International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam.

The Concept of Using Offset Piston to Improve the Engine Power Jarut Kunanoppadol Department of Mechanical Engineering, Faculty of Engineering and Industrial Technology, Silapakorn University, Nakornprathom 73000, Thailand E-mail: [email protected]

Abstract The research was aimed at studying the concept to modify the internal combustion engine by offsetting the piston to move in the reversing direction against the crank rotation, enabling the piston, the connecting rod, and the crank to form a quick return mechanism. The piston hence could reduce the moving time from the top-dead center to the bottom-dead center while the piston could increase the moving time from the bottom-dead center back to the top-dead center. The net indicated power of the engine then could be increased. Mathematic analysis was used to examine the result from offsetting the piston in order to compare with the normal engine movement. Engine specifications and thermodynamic assumptions were conducted to get initial data for analysis. The offsetting percentage was pinned at 0% before analysis for the normal engine. However, the percentage was set at 10%, 20%, 30%, and 40% for the engine with the offset piston while the crank ratio was changed from 3.0 to 3.5, and 4.0 respectively for all cases. According to the research result, offsetting the piston could increase movements of the piston mechanism, the connecting rod, and the crank. The stroke, the displacement volume, and the compression ratio then were also scaled up, leading to an increase in the thermodynamic cycle and the indicated work figure consequently. The quick return mechanism formed by movements of the piston, the connecting rod, and the crank could add up the indicated power. Based on the analysis, the indicated power of the engine could be enhanced by 0.278%, 0.196%, and 0.146% where the crank ratio was set at 3.0, 3.5, and 4.0, respectively while the offsetting percentage was increased to 1%. Keywords: offset-piston engine, internal combustion engine, indicated power

1. Introduction Offsetting the piston was to shift the moving direction of the piston from the center of the crank. The result of offsetting the piston in the same direction with the crank rotation could reduce the force between the piston and the side of the cylinder. This could cause friction, vibration, and noise while the engine was working; therefore, the engine would have more mechanism efficiency. (Haddad & Tjan, 1995). Considering the movement of the mechanism, offsetting the piston in the same direction with the crank rotation would help reduce the piston side force while it was moving from the topdead center to the bottom-dead center, or in

other words, the intake stroke and the exhaust stroke of the four-stroke engine. In the same time, the force at the other side of the piston would increase when the piston moved back from the bottom-dead center to the top-dead center, or in other words, the intake stroke and the exhaust stroke as well. According to related researches, the greater the offset length, the lesser the force on the anti-thrust side, but the greater the force on the thrust side. (Nakayama et al, 2000; Wakabayashi, 2003). Cho et al, 2003 also concluded that offsetting the length of the piston too much would instead increase the friction when the engine was working in high engine speed. Offsetting the piston in the same

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International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam. dead center increased. The net power could increase if the power from the engine gained during the power stroke was added while the power from the engine gained during the compression stroke was deducted. The research would use mathematical analysis to study the result from offsetting the movement of the piston to increase the engine power. Only the piston was changed during the experiment while the connecting rod and the crank would not be changed for convenience in actual use. Changing only the piston would alter the stroke figure, the displacement volume, and the compression ratio of the engine. The research therefore analyzed all changing value by mainly considering the net power or the result of the experiment gained from the engine. The analysis could be done by pinning the offsetting percentage at 0% for the normal engine. The percentage was set at 10%, 20%, 30%, and 40% for the engine with the offset piston while the crank ratio was changed from 3.0 to 3.5 and 4.0 respectively for all cases. Details of the research would be elaborated later on with engine specifications, thermodynamic assumptions, the quick-return mechanism theory, the Otto cycle, the analytical results, and the conclusions, respectively.

direction with the rotation of the piston therefore would be beneficial to the operation of the machine in reducing the friction, fuel consumption, and noise when the engine was working in low and middle engine speeds. The research made an analysis in another viewpoint by offsetting the piston to move in the opposite direction with the crank rotation to benefit the movement of the mechanism. Offsetting the piston could make the piston, the connecting rod, and the crank form a kind of a simple quick-return mechanism. (Mabie & Reinholtz, 1987). Offsetting the piston in the opposite direction with the crank rotation could increase the time that the piston used to move from the bottom-dead center to the top-dead center. However, it would reduce the time that the piston used to move back from the top-dead center to the bottom-dead center. The operation was conducted under the idea that when the work caused from the movement of the piston remained the same, the power at the intake stroke and the power stroke would increase if the time that the piston used to move from the top-dead center to the bottom-dead center reduced. Using the same idea, the power at the compression stroke and the exhaust stroke would decrease if the time that the piston used to move from the bottom-dead center to the top-

(1) INLINE

(2) OFFSET

BDC 1

d

b+a

b

d Stroke 2

BDC 2

a a

ß

Upward (ßٛٛٛٛDownward (a ٛ

a

d b-a

b-a

b+a

Stroke1

Anti-thrust side

Thrust side

TDC

F ?

