Variable Valve Timing In I.C. Engines
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20-09-2008, 10:54 PM


Valve timing is the regulation of the points in the combustion cycle, at which the valves are set to open and close. Since the valves require a finite period of time in which to open or close without abruptness, a slight lead-time is always necessary for proper operation. The design of the valve-operating cam provides for the smooth transition from one position to the other, while the cam setting determines the timing of the valve.

In a typical four-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. While the intake valve should open, theoretically at TDC, most engines utilise an intake valve opening, which is timed to occur a few degrees prior to the arrival of the piston at TDC on the exhaust stroke. This is because by the time the valve becomes fully open, the piston would have travelled considerably down the bore, and since the valve would have to be fully closed before BDC, the actual time the valve would be fully open would be minimal.Additionally, the inertia of the incoming mixture plays a big role.

Keeping the inlet valve open after BDC forces more mixture to pack into the cylinder, in spite of the fact that the piston is moving upwards. The exhaust valve is set to open before BDC, towards the end of the power stroke and close after TDC, at the beginning of the intake stroke. The reason the exhaust valve is opened before BDC is to prevent the exhaust gases from forming a high-pressure cushion, which would impede the movement of the piston and rob the engine of power. This also ensures that the valve is fully open at the start of the exhaust stroke. Keeping the exhaust valve open after TDC ensures that the entire burnt mixture is thoroughly scavenged..
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25-07-2010, 12:47 AM

could you please send me the entire report for Variable Valve Timing In I.C. Engines to my email-id mebin_89@yahoo.com
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27-07-2010, 05:50 PM

Variable Valve Timing
The concept behind this is simple :alter the timing and/or size of the intake and exhaust ports at different engine RPMs to ensure that the engine is as efficient as possible throughout it's range of operating speeds. In the normal cases , the the internal combustion engine works at the maximum efficiency at one particular range of speeds. Speeds higher or lower will reduce the fuel efficiency and power. variable valve timing solves this issue by adjusting the timing of valve to adjust the intake so that the engine has maximum efficiency over a wider range of speeds. Some engines that have this technology are:
Honda VTEC

VTEC is the acrronym for Variable Valve Timing and Electronic Lift Control.The VTEC allows the valves to remain open for two different durations:
1)for low-speed operation, it works at short opening time for give good torque and acceleration, and
2)a larger opening time for higher speeds

BMW Valvetronic

This technology by BMW alters the pivot point of the cam follower thereby providing different deflections of the tip of the follower. valve opening is increased by the arger movement .

Audi Valvelift

In this engine, choice is made between two differently-shaped cam lobes . a pair of sliding cam lobes is used on a splined part of the camshaft.

For more details, visit these links:
freepatentsonline7308877.html
carbiblesfuel_engine_bible_vvt.html
faqspatents/app/20090255495

There is also a ppt:


.ppt   Variable-Valve-Timing-Intelligent-system.ppt (Size: 2.43 MB / Downloads: 589)
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28-07-2010, 10:38 PM

:)I LIKE MOST
I WANT TO PRESENT ON THIS............................
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29-08-2010, 03:35 PM

i am requesting you to give more information on variable valve timing in I.C engine
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02-09-2010, 09:25 PM

Hi,
more details of this can be found in these links:
freepatentsonline7308877.html
freepatentsonline7599782.html
seminar and presentationabstractsformechanical.blogspot2008/07/variable-valve-timing-in-ic-engines.html
faqspatents/app/20090255495
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09-01-2011, 10:07 AM

ABSTRACT

VALVE TIMING (VT) is one of the most important aspects of consideration in the design of an automobile engine. Simply defined, it is the timing, or regulation of the opening and closing of the valves. In simpler terms, it is the way an engine ‘breathes’.

In an I.C.engine, usually the inlet valves open a few degrees (of crank angle) prior to TDC, and close after BDC. Similarly, the exhaust valves open a few degrees before BDC and close a few degrees after TDC. This is done to maximise:
 Intake of air/air-fuel mixture; and
 Scavenging, i.e. the exhaust of burnt gases.

