DESCRIPTION

Present invention relates generally to a variable valve timing arrangement for an internal combustion engine and more specifically to use epicyclic gear train for such an arrangement variable Valve Timing system controls the intake camshaft valve timing so as to obtain balance between the engine output, fuel consumption and emission control performance. The gctual intake side valve timing is feedback by means of the camshaft position sensor for constant control to the target valve timing. There are so many mechanisms are used to obtain Variable Valve timing. The mechanism is operated by planetary gear train to continuously and precisely change, the phase angle between camshaft and crank shaft with the help of electronically controlled stepper motor. This mechanism guarantees a precise and continuous camshaft phasing for intake and ^exhaust valves in internal combustion engine.

Analysis of epicyclic gear Train

Epicyclic trains usually have complex motions. Therefore comparatively simple methods are • used to analyses them which do not require accurate visualizations of the motions. Assume that the arm a is fixed. Turn S through x revolutions in the clockwise direction. Assuming clockwise motions of the wheel as positive and counterclockwise motion as negative,

Revolution made by a = 0
Revolution made by S = x
Revolution made by P = - (Ts/Tp)x

Now if the mechanism is locked together and turned through a number of revolutions, the relative motion between a, S and P will not alter. Let the locked system be turned through y revolutions in the clockwise direction, Then

Revolution made by a = y
Revolution made by S = x+y
Revolution made by P = y - (Ts/Tp)x

This implies that the ami a turns through y revolutions and S through (y+x) revolutions in the same direction then P will rotate through y - (Ts/Tp)x revolutions in space or relative to the fixed axis of S. Thus if the revolutions made by the third can be determined .Thus the procedure can be summarized as follows:

l.Lock the arm and assume the other wheels free to rotate

2.Turn any convenient gear through one revolution in the clockwise directions and record the number of revolutions made by each of the other wheels

3.Multiply all the above recording by x and write the same in the second row. This is equivalent to the statement that chosen wheel is given x revolution in the clockwise direction keeping the arm fixed.

4. Add y to all the quantities in the second row and make the reading in the Third row .This amount to the fact that by locking the whole system, it is turned through y revolutions in the clockwise direction. Thus the arm makes y revolutions ,the chosen wheel (y+x)revolutions and so on.

5.Applay the given conditions and find the values of x and y. Having known x and y ,the revolution made by any of the wheels can be known.

The number of revolutions of the wheel P given in the third row of the table is the number of revolutions in space or relative to the fixed axis of S and not above its own axis.

The method of the mechanism operation is easy and simple and it's described below:

1) When the stepper motor shaft is stationary, which is the prevailing case, the ring gear is also stationary. The rotation of the arm by the crank shaft causes the rotation of the planetary gear and sun gear. End of the Sun gear is connected to the cam shaft.

Revolution of Ring gear = 0
y-x/3 = 0 x = 3y let us assume
Number of teeth on Sun gear Ti = 20
Number of teeth on planetary gear Ti=20
Number of teeth on Ring gear = Ti + 2 T2
= 60

speed of the sun gear the U)i = (T3/T1+I) U)i
= 4 U)i

From the above table it is clear that for every one revolution of the arm Sun gear makes four revolutions. But velocity ratio between crank shaft and cam shaft should be two for the proper working of four stroke IC engine. Hence additional speed reduction gears(D & H) are used for speed reduction.

Number of Teeth on gear D =20 Number of Teeth on Gear H= 40
Nh = - Nd Td / Th
= - 1 x 20 / 40
= -0.5
Hence by introducing additional speed reduction gears we can make sure that velocity ratio between crank shaft and cam shaft should be two for the proper working of four stroke IC engine

Angle of Rotation Of Cam Shaft

When the stepper motor have a signal from the CPU it will rotate according to the required shift angle resulting in the rotation of the worm gear which is locked with External teeth of ring gear will cause the rotation of the ring gear and consequently an additional rotation of the planetary gears and planetary gear will transmit this additional rotation to Sun gear. This rotation resulting in additional rotation for the sun gear, which is connected with the camshaft, according to the following equation.

