05-09-2014, 12:30 PM
Variable Valve Timing
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ABSTRACT
Automobile is an industry where innovations are happening daily. Our project aims to implement one of those innovations -Variable Valve timing. We all know that the engine has undergone so many changes after its evolution in a constant effort to improve its performance. Variable valve timing technology is one such performance enhancing principle by which volumetric efficiency and so power output is increased.
Most of today's engines have fixed valve timing. That is the duration for which the valves, both inlet and exhaust are opened is same at all operating loads and speeds. Many performance test done have shown that as the speed increases the volumetric efficiency slightly increases until a critical speed and thereafter starts to decrease to reach a minimum value at top speed. This is attributed to decrease in time for which the charge is inducted in higher speeds.
New technology development related to VVT will directly help to increase the volumetric efficiency as more amount of charge will get induced in to the cylinder and so the power output will also increase. Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. Like VTEC and VVT-i systems were developed by Honda and Toyota respectively in order to improve the efficiency of car engines. VTEC (Variable Valve Timing and Lift Electronic Control) is a valve train system developed by Honda that allows engines to achieve turbo level specific output without the bad fuel efficiency that turbocharging normally introduces. VVT-i (Variable Valve Timing with intelligence) is a similar system developed by Toyota
INTRODUCTION
What is VVT?
In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems. Variable Valve Timing (VVT) is a generic term for an automobile piston engine technology. VVT allows the lift or duration or timing (some or all) of the intake or exhaust valves (or both) to be changed while the engine is in operation. Two stroke engines use a power valve system to get similar results to VVT.
There is two popular different technology developed by Toyota and Honda in case of Variable Valve Timing (VVT). VTEC (Variable Valve Timing and Lift Electronic Control) is a valve train system developed by Honda that allows engines to achieve turbo level specific output without the bad fuel efficiency that turbocharging normally introduces. VVT-i (Variable Valve Timing with intelligence) is a similar system developed by Toyota and has several variants among which VVTL-i (Variable Valve Timing and Lift intelligent system) is analogous to VTEC.
COMPONENTS OF IMPORTANCE
Valve and Actuation
Every good engine for proper performance has to breathe well i.e. the process of air intake and exhaust should be very smooth. So the design of valves, intake and exhaust manifold should be very accurate and precise. The shape of valve head, port and seat are developed either empirically or by Computational Fluid Dynamic analysis to produce minimum pressure drop across the valve section. For this purpose separate test rig are developed to find out the suitable design parameters for the valves and manifolds. The important parameters being valve size, port diameters and lengths, valve spring pressure, Valve seat angle and valve stem diameter. The valves are usually made of very hard materials since they are exposed to hot gases produced during combustion process. The temperature levels experienced by valves are in the range of 500°C at inlet to 800°C at exhaust. The valves undergo various heat treatment processes to add strength and toughness. The stem is also important and is usually friction welded to the valve head. The materials used for valves and stems should have a low thermal expansion coefficient so as to avoid deformation at such higher temperatures. The valve guides are annular spaces, which guides not only the valves but also helps to conduct heat from the valve to the cylinder head. Valve seats are areas where the valve gets seated when fully closed. Usually they are available in form of inserts, which have to be inserted in to the cylinder head. The seats undergo induction hardening to increase its surface hardness. The seat angle plays an important role in the determination of flow of charge in to the cylinder. When the lifts are low the jet fills the gap and adheres to both valve and seat. At an intermediate lift the jet will break away from one of the surfaces and at high lifts the jet breaks away from both surfaces to form a free jet. The transition point will depend whether the valve is closing or opening. Research has shown that a seat angle of 30° with minimum seat width gives best results. It has also been seen that it is better to round all corners on the valves and the seat rather than have a sharper one, which will affect flow turning.