Until recently, most engines around the world utilised ordinary or static VT, where the parameters of valve opening, lift, and closing (VO, VL and VC) were fixed. This was satisfactory at normal engine speeds, but posed problems at high and low speeds. Since the VT did not vary with speed, the additional requirements that arose at the extreme speeds could not be met with static VT. For example, at high speeds, the engine requires greater amounts of air. This implies that the IV should remain open for a longer period of time. This, though beneficial at high speeds, would be a menace at low speeds as it may lead to exhaust of unburnt fuel, which results in fuel wastage, increased emissions and lower performance.
This is where variable valve timing (VVT) comes into play. As the name suggests, the timing of the valves is not fixed, but varies, as per the demands of the situations. Therefore, the extra demands of the engine can be met, which in turn, results in improved engine performance.


.doc   Variable Valve Timing In I.C. Engines report.doc (Size: 158.5 KB / Downloads: 94)
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CONTENTS



CHAPTERS PAGE NO.

1. INTRODUCTION 1
1.1 Definition: Valve Timing 1
1.2 Inlet Valve Timing 1
1.3 Exhaust Valve Timing 1
1.4 Valve Overlap 2
1.5 Piston Engines 2
1.6 Ordinary Or Static Valve Timing 3

2. VARIABLE VALVE TIMING 4
2.1 Basic Mechanism 4

3. TYPES OF VVT MECHANISMS 5
3.1 Cam-Phasing VVT 5
3.2 Cam-Changing VVT 7
3.3 VVT-i (Variable Valve Timing–intelligent) 8
3.4 Cam-Phasing and Cam-Changing VVT 9
3.4.1 Toyota VVTL-i (VVT with Lift- intelligent) 9
3.4.2 Porsche Variocam Plus 12
3.4.3 Honda i-VTEC 13
3.5 VTEC (Variable Valve Timing and Lift Electronic Control) 14
3.6 Rover's unique VVC system 14
4. CONCLUSION 15
REFERENCES

LIST OF FIGURES
1. Fig 3-1 5
2. Fig 3-2 6
Fig 3-3


CHAPTER 1
INTRODUCTION


1.1 Definition: Valve Timing
Valve timing is the regulation of the points in the combustion cycle, at which the valves are set to open and close. Since the valves require a finite period of time in which to open or close without abruptness, a slight lead-time is always necessary for proper operation. The design of the valve-operating cam provides for the smooth transition from one position to the other, while the cam setting determines the timing of the valve.
In a typical four-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.

1.2 Inlet Valve Timing
While the intake valve should open, theoretically at TDC, most engines utilise an intake valve opening, which is timed to occur a few degrees prior to the arrival of the piston at TDC on the exhaust stroke. This is because by the time the valve becomes fully open, the piston would have travelled considerably down the bore, and since the valve would have to be fully closed before BDC, the actual time the valve would be fully open would be minimal.
Additionally, the inertia of the incoming mixture plays a big role. Keeping the inlet valve open after BDC forces more mixture to pack into the cylinder, in spite of the fact that the piston is moving upwards.

1.3 Exhaust Valve Timing
The exhaust valve is set to open before BDC, towards the end of the power stroke and close after TDC, at the beginning of the intake stroke. The reason the exhaust valve is opened before BDC is to prevent the exhaust gases from forming a high-pressure cushion, which would impede the movement of the piston and rob the engine of power. This also ensures that the valve is fully open at the start of the exhaust stroke. Keeping the exhaust valve open after TDC ensures that the entire burnt mixture is thoroughly scavenged.

1.4 Valve Overlap
Valve overlap refers to the time when both the intake and exhaust valves are open. It ensures that the exhaust gases rushing out of the cylinder create suction, in order to draw in fresh mixture, and the fresh mixture entering the cylinder pushes out the burnt fuel mixture.
Therefore, valve timing of any engine depends on:
 The amount of valve overlap
 Lag and lead, i.e. the degrees that the crankshaft turns between valve opening and TDC or BDC
 The intended usage of the engine.

1.5 Piston Engines
The absolute best valve timing for an engine varies with engine r.p.m. and the load on the engine. This means valve timing set to produce smooth idling may not deliver top performance at high speeds. And an engine timed to produce maximum horsepower at higher speeds will idle poorly, if at all.
For this reason, valve timing in most engines is a compromise between the two extremes. For most vehicles, this does not pose a problem. But on high performance vehicles, some top end performance is lost in order to maintain a workable idle speed.
Piston engines normally use poppet valves for intake and exhaust. Cams on a camshaft drive these, directly or indirectly. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important and the profile of these cams is optimized for a certain engine speed. This trade-off normally limits low-end torque or high-end power. VVT allows the cam profile to change, which results in greater efficiency and power.

