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Piston friction analysis using a direct-injection single-cylinder gasoline engine
JSAE Review 24 (2003) 53–58
Piston friction analysis using a direct-injection single-cylinder gasoline engine Takashi Kikuchia, Shinichiro Itoa, Yoshinori Nakayamab a
Component & Dynamics Department No. 1, Power Train Engineering Division 1, Power Train Development Center, Toyota Motor Corporation, 1, Toyota-cho, Toyota, Aichi 471–8572, Japan b Nippon Soken, Inc., 14, Iwaya, Shimohasumi-cho, Nishio, Aichi 445-0012, Japan Received 13 March 2002; received in revised form 15 April 2002
Abstract Reducing piston friction is an effective means for engine friction improvement. A recent report indicated that, indirect - injection engines, the fuel sprayed on the cylinder bore surface (which ‘‘wets’’ the cylinder) affects piston lubrication. The analyses of piston friction together with lubrication behavior will accordingly become increasingly important. This study focused on the influences of cylinder wetting on piston friction characteristic through the use of a direct-injection single-cylinder engine. The study revealed that cylinder wetting reduces piston friction during the compression stroke, and that this phenomenon can be explained by the lowered oil viscosity on the cylinder bore surface caused by fuel wetting. r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction
2. Single-cylinder piston friction measurement device
With increased demand for improved fuel economy, reducing engine friction loss is becoming important. The pistons of an engine are particularly important because their friction comprises 30–40 percent of the entire engine friction [1]. Although many studies on piston friction behavior have been made, this is an area in which there remain some difficulties in simulation approach due to the involvement of complex lubrication conditions, such as oil starvation [2–4]. Gasoline injected directly into the cylinder, which improves combustion and aims at improving fuel economy, affects the lubrication of the areas surrounding the pistons. Recent reports have revealed that the direct fuel injection into the cylinder creates a wet cylinder condition, which affects the oil film [5]. To further reduce the friction of the piston, it is necessary to analyze the lubrication characteristics around the piston. For the purpose of this study, a direct-injection, single-piston gasoline engine was developed to make precise comparisons of piston friction. With this engine, the influence on friction of the fuel injected into the cylinder was investigated. In addition, this device was used to investigate the effects of other friction reduction methods, like piston ring surface improvement.
2.1. Outline of device Fig. 1 shows the main configuration of the device. The measurement of piston friction was performed through the use of a movable cylinder liner [6]. This device adopted a method of the eight 3-component force sensors, placed on the movable cylinder liner in the thrust direction [7]. Movable liners require a special gas sealing method to prevent the influence of the cylinder pressure [8,9]. The device developed is able to utilize a
Fig. 1. Cross section of a single cylinder engine.
0389-4304/03/$30.00 r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 2 ) 0 0 2 4 2 - 4
JSAE20034008
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
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To facilitate a more precise analysis of the friction force during each stroke of the piston, the device incorporates a method to correct the inertial vibrations of the liner by means of an accelerometer shown in Fig. 1. The direct measurement of the inertial force of the liner, which is the product of its acceleration and mass, is difficult due to the presence of the coolant and connecting parts. The accelerometer output (a) was employed to obtain the inertial force (M a) so that the difference between the 3-component force sensor output and the inertial force was considered to be the piston friction force. Fig. 3 indicates the following: Fig. 2. Piston friction measurement system.
mass-produced cylinder head by employing a mechanism with 0-ring which can cancel the internal cylinder pressure. In addition, a mass-produced piston and connecting rod were used. These parts are used in a 2-l direct-injection gasoline engine for passenger cars (F 86 mm bore diameter, 86 mm stroke). Fig. 2 indicates the measurement system. The outputs of the eight 3-component sensors in the friction force direction are connected in parallel to a data recorder and a charging amplifier, and were recorded at 11 crankshaft angle intervals. The outputs of the thrust force were also measured. To minimize the variances created by combustion fluctuations, all analyses were conducted by taking the average of ten cycles as a single piece of data, with six pieces of data obtained per condition.
