DESC:FIELD OF THE INVENTION
[001] The present invention relates to a zero-overlap system and method of manufacturing thereof, particularly, it relates to a zero-overlap system for CNG engine.
BACKGROUND
[002] CNG vehicles engines mostly are the upgraded and/or modified version of the diesel engine. Therefore, there is a limitations or disadvantages of long valve overlap in the diesel cam-shaft engine system. Such system has adverse impact on CNG based vehicle system. The internal EGR is the one of the adverse impacts of valve overlap on the CNG engines, because at part load, CNG engines have negative pressure (vacuum) in the intake manifold and the exhaust gases are sucked back into the intake manifold via combustion chamber and mixed with air and fuel. Hence due to mixing of exhaust gases with intake charge (air-fuel), precise air fuel ratio control via EMS is difficult, results in higher engine out emissions, which leads to higher EATS cost. Additionally, during valve overlap period small amount of unburned fuel, coming with intake air, directly goes to exhaust manifold, results in loss of engine performance and fuel economy.
[003] Hence there is an unmet need of minimal overlap system for CNG system to avoid or minimize aforementioned limitations.
BRIEF SUMMARY
[004] The present invention relates to a zero-overlap system and method of manufacturing thereof. Particularly, it is relating to zero-overlap system for CNG engine.
[005] The system of present invention comprising below components:
a. Intake valve: It is a poppet valve in the cylinder head of an internal-combustion engine that opens at the pre-defined time for certain crank angle rotation of the cycle to allow air (in diesel engines) and air-fuel mixture (in CNG engines) to be drawn into the combustion chamber.
b. Exhaust valve: Similar to intake valve, exhaust valve also is a poppet valve in the cylinder head of an internal-combustion engine that opens at the pre-defined time for certain crank angle rotation to provide path to release burned combustion gases from combustion chamber to exhaust manifold.
c. Valve movement: The exhaust valve closes during the initial part of the suction stroke. The inlet valve opens a little before top dead center and the exhaust valve remains open a little after top dead center. Movement of both intake and exhaust valves depend upon camshaft profile.
d. Camshaft: Camshaft is a rotating shaft with attached disks of irregular shape, known as cams, which actuate the intake and exhaust valves of the cylinders and hence guide their motions. The cams and the camshaft are usually formed as a unit, with the cams set at angles so as to open and close the valves in a prescribed sequence as the cams rotate. A separate camshaft for each row of cylinders is driven by gears or chains from the crankshaft.
e. Cam profile: The shape of the contoured cam surface by means of which motion is communicated known as pitch line. The cam profile shape determines when the valves open or close.
f. Valve lash: It is also known as tappet gap or tappet clearance. It is the mechanical clearance in the valve train between camshaft and valve in an internal combustion engine. Valve lash is usually about 0.2 to 0.8 mm depending on the engine design specifications. Valve lash is intended to provide the greatest amount of valve opening on the high point of the camshaft lobe and assure that the valve is tightly closed on the low segment of the camshaft lobe. Valve lash adjustment is necessary maintenance for engines without hydraulic valve lifters.
g. Valve overlap: Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC at the time of end of exhaust stroke and beginning of intake stroke. Typical duration of valve overlap is between 20° - 80° of crankshaft rotation, depending on the engine design.
[006] In order to understand the present subject, we need to first understand brief functionality of conventional four-stroke CNG engines.
[007] The four-stroke engine (also known as four-cycle engine) is an internal combustion (IC) engine in which the piston completes four separate strokes, while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes of CNG engines as follows:
1. Intake (or suction) stroke: This stroke of the piston begins at top dead center (TDC) and ends at bottom dead center (BDC). In this stroke the intake valve must be in the open position, while the piston pulls an air-fuel mixture into the combustion chamber by producing vacuum pressure into the cylinder through its downward motion.
2. Compression stroke: This stroke begins at BDC, or just at the end of the suction stroke, and ends at TDC. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both the intake and exhaust valves are closed during this stroke.
3. Expansion stroke: This is also known as power, combustion or ignition stroke. This starts at the start of second revolution of the four-stroke cycle. At this point the crankshaft has already completed a full 360-degree revolution. While the piston is at TDC the compressed air-fuel mixture is ignited by a spark plug, results in energy generated due to combustion which forcefully returning the piston to BDC. This stroke produces mechanical work from the engine to turn the crankshaft.
4. Exhaust Stroke: During the exhaust stroke, the piston, once again, returns from BDC to TDC while the exhaust valve is open. This action expels the burned gases through the exhaust valve into exhaust manifold.
[008] The challenges to achieve zero valve overlap is to achieve almost zero value in the present invention is illustrated with one of the examples as below:
[009] The valve tip (top surface) damaged due to high tappet closing velocity to achieve zero value. Further due to higher tappet gap, the rocker arm will hit the valve tip with higher velocity and momentum. This may damage valve top surface as well as rocker arm. Additionally, higher valve closing velocity can damage valve seating area on engine cylinder head. This issue was resolved by doing 1-D simulation on GT Power tool to evaluate max valve lash to keep valve closing velocity lesser the limit. As per simulation results maximum valve lash is limited by valve closing velocity and can be considered upto 0.85 mm max. As there is higher thermal expansion in exhaust valve (due to higher exhaust gas temperature) and thus it is considered to keep higher valve lash for exhaust valve lash than intake valve lash. And this in initial trials intake and exhaust valve lash were kept 0.6 mm and 1.0 mm. However, considering max valve lash limited (i.e. 0.85mm) due to valve closing velocity via 1-D simulation results, it was decided to keep both intake and exhaust tappet same as 0.8 mm.
[0010] Further minimal lubrication near tappet area due to higher tappet gap and due to higher valve lash, there is very high possibility of oil film breakdown around tappet area. The higher valve closing velocity, will further increases chances of oil film breakdown. In order to verify possible impact, lubrication path around rocker arm verified thoroughly with new 0.8 valve lash and it was found all right. Additionally, an engine durability of 1000 hours done at test cell. After completion of engine durability, the wear and tear as well as tolerance of all the components were checked and these were found within limit.