1.6 Ordinary or Static VT
In static VT, one or more camshafts are used (plus pushrods, lifters and rocker arms) to open and close an engine's valves. The camshaft/camshafts are turned by a timing chain that is connected to the crankshaft. As engine r.p.m. rises and falls, the crankshaft and camshaft turn faster or slower to keep valve timing relatively close to what is needed for engine operation.
Unfortunately, the dynamics of airflow through a combustion chamber change radically between 2000 r.p.m. and 6000 r.p.m.. Despite the manufacturer’s best efforts, there was no known way to maximise valve timing for high and low r.p.m. with a simple crankshaft-driven valve train. Instead, engineers had to develop a ‘compromise’ system that would allow an engine to start and run when pulling out of the driveway but also allow for strong acceleration and highway cruising at high speeds.
Though static VT was quite successful, because of the ‘compromise’ nature of standard valve train systems, few engines were ever in their ‘sweet zone’, which resulted in wasted fuel and reduced performance.

CHAPTER 2
VARIABLE VALVE TIMING


Around the early 1990s, Variable Valve Timing [VVT] started to become popular on gasoline passenger cars. Using this technique, engine manufacturers were enabled to control the extent and duration for which the poppet valves were open. In addition, the opening and closing of the valve could also be varied depending on the crank angle. In the absence of 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 optimise 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 optimised for a particular speed.

2.1 Basic Mechanism
With VVT, a sensor is used to detect the engine's speed. An electronic system then uses this information to adjust the valve opening and closing timings accordingly. This avoids the problems associated with static valve timing, and also allows for maximum torque at all engine speeds.


CHAPTER 3
TYPES OF VVT MECHANISMS

3.1 Cam-Phasing VVT
Cam-phasing VVT is at present, the simplest, cheapest and most commonly used mechanism. However, its performance gain is also the least.
Basically, shifting the phase angle of camshafts varies the valve timing. For example, at high speeds, the inlet camshaft will be rotated in advance by 30° so as to enable earlier intake. This movement is controlled by an engine management system according to need, and actuated by hydraulic valve gears.

Fig 3-1: Lift v angle diagram (Toyota VVT-i)

Cam-phasing VVT cannot vary the duration of valve opening. It only allows earlier or later valve opening. Earlier opening results in earlier close. It cannot vary the valve lift. However, cam-phasing VVT is the simplest and cheapest form of VVT because each camshaft needs only one hydraulic phasing actuator, unlike other systems that employ individual mechanism for every cylinder.
Better systems have continuous variable shifting, say, any arbitrary value between 0° and 30°, depending on r.p.m. This provides the most suitable valve timing at any speed, thus greatly enhancing engine flexibility. Moreover, the transition is so smooth that it is hardly noticeable.

