A new method for assessing the lubricating

ability of two-stroke engine oil

V.K. Jain

Evaluating the lubricating ability of two-stroke engine oils in laboratory test machines is difficult because of the lack of correlation with the engine test results. A high speed disc machine has proved its value as an effective screening method for this purpose: this paper describes the technique and its correlation with engine performance

In small air-cooled two-stroke engines, the piston ring, piston skirts, and engine bearings require lubrication. It is achieved by mixing a proportion of lubricating oil with the fuel.

In high output two-stroke engines, higher temperatures lead to piston tightening which in turn increases friction and reduces the engine speed. Here, good film strength of oil is important: it is achieved by using a suitable proportion of the optimum lubricant viscosity and type. There is a tendency to decrease the oil proportion to reduce pollution problems 1 . This reduction in oil per- centage, however, places greater demand on the lubricant to prevent film failure leading to seizure. Laboratory machines, such as Timken, Almen and 4-Ball, have been used by Ghandhi 2 to assess the lubricant performance, but no correlation was obtained with engine tests. The most widely used test is 'piston tightening test '2"s. In this test, the cooling air is to cut off and the rise in cylinder temperature experienced for a specified drop in speed is noted. To obtain reliable data, however, the test is expensive and time consuming. This paper describes a simpler and cheaper test that correlates with piston tightening tests.

Test method A disc machine was used to find the scuffing load and temperature and this data was correlated with engine tests on a 50 cc, 1.8 HP Motobecone engine running at 4000 rpm at full load. Additional information obtained from the literature was used for correlation. During the disc machine tests frictional torque and percentage metallic contact 6 were also noted.

Aluminium alloy and chrome plated cast iron discs formed the test pair. The aluminium alloy disc corres- ponded to the piston material of the Motobecone engine and the chrome plated cast iron disc to the cylinder material, The discs were 45 mm in diameter and 10 mm wide. Each disc was coupled to a separate electric motor. The lower chrome plated disc rotated at 2250 rpm and the aluminium alloy disc at 2000 rpm, both in the same direction. The relative sliding speed under this condition was 1000 cm/s, which corresponds to the sliding speed of Motobecone engine at 4000 rpm at full load. The disc pair was loaded from 4 kg and the load increased in 2 kg steps until scuffing occured. During the test, disc tempera-

Indian Institute of Petroleum, Dehradun, India

Tonk for mixture of fuel and oil

Aluminium alloy disc

come~O ~ piece

"Focussing

I Recorder

Chromium pioted disc

Fig i Arrangement o f the infra-red camera used to measure the temperature o f the discs

ture, frictional torque, and percentage metallic contact were continuously recorded. The duration of test at each load was 5 min. Onset of scuffing was indicated by the sudden rise of disc temperature, frictional torque, and metallic contact. The flow of fuel and oil mixture on the lower disc was maintained drop by drop at a rate of 0.5 ml/min (Fig 1).

The frictional torque was recorded by strain gauges. The temperature of the disc was recorded using an infra-red camera, and metallic contact was measured by the voltage drop method 6.

The infra-red thermometer was calibrated following the manufacturer's prescribed procedure. The emissivity was selected on the basis of the colour of oil film on the disc: an emissivity of 0.95 was chosen for the black-brown colour observed. Comparison of temperature with a thermocouple lightly loaded one centimetre away from the contact zone on the lower disc with four different oils showed infra-red measurements were 15-20°C higher than those observed with the thermocouple.

The surfaces of the discs were prepared on the machine itself by grinding with emery paper at 4 kg load and slow speed. This method provides true line contact and ensures uniform wear on the surfaces. The discs were prepared to be as similar to each other as possible.

0301-679X/78/1104-0243 $~02.00 © 1978 IPC Business Press TRIBOLOGY international August 1978 243

Table 1 Oils and additives tested

Fluid Oil and additives

A B C D E

F G H A~ B]

Commercial oil, Mineral base Commercial oil, Mineral base Commercial oil, Mineral base Synthetic oil containing ashless detergent additive Synthetic oil + 20% additive B, + 10% Polar compound Synthetic oil Synthetic oil Synthetic oil + 5% additive B Calcium sulphonate type detergent Amide type detergent