Angle of rotation of Cam shaft 0c = (Tw Tr / Te Th) 8W
Tw = No of teath of Worm gear
Tr = Number of teath on internal ring gear
Te = Number of teeth on External Ring Gear
Th - Number of teeth on Gear H
9w= Angle of rotation of worm

. .Above Drawing is not in Scale , Sun and planetary gears and Sun gear and gear "DT?are of same size ,Worm is Connected with External worm teeth of Ring gear and Planetary gear is connected ' with internal teeth of ring gear.

Theoretical Valve Timing Diagram

The exact moment at which the inlet and outlet valve opens and closes with reference to the position of piston and crank shown diagrammatically is known as Valve Timing Diagram. The timing is expressed in terms of degrees of crank rotation.

Suction Stroke: Inlet valve is open. Piston moves from the Top Dead Centre (TDC) to Bottom Dead Centre (BDC). Air-fuel mix is sucked in by negative pressure in cylinder.

Compression Stroke: Inlet and outlet valves closed. Piston moves upwards from BDC to TDC. Air-fuel mix is compressed.

Expansion/Power Stroke: Inlet and outlet remains closed here also. Piston moves from down from TDC to BDC. This happens as a result of ignition of the mixture inside the cylinder. Ignition is started by spark plug.

Exhaust Stroke: Exhaust valve opens. Piston moves up from BDC to TDC. Exhaust gases are pushed out of the cylinder.

Actual Valve Timing Diagram

The Actual Valve Timing Diagram has slight variations with respect to the Theoretical Valve Timing Diagram. The variations are made in order to maximize the engine performance.

Opening and closing of Inlet Valve:

The inlet valve is made to open 10degree to 30degree before the piston reaches the Top Dead Center (TDC) during Suction Stroke and is allowed to close only after 30degree to 40degree after the piston reaches and leaves the BDC in the beginning of compression stroke.

Reason - The reason for doing this is to facilitate silent operation of the engine under high speeds. The inlet valves are made to operate slowly to avoid noise and hence sufficient time should be provided for the air-fuel mix to get into the cylinder. Thus valves are made to open before the actual BDC. Since the inlet valve is a small opening sufficient mixture doesn't enter the cylinder in such short time, as the piston reaches BDC. Thus the inlet valve is kept open for some time period of time after BDC, to facilitate sufficient flow of charge into the cylinder.

Opening and closing of Exhaust Valve: The exhaust valve is made to open 30degree to 60degree before the TDC in the exhaust stroke and allowed to close only after 80 to 10 inO the beginning of the suction stroke.

Reason - The gases inside the cylinder posses a higher pressure even after the expansion stroke. This higher pressure enables it to move out of the cylinder throughthe exhaust valve reducing the work that needs to be done by the engine piston in pushing out these gases. Thus the exhaust valve is made to open before the piston reaches the BDC thus enabling the gases to escape outside on its own and the remaining gases are pushed out by the upward motion of the piston. When the piston reaches the TDC, if the exhaust valve is closed like in actual timing diagram, a certain amount of exhaust gases will get compressed and remain inside the cylinder and will be carried to the next cycle also. To prevent this, the exhaust valves are allowed to close only a certain time after the piston reaches the TDC.

Claims

1 This mechanism guarantees a precise and continuous camshaft phasing for intake and exhaust valves in internal combustion engine.

2The worm gear, which is connected to the stepper motor and meshing with ring gear, offers a self-locking mechanism for ring gear. That will guarantee a constant speed ratio between the camshaft and crank shaft for specific phase angle, which is necessary for good engine operation.

3.In this mechanism there is no limitation for phase angle changing value, except the limitation imposed by the engine's performance envelop.

4.Less Complex operation Comparing with Hydraulic Systems.