formation of tribofilms in the cast iron. Ferreira [47] and Liu [48] showed the importance of a good surface finish on DLC rings to reduce wear. However, despite the advances in material and coating characterization, as well as in wear simulation and durability prediction [17,19,34,35,49,50,51,52,53,54,55,56], actual engine tests are still needed to validate new technologies.

In this work, the use of the more advanced CrN and ta-C DLC ring coatings is investigated through tribological rig tests and engine durability tests on a small agricultural diesel engine. The ring and liner wear are compared with a previous engine version using GNS rings and chromed-coated liners. Some additional potential improvements, allowed by the very low wear, are discussed to reduce friction and fuel consumption.

2. Materials and Methods

2.1. Rig Tribological Tests

Friction, wear, and scuffing tests were performed in an in-house-built reciprocating test using piston ring specimens (Figure 6). CoF was defined as the cycle average. For scuffing, the liner was lubricated with a few oil drops, and the test duration was a minimum of 3 h or when CoF reached 0.3. Time to scuffing was defined as the minimum of three replications.

2.2. Engine Tests

Two agricultural diesel engines were used.

Table 1. Main engine characteristics.

	Production	New
Displacement volume [L]	3.5	3.6
Bore × stroke [mm]	98 × 115	98 × 120
Max. Power [kW]	95 @ 2600 rpm	118 @ 2400 rpm
Specific Power [kW/l]	27.1	32.8
Max. Torque [Nm]	450 @ 2000 rpm	550 @ 1700–1800 rpm
Cylinder	Chrome Coated	Cast Iron
Lubricant	20W-50	20W-50

gives the main engine characteristics. Notice that the new engine, where the advanced piston ring coatings were tested, has 21% higher specific power. Both engines are marketed to small tractors and harvest machinery like the one shown in Figure 7. Similar small-power diesel engines are used in developing countries for irrigation, pumping, and other agricultural activities.

The main characteristics of the piston rings are described in

Table 2. Piston rings—engine in production.

	Top	2nd	OCR
Coating	Nitrided	Phosphate	Nitrided
material	Steel	Cast Iron	Nodular Cast Iron
Axial width [mm]	2.5	2.0	3.0
Radial [mm]	3.80	3.6	3.5
type	Keystone	taper-faced	2-piece
Tangential Force [N]	19.6	14.5	30.0

and

Table 3. Piston rings—engine under development.

	Top	2nd	OCR
Coating	CrN	Phosphate	ta-C, DLC
material	GNS	Cast iron	Nodular Cast Iron
Axial width [mm]	2	2	3
Radial [mm]	3.55	4.0	2.54
type	Keystone	taper-faced	2-piece, “LKZ” profile
Tangential Force [N]	15.1	15.9	41.8

.

Figure 8, Figure 9, Figure 10 and Figure 11 and

Table 4. Piston ring coatings—main characteristics (as finished).

	GNS	CrN	ta-C
Deposition method	Gas Nitriding	PVD—Cathodic Arc	FCVA
Hardness [Hv]	900 min. at 10 µm	1100 min.	1700–3500
Thickness [µm], min.	100	100	20
Roughness [µm], max.	Rz 3.0	Rz 1.6	Rpk 0.2, Rk 0.6

show more details about the investigated ring coatings. The more wear-resistant CrN and DLC coatings were applied only to the running face over a nitriding face. The nitriding layer worked to protect the ring side faces and, in less demanding cases, acted as the wear-resistant surface treatment in the running face, as the engine in production used as a reference. An adhesion layer, usually chrome and around 1 µm thick, was used to promote good adhesion between the CrN and DLC coatings with the base material. Both the CrN and the ta-C DLC were deposited by Physical Vapor Deposition (PVD), also known as Ion Plating (IP). Process details are out of the scope of this paper, but for ta-C DLC, a filtered cathodic vacuum arc (FCVA) was used to ensure a good coating quality and a reduced number of pores and droplets. Details about FCVA can be found in [57,58].

Table 5. Cylinder liner material and roughness.

	Chromed	Cast Iron
Hardness, Hv	800 min.	400
Sa [µm]	0.28	0.62
Spk [µm]	0.21	0.19
Sk [µm]	0.66	0.28
Svk [µm]	0.77	2.62
Mr1 [%]	5.4	7.4
Mr2 [%]	81.3	72.6

describes the cylinder liner’s main roughness parameters. Figure 12 shows the topography measured by the Taylor-Hobson CCI-MP optical profilometer, and measurements were carried out using a 20X lens. Roughness parameters were calculated by the equipment software, TalyMap Gold 6.2.6613.

To simulate field operation conditions, the dynamometer test was conducted as follows:

(a)

0 to 300 h: full power.

(b)

300 to 800 h: full power.

(c)

800 to 1100 h: thermal shock, alternating every 6 min at full power with 95 °C cooling water and 4 min at idle with 30 °C cooling water.

The prototype fuel injector caused excessive piston carbon buildup. An improved fuel injector and new pistons, cylinder liners, and piston rings were assembled. Lubricant oil was changed every 300 h.