Camshafts
Camshaft is a main component in the valve actuation mechanism. It determines when, how much and how long the valves will be open. As explained earlier the crankshaft drives the camshaft through a set of gears in reduction. In four stroke engines for every complete cycle the crankshaft rotates twice but the valves open only once. So the camshaft must make only one revolution so as to actuate the valves. So the general gear reduction ratio is 2:1. Camshafts are typically made from hardened steel, hardened alloy cast iron or chilled cast iron. They undergo a variety of surface hardening processes like induction hardening and flame hardening. The camshaft rotates on well-lubricated bearings. Camshafts can be of two types one being Over Head Cam and the other, the conventional type. The OHC is directly connected to tappet and spring assembly thereby avoiding pushrod and follower assembly. A normal camshaft has two cams one for the Inlet valve and the other for the Exhaust. Diesel engines have an additional cam for the fuel pump. Very high consideration is given for the development of the cams, since they actually control the valves. The cam profile is generated in an elaborate process, which involves profile cutting and grinding. The profiles of both the cams are usually the same. In some cases the exhaust cam will have a short lift but the duration remains the same. One important thing is that each of these cam lobes has a phase difference and this determines parameters like valve overlap and at what time each of the valves gets actuated.
CHARACTERISTICS PARAMETER
Valve Timing
Valve timing is one of the most important characteristics of valves that will affect the performance of an engine. Valve timing is defined as the duration for which the valves are actuated. Every engine has a definite period for which the valves get actuated and the cam as described earlier does this. The timing of the valves in an engine till date are mostly fixed. This timing is achieved with the help of timing gears, which are connected to camshaft and crankshaft. Since the timing is fixed the valves open and close for the same crank angle at all times. With the valve timing is fixed the factor that actually contributes to teaser volumetric efficiency is the amount of time and not the angle, that the valves are open at different speeds. To start with at low speeds the duration of valves will take a definite time, which will directly depend on camshaft speed. As the speed increases the camshaft speed also increases and this reduces the duration for which the valves will open. Since there is a decrease in the valve duration times at higher speeds the amount of charge entering the cylinder is less than the amount it could take at slow speeds. The air-fuel mixtures at high speeds are usually maintained rich to produce sufficient power. If the valve had had the same duration as at low speed the amount of charge entering would be high and the air-fuel mixture can be made light rich. This will directly aid for low fuel consumption even at high speeds.
Valve Overlap
We have seen that inlet and exhaust valve open at different times of operation of the engine. They both have a corresponding crank angle duration during which they are actuated. But there is duration of crank angle when both the valves are open. This duration is called as the VALVE OVERLAP period. Valve overlap occurs at the end of exhaust stroke during which the inlet valve also begins to open. This duration significantly affects the performance of engines. Most engines have different valve overlap settings. Valve overlap is usually provided so as to increase the scavenging efficiency. The fresh charge comes in and pushes out the exhaust gas. There is also considerable mixing of some part of residual gas with the fresh charge, which will aid in rising the temperature of charge and also making the mixture a more combustible one. The diesel engines have a very short valve overlap because compression ratio is a very important parameter. A higher valve overlap will make the cold starting conditions very difficult. Usually the piston to cylinder head clearance limits the valve overlap for CI engines. Turbocharged diesel engines use a large overlap since operating pressure is large and they completely use the purpose of exhaust gas re-circulation. Spark ignition engines have variable valve overlap. Compression ratio is not an important parameter for proper combustion as is for CI engines. High performance engines, which operate at high speeds, have large overlap while other conventional engines prefer small overlap. Conventional spark ignition engines operate at low speeds and if there is high valve overlap the fresh charge entering the cylinder will directly be carried into exhaust. This will result in the wastage of charge and also aid to emission of unburned hydrocarbons. With the short overlap they show high performance in terms of power output and specific fuel consumption. The greater valve overlap in high performance takes full advantage of the pulse effects that can be particularly beneficial at higher speeds. But if the engine is operated at part throttle or idling conditions the performance suffers because the reduced induction manifold pressure uses back flow. The overlap is always optimized for an engine to the successful operation of the engine. Since the engines operate at fixed valve timing this becomes an inevitable factor. Valve overlap has to be suitably selected such that it will aid cold starting and also part throttling or full throttling performance.