ß

Upward (ßٛٛٛٛDownward (a ٛ (Quick Downward)

Fig.1: Inline and offset piston engine layout

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International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam. Table 1: Engine specification information and thermodynamics assumption

Bore and Stroke

4-Stroke, 4-Inline Cylinder Gasoline Engine 86 x 86 mm

Compression ratio

10:1

Engine speed

3000 rpm

Crank ratio (OR)

Intake pressure

3.0, 3.5, 4.0 0%, 5%, 10%, 15%, 20% 101.325 kPa

Intake temperature

333 K

Combustion efficiency

80%

Ratio of specific heat (k)

1.35

Specific heat (Cv)

0.821 kJ/kg-K

AF ratio (gasoline)

14.6

Engine type

Offset percentage (OP)

2. Engine Specification Information and

thermodynamics assumptions In mathematic analysis, the researcher indicated engine specifications information and thermodynamics assumptions to use in calculation as illustrated in Table 1. The crank ratio was the length proportion between the connecting rod by the crank. Normally the ratio was about 3.0 to 4.0 for small engines. (Pulkrabek, 2004; Wakabayashi, 2003) The offsetting percentage was the length percentage of the offset by the crank. Related researches had indicated the offset length in units of length; however, the researcher viewed that it should be related with the length of the crank to prevent the offset length from exceeding. (Cho et al, 2003) The offset length therefore was set in percentage and in proportion with the length of the crank for convenience in using with engines of other sizes. Offsetting was made in the opposite direction with the rotation of the crank as shown in Figure 1. The researcher indicated the intake pressure, the intake temperature, the combustion efficiency, the specific heat, the specific heat ratio, the air-fuel ratio, and the heating value. The specifications of initial values for calculation were prepared to ensure that analysis of the differences between the engine with the offset piston and the normal one were made in the same bases, and only the differences caused by offsetting the piston were considered.

3. Quick-return mechanism theory Offsetting the movement of the piston enabled the piston, the connecting rod, and the crank to form a quick return mechanism, making the time that the piston used to go and come back unequal. Only the piston was changed to modify the normal engine to become the one with the offset piston while the connecting rod and the crank remained the same. This could alter the length that the piston moved as illustrated in Figure 1. Therefore, the movement of the piston must be calculated again as follows; Crank length a = S /2 (1) Connecting rod length b = a × CR (2) Offset d = a × OP% (3) Axial piston distance at TDC

xTDC =

( b + a )2 + d 2

(4)

Axial piston distance at BDC

xBDC =

( b − a )2 − d 2

(5)

New piston stroke S ′ = xTDC − xBDC (6) New displacement volume π Vd′ = B 2 S ′ (7) 4 New compression ratio V′ (8) rc = d − 1 Vc Whereas; a is the length of the crank, b is the length of the connecting rod, d is the offset length from the original direction, S is the stroke, CR is the crank ratio, OP% is the offsetting percentage, B is the diameter of the cylinder, and Vc is the clearance volume. The movement to go and come back of the quick-return mechanism could be analyzed from the forward angle, the backward angle, and the time ratio. As shown in Figure 1, the upward angle could be defined as the rotation angle of the crank when the piston moved from the bottom-dead center to the top-dead center. Meanwhile, the downward angle could be defined as the rotation angle of the crank when the piston moved back from the top-dead center to the bottom-dead center. (Mabie & Reinholtz, 1987). The analysis could be done as follows;

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International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam. Upward angle α = φ + 180 − ψ (9) Downward angle β = ψ + 180 − φ (10) Time ratio α rt = (11) β Whereas; ⎛ d ⎞ (12) ψ = sin −1 ⎜ ⎟ ⎝b−a⎠ ⎛ d ⎞ (13) φ = sin −1 ⎜ ⎟ ⎝b+a⎠ If this time ratio ( rt ) is greater than 1.0, the mechanism could be classified as the quickreturn one.

4. Otto-cycle theory The Otto-cycle theory is a thermodynamic cycle used to analyze the function of four-stroke engine using gasoline fuel. The intake air, comprising air and fuel in stoichiometric form, was sent into the cylinder during the intake stroke under stable pressure. In the compression stroke, the volume of intake air was compressed and reduced, prompting the intake pressure and temperature to increase while the compression was made in constant entropy. The combustion caused in the Otto-cycle had constant volume. The heat caused from the combustion process could increase both the pressure and the temperature of the gas rapidly. Expansion of the heat gas caused by the combustion pushed the piston to move from the top-dead center to the bottom-dead center, creating the work in the power stroke while the compression was made in constant entropy as well. After the completion of the heat gas combustion, exhaust was pushed out from the cylinder in the constant pressure during the exhaust stroke. It could be seen that work must be put to the engine during the intake, compression, and exhaust strokes, and the engine would return the works back only during the power stroke. Calculations could be made as follows; Work in intake and exhaust stroke W = PΔV (14) Work in compression and power stroke Δ ( PV ) W= (15) 1− k