~2

30

28

26

24

" 22 o 2o

14

I0

230

=e

180

q3

150

8

Fuel oil ratio 200:1

t ~ E l laJ la. (.9 " r

H

Fig 2 Variation o f scuffing load and scuffing temperature for some of the oils tested at a fuehoil ratio of 200.'1 (Cross-hatched area indicates repeatability)

T e s t resul ts

Many factors affect the lubricating ability of an oil. Various Commercial oils and synthetic oils, with or without deter- gent additives, were studied (Table 1). Some of these oils were tested on the Motobecone test bench engine 7 for their lubricating ability. The results observed were in the following order:

Synthetic oil D at 1.0% > commercial oil B at 2.0% -~ oil E at 1.0% > oil B at 1.0% > oil H at 1.0% ~ oil A at 3.0% - oil C at 3.0%.

Systematic studies conducted with various oils, including those mentioned with regard to Motobecone engine tests, are d e s c r i b e d in detail below. Fig 4 shows that the rating for scuffing load with the proposed screening technique is qualitatively similar to the engine test data.

Fuel ail ratio I00 I

32 -

30 -

28 ~

2 16

14

12

o 8 8

o, 26

x; 24

E 22

~ zo (,9

18

IO

/

3 o

50 25 !

t . ) t..)

8

1(30!

23O

,o j 205

E

180

O3

155

BO

I

I

7Z~

]

" r

o

f Fig 3 Scuffing load and scuffing temperature are a function o fo i l concentration

With reduced oil quantity in the fuel, the reduction in scuffing load and temperature experienced was somewhat as expected (Figs 2 and 3). The overall data in Fig 4 clearly shows the effect of oil concentration in the fuel. For example, the scuffing load of oils B, D, and E increased significantly with minor increase of oil concen- tration in gasoline. Further addition of oil B improved this characteristic but at a slower rate. It has been the experience with the piston tightening test conducted on the Motobecone engine that none of the oils except oil D at a concentration of 0.5% were suitable because the piston tended to scuff as soon as cooling air of the engine was shut off. However, oil D at 1.0% was found superior to oil B at 2.0%, but both passed the engine test. These were taken as reference oils for comparison with others, assuming oil D as excellent. Also, oils B and E at a concentration of 2.0% and 1.0% respectively, had more or less the same piston tightening temperatures but these results did not agree with our results. Our results indicated that oils B and E at 1.0% were similar. Further analysis of our results revealed that the frictional properties of these oils were also important because the coefficient of friction rose at loads greater than 12 kg and then decreased again with higher load (Fig 5). In contrast, most of the oils tested showed an initial decrease followed by a steady friction coefficient with increasing load. While oil B at 1% showed this different situation and although the temperature increases during the rising portion of the

244 T R I B O L O G Y international August 1978

frictional curve may reach values normally associated with onset of scuffing, there was no visible scuffing on the discs: testing should be continued till visible scuffing occurs. It is quite possible that oil B at 1.0% functioned well because the piston tightening test was not carried out to de st ruction.

With oil C an increase in oil concentration did not improve lubrication significantly compared to the other oils. However, the studies indicated a good correlation between our results and piston tightening results obtained on the Motobecone engine. In some cases, our technique gave better discrimination and repeatability (shown as the cross-hatched area), when two oils have comparatively similar characteristics, for example oils B and E. These tests give a means of rapid screening of various materials: final conclusions must, however, be drawn from full scale engine tests.

Clearly, the reduction of oil concentration caused a reduction in scuffing temperature and load in these experiments. Harting and Savin a also showed from their piston tightening test that the reduction of oil quantity in gasoline causes a reduction in piston tightening temperature. Excess oil will reduce the probability of scuffing, but may increase adverse effects 9'~° such as deposition, spark plug failure, crankcase pin and piston scoring, etc. On the other hand, some authors consider that the reduced oil quantity may increase these effects ~l . This divergent view appears to depend on the engine design. Nevertheless, the optimum condi- tions should be determined by actual engine tests. Results from one engine test bench, however, must not be the basis for recommending the proportion of oil for other engines.

Effect of base stocks

The effect of base stocks on scuffing load and temperature was studied at two different oil concentrations, ie 2.0% and 4.0%, with or without detergent additives. The viscosity of all the oils was brought to 11 cs at 210°F by suitable blending.