1. Original friction force (the 3-component sensor output); 2. Inertial force of the movable liner through the accelerometer; 3. Compensated friction force obtained from 1–2. Because it is not possible to detect the complete inertial force due to the effect of the position of the accelerometer and the elastic behavior of the cylinder liner, the vibrations could not be completely eliminated. However, the large inertial force that would have grown up after the combustion was reduced by more than 50
2.2. Improving the precision of piston friction analysis In the case of movable liners, proper measures are necessary, especially for inertial vibration and the rigidity of the liner support. However, the inertial vibration, generated in a movable liner through piston friction, cannot be prevented entirely. Formula (1) shows the forces which can be detected by a 3-component sensor. The liner weight M and the gravity g; whose value are constant, are not detected because of the characteristic of the 3-component sensor. The cylinder pressure Fc is canceled by the pressure cancellation system as stated previously. However the liner inertial force M a in the axial direction remains in the 3-component sensor detection value Fx in addition to the piston friction Fp : Fx ¼ Fp þ M g þ M a þ Fc ;
ð1Þ
Fx: 3-component sensor detection value, Fp : Piston friction, M: Liner weight, g: Gravity, a: Liner inertial acceleration, Fc : Cylinder pressure.
Fig. 3. Compensation of inertia force.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
percent. The high-frequency vibrations were corrected through the moving average method [9].
3. Investigation of the influences of the direct-injection fuel on piston friction 3.1. Test conditions 1. Measurement conditions Table 1 shows the conditions for friction measurement. To represent general driving conditions in vehicles, the engine speed and load were set to a lowto-medium speed, and a low-to-high load range. To observe the influence of the wetting of the cylinder with fuel, a lower water and oil temperature were also tested. 2. Combustion and injection timing In this study, measurement was made in homogenous combustion at an air–fuel ratio of 1:14.5. For the purpose of comparing friction, the indicated mean effective pressure (IMEP) was kept constant. To investigate the influence of injection start timing, injection start timing was selected at 360 to 1801 ATDC (after top-dead-center).
3.2. Test components Table 2 shows the specifications of piston set and cylinder bore tested. Both piston and piston rings were identical to that of production. The cylinder bore material was cast iron.
55
3.3. Influences on the piston friction by changing water temperature, load and injection start timing Fig. 4 shows the friction mean effective pressure (FMEP), measured at 3601 ATDC (after top-deadcenter) injection start timing, and in Fig. 5 at 2401 ATDC injection start timing. The obtained FMEP has a clear tendency to increase with a lower water temperature and a higher load. However, the FMEP value is lower at 2401 than at 3601 injection timing. Reports [5,10] indicate that 2401 is a region susceptible to creating the wet cylinder condition. Hence it was assumed that this region affects piston friction. Fig. 6 shows the piston friction at various water temperatures. The friction decrease by changing injection timing 360 to 2401 appeared particularly in the compression and expansion strokes. During the compression stroke, this effect was increased by lowering water temperature. This is thought to be caused by the wetting of the cylinder along the piston sliding surface during the compression stroke, in the anti-thrust direction. Immediately after the combustion, however, friction decreases in a similar manner at all water temperatures. This occurs because even if the IMEP remains constant, the difference in the injection timing changes the combustion, thus resulting in a decrease in the maximum combustion pressure. Fig. 7 shows the amount of friction reduction at each piston stroke. At a water temperature of 401C, which exhibits a significant reduction, friction is reduced in the areas other than the compression stroke, indicating that the injected fuel remains in the cylinder bore without evaporating. This phenomenon appeared at the high
Table 1 Test condition of piston friction measurement Water and oil temp. (1C)
40 60 80 90
IMEP (kPa) 300
650
900
A A A A
B A B, C A
A A A A
A B C
Injection start timing (deg.ATDC)
Engine speed (r/min)
360, 240 360, 300, 240, 180 360, 240
2000 2000 800, 3000
Table 2 Specifications of test engine Pistona
Type Skirt
Deep cavity Resin coated
Piston ringa
Top Second Oil (2-piece)
h1: 1.2 mm, a1: 2.9 mm, Tangential force: 7N, Material: nitrized steel h1: 1.2 mm, a1: 3.4 mm, Tangential force: 7N, Material: Cr plated FC h1: 2.0 mm, a12: 2.75 mm, Tangential force: 30N, Material: nitrized steel
Cylinder bore
Roughness: Rz3.5 mm(initial), Material: FC230
56
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
Fig. 4. FMEP at 3601 ATDC injection start timing.
