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National Conference in Mechanical Engineering Research and Postgraduate Students (1stNCMER 2010)26 -27 MAY 2010, FKM Conference Hall, UMP, Kuantan, Pahang, Malaysia; pp. 249-260

ISBN: 978-967-5080-9501 (CD ROM); Editors: M.M. Rahman, M.M. Noor and K. Kadirgama

Universiti Malaysia Pahang

249

THEORETICAL TORQUE COMPARISON BETWEEN REVETEC ENGINE

AND CONVENTIONAL INTERNAL COMBUSTION ENGINE

Maisara Mohyeldin Gasim, M.M.Noor and K.Kadirgama

Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Tun Abdul Razak

Highway, 26300 Kuantan, Pahang, Malaysia

Email: [email protected]

ABSTRACT

Revetec engine is a new engine arrangement used to increase engines efficiency by

replacing the crankshaft and the connecting rod used in conventional engines by cams.In this research a computational work done to measure the torque given by the

conventional engine (engine with crankshaft) and the new one which uses cams. Aftercalculating the cylinder pressure - supposing that the two engines have the same

cylinder bore, speed, piston displacement, piston mass and the same heat addition- the

torque in each engine calculated. The results show that the new engine has a biggertorque than the conventional engines, and that means a bigger efficiency.

Keywords: Revetec, internal combustion engines, torque calculation, cam profile.

INTRODUCTION

Crankshafts are devices used to convert the reciprocating piston motion, to rotating

motion at the driveline, but the efficiency of these devices is very poor, with losses that

can approach 36%. At the top of the piston stroke, where gas pressure is highest, force

transfer efficiency is at its lowest, though it rises as the piston descends and theconnecting rods leverage increases. Peak efficiency happens about 40% through the

piston stroke, and then drops at an exponential rate that mirrors its rise. In addition, the

piston doesnt travel a path that is strictly parallel to the bore, so an angular force equal

to the pressure on the top of the piston is transferred to the cylinder wall. This increases

friction, wear, and fuel consumption (Christopher , 2010). Efforts were done to replacethe crankshaft with other devices in internal combustion engines, which could transfer

the power in more efficient way. Two samples of these efforts are Shane engine and

Revetec engine. In these new engines a cam replaces the crankshaft.Revetec engine is solution for some conventional engines problems such as

vibration, side thrust force, liners wear and as a result of all those factors engines poorefficiency. Revetec Engine, Figure 1, design consists of two counter-rotating (three

lobed) cams geared together, so both cams contribute to forward motion. Two bearings

run along the profile of both cams (four bearings in all) and stay in contact with the

cams at all times. The bearings are mounted on the underside of the two inter-connected

pistons. The two cams rotate and raise the piston with a scissor-like action to thebearings. Once at the top of the stroke , Figure 2, the air/fuel mixture is fired. The

expanded gas then forces the bearings down the ramps of the cams spreading them apart

ending the stroke. The effective cranking distance is determined by the length from the

point of bearing contact to the centre of the output shaft (not the stroke). A conventionalengine's turning distance is half of the piston stroke. The piston acceleration throughout


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Theoretical Torque Comparison Between REVETEC Engine And Conventional Internal Combustion Engine

250

the stroke is controlled by the cam. The piston assembly slides rigidly through the block

via an oil pressure fed guiding system eliminating piston to cylinder-bore contact. This

reduces wear and lubrication requirements in the cylinder, and also reduces piston side

thrust force (Bradley, 1996).

Figure 1: Revetec engine with four cylinder (Bradley, 1996)

Figure 2: Revetec engine (piston at TDC )


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Maisara Mohyeldin Gasim, M.M.Noor and K. Kadirgama

251

In this paper, calculating the torque generated when using cams instead of

crankshafts was done, y using simple harmonic motion to construct the profile for cam,

because it is similar to the conventional engine piston motion, after that the cylinder

pressure was calculated starting from the equation of the state for an ideal gas, using

Wiebe function for heat release fraction, using piston force balance then, finally theengines torque calculated.

REVETEC ENGINE CAM DESIGN

When the cam turns through one motion cycle, the follower executes a series of events

consisting of rises, dwells and returns. Rise is the motion of the follower away from the

cam center, dwell is the motion during which the follower is at rest; and return is the

motion of the follower toward the cam center. There are many follower motions that can

be used for the rises and the returns. The following section is a description for thesimple harmonic motion.

Simple H armonic M otion

A cam mechanism with the basic curve like (g) in Figure 3. will impartsimple harmonicmotion to the follower.

