Variable valve timing in internal combustion engine
- 1. VARIABLE VALVE TIMING IN INTERNAL COMBUSTION ENGINE SUBMITTED BY :- RISHABH USN :- 1BI12ME080 PROJECT GUIDE :- Mr PREM KUMAR H. N. TECHNICAL SEMINAR ON :-
- 2. CONTENTS Serial No. Topic Slide No. 1. Valve 3 2. Valve Functions 4 3. Valve Operations 5 4. Valve Timing 6-9 5. Multi-valve Technology 10 6. Variable Valve Timing 11-12 7. Types of VVT 13-17 8. Benefits of VVT 18 9. Present Development in VVT Systems 19 10. Conclusion 20
- 3. What is a Valve? • In Internal Combustion Engine, valves are used to control the flow of the intake and exhaust gases into and out of the combustion chamber. • The timing, duration and lift of these valve events has a significant impact on engine performance. • Valves allow the engine to breathe.
- 4. What are the Valve Functions? • INTAKE :- The Intake valve allows the air-fuel mixture from the carburettor to enter into the combustion chamber. • EXHAUST :- The exhaust valve releases the exhaust gases and spent fuel to escape out of the engine cylinder into the atmosphere.
- 5. How does the valves operate? • CAMS :- These provide the force to push the valves open, usually with the help of a rocker arm. • ROCKER ARMS :- These are the links between the cam and the valve. • SPRINGS :- These are used to keep the valves in place and to provide the force required to close the valve.
- 6. What is Valve Timing? • It’s the regulation of the points in the combustion cycle, at which valves are set to open and close. • The design of the valve-operating cam provides for the smooth transition from one position to other. • The cam setting determines the timing of the valve. • In a typical 4-stroke engine, the inlet valve is set to open before TDC (Top Dead Centre), towards the end of the exhaust stroke and close after BDC (Bottom Dead Centre), at the start of the compression stroke.
- 7. Valve Timing…..Continued • Inlet Valve Timing :- Theoretically, the inlet valve should open at TDC. But, its timed to occur a few degrees prior to the arrival of piston at TDC on the exhaust stroke. This is done to induct as much as possible amount of charge into the cylinder before the piston reaches BDC.
- 8. Valve Timing…..Continued • Exhaust Valve Timing :- The exhaust valve is set to open before BDC, towards the end of the power stroke. This is done to prevent the exhaust gases from forming a high-pressure cushion, which would impede the movement of the piston. The valve closes after TDC which ensures that the entire burnt mixture is thoroughly scavenged.
- 9. Valve Timing…..Continued • Valve Overlap :- This refers to the time when both the intake & exhaust valves are open. It ensures that the exhaust gases rushing out of the cylinder create suction. The fresh mixture entering the cylinder pushes out the burnt fuel mixture. • Valve timing of any engine depends on :- The amount of valve overlap. Lag & lead, i.e. the degrees that the crankshaft turns between valve opening and TDC or BDC.
- 10. What is Multi-Valve Technology? A multi-valve engine is one where each cylinder has more than two valves. Adding more valves increases valve area and improves the flow of intake and exhaust gases, thereby enhancing combustion and power output. Multi-valve geometry allows the spark plug to be ideally located within the combustion chamber for optimal flame propagation. Some engines are designed to open each intake valve at a slightly different time, which increases turbulence, improving the mixing of air and fuel at low engine speeds.
- 11. What’s Variable Valve Timing (VVT) and it’s Need? • In internal combustion engines, Variable valve timing is the process of altering the timing of a valve lift event. • It has the task of setting the most advantageous valve timing for the particular engine for different operating modes. • When the engine rev increases, the duration of intake and exhaust stroke decreases so that fresh air becomes not fast enough to enter the combustion chamber, while the exhaust becomes not fast enough to leave the combustion chamber. • The best solution is to open the inlet valves earlier and close the exhaust valves later. Hence, Overlapping between intake period and exhaust period should be increased as rev increases.
- 12. VVT’s necessity….Continued • Without Variable Valve Timing technology, engineers used to choose the best compromise timing. • For example, a van may adopt less overlapping for the benefits of low speed output. • A racing engine may adopt considerable overlapping for high speed power. • An ordinary sedan may adopt valve timing optimize for mid-rev so that both the low speed drivability and high speed output will not be sacrificed too much. • No matter which one, the result is just optimized for a particular speed.
