piston ring lubrication in a two-stroke diesel engine

Wear, 72 (1981) 353 - 369 0 Elsevier Sequoia S. A., Lausanne - Printed in The Netherlands

353

PISTON RING LUBRICATION IN A TWO-STROKE DIESEL ENGINE

S. L. MOORE

Department of Engineering, University of Reading, Whiteknights, Reading RG6 2AY (Gt. ~r~tuin~

(Received May 20,1981)

Summ~y

Miniature capacitance film thickness transducers were fitted to the cylinder liner of an opposed piston two-stroke diesel engine. The minimum fiim thickness between the rings and liner was measured and the attitude of the rings as they passed the induction ports was determined.

1. Introducti.on

Capacitance transducers have been used recently by several workers to measure the oil film thickness between the piston rings and the cylinder liner of a working engine. Sreenath and Venkatesh [l] and Parker et al. [2] have fitted gauges into the piston rings. These gauges enabled the film thickness to be measured throughout the stroke, although owing to the ability of the ring to tilt in the groove this was not necessarily the minimum film thickness.

Hamilton and Moore [ 3,4] have shown that by fitting the gauges into the cylinder liner, instead of into the piston rings, both the minimum film thickness and the ring profile can be measured as the ring passes the gauge. As the gauge is stationary in the cylinder liner with this method it is of course necessary to fit the instrumentation at the part of the stroke which is being investigated.

The general conclusions from measurements carried out using this tech- nique have been that over most of the stroke the rings are adequately lubricated by an oil film 1 - 2 pm thick. Also, except near the top dead centre on the firing stroke, the operating profile of the rings is almost constant throughout each stroke. These results have all been obtained from single-cylinder four-stroke engines, It was not certain whether the same pattern of behaviour would be observed in a ported two-stroke engine where the ports in the liner surface interfere with both the specific loading and the lubrication conditions of the piston rings.

3 54

In the present paper the results from such an engine, a Rootes TS3 opposed piston ported two-stroke, are described. By using capacitance transducers clustered around the ports, the ring profile and film thickness were measured over a range of operating conditions. It was found that as the rings traversed the ports, the film thickness remained almost constant while the attitude of the rings changed completely. By interrelating the results from all the gauges it is suggested that this tilting is due to the movement of individual rings in their grooves.

2. The engine

A Rootes TS3 engine was used for these experiments. This is a three- cylinder horizontally opposed piston engine working on a two-stroke diesel cycle and pressure charged from a Rootes blower. The cylinder bore diameter was 3.25 in with a piston stroke of 4 in. The maximum output power was approximately 85 b.h.p. at 2500 rev min.-r. The engine was mounted on a fully instrumented test bed at the National Gas Turbine Establishment, Cobham. The output power was absorbed in a Heenan and Froude DPX4 dynamometer.

3. The piston and rings

A two-part piston is used in the Rootes TS3 engine; the piston consists of a cast iron body with a steel crown. One novel feature is the use of a fire ring as the top ring. The principle of operation of this ring is that the large surface area provides a good thermal path to conduct heat away to the liner. In addition, owing to its shape it acts as a Dykes ring [ 51 and gives good gas- sealing properties. A full description of fire rings has been given by Englisch

[61. The fire ring consists of two individual rings: a large area portion called

the chamber ring and a smaller ring which surrounds it at its lower edge called the sealing ring. Below the fire ring two keystone-section scraper rings are fitted. These are pegged at the back of the ring groove to prevent their rotation. Two rings are mounted below the gudgeon pin. The first of these rings is an externally stepped scraper and the second is a slotted oil-control ring which consists of two separate lands fitted in the same groove.

The surface profiles of the piston rings, measured on a Talysurf instru- ment, are shown in Fig. 1. These profiles were recorded opposite the ring gap with the rings in the uncompressed condition. The profile of the externally stepped scraper will almost certainly be modified when the ring is closed in the liner. This is also true of the chamber section of the fire ring which has a complicated double-L section. The surface finish on the chamber ring and its sealing ring was extremely rough (50 pm peak to peak) while on the re- maining rings it was relatively smooth at 1 - 2 pm peak to peak.