Limited information is available in the literature on the effect of bright stock. With a low percentage of bright

o3

34

30

26

22

14

I0

Oil D ~ Oil B

ilA

I I I I 2 3

Oil concentration, %

Fig 4 b.ffect of oil concentration on scuffing load

Oil C

stock in paraffinic oil no proper distinction was seen (Fig 6). However, its effect was clearly indicated at higher concentrations. It is interesting to note that at a higher fuel oil ratio of 4%, the role of bright stock was not significant. It is possible that its use may be more important when the oil quantity in gasoline is reduced. At higher Concentrations paraffinic oil itself is enough to take care of scuffing characteristics.

No significant improvement in scuffing load was observed when the concentration of naphthenic oil in gasoline was changed from 2.0 to 4.0%. Also, addition of bright stock in the range of 5 to 10% was not very helpful (Fig 7). On the other hand, increased oil concentration of paraffinic oil, ie from 2.0 to 4.0%, improved the scuffing character- istics. Thus, it is possible that the paraffinic oil is superior to naphthenic oil with regard to scuffing at a higher concentration level in gasoline. A typical formulation containing 45% naphthenic and 45% paraffinic oils along with 10% bright stock was also tried and the improvement in scuffing load was nominal. From this information it appears that naphthenic base oils might be unsuitable for 2-stroke engines with regard to scuffing at higher concen- trations. At lower concentrations, ie 2%, both the base stocks had similar scuffing characteristics. It has been shown 8 that oils with paraffinic tendency perform better than those with naphthenic tendency having the same viscosity, and increase in bright stock concentration can increase the tightening temperature. Similar results were obtained with the high speed disc machine.

0.07 f u "6 I00:1

:~ 0.05

I I 5011 1 I

0.03 8 ~O 12 14 16 18 20 22 24 26 Load, kg

Fig 5 Effect o f load on friction with a commercial mineral oil (oil B)

28

26

~; z4

22

20

16

14

r i ~ . .~ Poraffinic air

Poraffinic oil+ IO% BS Poraffinic oil + i5 % BS

Fuel oil ratio o - -o5OI

25:1

IO I I I I 1 I O I 2 3 O 5 IO 15

Oil concentration, % Bright stock concentration, %

Fig 6 Oil and bright stock concentration affect scuffing load

TRIBOLOGY international August 1978 245

25 : I Fuel oil rot io V iscos i ty o f oits ; I I Cs at 210 o F

Effect of detergent additives The tests described earlier a t tempted to evaluate the lubricating ability of an oil. The effect of detergent additives on the lubricating ability of an oil was also investigated. Calcium sulphonate additive A1 was tested at concentrations ranging from 3 to 12% in oil K (90% paraffinic oil + 10% bright stock) and the maximum scuffing load observed in all cases was the same. With a 3.0% concentration of additive A1, the repeatability observed was better than for other concentrations. Also, the maximum scuffing temperature observed was higher (Fig 8). The reason for this behaviour is not clear.

Additive B~, an amide type detergent, had an effect on load carrying capacity of oil K, although the scuffing temperature was more or less the same. This is in agree- ment with the supplier's information. It was observed during the study that oils with or without calcium sulphonate type detergent, developed chatter when the temperature of discs reached 150-160°C. With the amide type detergent, chatter was observed at a higher load and temperature. In certain cases no chatter was observed until scuffing.

Discussion The disc machine data shows the effect of oil proportion in gasoline on performance. Oil concentration as low as 1.0% can be used 4'12. Oil D, a synthetic type, at a 1.0% concentration was found to be the best oil tested. The same phenomena was observed on Motobecone engine test bench. Without bright stock, the paraffinic oils performed better than naphthenic oils. The use of bright stock may improve scuffing characteristics, depending upon the percentage in the oil and the nature of the base stock.

The results indicate that oils can be distinguished in a qualitative way by this test. The hatched areas show that repeatability is better than the engine test results. The method is useful in predicting the lubricating ability during initial stages of development of two-stroke engine

22C ~Z = 20C

~. 180 E

160 g

140

=20

I00

30

28

~ 26 -o"

~ 22

18

16

F u e l o i l m t i o 5 0 : 1 2 5 : 1

¢n '_g #-I ,~-

7"P

/ / ,

gl

I.// , // 4 / / "i.,/

- - / / ' /

Fig 7 Effect o f various base stocks on scuffing load and scuffing temperature

" o

g

30

28 -

26 -

22 ~

2 0 -

N 16 - e n

12 - ÷

(3_

10

" o

to) ff~

i + ÷ +

g

i

/ /

/ /

/ z "

,'n

"o "o

4- v"

/ / S

¢rl

"0 30

0 CM +

2°i ~d ~ 175

.E °' 150

125

~oo Fig8 Variation of scuffing load and scuffing temperature with mcreasing detergent additive concentration

oils. Once the conditions for the engine test bench are established on the high speed disc machine, the oils can be screened rapidly for lubricating ability.