Fig. 5. FMEP at 2401 ATDC injection start timing. Fig. 6. Effect of water temperature on piston friction.
load, when the fuel is injected in large quantities, even at a high temperature. Fig. 8 shows the amount of friction reduction during each stroke, in which the injection timing was staggered 60 degrees at water temperatures of 401C and 801C. Friction decreases during the compression stroke when the injection timing is in the vicinity of 3001 to 2401 ATDC. This region is practically identical to the region in which the wetting of the cylinder increases in this engine [5,10]. Fig. 9 shows the FMEP at 800, 2000 and 3000 r/min at a water temperature of 801C. Unlike at 2000 r/min, no changes were observed at 3000 r/min; a high speed range in which the wetting of the cylinder is unlikely to occur. No changes were also observed at 800 r/min; a lower range in which the lubrication condition was thought to become worse by the wetting of the cylinder.
Fig. 7. Piston friction decrease at each piston stroke.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
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Fig. 8. Effect of the injection timing on piston friction. Fig. 10. Effect of cylinder bore temperature on piston friction.
Fig. 9. Effect of the engine speed on piston friction.
3.4. Relationship between the piston friction and the oil viscosity on the cylinder bore surface Fig. 10 shows the relationship between the temperature of the cylinder bore in the middle of the stroke and piston friction. While friction decreases with a rise in the bore temperature, the major factor for this phenomena is a decrease in the viscosity of the oil on the cylinder bore surface. However, when the injection timing is changed, the result does not remain on the same line. This is thought to be primarily the effect of the fuel on the cylinder bore surface. Fig. 11 shows the relationship between the estimated oil viscosity on the cylinder bore surface and the piston friction. The oil viscosity was calculated in accordance with the following data: (1) Fuel adhesion and oil dilution around the piston in this engine [5,10]; (2) Measurement of the relationship between oil dilution and oil viscosity; (3) Measurement of cylinder bore temperature (Fig. 10). The piston friction has a relation to the estimated oil viscosity. Consequently, the major factor for the changes in friction is thought to be the oil viscosity that is reduced by fuel.
Fig. 11. Effect of oil viscosity on piston friction.
In accordance with the aforementioned factors, it is necessary to take the oil viscosity reduced by fuel into consideration in order to reduce friction in a directinjection gasoline engine.
4. Effects of the typical methods for friction reduction 4.1. Piston rings for test Reducing the ring tension [9] and providing a lowfriction surface finish are effective approaches for reducing the friction of the hydrodynamic lubrication and the boundary lubrication of piston rings. In terms of the ring tangential force indicated in Table 2, piston rings with the following conditions were tested: oil ring with reduced tension (10N) and top ring sliding surface coated with CrN-PVD. 4.2. Measurement conditions The engine was operated at an oil and water temperature of 801C, and at an engine speed of 2000 r/ min. Fuel was injected at 3601 ATDC in order to prevent the effect of the wetting of the cylinder with fuel.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
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Fig. 12. Effect of ring specifications on piston friction.
reduced) for measuring piston friction, was developed. (2) As a result of using this device for investigating the influence exerted on piston friction by the fuel injected into the cylinder, the following facts were revealed: * Compared with the injection at 3601 ATDC, the 2401 ATDC injection reduces friction primarily during the compression stroke. * Friction at 2401 ATDC injection decreases significantly during high-load and low watertemperature conditions. * The primary factor for this reduced friction is the reduction of oil viscosity by fuel. (3) The oil ring tension is effective in reducing friction during all strokes. The addition of the CrN-PVD coating on the top ring has not exhibited any significant contribution to reducing friction of this engine.
Acknowledgements The authors wish to thank Mr. Inagaki of Toyota Central R&D Labs, Inc. and all others for their cooperation in this study.
Fig. 13. Effect of ring specifications on FMEP.