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fire ring [chamber s&ion)

externally stepped scraper ring

Fig. 1. Surface profiles of new piston rings for the Rootes TS3 engine measured opposite the ring gap.

4. The engine instrumentation

The aim of instrumenting the engine was to investigate the lubrication conditions of the rings in the region of the liner occupied by the ports. The high exhaust temperature (approximately 500 “C) ruled out the use of the film thickness gauge in this position. This was because the epoxy resin (Eco- bond 104) used in the construction of the gauge was unsuitable for use above 280 “c. It was also felt that at such high exhaust temperatures the lubrication would almost certainly be of the mixed boundary type and that it might prove impossible to make sensible measurements. Consequently it was decided to examine the lubrication conditions around the induction ports of the engine. Access to these ports was relatively easy through the air chest, and as there was no water cooling in this region the sealing of the gauges was simplified.

Four capacitance gauges were fitted around one of the ports of the no. 2 cylinder liner, as shown in Fig. 2. Gauges 1 and 4 were on opposite sides of the port (gauge 1 towards the centre of the engine, gauge 4 towards the outside of the engine). Gauge 2 was fitted into the centre of a port-con-

' secto-d view thrqh inlet pats

Fig. 2. Cross-sectional view of the Rootes TS3 cylinder liner showing the position of the capacitance gauges around one of the induction ports.

netting web and gauge 3 was in line with gauge 2 but on the same circum- ferential position as gauge 4. The exact position of the gauges with respect to the individual piston rings is shown in Table 1. The oil-control ring, which is at the lower end of the piston skirt, does not travel far enough into the bore to be visible on any of the gauges.

All the gauges were fitted so that they passed radially through the liner wall. This caused a difficulty with gauge 2, which was positioned in the centre of one of the port webs. Owing to the cross-sectional shape of the webs (see Fig. 2) it was necessary to machine away a section so that a hole could be drilled into the centre of the inside surface of the web. In addition to the film thickness gauges, a thermocouple was fitted alongside gauge 4 to monitor the inside surface temperature of the liner.

5. The capacitance gauge

A full description of the construction and use of this gauge has been given recently [ 3,4] so only those details necessary for an understanding of the present results will be given here. The capacitance is measured between the end of a wire 0.25 mm in diameter, fitted into the cylinder liner, and the opposing piston ring. A thin steel tube of 0.5 mm inside diameter and 0.75 mm outside diameter surrounds the wire and acts as an electrical screen. As a guide to the calibration a capacitance of 1 pF is obtained for a film thickness of approximately 1 pm. The capacitance is measured on a Wayne Kerr B224 universal bridge and the output from the bridge displayed on a Tektronix 5113 storage oscilloscope. It is necessary to operate the bridge from an external frequency source of 50 kHz so that a sufficiently detailed

TABLE 1

Angular position of the piston rings for the capacitance gauges in the Rootes TS3 engine

Ring Angular position from outer dead centre to ring

Gauge 1 Gauge 2 Gauge 3 -

Chambera 57.6” 41.4” Opposite gauge Sealinga 64.8” 51.3” 30.6” 1st scraper 82.5” 71.4” 59.4” 2nd scraper 91.5” 80.4” 69.9” Step scraper Not visible Not visible 175.5” Oil control Not visible Not visible Not visible

aThe chamber ring and the sealing.ring comprise the fire ring.

Gauge 4

Opposite gauge 30.6” 59.4” 69.9”

175.5” Not visible

oscilloscope trace can be obtained; this enables the ring profile to be deduced accurately.

The method devised previously for fitting gauges into engines involved cementing the gauge into the liner with epoxy resin after the liner had been pressed into the cylinder block. To cure the epoxy the complete block was then heated to 150 “C. Although this was feasible with small cylinder blocks it was totally impracticable to heat the whole of the Rootes crankcase to this temperature. To overcome this problem a new capacitance gauge which con- sisted of two parts was designed. The first part was fitted into the liner and cemented in position before the liner was assembled into the engine. The second part was then screwed on through an opening in the top of the engine casing after the liner had been fitted.

The mounting holes for the gauges had to be recessed into the liner so that when the first part of the gauge was fitted it did not protrude above the outside diameter of the inlet ports. This was necessary to avoid the gauges being sheared off as the liner was pressed into the crankcase.