In the present paper no consideration has been given to the engine cleanliness characteristics of lubricants. The authors believe that friction is a useful criteria in this respect. The relationship between engine cleanliness and friction will be described in a future publication.

Conclusion A bench test method utilising a disc machine was developed for evaluating the lubricating ability of two-stroke engine oils. It was found that: 1. The scuffing load and temperature correlate well with

the piston tightening tests. 2. The technique is a useful, cheap and quick method for

screening the lubricants. 3. Synthetic oils appear to be better than commercial

mineral base oils at lower concentrations. 4. Reduced oil concentration in gasoline results in

decreased scuffing load and temperature. 5. Paraffinic oil appears to be better than naphthenic oil

at higher concentrations. 6. Some ashless detergent additives may improve scuffing

characteristics. 7. Commercial oils recommended for use at 4 - 6 % are not

likely to be satisfactory at lower concentrations, except oil B. Formulations can be developed that will perform satisfactorily with concentration as low as 1% in gasoline, for example oil D.

2 4 6 T R I B O L O G Y i n t e r n a t i o n a l A u g u s t 1 9 7 8

References 1. Schilling A. Automobile Engine Lubrication, Scientific

Publication G.B.Ltd., 1972, London

2. Ghandhi B.K. Lubrication requirements for todays out board, SA.E. Fuel and Lubricants Meeting, November 1-3, 1966

3. Dyson A. Misc an point d'une Methods d'essar paramettant de diterminer & L'influence des lubricants sux le grippage des pistons dans les moteurs a 2 temps a essence, Revue Inst. France du Pet. xi, 11 P.1489-1495, 1956

4. Savin J.W. Some Lubrication Problems in Air Cooled Two Cycle Engines, Lub, Engg. Feb. 1962, 78-82

5. Abbotts J . , Beers R,L. and Swanson J.W. Frictional Characteristics of Two-cycle Engines Oils, SAE National Fuels and Lubricants Meeting, Oct 28-31, 1968

6. Jain V.K. Traction and Metallic Contact in Elastohydrodynamic Contacts, World Conference on Industrial Tribology, 11-18 Dec. 1972, Delhi

7. Essai de serrage sur Moteur 2 Temps de 50 cm 3 , Dept. Essai Mechaniques IFP

8. Hatting H.A. and Savin J.W. Piston Lubrication in Small Air-cooled Two-cycle Engines, ASME, JarL 22, 1962

9. Towle A. Some Factors contributing to successful crank lubrication of small European two-stroke petrol engine, SAE Summer Meeting, June 14-19, 1959

10. Miller G.E. and Pollan H.M. Two-stroke cycle, liquid cooled engine lubricating oil, SA.E.

11. Colyer C.C. and Sieker W.L. Two-cycle engines require special oil, 1961 SAE Summer Meeting

12. Cow D.W. and Trantman C.E. Lean Mixture Lubrication of Two-cycle Gasoline Engines, SAE, Fuels and Lubricants Meeting, Nov. 1-3, 1966

S e e a n d b e s e e n a t 26--28 September 1978 Leeds

TRWO4NTERNATIONAL'78 The third international exhibition of bearings, lubricants, lubrication, gears, seals and novel mechanisms.

and

TIUBO4LOGV IN INDWTRV .rdlCIE Three one day conferences are being run simultaneously with the exhibition. These are run on strictly practical lines, being run by industrialists for people from industry. Topics to be covered are

• LUBRICATION FOR THE PRODUCTION MAN • DESIGN AND SELECTION OF BEARINGS, GEARS AND SEALS • MAINTENANCE ENGINEERING

Contact Don Mitchell, Industrial Unit of Tribology, University of Leeds, Leeds LS2.9JT, UK. Telephone: 0532 31751 Ex:ension 7073

TRIBOLOGY international August 1978 247