3. Results

3.1. Rig Tribological Tests

Figure 13 compares the friction and wear of the different ring coatings versus the cast-iron liner. The GNS ring has the same nitriding treatment but no CrN PVD overcoating (see Figure 8).

Figure 14 shows the scuffing resistance of chrome, CrN, and ta-DLC coatings with oils equivalent to SAE 40 (HTHS (high-temperature high-shear): 3.5 mPa.s) and SAE 30 (HTHS 2.9 mPa.s). Under the test conditions, the use of lower-viscosity oils decreased the scuffing resistance for both chrome and CrN coatings, while the DLC concluded the 250 min tests without scuffing. Similar trends were reported by [25].

3.2. Engine Performance

Engine performance remained almost constant throughout the test (see Figure 15, Figure 16 and Figure 17). Before the thermal shock, the engine had its calibration fine-tuned and underwent minor maintenance. The two values at 800 h show the values before and after the minor maintenance. Only maximum blow-by showed some increase along the abused thermal shock.

Due to very low wear and engine degradation, emissions at the test end were similar to those at the test start (see Figure 18 and Figure 19).

3.3. Ring Wear

Ring radial wear was measured by superimposing the face profile before and after the test. On the GNS top ring, tested for 1000 h, the wear at ring tips was 6.5 µm. On the CrN, with an accumulated 800 h, wear was less than 2 µm (see Figure 20).

The second ring, not directly exposed to the combustion pressure and temperature, is the lowest tribologically loaded. Being only phosphate-coated, the second ring presented larger radial wear, between 8 and 10 µm (see Figure 21). Figure 22 shows the wear rate of the different rings. The wear rate of the second ring was very similar in both engines.

Due to the almost flat running profile, the direct measurement of wear on the oil control rings was not possible. The DLC oil control rings were measured in five angular positions in the upper and lower lands (see Figure 23). Very low wear was evidenced by the fact that the minimum land height in all measured positions was 24 µm. The land height has dimensions similar to the DLC thickness (see Figure 11).

3.4. Cylinder Liner Wear

As shown in Figure 13, PVD and DLC ring coatings are highly wear-resistant and cause lower wear on the cylinder liner. While the engine production uses chrome-coated liners, the one in development uses uncoated liners. The use of smoother slide honing also contributes to low liner wear. Unfortunately, during the development of the new engine, liners of the 1000 h test in the production engine were not found to be measured. For the development engine, as mentioned for the rings, measurements were made at the end of the test, with rings and liners having accumulated 800 test hours. Liner honing was well preserved with no significant polishing or wear scratches. Honing grooves are visible even at the TDC (see Figure 24).

Liner radial wear was measured on the piston ring reversal points top dead center (TDC) and bottom dead center (BDC) in four angular positions (front, thrust side, back, and anti-thrust side) by a profilometer, see example in Figure 25 and except for one position, all values were below 4 µm (see Figure 26 and Figure 27).

4. Discussion

This paper was offered to the Lubricants Special Issue “Tackling Emissions from the Internal Combustion Engine: Advances in Piston/Bore Tribology”, for which one of the authors is co-editor. In the special edition, more detailed and rigorous investigations are described. The Special Edition articles covered themes from very detailed computer models [59,60,61], fuel consumption on emission tests, and the associated experimentation uncertainties [39,62] to long real-world vehicle tests using low-viscosity oils [63]. Stellljess [64] showed a reduction in friction losses from 9% to 17.3% with the use of a profiled cylinder bore.

Technology advances need to be implemented in mass production products in order to generate the expected benefits to society. In the current work, we have tried to demonstrate the benefits of some advanced ring coatings on small agricultural diesel engines produced for emerging countries. Compared with the GNS rings tested in a similar engine already in production, the CrN top ring presented a wear rate five times lower. The ta-C oil ring presented wear below measurable capabilities. Despite a direct comparison not being possible since the engines and liners were different, the engine wear results correlated well with the rig tribological tests.

The very low wear found in the piston rings and liners creates opportunities for further improvements to reduce friction and fuel consumption:

• The use of lower-viscosity oils: lower oil film thickness and higher boundary contact would be acceptable.
• Smoother cylinder liners: honing grooves were preserved at the end of the test.
• Reduced tangential load on oil control ring tangential: contact pressure was not reduced along the engine life due to ring face wear.

5. Conclusions

CrN PVD top rings and ta-C DLC oil control rings showed very low wear after the 800 h engine test. In combination with the ring coatings, the use of smoother slide honing on the cylinder also caused very low wear on the cylinder. Cylinder wear was measured by radial profiles at TDC and BDC, and it is in the order of 3 µm. No polishing or scratch marks were observed even in the ring reverse regions.

Except for the blow-by increase in the final hours of thermal shock, all engine parameters remain almost unchanged throughout the test. At test end, engine performance and emissions are almost identical to those at test start. This brings us to the conclusion that the integration of advanced piston ring coatings and optimized cylinder liner surfaces significantly reduces wear in agricultural diesel engines. The results from the reliability test indicate that these technological advancements not only maintain engine performance but also contribute to lower emissions, supporting sustainability in the agricultural sector.