6. The oil

All the re$ults in the present paper are for Admiralty OMD113 refer- ence oil ER3/E. This is an SAE 30 oil formulated from mineral oil and con- ventional detergent additives. The viscosity was 119.5 cSt at 37.8 “C and 11.87 cSt at 98.9 “C; the viscosity index was 96. An important property of the oil for making capacitance measurements is the dielectric constant E. For clean oil E was measured as 2.34 at 23 “C. Tests on used oil after prolonged engine operation showed the value of E to be almost unchanged at 2.37.

7. Running-in procedure

An abrasive running-in compound is usually added to the fuel when running in the Rootes engine on the test bed. It was decided not to use this

for the present experiments because of the unknown effects that the abrasive might have on the epoxy resin in the gauges.

The output from the capacitance gauges was monitored continuously during the initial period of running although for the first few hours no capacitance signal could be recorded owing to the amount of electrical breakdown. This breakdown was very similar to that observed during running-in on other engine experiments [7]. After approximately 5 h an oil film could occasionally be detected beneath the rings on all the gauges; this film was always very thin (usually less than 0.4 pm). The engine operating conditions during this initial period of running were 1 b.h.p. at 600 rev min ’ _ The liner temperature was 63 “C and the iI~duction boost pressure was 3.8 cmIIg

(gauge ). To advance the running-in process, the power was increased after 7 h

to 4.2 b.h.p. at 1000 rev min ’ and then, for a short period, the power was increased further to 12.5 b.h.p. at 1500 rev min.-l, No significant improvement in the breakdown was observed and so, after a running time of 23 h, it was decided to change the oil. It had previously been found on the other engines that owing to the accumulation of running-in debris in the oil an oil change after about 20 h of running greatly improved the quality of the capacitance traces. However, after the oil change on the Rootes engine only a slight improvement in the breakdown conditions occurred and only after a total running time of 54 h did conditions improve sufficiently to enable the minimum film thickness to be continu~ly monitored.

The engine power was then increased to 21.9 b.h.p. at 1500 rev min ‘. This resulted in a liner temperature of 86 “C. The inlet boost pressure, which rose automatically as the engine speed increased, was 14.6 cmHg. This increase in power resulted in severe breakdown on the capacitance traces; this indicated that the running-in of the rings was not yet complete. In addition, the oil consumption during this period was heavy and averaged 0.1 1 h--l; this also suggested that the rings were not fully run in.

After a total running time of 100 h the conditions on all rings and gauges improved considerably and after 135 h traces could be obtained with sufficient clarity to enable the operating attitude of the rings to be deduced. The oil consumption also dropped after this time period and no extra oil needed to be added for the remainder of the test. Once this stage in the running had been reached it was found that the engine power could be varied over a wide range without serious deterioration in the quality of the traces. After 150 h of running gauge 3 failed and it proved impossible to measure the film thickness on this gauge for the remainder of the test. The other gauges operated successfully for the whole test period and were still in working order when the test was concluded.

8. Description of the gauge outputs

The terminology “inward” and “outward” scraping will be defined before the results from the engine are described. An inw~d-scraping ring is

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one in which t,he oil film thickness at the top edge of the ring (i.e. the edge nearest to the piston crown) is thinner than the oil film thickness at the lower edge of the ring. On an in-stroke such a ring will tend to scrape oil towards the centre of the engine, An outward-scraping ring has the opposite charac- teristics and when the piston is travelling outwards it tends to scrape the oil to the outside of the engine. As the definition applies only to the shape of the ring, it means that a ring can be of inward-scraping form on an out-stroke or of outw~d-scraping form on an in-stroke.

Figure 3 shows a set of out-of-balance capacitance traces from gauge 1 for the main ring pack on both the in-stroke and the out-stroke of the piston. The engine conditions were as follows: speed, 750 rev min-‘; power, 3.13 b.h.p. The individual photographs in this set have been positioned so that the top face of each ring is always on the left-hand side irrespective of the direction of travel of the piston. On the in-stroke, the first ring seen passing, from the left-hand side, the gauge is the fire ring (Fig. 3(a)). This consists of the chamber ring and the sealing ring. Although these two rings are mounted directly on top of each other they behave as two separate rings in the engine. Following the fire ring assembly the two scraper rings pass the gauge (Figs. 3(b) and 3(c)). These rings are nominally outward scraping but in Fig. 3 they are acting in an inw~d-scraping manner, i.e. the largest capacitance signal, which represents the thinnest oil film, is at the top edge of the ring. The same effect is also true on the fire ring and this is discussed in more detail below. On the out-stroke (Figs. 3(d) - 3(e)) the rings are behaving in a manner almost identical to that of the in-stroke, they are still in an inward- scraping form and the individual film thicknesses, indicated by the height of the out-of-balance signals, are similar.