4.3. Measurement results Fig. 12 shows the piston friction and Fig. 13 shows the FMEP. The effects of the reduced tension were exhibited not only in the vicinity of the middle of the stroke where hydrodynamic lubrication occurred, but also in the boundary lubrication conditions at the top and bottom dead centers. The FMEP accordingly decreased significantly. This is thought to have been caused by a partial boundary-to-hydrodynamic transfer that is associated with the oil film that thickened at the top and bottom dead centers as a result of the reduced oil ring tension. Furthermore, the addition of the CrNPVD coating on the top ring did not result in any significant effect on the FMEP. A probable cause is an increase in the feeding volume of oil that resulted in the reduced oil ring tension, which decreased the boundary lubrication area.
5. Conclusion (1) A direct-injection, single-cylinder engine (in which the inertial vibration of the floating liner was
References [1] M. Nakada, Piston and piston ring tribology and fuel economy, Proceedings of International Tribology Conference, Yokohama, 1995. [2] M. Takiguchi, Present states and future tasks of piston lubrication (in Japanese with English Summary), J. JSAE (2000) 20004222. [3] M. Murakami, et al., Analysis of oil film behavior between piston and cylinder liner by using scanning-LIF (in Japanese with English Summary), Transactions of 11th Internal Combustion Engine Symposium 1993, pp. 363–368. [4] Y. Wakuri, et al., Studies on friction characteristics of reciprocating engines, SAE Paper No. 95247, 1995. [5] H. Inagaki, et al., Influence of cylinder wetting in a directinjection gasoline engines on piston ring lubrication (in Japanese with English Summary), Trans. JSAE (2001) 20014157. [6] S. Furuhama, et al., New device for the measurement of piston frictional forces in small engines, SAE Paper No. 831284, 1983. [7] T. Ito, et al., Piston friction analyses during actual engine operation in Japanese, Transactions of Sixth. Int. Combust. Engine Symposium, 1987, pp. 373–378. [8] M. Takiguchi, Piston friction force in actual operating engine, Monthly Int. Combust. Engine (in Japanese) 27 (342) (1988). [9] M. Murakami, et al., Analyses of piston frictional force at engine firing condition (in Japanese with English Summary), Transactions of Ninth Int. Combust. Engine Symposium, 1991, pp. 213–217. [10] M. Sugiyama, et al., Oil dilution reduction study with direct injection S.I. Engine (in Japanese with English Summary), Proceedings of JSAE No. 9732586, 1997.
Piston friction analysis using a direct-injection single-cylinder gasoline engine Takashi Kikuchia, Shinichiro Itoa, Yoshinori Nakayamab a
Component & Dynamics Department No. 1, Power Train Engineering Division 1, Power Train Development Center, Toyota Motor Corporation, 1, Toyota-cho, Toyota, Aichi 471–8572, Japan b Nippon Soken, Inc., 14, Iwaya, Shimohasumi-cho, Nishio, Aichi 445-0012, Japan Received 13 March 2002; received in revised form 15 April 2002
Abstract Reducing piston friction is an effective means for engine friction improvement. A recent report indicated that, indirect - injection engines, the fuel sprayed on the cylinder bore surface (which ‘‘wets’’ the cylinder) affects piston lubrication. The analyses of piston friction together with lubrication behavior will accordingly become increasingly important. This study focused on the influences of cylinder wetting on piston friction characteristic through the use of a direct-injection single-cylinder engine. The study revealed that cylinder wetting reduces piston friction during the compression stroke, and that this phenomenon can be explained by the lowered oil viscosity on the cylinder bore surface caused by fuel wetting. r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction
2. Single-cylinder piston friction measurement device
With increased demand for improved fuel economy, reducing engine friction loss is becoming important. The pistons of an engine are particularly important because their friction comprises 30–40 percent of the entire engine friction [1]. Although many studies on piston friction behavior have been made, this is an area in which there remain some difficulties in simulation approach due to the involvement of complex lubrication conditions, such as oil starvation [2–4]. Gasoline injected directly into the cylinder, which improves combustion and aims at improving fuel economy, affects the lubrication of the areas surrounding the pistons. Recent reports have revealed that the direct fuel injection into the cylinder creates a wet cylinder condition, which affects the oil film [5]. To further reduce the friction of the piston, it is necessary to analyze the lubrication characteristics around the piston. For the purpose of this study, a direct-injection, single-piston gasoline engine was developed to make precise comparisons of piston friction. With this engine, the influence on friction of the fuel injected into the cylinder was investigated. In addition, this device was used to investigate the effects of other friction reduction methods, like piston ring surface improvement.