Volumetric Efficiency
We have earlier defined volumetric efficiency as the ratio of amount of charge inducted into the cylinder to that of the maximum possible charge that can be induced in to the cylinder during a cycle. From the definition of volumetric efficiency it is clear that it measures the breathing efficiency of an engine i.e. how well an engine takes in air. Various experimental results on different engines have shown that the volumetric efficiency is high at slow speeds and it gradually increases till a critical speed before it starts to drop to a minimum at high speed. Some of the parameters that cause this decrease in efficiency. The decrease in volumetric efficiency is attributed to the decrease in duration for which the charge is inducted. Volumetric efficiency is largely affected by intake manifold designs. Many engines use a short intake manifold to increase the amount of charge that they can induct and supply to the engine. Experimental results have shown that long intake manifold generally deliver less charge than shorter ones as the speed increases. Another important parameter that will affect is manifold diameters. Proper diameters are needed to produce the minimum pressure drop across the valve. Excessive pressure drop can cause back flow in exhaust valves, which will affect engine performance. For the design of proper manifold, model test rigs are build that will resemble and produce the same flow performance as that of an inlet passage. Some of the important parameters that are found using this modeling are discharge coefficient, effective flow area. It has been seen that discharge coefficient usually varies with valve lift and it decreases with higher values
VARIABLE VALVE TIMING
Theory:
One of the revolutionary inventions in the automobile industry is Variable Valve Timing technology. Till now all the engines have been operating on fixed valve timing and this limited their operating performance to some limit. With the invention of this technology the same engines have been able to produce more power than before for the same operating load and speed. It has also improved emissions levels and fuel economy.
Variable valve timing when first developed was in the form of Cam phasing VVT. It was one of the simplest, cheapest and most used forms of this technology. It basically varies the valve timing by shifting the phase angle of camshafts. To be more precise at higher speeds the camshaft will be rotated in advance by some already calibrated angle. All these control are completely achieved by engine management system and a set of hydraulic valve gears. It must be noted here that cam phasing cannot alter the duration of valve opening. It also cannot vary valve lift it just allows earlier or later valve opening.
Cam phasing can be done in two ways either in discrete steps or has a continuous one. In discrete one the cam is phased to certain already set values depending upon the speed. While the continuous type phases continuously with change in speed. For better control both inlet and exhaust are sometimes phased to obtain higher efficiency. Many car manufacturers have employed this system because of its cheaper, simple arrangement. It has also proved to be fuel-efficient and is relatively good for low to medium torque delivery.
The other form of VVT is the Cam-Changing type. Unlike the compassing VVT it allows for variable valve duration and also variable lift. It basically involves the use of two sets of cam profiles and a mechanism for switching between them. One cam has shorter duration while the other has a long duration and also increased lift. The shorter duration cam is optimized to produce low-end power and torque. It is suitable to operate this cam for slow to medium speeds. At a critical speed the electronically controlled hydraulic mechanism switches the control to the longer duration cam. The longer duration cam is optimized to produce good power and torque at high speeds. Since the lift of the longer duration cam is higher than the short duration cam it offers variable valve lift, which is absent in cam-phasing VVT. Many
Cam Phasing
Cam phasing advances or retards valve lift events by rotating the camshaft, typically over a range of about 60 degrees relative to the crankshaft angle. Let's say our intake valve normally opens 5 crankshaft degrees before top dead center and closes 185 crankshaft degrees after top dead center (5 degrees after bottom dead center). "Retarding" valve timing by 10 degrees means the valve opens and closes 10 degrees later, that is, it opens 5 degrees after top dead center and closes 195 degrees after top dead center. By retarding the camshaft’s timing, the engine achieves better high RPM torque, whereas advancing the intake camshaft’s timing produces better power at low RPM.
CHALLENGES
The main factor preventing this technology from wide use in production automobiles is the ability to produce a cost effective means of controlling the valve timing under the conditions internal to an engine. An engine operating at 3000 revolutions per minute will rotate the crankshaft 25 times per second, so the valve timing events have to occur at precise times to offer performance benefits. Electromagnetic and pneumatic camless valve actuators offer the greatest control of precise valve timing, but are not cost effective for production vehicles at this time
EPILOGUE
I have learnt characteristics parameter of valve, working of valve, cam phasing and cam changing in this project. Any 'variable' ability in valve timing or lift can be used to better suit the breathing capability of an engine to the job it is performing at any one time, resulting in improved performance. The more finely that adjustments are made, and the more valves on which these alterations act, the better will be the end result.Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems but all of these means only once to increase engine performance