Figure 4 shows a comparison between gauges 1 (Fig. 4(a)) and 2 (Fig. 4(b)) for the main ring pack on an out-stroke. The engine conditions were as

(a) (b)

(d)

Fig. 3. Photographs showing the output from capacitance gauge 1 for the main ring pack: (a) fire ring, in-stroke; (b) first scraper ring, in-stroke; (c) second scraper ring, in-stroke; (d) fire ring, out-stroke; (e) first scraper ring, out-stroke; (f) second scraper ring, out-stroke.

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outer dead centre

I

tbf tb)

Fig. 4. The main ring pack measured on gauges 1 and 2 showing the change in the attitude of the rings between the two gauges: (a) gauge 1; (b) gauge 2.

Fig. 5. The main ring pack measured on gauges 3 and 4 showing the variation in the film thickness between the two gauges: (a) gauge 3; (b) gauge 4.

follows: speed, 1000 rev mini’; power, 8.3 b.h.p. An obvious difference between the gauges is that, whereas the rings are in an inward-scraping form on gauge I (as in Fig. 3), their attitude has changed on gauge 2 so that the scraper rings have become symmetric~ and the fire ring has become outward scraping.

A comparison betwen gauges 3 and 4 is shown in Fig. 5. The two scraper rings, starting from the left-hand side of the photographs in Fig. 5, are seen passing gauge 3 (Fig. 5(a)) on an out-stroke; the low signal level indicates a relatively thick oil film beneath the rings, The next ring seen is the sealing section of the fire ring combination; the increased signal level indicates that the oil film has decreased considerably. The lower edge of the chamber ring then passes the gauge with a very thin oil film beneath it. In fact there is some indication that this edge is breaking down electrically against the cylinder liner. Whilst the chamber ring is opposite the gauge the piston reaches its m~imum outer travel and reverses its direction into the in-stroke. This means that the top edge of the chamber ring is not visible. Once the lower edge of the chamber ring has repassed the gauge on the in-stroke, the sealing ring is again visible; the film thickness has increased slightly over the preceding out- stroke value. Finally, the two scraper rings are seen again; they still have a

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markedly thicker oil film beneath them than the fire ring. The same sequence of rings is shown in Fig. 5(b). The main difference between Fig. 5(a) and Fig. 5(b) is that the scraper rings have decreased in film thickness, in comparison with the gauge 3 results, while the fire ring has increased in film thickness.

9. Detailed ring profiles

One of the most interesting results obtained from the engine has been the change in ring attitude observed on the different gauges. This has already been shown on the oscilloscope traces of Fig. 4 where the rings were inward scraping on gauge 1 and symmetrical on gauge 2.

In Fig. 6 the ring profiles on gauges 1,2 and 4 are shown in detail for the main pack on an in-stroke. The engine conditions were as follows: speed, 7 50 rev mine1 ; power, 3.13 b.h.p.; liner temperature, 65.5 “C; inlet boost

fire ring 1st scr*per 2nd scraper 9

Fig. 6. Ring profile and minimum film_;hickness measurements for the main ring pack on an in-stroke of the engine (750 rev min ; 3.13 b.h.p.): (a) gauge 1; (b) gauge 2; (c) gauge 4.