2.1. Outline of device Fig. 1 shows the main configuration of the device. The measurement of piston friction was performed through the use of a movable cylinder liner [6]. This device adopted a method of the eight 3-component force sensors, placed on the movable cylinder liner in the thrust direction [7]. Movable liners require a special gas sealing method to prevent the influence of the cylinder pressure [8,9]. The device developed is able to utilize a
Fig. 1. Cross section of a single cylinder engine.
0389-4304/03/$30.00 r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 2 ) 0 0 2 4 2 - 4
JSAE20034008
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
54
To facilitate a more precise analysis of the friction force during each stroke of the piston, the device incorporates a method to correct the inertial vibrations of the liner by means of an accelerometer shown in Fig. 1. The direct measurement of the inertial force of the liner, which is the product of its acceleration and mass, is difficult due to the presence of the coolant and connecting parts. The accelerometer output (a) was employed to obtain the inertial force (M a) so that the difference between the 3-component force sensor output and the inertial force was considered to be the piston friction force. Fig. 3 indicates the following: Fig. 2. Piston friction measurement system.
mass-produced cylinder head by employing a mechanism with 0-ring which can cancel the internal cylinder pressure. In addition, a mass-produced piston and connecting rod were used. These parts are used in a 2-l direct-injection gasoline engine for passenger cars (F 86 mm bore diameter, 86 mm stroke). Fig. 2 indicates the measurement system. The outputs of the eight 3-component sensors in the friction force direction are connected in parallel to a data recorder and a charging amplifier, and were recorded at 11 crankshaft angle intervals. The outputs of the thrust force were also measured. To minimize the variances created by combustion fluctuations, all analyses were conducted by taking the average of ten cycles as a single piece of data, with six pieces of data obtained per condition.
1. Original friction force (the 3-component sensor output); 2. Inertial force of the movable liner through the accelerometer; 3. Compensated friction force obtained from 1–2. Because it is not possible to detect the complete inertial force due to the effect of the position of the accelerometer and the elastic behavior of the cylinder liner, the vibrations could not be completely eliminated. However, the large inertial force that would have grown up after the combustion was reduced by more than 50
2.2. Improving the precision of piston friction analysis In the case of movable liners, proper measures are necessary, especially for inertial vibration and the rigidity of the liner support. However, the inertial vibration, generated in a movable liner through piston friction, cannot be prevented entirely. Formula (1) shows the forces which can be detected by a 3-component sensor. The liner weight M and the gravity g; whose value are constant, are not detected because of the characteristic of the 3-component sensor. The cylinder pressure Fc is canceled by the pressure cancellation system as stated previously. However the liner inertial force M a in the axial direction remains in the 3-component sensor detection value Fx in addition to the piston friction Fp : Fx ¼ Fp þ M g þ M a þ Fc ;
ð1Þ
Fx: 3-component sensor detection value, Fp : Piston friction, M: Liner weight, g: Gravity, a: Liner inertial acceleration, Fc : Cylinder pressure.
Fig. 3. Compensation of inertia force.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
percent. The high-frequency vibrations were corrected through the moving average method [9].
3. Investigation of the influences of the direct-injection fuel on piston friction 3.1. Test conditions 1. Measurement conditions Table 1 shows the conditions for friction measurement. To represent general driving conditions in vehicles, the engine speed and load were set to a lowto-medium speed, and a low-to-high load range. To observe the influence of the wetting of the cylinder with fuel, a lower water and oil temperature were also tested. 2. Combustion and injection timing In this study, measurement was made in homogenous combustion at an air–fuel ratio of 1:14.5. For the purpose of comparing friction, the indicated mean effective pressure (IMEP) was kept constant. To investigate the influence of injection start timing, injection start timing was selected at 360 to 1801 ATDC (after top-dead-center).
3.2. Test components Table 2 shows the specifications of piston set and cylinder bore tested. Both piston and piston rings were identical to that of production. The cylinder bore material was cast iron.