Fig 3-2: Lift v angle diagram (BMW Double Vanos)
Operation:
The end of camshaft incorporates a gear thread. The thread is coupled by a cap, which can move towards and away from the camshaft. Because the gear thread is not in parallel to the axis of camshaft, phase angle will shift forward if the cap is pushed towards the camshaft. Similarly, pulling the cap away from the camshaft results in shifting the phase angle backward. The hydraulic pressure determines push or pull. There are two chambers right beside the cap and they are filled with liquid. A thin piston separates these two chambers, the former attaches rigidly to the cap. Liquid enter the chambers via electromagnetic valves, which controls the hydraulic pressure acting on which chambers. For instance, if the engine management system signals the valve at the green chamber open, then hydraulic pressure acts on the thin piston and push the latter, accompany with the cap, towards the camshaft, thus shift the phase angle forward. Continuous variation in timing is easily implemented by positioning the cap at a suitable distance according to engine speed.
Advantages: Cheap and simple; continuous VVT improves torque delivery across the whole rev (speed) range.
Disadvantages: Lack of variable lift and variable valve opening duration, thus it produces less top end power.
Applications: Most car makers, such as:
 Audi 2.0-litre - continuous inlet
 Audi V8 - inlet, 2-stage discrete
 BMW Double Vanos - inlet and exhaust, continuous
 Ferrari 360 Modena - exhaust, 2-stage discrete
 Fiat (Alfa) SUPER FIRE - inlet, 2-stage discrete
 Ford Puma 1.7 Zetec SE - inlet, 2-stage discrete
 Ford Falcon XR6's VCT - inlet, 2-stage discrete
 Jaguar AJ-V6 and updated AJ-V8 - inlet, continuous
 Lamborghini Diablo V12 since SV - inlet, 2-stage discrete
 Mercedes V6 and V8 - inlet, 2-stage
 Nissan VQ V6 - inlet, continuous
 Nissan VQ V6 since Skyline V35 - inlet, electromagnetic
 Porsche Variocam - inlet, 3-stage discrete
 Toyota VVT-i - continuous, mostly inlet but some also exhaust
 Volvo 4 / 5 / 6-cylinder modular engines - inlet, continuous
 Volkswagen VR6 - inlet, continuous
3.2 Cam-Changing VVT
Honda pioneered road car-used VVT in the late 80s by launching its famous VTEC system (Valve Timing Electronic Control). First appeared in Civic, CRX and NS-X, and then became standard in most models.
Two sets of cams having different shapes to enable different timing and lift. One set operates during normal speed, say, below 4,500 r.p.m.. The other set substitutes at higher speeds. Such layout does not allow continuous change of timing, therefore the engine performs modestly below 4,500 r.p.m. but above that it will suddenly transform into a wild animal.
This system does improve peak power - it can raise red line to nearly 8,000 r.p.m. (even 9,000 r.p.m. in S2000), just like an engine with racing camshafts, and increase top end power by as much as 30 hp for a 1.6-litre engine. However, to exploit such power gain, the engine must be kept boiling at above the threshold r.p.m., therefore frequent gear change is required. As low-speed torque gains too little (remember, the cams of a normal engine usually serves across 0-6,000 r.p.m., while the "slow cams" of VTEC engine still need to serve across 0-4,500 r.p.m.), drivability won't be too impressive. In short, cam-changing system is best suited to sports cars.
Advantages: Powerful at top end
Disadvantages: only 2 or 3 stages; non-continuous; only slight improvement in torque; complex structure
Applications:
 Honda VTEC,
 Mitsubishi MIVEC,
 Nissan Neo VVL.
3.3 VVT-i (Variable Valve Timing–intelligent)
Toyota originally introduced the VVT-i (Variable Valve Timing - intelligent) as a revolutionary design that increases engine torque and output while addressing environmental issues. By adjusting the intake valve opening timing according to the engine speed, more oxygen is supplied through the air intake valve as more fuel is injected into the combustion chamber. Power and torque is maximized due to larger scale combustion. This optimised fuel to air ratio ensures the air-fuel mixture is combusted more thoroughly.
The VVT-i portion of the system continuously varies intake valve timing throughout the rev range by hydraulically rotating the camshaft relative to its drive gear. Note that VVT (without the "i") did not do this continuously. The VVL portion of the system incorporates two distinct cam profiles. However, the actual mechanism is quite different. Both cam lobes operate a single, wide rocker arm that acts on both intake or both exhaust valves. A needle-bearing roller on the arm follows the low-r.p.m., short-duration, low-lift lobe, forcing both valves to open and close on that profile. The roller design and roller bearings on the rocker arm pivot help to minimize valve train friction. The high-r.p.m., higher-duration, longer-lift lobe rubs on a hardened steel slipper follower mounted to the rocker arm with a spring. Even though the high-r.p.m. lobe is pushing down further than the low-r.p.m. lobe, the spring absorbs the extra movement. At 6000 r. p.m., the ECU sends a signal to an oil control valve at the end of the camshaft that puts oil pressure behind a lock pin in the rocker arm, sliding the pin under the spring-loaded slipper follower, locking it to the rocker arm and forcing the arm to follow the high-r.p.m. cam profile.
3.4 Cam-Phasing and Cam-Changing VVT
Combining cam-changing VVT and cam-phasing VVT could satisfy the requirement of both top-end power and flexibility throughout the whole rev range, but it is inevitably more complex. Presently, only Toyota and Porsche have such designs. However more and more sports cars are believed to adopt this VVT mechanism in the future.