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pressure, 5.3 cmHg. There is no fire ring profile on gauge 4 because, as ex- plained above, at this position on the stroke the ring does not completely pass the gauge. The top face of each ring is on the left-hand side of the figure. It can be seen that on gauge 1 all the rings are in an inward-scraping mode, including the sealing ring of the fire ring combination. On gauge 2 the fire ring has tilted to become outward scraping while the two keystone scraper rings are symmetrical. On gauge 4, which is on the opposite side of the port opening to gauge 1, the two scraper rings have switched over completely to become outward scraping. The minimum film thickness ranges from 0.75 I.rrn for the sealing and first scraper rings on gauge 2 to 3.8 pm for the first scraper ring on gauge 1. Figure 7 shows a similar series of profiles to Fig. 6 but this time for an out-stroke. The same pattern of behaviour is seen, with inward- scraping rings on gauge 1 and outward-scraping rings on gauge 4.

To observe the effect of changes in engine power on the minimum film thickness a series of experiments was undertaken at the following three dif-

‘D fire ring 1st xmper 2nd xraper

1

Fig. 7. Ring profile and minimum film thickness measurements for the main ring pack on an out-stroke of the engine (750 rev min -l; 3.13 b.h.p.): (a) gauge 1; (b) gauge 2; (c) gauge 4.

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’ ferent power ranges: (i) 750 rev mm -‘I, 3.13 b.h.p., 5.1 cmHg; (ii) 1000 rev

min-I, 8.33 b.h.p., 8.1 cmHg; (iii) 1250 rev min-‘, 15.63 b.h.p., 10.7 cmHg. Because of the limited temperature control facilities on the engine, the liner temperature was allowed to increase as the power increased and ranged from 64 “C at low power to 81 “C at high power. The results of some of these experiments are shown in Fig. 8 for the second scraper ring on gauges 1, 2 and 4. It is seen that the increase in load has had virtually no effect on the minimum film thickness, except for a slight decrease on gauge 1 at high power. However, this is consistent with the positioning of the gauges in the liner. The exhaust ports have been uncovered when the ring reaches gauges 2 and 4 so the effect of increased gas pressure in the cylinder has largely been dissipated. Any further decrease in film thickness due to an increase in operating temperature will be offset by the increase in piston speed which tends to increase the film thickness.

750 rev rnltil 1000 rev mu-+ 12501-w mid 3.13 b.h p 8.33 b.h.p. 15.63

L" bhp.

0.5 0 05 0 05 1.0 acnxs ring xia

Fig. 8. Ring profile and minimum film thickness measurements for the second scraper ring at three power levels on an in-stroke of the engine: (a) gauge 1; (b) gauge 2; (c) gauge 4.

364

b

?I 2

5 I------ (d)

r----l (0

!-I I I

(h)

r--- l (e)

lOwI ,Imm,

Fig. 9. Talysurf profiles of used rings from the Rootes engine after 185 h of running: (a) - (h) are discussed in Section 9. (The profiles were obtained with the rings in the uncompressed condition.)

After a total running time of 185 h the test was halted and the pistons and rings were examined. The rings were removed from the piston and their surface profiles remeasured. These profiles are shown in Fig. 9 and can be compared with the original profiles in Fig. 1. Figure 9(a) shows the profile of the worn chamber section of the fire ring. This profile was recorded opposite the ring gap and it is seen that the ring has developed a larger taper than the new ring had. In Fig. 9(b) the profile measured at 90” to the ring gap is shown and, in addition to the reduction in slope, some of the original machining marks on the surface of the ring are still visible. In Fig. 9(c) the profile of the sealing section of the fire ring is shown. The ring has become slightly inward scraping and almost all the original coarse machining marks have been removed.

As the scraper rings are pegged in their grooves it is possible to measure their profiles at the precise point which passes over the gauges (either gauges 1 and 4 or gauges 2 and 3). In practice it was found that because the distance between the gauges was small compared with the circumference of the rings, their profiles at the two gauge positions were almost identical. In Fig. 9(d) the first scraper profile has been measured near that part of the ring which

traverses gauges 2 and 3. The profile is very similar to that recorded when the ring was new. For the second scraper ring (Fig. 9(e)), the worn ring is slightly flatter than when new.

The stepped scraper on the piston skirt is a deflector-type ring: when it is compressed in the cylinder liner the asymmetric cross section causes the ring to twist into an outward-scraping form. As the ring was measured here with the gap open the profile differs from that which would be measured in the engine. It is seen in Fig. 9(f) that the profile has been greatly modified compared with when the ring was new. In Fig. 9(g) the profile of the same ring is measured near the locating spigot on the inside surface. At this position some of the original machining marks on the running surface are still visible. The two separate lands of the oil-control ring are shown in Fig. 9(h). When new these lands had outward-scraping profiles which have now been modified to inward-scraping profiles. As the oil-control ring is not visible on any of the gauges, it is not possible to see how this ring operates in the engine.