Figure 3: follower displacement, velocity and acceleration

The velocity diagram at h indicates smooth action. The acceleration, as shown at i, is

maximum at the initial position, zero at the mid-position, and negative maximum at the

final position.

(1)

Displacement

Velocity

acceleration


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252

(2)

(3)

Design Principle

The basic principle of designing a cam profile with the inversion method is still used.

However, the curve is not directly generated by inversion. This procedure has two steps:

1. Imagine the center of the roller as a knife edge. This concept is important in cam

profile design and is called the trace point) of follower. Calculate the pitch

curveaa, Figure 4, that is, the trace of the pitch point in the inverted mechanism.

2. The cam profilebb is a product of the enveloping motion of a series of rollers.

Figure 4: The trace point of the follower on a disk cam

Design Equations

The problem of calculating the coordinates of the cam profile is the problem of

calculating the tangent points of a sequence of rollers in the inverted mechanism. At themoment shown Figure 5, the tangent point isPon the cam profile.

Figure 5: The tangent point, P, of a roller to the disk cam


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253

The calculation of the coordinates of the pointPhas two steps:

1. Calculate the slope of the tangent ttof pointKon pitch curve,aa.

2. Calculate the slope of the normalnn of the curveaa at pointK.

Since the coordinates of point K: (x1, y1 ) are

(4)

(5)

d: the radius of basic circle for the cam.

so, the expression of the coordinates of pointPare:

(6)

(7)

By appling this on Revetec cam, the torque arm at any cam angle can be calculated,Figure 6 shows the torque arm.

Figure 6: torque arm in Revetec engine cams.

Calculating Gas Pressure


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254

The equation of state for an ideal gas is (Ferguson, 1986):

mRTpv (8)

Taking the logarithm of both sides and differentiating with respect to crank angle gives

d

dT

Td

dV

Vd

dp

p

111 (9)

The first law of thermodynamics in differential form for an ideal gas with constant

specific heat is

d

dVp

dQdTmcv (10)

Dividing the left hand side by mRTand the right side bypV, and rearranging thenyields

d

dV

Vd

dQ

pVd

dT

T

111

1(11)

Combining equation (9) and equation (11) and introduction ofdQ = Q indxproduces

ddx

V

Q

d

dV

V

p

d

dp in

1 (12)

In practice, it is convenient to normalize the equation by letting

1/ ppp 1/ VVV 11/ VpQQ in (13)

In which case, equation (12) becomes

d

dx

V

Q

d

Vd

V

p

d

pd~

~

1~ (14)

Equation (14) is a linear first-order differential equation and easily can be solved by

numerical integration methods.

The volumeVand its derivative dV/dare a known function of the crank angle

rrV /cos12

11

~

(15)

r

rVd

sin

2

1

(16)


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255

r= the compression ratio =V()/V(0)

The fraction of heat release x (Wiebe function)

n

b

sx

1exp1 (17)

b

n

b

sxnd

dx

/1

1

( 18)

To solve equation (14) and obtaining the values of p for both the compression

and power strokes, a FORTRAN program was made by taking the following prior

values and assuming a spark ignition engine:

r= 10, = 1.3,s=-40o,b= 40

o,n = 4,Q = 20

The results after running this program are in Table 1.

Table 1: Cylinder pressure

Crank angle (deg) Pressure (bar) Crank angle (deg) Pressure (bar)

0 108.6028 39 55.71481

3 113.0503 42 51.11672

6 112.0601 45 46.916819 110.792 48 43.0992512 107.0521 51 39.64182

15 102.1803 54 36.51876

18 96.53259 57 33.70288

21 90.4341 60 31.16687

24 84.15776 63 28.8843127 77.91665 66 26.83016

30 71.86614 69 24.98114

33 66.11131 72 23.31586

36 60.71648 75 21.81489

Conventi onal Engi ne Perf ormance

Stroke =0.046 m, Bore = .076 mLength of connecting rod = 0.092 = b

Crank arm = 0.023 m = r

Piston and part of connecting rod mass = 0.3 kg

Speed of the engine = 2500 rpm

Piston force balance:


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256

Figure 7: piston force balance

Now:

The net force applied on the piston, Figure 7:

Fc=[ P*A - m*a] / cos (19)

Acceleration (J. Hannah and Stephens, 1984):

rbra

/

2coscos

2 (20)

But:

Sin =r*sin/b (21)

Then

br

amAPFc

/sinsincos

**

1

(22)

By taking the reaction of Fc:

Fa= Fc*cos (23)

Fais the vertical component of Fc, which causes the torque.