55
3.3. Influences on the piston friction by changing water temperature, load and injection start timing Fig. 4 shows the friction mean effective pressure (FMEP), measured at 3601 ATDC (after top-deadcenter) injection start timing, and in Fig. 5 at 2401 ATDC injection start timing. The obtained FMEP has a clear tendency to increase with a lower water temperature and a higher load. However, the FMEP value is lower at 2401 than at 3601 injection timing. Reports [5,10] indicate that 2401 is a region susceptible to creating the wet cylinder condition. Hence it was assumed that this region affects piston friction. Fig. 6 shows the piston friction at various water temperatures. The friction decrease by changing injection timing 360 to 2401 appeared particularly in the compression and expansion strokes. During the compression stroke, this effect was increased by lowering water temperature. This is thought to be caused by the wetting of the cylinder along the piston sliding surface during the compression stroke, in the anti-thrust direction. Immediately after the combustion, however, friction decreases in a similar manner at all water temperatures. This occurs because even if the IMEP remains constant, the difference in the injection timing changes the combustion, thus resulting in a decrease in the maximum combustion pressure. Fig. 7 shows the amount of friction reduction at each piston stroke. At a water temperature of 401C, which exhibits a significant reduction, friction is reduced in the areas other than the compression stroke, indicating that the injected fuel remains in the cylinder bore without evaporating. This phenomenon appeared at the high
Table 1 Test condition of piston friction measurement Water and oil temp. (1C)
40 60 80 90
IMEP (kPa) 300
650
900
A A A A
B A B, C A
A A A A
A B C
Injection start timing (deg.ATDC)
Engine speed (r/min)
360, 240 360, 300, 240, 180 360, 240
2000 2000 800, 3000
Table 2 Specifications of test engine Pistona
Type Skirt
Deep cavity Resin coated
Piston ringa
Top Second Oil (2-piece)
h1: 1.2 mm, a1: 2.9 mm, Tangential force: 7N, Material: nitrized steel h1: 1.2 mm, a1: 3.4 mm, Tangential force: 7N, Material: Cr plated FC h1: 2.0 mm, a12: 2.75 mm, Tangential force: 30N, Material: nitrized steel
Cylinder bore
Roughness: Rz3.5 mm(initial), Material: FC230
56
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
Fig. 4. FMEP at 3601 ATDC injection start timing.
Fig. 5. FMEP at 2401 ATDC injection start timing. Fig. 6. Effect of water temperature on piston friction.
load, when the fuel is injected in large quantities, even at a high temperature. Fig. 8 shows the amount of friction reduction during each stroke, in which the injection timing was staggered 60 degrees at water temperatures of 401C and 801C. Friction decreases during the compression stroke when the injection timing is in the vicinity of 3001 to 2401 ATDC. This region is practically identical to the region in which the wetting of the cylinder increases in this engine [5,10]. Fig. 9 shows the FMEP at 800, 2000 and 3000 r/min at a water temperature of 801C. Unlike at 2000 r/min, no changes were observed at 3000 r/min; a high speed range in which the wetting of the cylinder is unlikely to occur. No changes were also observed at 800 r/min; a lower range in which the lubrication condition was thought to become worse by the wetting of the cylinder.
Fig. 7. Piston friction decrease at each piston stroke.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
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Fig. 8. Effect of the injection timing on piston friction. Fig. 10. Effect of cylinder bore temperature on piston friction.
Fig. 9. Effect of the engine speed on piston friction.
3.4. Relationship between the piston friction and the oil viscosity on the cylinder bore surface Fig. 10 shows the relationship between the temperature of the cylinder bore in the middle of the stroke and piston friction. While friction decreases with a rise in the bore temperature, the major factor for this phenomena is a decrease in the viscosity of the oil on the cylinder bore surface. However, when the injection timing is changed, the result does not remain on the same line. This is thought to be primarily the effect of the fuel on the cylinder bore surface. Fig. 11 shows the relationship between the estimated oil viscosity on the cylinder bore surface and the piston friction. The oil viscosity was calculated in accordance with the following data: (1) Fuel adhesion and oil dilution around the piston in this engine [5,10]; (2) Measurement of the relationship between oil dilution and oil viscosity; (3) Measurement of cylinder bore temperature (Fig. 10). The piston friction has a relation to the estimated oil viscosity. Consequently, the major factor for the changes in friction is thought to be the oil viscosity that is reduced by fuel.
Fig. 11. Effect of oil viscosity on piston friction.