10. Discussion

It has been shown that it is possible to measure both the minimum oil film thickness between the piston ring and cylinder liner and the operating attitude of the rings as they pass over the induction ports. One of the most surprising results has been the observation that as the rings pass the ports they tilt; they change from an inward-scraping form on the inside section of the liner to an outward-scraping form on the outside section. It can be seen from Fig. 3 that this tilting is independent of the direction of piston travel; the capacitance traces from the individual rings are almost identical on the in-stroke and the out-stroke. This is in contrast to the results in Fig. 4 where the profiles are measured on two different gauges on the same stroke; a con- siderable tilting effect which has occurred in only 2.5 ms is seen.

In Fig. 5 the output from gauges 3 and 4 was compared. These gauges are in line circumferentially; gauge 3 is opposite one of the porthole webs and gauge 4 is opposite one of the portholes (see Fig. 2). As the gauges are in line, Fig. 5(a) and 5(b) can be interrelated directly. The most noticeable difference between them is that on gauge 3 the scraper rings are lubricated by a very thick oil film while on gauge 4 the oil film is greatly reduced. Again, however, the ring attitudes are the same on both the in-stroke and the out- stroke.

A detailed comparison between the ring profiles from the individual gauges was given in Figs. 6 and 7 for both the in-stroke and the out-stroke. In addition to the fact that the ring attitudes are identical on both strokes, the values of the minimum film thicknesses were comparable. For example, the gauge 1 results between the fire ring and second scraper ring or the gauge 2 results between the individual scraper rings can be compared: the thinnest oil films were always recorded on gauge 2. This is consistent with the position of this gauge in the liner where, owing to the interruption in the surface

caused by the ports, the ring loading is increased in comparison with that of the other gauges.

It might be expected that the minimum film thickness on gauges 1 and 4 would depend to some extent on the direction of piston travel. On an out- stroke, for example, the rings approach gauge 1 from an uninterrupted liner surface with a well-established oil film lubricating them, whereas on an in- stroke the oil film will need to be re-established because the rings have just passed the ports. However, this effect was not observed and it is surprising, considering that they are on opposite sides of a port opening, how similar the film thickness between gauges 1 and 4 is.

It was originally thought that the change in ring attitude was caused by piston movement and that the change was due either to inertia effects near the end of the stroke or to thermal deformation of the cylinder liner but it can be shown that this is not so. As the distance between some of the rings is almost the same as the distance apart of some of the gauges it is possible to obtain oscilloscope traces at a constant-time-base speed on which the rings and gauges can be interrelated. This is shown in Fig. 10 for the main ring pack on gauges 1,2 and 4. Vertical lines have been drawn on the figure to show the different rings that can be compared. When the first scraper is opposite gauge 1, the second scraper is opposite gauge 2, and when the first scraper is opposite gauge 2, the second scraper is opposite gauge 4. In addition, while the fire ring is opposite gauge 1, the first scraper is opposite gauge 4. This means that if, for example, the attitude of the fire ring on gauge 1 was caused by piston movement then the same piston movement should be reflected in the first scraper profile on gauge 4; however, this is not so. The same argu- ment can be applied to the first scraper profile on gauge 1 and the second scraper profile on gauge 2 or to t.he first scraper on gauge 2 and the second scraper on gauge 4. Thus it can be shown that the change in profile must be due to the individual rings moving in their grooves and not to the movement of the piston.

One possible reason for the tilting is that it is caused by air pressure in the induction chest. This would act on a different part of the ring depending on the position of the piston in the bore. When the rings are near gauge 1 the air pressure is acting on the lower face. When they are near gauges 3 and 4 the pressure is towards the top face and when the rings are near gauge 2 the pressure is symmetrical to either side and the rings are seen in the process of tilting from one position to the other. An advantage of this explanation is that it would explain why the tilting is the same on both the in-stroke and the out-stroke. The amount of tilting might be expected to change as the induction pressure is varied, however, and this has not been observed.