Whereas; P is the pressure, V is the volume, k is the specific heat ratio used in this analysis, set at 1.35. This research analyzed the thermodynamic cycle by specifying initial values prior to the analysis, including the intake pressure, the intake temperature, the combustion efficiency, the air-fuel ratio, and the fuel heat. The analysis on the differences from offsetting the moving direction of the piston was carried out by considering the indicated work gained from the cycle and the net indicated work at the engine speed of 3,000 rpm. The indicated work in each stroke could be calculated as follows; (Pulkrabek, 2004) Power in intake and power stroke ⎛ 1 rpm α + β ⎞ W = W ⎜ (16) ⎟ nc ⎝ 4 60 β ⎠ Power in compression and exhaust stroke ⎛ 1 rpm α + β ⎞ (17) W = W ⎜ ⎟ nc ⎝ 4 60 α ⎠ Whereas; rpm is the engine speed, and nc is the number of pistons of the engine. After having the indicated work for each stroke, the calculation for the net indicated work could be done to analyze the differences between the engine with the offset piston and the normal engine.

5. Analytical results In the mathematical analysis, the process analyzed the changing movement of the piston mechanism, the connecting rod, and the crank, and their effects to the stroke length, the displacement volume, and the compression ratio. The analysis was also made on thermodynamic cycle and its effects to the pressure, the temperature, the indicated work, and notably the indicated power gained from the engine. In the analysis, the crank ratio was changed into three values, including 3.0, 3.5, and 4.0. (Wakabayashi, 2003) The offsetting percentages were prepared in five figures. The percentage was pinned at 0% before analysis for the normal engine. However, the percentage was set at 10%, 20%, 30%, and 40% for the engine with the offset piston. According to the analysis of the movement of the piston mechanism, the connecting rod, and the crank as illustrated in Table 2, it could be seen that offsetting the piston could increase the

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International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam. stroke length of the engine further. This change could be noticed clearer in the engine with the lesser crank ratio. Considering the offsetting, it is found that the greater the offsetting percentage, the longer the stroke length. The changes of the stroke length would change in relation with the displacement volume of the cylinder and the compression ratio. It could be seen that the greater the direction offset, the greater the displacement volume and the compression ratio of the engine. The change could be noticed clearer for the engine with the lesser crank ratio as well. Table 2: Results of mechanism motion Description Offset (mm)

Offset percentage 0%

10%

20%

30%

40%

0.00

4.30

8.60

12.90

17.20

86.00

86.05

86.22

86.49

86.88

Strike (mm) CR = 3.0 CR = 3.5 CR = 4.0

86.00

86.04

86.15

86.35

86.62

86.00

86.03

86.11

86.26

86.46

Displacement volume (cc) CR = 3.0 CR = 3.5 CR = 4.0

499.76

500.07

501.01

502.60

504.85

499.76

499.98

500.65

501.77

503.36

499.76

499.93

500.43

501.27

502.45

10.00

10.01

10.02

10.05

10.09

10.00

10.00

10.02

10.04

10.06

10.00

10.00

10.01

10.03

10.05

Compression ratio CR = 3.0 CR = 3.5 CR = 4.0

As seen in the analysis on thermodynamic cycle of the engine with the offset piston as illustrated in Table 3 and Table 4, the increase of indicated work was in line with the increase of the offsetting percentage. The change could be observed clearer in the engine with the lesser crank ratio. In Table 4, it could be seen that the combination of the increase of indicated works and properties of the quick-return mechanism could increase the time that the piston used to move from the bottom-dead center to the topdead center. In the meantime, the time that the piston used to move back from the top-dead center to the bottom-dead center could be decreased. As a result, the net indicated power of the engine could be increased. The changing percentage of the net indicated power of the engine with offsetting piston was equal to the normal engine as illustrated in Figure 2. It showed that the lesser the crank ratio and the greater offsetting percentage, the greater the net indicated power of the engine. An

example could be seen from the analysis under the conditions and assumptions specified in Table 1. When the crank ratio was set at 3.0 while the offsetting percentage was set at 40% with the offsetting length of 17.20 mm, the net indicated work could increase from original by 11.468%. Table 3: Thermodynamics results of normal engine and offset-piston engine; Indicated work Description CR = 3.0 Intake Compress Power Exhaust Total CR = 3.5 Intake Compress Power Exhaust Total CR = 4.0 Intake Compress Power Exhaust Total