- 13. Types of Variable Valve Timing System:- 1. Cam-Changing VVT • It uses different cam profiles to lift the valves depending on engine load and speed (RPM). • One common system uses two rocker arms for normal operation on its two intake valves, with a third, higher profile, rocker arm between the other two arms. • At engine speeds above 5000-6000 rpm, the engine ECU activates an oil pressure controlled pin that locks the three rocker arms together. • The centre rocker arm follows a larger and more aggressive profile, transferring its movement to the intake valves which now open further and for a longer period. • When engine speed fall below the threshold speed, oil pressure is removed from the pin and a spring deactivates the pin.
- 14. Cam-Changing VVT Applications Honda’s latest 3-stage VTEC (Valve Timing Electronic Control) has been applied in CIVIC SOHC engine in Japan. The mechanism has 3 cams with different timing and lift profile. • Stage 1 (Low Speed) :- The 3 pieces of rocker arms move independently. Both cams' timing is relatively slow compare with the middle cam, which actuates no valve now. • Stage 2 (Medium Speed) :- Hydraulic pressure connects the left and right rocker arms together, driven by the right cam. As a result, both inlet valves obtain slow timing but medium lift. • Stage 3 (High Speed) :- Hydraulic pressure connects all the 3 rocker arms together. Since, middle cam is largest, both inlet valves are actually driven by that cam. Therefore, fast timing and high lift are obtained in both valves.
- 15. Types of VVT….Continued 2. Cam-Phasing VVT • It is the simplest, cheapest and most commonly used mechanism at this moment. • It varies the valve timing by shifting the phase angle by shifting the phase angle of camshafts. • At high speed, the inlet camshaft will be rotated in advance by 30 degrees so to enable earlier intake. This movement is controlled by engine management system according to need, and actuated by hydraulic valve gears. • It cannot vary the duration of valve opening. It just allows earlier or later valve opening. Earlier open results in earlier close. It also cannot vary the valve lift, unlike cam-changing VVT.
- 16. Cam-Phasing VVT Applications • Toyota's VVT-I (Variable Valve Timing - Intelligent) has been spreading to more and more of its models, from the tiny Yaris (Vitz) to the Supra. Its mechanism is based on the working of Cam-Phasing VVT, it is also a continuously variable design. • However, the word "Intelligent" emphasize the clever control program. Not only varies timing according to engine speed, it also consider other conditions such as acceleration, going up hill or down hill.
- 17. Types of VVT….Continued 3. Cam-Changing + Cam-Phasing VVT • Combining cam-changing VVT and cam-phasing VVT. • It could satisfy the requirement of both top- end power and flexibility throughout the whole rev range, but it is inevitably more complex. • At the time of writing, only Toyota and Porsche have such designs.
- 18. Benefits of Variable Valve Timing(VVT) • EGR (Exhaust gas recirculation) is a commonly adopted technique to reduce emission and improve fuel efficiency. However, it is VVT that really exploits the full potential of EGR. • Variable valve timing systems can provide better torque for an engine. • With more precise handling of engine valves, automakers have shown that VVT can produce better fuel economy for vehicles. • Reduced nitrogen oxide and Hydrocarbon emissions .
- 19. Present Development in VVT Systems Koenigsegg’s FreeValve Technology :- The FreeValve technology is equipped with pneumatic spring and position sensor so that the valve can reciprocate in & out of the engine cylinder. It provides full control of gas exchange, variable valve lift and accurate valve opening and closing time. The FreeValve technology is connected to the ECU of the engine which feeds the system with the timing and amount of valve lift required. FreeValve’s primary goal is to reduce emissions and fuel consumption. Secondary goals are downsizing, torque and power increases.
- 20. Conclusion • Variable valve timing technology has grown as technological availability increases. The need for ever more efficient, clean, powerful engines has pushed the bar higher within production automotive engines. • The trend toward computer controls is rapidly approaching a limit within the cam engine world and the jump into camless designs is something of the future that holds much promise. • The problem of transition time has been virtually solved and the seating speed will soon be under control. • Despite the problems, the camless engine is the future. The level of control (lift, duration, phase) is practically unheard of in mechanical systems.
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