In accordance with the aforementioned factors, it is necessary to take the oil viscosity reduced by fuel into consideration in order to reduce friction in a directinjection gasoline engine.
4. Effects of the typical methods for friction reduction 4.1. Piston rings for test Reducing the ring tension [9] and providing a lowfriction surface finish are effective approaches for reducing the friction of the hydrodynamic lubrication and the boundary lubrication of piston rings. In terms of the ring tangential force indicated in Table 2, piston rings with the following conditions were tested: oil ring with reduced tension (10N) and top ring sliding surface coated with CrN-PVD. 4.2. Measurement conditions The engine was operated at an oil and water temperature of 801C, and at an engine speed of 2000 r/ min. Fuel was injected at 3601 ATDC in order to prevent the effect of the wetting of the cylinder with fuel.
T. Kikuchi et al. / JSAE Review 24 (2003) 53–58
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Fig. 12. Effect of ring specifications on piston friction.
reduced) for measuring piston friction, was developed. (2) As a result of using this device for investigating the influence exerted on piston friction by the fuel injected into the cylinder, the following facts were revealed: * Compared with the injection at 3601 ATDC, the 2401 ATDC injection reduces friction primarily during the compression stroke. * Friction at 2401 ATDC injection decreases significantly during high-load and low watertemperature conditions. * The primary factor for this reduced friction is the reduction of oil viscosity by fuel. (3) The oil ring tension is effective in reducing friction during all strokes. The addition of the CrN-PVD coating on the top ring has not exhibited any significant contribution to reducing friction of this engine.
Acknowledgements The authors wish to thank Mr. Inagaki of Toyota Central R&D Labs, Inc. and all others for their cooperation in this study.
Fig. 13. Effect of ring specifications on FMEP.
4.3. Measurement results Fig. 12 shows the piston friction and Fig. 13 shows the FMEP. The effects of the reduced tension were exhibited not only in the vicinity of the middle of the stroke where hydrodynamic lubrication occurred, but also in the boundary lubrication conditions at the top and bottom dead centers. The FMEP accordingly decreased significantly. This is thought to have been caused by a partial boundary-to-hydrodynamic transfer that is associated with the oil film that thickened at the top and bottom dead centers as a result of the reduced oil ring tension. Furthermore, the addition of the CrNPVD coating on the top ring did not result in any significant effect on the FMEP. A probable cause is an increase in the feeding volume of oil that resulted in the reduced oil ring tension, which decreased the boundary lubrication area.
5. Conclusion (1) A direct-injection, single-cylinder engine (in which the inertial vibration of the floating liner was
References [1] M. Nakada, Piston and piston ring tribology and fuel economy, Proceedings of International Tribology Conference, Yokohama, 1995. [2] M. Takiguchi, Present states and future tasks of piston lubrication (in Japanese with English Summary), J. JSAE (2000) 20004222. [3] M. Murakami, et al., Analysis of oil film behavior between piston and cylinder liner by using scanning-LIF (in Japanese with English Summary), Transactions of 11th Internal Combustion Engine Symposium 1993, pp. 363–368. [4] Y. Wakuri, et al., Studies on friction characteristics of reciprocating engines, SAE Paper No. 95247, 1995. [5] H. Inagaki, et al., Influence of cylinder wetting in a directinjection gasoline engines on piston ring lubrication (in Japanese with English Summary), Trans. JSAE (2001) 20014157. [6] S. Furuhama, et al., New device for the measurement of piston frictional forces in small engines, SAE Paper No. 831284, 1983. [7] T. Ito, et al., Piston friction analyses during actual engine operation in Japanese, Transactions of Sixth. Int. Combust. Engine Symposium, 1987, pp. 373–378. [8] M. Takiguchi, Piston friction force in actual operating engine, Monthly Int. Combust. Engine (in Japanese) 27 (342) (1988). [9] M. Murakami, et al., Analyses of piston frictional force at engine firing condition (in Japanese with English Summary), Transactions of Ninth Int. Combust. Engine Symposium, 1991, pp. 213–217. [10] M. Sugiyama, et al., Oil dilution reduction study with direct injection S.I. Engine (in Japanese with English Summary), Proceedings of JSAE No. 9732586, 1997.