Offset percentage 0%

10%

20%

30%

-0.0506

-0.0507

-0.0508

-0.0509

-0.1991

-0.1993

-0.1999

-0.2008

0.8929

0.8936

0.8956

0.8991

-0.2271

-0.2272

-0.2275

-0.2280

0.4161

0.4164

0.4175

0.4194

-0.0506

-0.0507

-0.0507

-0.0508

-0.1991

-0.1993

-0.1997

-0.2003

0.8929

0.8934

0.8948

0.8973

-0.2271

-0.2271

-0.2274

-0.2277

0.4161

0.4163

0.4171

0.4184

-0.0506

-0.0507

-0.0507

-0.0508

-0.1991

-0.1992

-0.1995

-0.2000

0.8929

0.8933

0.8944

0.8962

-0.2271

-0.2271

-0.2273

-0.2276

0.4161

0.4163

0.4168

0.4178

Table 4: Thermodynamics results of normal engine and offset-piston engine; Indicated power Description CR = 3.0 Intake Compress Power Exhaust Total CR = 3.5 Intake Compress Power Exhaust Total CR = 4.0 Intake Compress Power Exhaust Total

Offset percentage 0%

10%

20%

30%

-5.0638

-5.1076

-5.1588

-5.2179

-19.9131

-19.7740

-19.6728

-19.6085

89.2897

90.0751

91.0165

92.1242

-22.7059

-22.5372

-22.3916

-22.2683

41.6069

42.6562

43.7933

45.0295

-5.0638

-5.0949

-5.1310

-5.1724

-19.9131

-19.8140

-19.7415

-19.6953

89.2897

89.8468

90.5107

91.2863

-22.7059

-22.5857

-22.4816

-22.3932

41.6069

42.3522

43.1565

44.0254

-5.0638

-5.0871

-5.1140

-5.1448

-19.9131

-19.8387

-19.7841

-19.7492

89.2897

89.7068

90.2022

90.7786

-22.7059

-22.6156

-22.5373

-22.4706

41.6069

42.1655

42.7668

43.4140

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Indicated Power Improvement (%)

International Conference on Science, Technology and Innovation for Sustainable Well-Being II (STISWB II), 13-14 August 2010, Quang Binh University, Viet Nam.

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CR 3.0 : y=0.278x , R2=0.993 CR 3.5 : y=0.196x , R2=0.994 CR 4.0 : y=0.146x , R2=0.994

12

CR 3.0

10 8

CR 3.5

6

CR 4.0

4 2 0 0

10

20

30

40

50

Offset Percentage (%)

Fig. 2: Indicated power improvement of

offset-piston engine base on normal engine

doi:10.1016/0094-114X(94)00035-J. [3] Nakayama, K., Tamaki, S., Miki, H., and Takiguchi, M., 2000. “The effect of crankshaft offset on piston friction force in a gasoline engine” SAE Paper 2000-01-0922. [4] Mabie, H. H., and Reinholtz, C.F., 1987. Mechanisms and dynamics of machinery. 4th edition, John Wiley & Sons, ISBN: 0-471-80237-9. [5] Pulkrabek, W.W., 2004. Engineering fundamentals of the internal combustion engine. 2nd Edition, Person. Educ, ISBN: 0-13-191855-9. [6] Wakabayashi, R., Takiguchi, M., Shimada, T., Mizuno, Y., and Tamauchi, T., 2003. “The effects of crank ratio and crankshaft offset on piston friction losses,” SAE Paper 2003-010983.

6. Conclusions Modifying the engine by offsetting the piston to move in the reversing direction against the crank rotation, could enable the piston, the connecting rod, and the crank to form a quick return mechanism. This would be beneficial as it could increase the stroke of the engine while the displacement volume and the compression ratio also increased as well. A thermodynamic effect then was created as the indicated work gained from the engine increased. The movement of the quick-return mechanism could reduce the time used in the intake stroke and the power stroke but increase the time used in the compression stroke and the exhaust stroke, leading to an increase in the net indicated power. By using engine specifications and thermodynamic assumptions as detailed above, the net indicated power of the engine increased by 0.278%, 0.196%, and 0.146% when the crank ratio was set at 3.0, 3.5, and 4.0 respectively, under the offsetting percentage of 1%.

6. References [1] Cho, M. R., Oh, D. Y., Moon, T. S., and Han, D. C., 2003. “Theoretical evaluation of the effects of crank offset on the reduction of engine friction,” Proc. Instn Mech. Engrs, Vol. 217 Part D: J. Automobile Engineering, pp. 891-898, doi: 10.1243/095440703769683298. [2] Haddad, S. D., and Tjan, K., 1995. “ An analytical study of offset piston and crankshaft designs and the effect of oil film on piston slap excitation in diesel engine,” Mech. Mach. Theory, Vol. 30, Issue 2, pp. 271-284,

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