DESCRIPTION OF DRAWINGS
[0011] The present invention relates to relates to a zero-overlap system and method of manufacturing thereof. Particularly, it is relating to zero-overlap system for CNG engine.
[0012] FIG. 1 illustrates CAM profile of present invention.
[0013] FIG. 2 show valve timing diagram of 4-Stroke CNG engine of the existing or conventional system.
[0014] FIG. 3 shows conventional mechanism of CNG engines valve timing with valve overlap.
[0015] FIG. 4 shows CNG engines valve timing without valve overlap
[0016] The FIG. 1 describes CAM profile of present invention. Here the shape of the contoured cam surface by means of which motion is communicated known as pitch line. The cam profile shape determines when the valves open or close
[0017] FIG. 2 show valve timing diagram of 4-Stroke CNG engine of the existing or conventional system.
[0018] In order to understand the present subject, we need to first understand brief functionality of conventional four-stroke CNG engines in FIG. 2.
[0019] The four-stroke engine (also known as four-cycle engine) is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes of CNG engines as follows: The intake (or suction) stroke: This stroke of the piston begins at top dead center (TDC) and ends at bottom dead center (BDC). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the combustion chamber by producing vacuum pressure into the cylinder through its downward motion.
[0020] Compression stroke: This stroke begins at BDC, or just at the end of the suction stroke, and ends at TDC. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both the intake and exhaust valves are closed during this stroke.
[0021] Expansion stroke: This is also known as power, combustion or ignition stroke. This starts at the start of second revolution of the four-stroke cycle. At this point the crankshaft has already completed a full 360-degree revolution. While the piston is at TDC the compressed air-fuel mixture is ignited by a spark plug, results in energy generated due to combustion which forcefully returning the piston to BDC. This stroke produces mechanical work from the engine to turn the crankshaft.
[0022] Exhaust Stroke: During the exhaust stroke, the piston, once again, returns from BDC to TDC while the exhaust valve is open. This action expels the burned gases through the exhaust valve into exhaust manifold.
[0023] A spark-ignition (SI) engine is an internal combustion engine, where the combustion of the air-fuel mixture is ignited by spark from spark plug. SI engines are different compare to compression-ignition (CI) engines (typically diesel engines), where the combustion start by auto-ignition of fuel, injected directly into the compressed hot air inside combustion chamber, without needing any external spark. The CNG engines comes in SI engine categories.
[0024] As mentioned in the FIG. 3, the conventional valve timing has valve overlap. The valve overlaps system of present system almost reduced to zero, results in improved engine performance and better fuel economy.
[0025] The methodology of achieving zero valve overlap show by experimental data in FIG. 4, wherein experimental trials related to this innovation has been done on E483 NA CNG BS-VI (70kW) engine. The zero-valve overlap was achieved by increasing tappet gap (or Valve lash) form existing setting of 0.4 mm to 0.8 mm. As the tappet gap increases, the rocker arm will take more time to come into contact with valve tip and hence valve opening will be delayed. Refer Figure.5 for valve & tappet mechanism details. On the other hand, due to increase in tappet gap, the contact between rocker arm and valve tip be broken early, results in early valve closing. So, higher tappet gap results in late valve opening and early valve closing. As described above, valve overlap is the time period between closing of exhaust valve and opening of intake valve. Higher tappet gap of intake & exhaust valve, results in earlier closing of exhaust valve and late opening of intake valve and therefore the valve overlap has been reduced. In E483 NA CNG BS-VI engine, existing mechanism has valve overlap of about 68-degree crank angle (deg CA). With 0.1 mm higher valve lash setting, there is about 8.5 deg, CA delay in valve opening and 8.5 deg CA early valve closing. So, with 0.8 mm valve lash, there is 34 deg CA early closing of exhaust valve and 34 deg CA delay opening of intake valve, results in 68 deg CA reduction in valve overlap. Since, E483 NA CNG BS-VI engine, existing valve overlap was only 68 deg CA, with new mechanism valve overlap reduced by 68 deg CA and hence it was completely eliminated.
[0026] Although the present disclosure is described in terms of certain preferred embodiments and examples, other embodiments and examples will be apparent to those of ordinary skill in the art, given the benefit of this disclosure, including embodiments and examples that do not provide all of the benefits and features set forth herein, which are also within the scope of this disclosure. It is to be understood that other embodiments may be utilized, without departing from the true spirit and scope of the present invention.
[0027] It is to be understood that other embodiments of the present disclosure will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only various embodiments of the disclosure by way of illustration. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.
[0028] It would become abundantly clear to a person in the art, after reading this specification that the present subject matter also provides a zero-overlap system for CNG engine and method of manufacturing thereof. More specifically, the present subject matter discloses a zero-overlap system for CNG engine to prove zero overlap effectively and without departing from the spirit of the present subject matter. While the subject matter may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described herein. Alternate embodiments or modifications may be practiced without departing from the spirit of the present subject matter. The drawings shown are schematic drawings and may not be to the scale. While the drawings show some features of the subject matter, some features may be omitted. Alternatively, in some other cases some features may be emphasized while others are not. Further, the methods disclosed herein may be performed in manner and/or order in which the methods are explained. Alternatively, the methods may be performed in manner or order different than what is explained without departing from the spirit of the present subject matter. It should be understood that the subject matter is not intended to be limited to the particular forms disclosed. Rather, the subject matter is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the subject matter.
Yours Faithfully