But

= 90 (+ ) (24)

ThenFa=Fccos(90 (+ sin

-1[r*sin/b])) (25)

FG

h

FC


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The torque:

T = Fa * r (26)

A Fortran computer program, also, made to calculate the torque, Supposing that:

1-P1from equation (13) = 1 bar, so the pressure values from table 1, will multiply by 100000.

2-The values of the torque are through the compression and expansion strokes, so the crankangles are between 180 and 180 degrees.

Revetec Engin e Per for mance

Stroke = .046 mBore = .076 m

Figure 8: piston force balance when using cam instead of crankshaft

Using the fundamental of geometry to describe the profile of the cam, Figure 8:

Now:

Fc= [P*A - m*a] / cos (27)

Acceleration from equation (3):

(28)

(0 ,0)

(x ,y)

(xp ,yp)

(x1 ,y2)

O

P

K

LM

60-

FG

FG

FC FA


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And:= (-(60-)) (29)

Then

))-(60-cos(

**

amAPFc

(30)

By taking the reaction of Fc:

Fa= Fc*sin (31)

A computer program was written in Fortran language to calculate the torque.

RESULTS AND DISCUSSION

The results obtained when running the program are shown in Table 2 and Figure 9. The

torque for the two engines arrangements calculated through the compression and expansionstrokes. Here are the results:

Table 2: Revetec and conventional engines torque

CrankAngle

CAMAngle

Cyl .Presbar

Revetec Torque(Nm)

Conventional Torque(Nm)

0 0 108.6028 0 03 1 113.0503 104.0717 76.89117

6 2 112.0601 201.0236 162.1311

9 3 110.792 291.0042 246.1328

12 4 107.0521 366.032 320.6724

15 5 102.1803 426.3264 380.770118 6 96.53259 471.7084 424.9465

21 7 90.4341 503.0412 454.2347

24 8 84.15776 521.8611 470.8581

27 9 77.91665 530.0377 477.2629

30 10 71.86614 529.5211 475.672133 11 66.11131 522.1685 467.9778

36 12 60.71648 509.6456 455.7579

39 13 55.71481 493.3837 440.31

42 14 51.11672 474.5694 422.681745 15 46.91681 454.1631 403.699848 16 43.09925 432.9175 384.0024

51 17 39.64182 411.4122 364.0718

54 18 36.51876 390.0795 344.2639

57 19 33.70288 369.2331 324.8352

60 20 31.16687 349.09 305.965663 21 28.88431 329.7933 287.7753

66 22 26.83016 311.4278 270.3403

69 23 24.98114 294.0343 253.7032

72 24 23.31586 277.6218 237.8822

75 25 21.81489 262.1757 222.8779Total torque 9826.343 8675.717


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Average torque 393.0537 347.0287

Percentage of increase in torque 13.26%

Figure 9: torque comparison between Revetec and conventional engine

It is clear from the Table 1 and Figure 9 that by replacing the crankshaft with cam

there is a sensitive increase in torque obtained. Because when using crankshaft the crank armis fixed, but by using cam the torque arm increases with the increase of cam angle. The

effective cranking distance is determined by the length from the point of bearing contact tothe centre of the output shaft (not the stroke).

CONCLUSION

Revetec engine is a new internal combustion engine arrangement, where in this new engine

cams are used instead of crankshaft to convert the reciprocating piston motion, to rotating

motion.In this paper a theoretical comparison between the torque generated by conventional

engine and Revetec engine is done. From the results obtained, the increase of torque whenusing Revetec engine is equal to13.26% during the maximum cylinder pressure range.

REFERENCES

Christopher, A. Sawyer. Automotive Design & Production,. www.autofieldguide.com,accessed on 19.03.2010

Bradley, H.S. 1996. Revetec Holdings Limited. X4v2 Testing.

Ferguson, C.R. 1986. Internal Combustion Engines-Applied Thermosciences. John Wiley &

Sons.

Hannah, J. and Stephens, R.C. 1984. Mechanics of Machines. Edward Arnold Ltd.

NOMENCLATURES


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A piston cross-section areac specific heat

F vertical component of F

F connecting rod force

F gas pressure forceF h side thrust forcem mass

n a parameter used to curve fit experimental data

p pressure

P cylinder pressure

Q heat additionR gas constant

T temperature

V volume

x fraction of heat release

specific heat ratio crank angle

time scale of heat release

start of heat release

the angle between the connecting rod and the line joining the center of the crankshaft to

the piston the angle between the force Fa and extension of force Fc

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