In order to compare the Talysurf profiles with the operating profiles recorded in the engine it is necessary to plot the Talysurf traces on the same scale as the capacitance trace profiles. This has been done in Fig. 11 for the fire ring and the first and second scraper rings. The fire ring chamber and sealing sections have been plotted alongside each other (as they appear on the oscilloscope). The larger slope on the chamber ring (Fig. 11(a)) is almost

361

(b)

across ring xh

(cl Fig. 10. Synchronized oscilloscope outputs showing the rings tilting between the different gauges: (a) gauge 1; (b) gauge 2; (c) gauge 4.

Fig. 11. Talysurf profiles of the used piston rings plotted on the same scale as the capacitance trace profiles: (a) curve 1, chamber ring with gap open; curve 2, sealing ring with gap open; (b) first scraper; (c) second scraper.

certainly an exaggerated profile because the ring was measured with the gap open; in this situation the asymmetric section behaved like the stepped scraper ring and twisted when the ring was closed in the liner. This was checked by closing the ring gap and repeating the measurement. The resulting profile is shown in Fig. 12. It is seen that the shape has been reduced from the large single sloping surface to a more rounded profile; this confirms that the ring is twisting in the engine.

A comparison between the capacitance trace profiles and the Talysurf profiles shows a good agreement between the fire ring measured on gauge 2 on the in-stroke (Fig. 6) and the Talysurf profile of this ring measured with the gap held closed (Fig. 12). This suggests that the fire ring is sitting relatively square in the bore at this position in the stroke. The scraper rings, however, agree best with the capacitance trace profiles recorded on gauge 4 (Figs. 6 and 7). It is of course not possible to measure the fire ring profile in the engine on this gauge so no comparisons can be made with this ring at this position in the stroke. However, it is clearly evident from these results that

368

Fig. 12. Talysurf profile of the chamber section of the fire ring measured with the ring gap closed.

the capacitance profiles from gauge 1 are vastly different in attitude to the Talysurf profiles measured with the rings held square. This suggests that at the gauge 1 position in the liner the rings are operating in a highly tilted condition.

11. Conclusions

The oil film thickness between the piston rings and cylinder liner of an opposed piston two-stroke diesel engine was measured in the vicinity of the inlet ports. The value of the minimum film thickness ranged from 0.4 to 3.8 pm. It was found that the attitude of the rings changed as they traversed the ports from inward scraping near the inside edge of the ports to outward scraping near the outside edge of the ports.

Acknowledgments

The engine testing facilities were kindly made available by Mr. R. P. Langston of the National Gas Turbine Establishment, Cobham. Assistance given by the staff of that laboratory is gratefully acknowledged. The work was funded by a grant from the Ministry of Defence Procurement Executive.

References

1

2

A. V. Sreenath and S. Venkatesh, Piston ring lubrication in I. C. engines, 1st World Conf. in Industrial Tribology, New Delhi, 19 72. D. A. Parker, J. V. Stafford, M. Kendrick and N. A. Graham, Experimental measurements of the quantities necessary to predict piston ring-cylinder bore oil film thickness,

and of the oil film thickness itself, in two particular engines, Proc. Conf. on Piston Ring Scuffing, London, 1975, Institution of Mechanical Engineers, London, pp. 79 - 98.

369

3 G. M. Hamilton and S. L. Moore, Measurement of the oil film thickness between the piston rings and liner of a small diesel engine, Proc., Inst. Mech. Eng., London, 188 (1974) 253 - 261.

4 S. L. Moore and G. M. Hamilton, The starved lubrication of piston rings in a diesel engine, J. Mech. Eng. Sci., 20 (6) (1978) 345 - 352.

5 P. de K. Dykes, Piston ring movement during blow-by in a high speed petrol engine, Proc., Inst. Mech. Eng., London (Automob. Div.), (1948) 71 - 78.

6 C. Englisch, Kolbenringe, Springer, Vienna, 1958. 7 G. M. Hamilton and S. L. Moore, Measurement of piston ring profile during running-in,

Proc. Conf. on Piston Ring Scuffing, London, 19 75, Institution of Mechanical Engi- neers, London, pp. 61 - 70.

piston ring lubrication in a two-stroke diesel engine

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