ANIL KUMAR PANDEY
(IN P/A 2359)
AGENT FOR THE APPLICANT(S)

,CLAIMS:We claim:

1) A zero-overlap system for CNG engine comprising:
a) an intake valve opens at the pre-defined time for certain crank angle rotation of the cycle to allow air (in diesel engines) and air-fuel mixture (in CNG engines) to be drawn into the combustion chamber;
b) an exhaust valve opens at the pre-defined time for certain crank angle rotation to provide path to release burned combustion gases from combustion chamber to exhaust manifold;
c) a valve movement system to co-ordinate movement of the intake valve and exhaust valve;
d) a Camshaft configured to engage with disks to actuate the intake valve and exhaust valve of the cylinders to guide the motion of intake valve and exhaust valve.
e) a cam profile shape to determine the opening and closing of the intake valve and exhaust valve.
f) A valve lash to provide mechanical clearance in the valve train between camshaft and valve in an internal combustion engine.
g) a valve overlap system to operate opening and closing of the intake valve and exhaust valve at the same time. The valve overlap occurs when the piston nears TDC at the time of end of exhaust stroke and beginning of intake stroke.

2) The zero-overlap system for CNG engine as claimed in Claim 1, wherein said system is for four stroke engine.
3) The zero-overlap system for CNG engine as claimed in Claim 1, wherein said duration of valve overlap is between 20° - 80° of crankshaft rotation, depending on the engine design.

Yours Faithfully

ANIL KUMAR PANDEY
(IN P/A 2359)
AGENT FOR THE APPLICANT(S)