3.4.1 Toyota VVTL-i (VVT with Lift- intelligent)
The VVTL-i system comprises four major components:
1. The Electronic Control Unit (ECU), which calculates optimum intake valve timing and decides whether or not to operate the cam changeover mechanism based on engine operating conditions.
2. The Oil Control Valve (OCV) for variable valve timing, which controls hydraulic pressure, and in turn the VVT Pulley, under instruction from the ECU.
3. The Oil Control Valve (OCV) for variable valve timing and lift, which controls hydraulic pressure to operate the cam changeover mechanism.
4. The VVT Pulley, which continuously changes the intake valve timing using hydraulic pressure.
Toyota’s VVTL-i is the most sophisticated VVT design yet. Its powerful functions include:
 Continuous cam-phasing variable valve timing
 2-stage variable valve lift plus valve-opening duration
 Applied to both intake and exhaust valves
The system could be seen as a combination of the existing VVT-i and Honda’s VTEC, although the mechanism for the variable lift is different from Honda.
Like VVT-i, the variable valve timing is implemented by shifting the phase angle of the whole camshaft forward or backward by means of a hydraulic actuator attached to the end of the camshaft. The timing is calculated by the engine management system with engine speed, acceleration, going up hill or down hill etc. taking into consideration. Moreover, the variation is continuous across a wide range of up to 60°, therefore the variable timing alone is perhaps the most perfect design up to now.
Based on the VVT-i system, the VVTL-i system has also adopted a cam changeover mechanism that changes the amount of lift and duration of the intake and exhaust valves while the engine is operating at high speeds. In addition to achieving higher engine speeds and higher outputs, this system enables the valve timing to be optimally set, resulting in improved fuel economy.
When the engine is operating in the low-to-mid-speed range, the low/medium-speed cam lobes of the camshafts operate to move the two valves via the rocker arms. Then, when the engine is operating in the high-speed range, the signals from the sensors cause the ECM (Electronic Control Module) to change the hydraulic passage of the oil control valve, thus changing to the high-speed cam lobes. Now, the lift and the duration of the intake and exhaust valves increases, allowing a greater volume of the air/fuel mixture to enter the cylinder, and a greater volume of the exhaust gases to leave the cylinder. As a result, the engine produces more power over a wider RPM range.
Operation
The construction and the operation of the valve timing control are basically the same as in the V VT-i system.
The main components of the rocker arm assembly are the rocker arm, rocker arm pad, rocker arm pin, and the rocker shaft. This assembly is used for both the intake and exhaust camshafts, with each connected to its respective rocker arm shaft. Both the intake and exhaust camshafts contain low and medium-speed cams and high-speed cams.
When the engine coolant temperature is higher than 60°C (140°F) and the engine speed is higher than 6000 RPM, this system switches from the low/medium speed cams to the high-speed cams.
When the engine is operating in the low-to-mid-speed range, the oil control valve is open on the drain side so that the oil pressure will not be applied to the cam changeover mechanism. Then, when the engine reaches a high speed, the oil control valve closes on the drain side in order to apply the oil pressure to the high-speed cam of the cam changeover mechanism.
Advantages: Continuous VVT improves torque delivery across the whole rev range; Variable lift and duration lift high rev power.
Disadvantage: More complex and expensive
Applications:
 Toyota 1.8-litre 190hp for Celica GT-S and hot Corolla
3.4.2 Porsche Variocam Plus
Porsche’s Variocam Plus was said to be developed from the Variocam which serves the Carrera and Boxster. The Variocam was first introduced to the 968 in 1991. It used timing chain to vary the phase angle of camshaft, thus provided 3-stage variable valve timing. 996 Carrera and Boxster also use the same system. This design is unique and patented, but it is actually inferior to the hydraulic actuator favoured by other car makers, especially it doesn’t allow as much variation to phase angle.
However, the most influential changes of the "Plus" are the addition of variable valve lift. It is implemented by using variable hydraulic tappets. As shown in the picture, each valve is served by 3 cam lobes - the center one has less lift (3 mm only) and shorter duration for valve opening. In other words, it is the "slow" cam.

Fig 3-3: Cams in the Porsche Variocam Plus
The outer two cam lobes are exactly the same, with fast timing and high lift (10 mm). Selection of cam lobes is made by the variable tappet, which actually consists of an inner tappet and an outer (ring-shape) tappet. They could by locked together by a hydraulic-operated pin passing through them. In this way, the "fast" cam lobes actuate the valve, providing high lift and long duration opening. If the tappets are not locked together, the valve will be actuated by the "slow" cam lobe via the inner tappet. The outer tappet will move independent of the valve lifter.
The variable lift mechanism is unusually simple and space-saving. The variable tappets are just marginally heavier than ordinary tappets and engage nearly no more space.
Advantages: VVT improves torque delivery at low / medium speed; Variable lift and duration lift high rev power.
Disadvantage: More complex and expensive
Applications:
 Porsche 911 Turbo
 911 Carrera 3.6
3.4.3. Honda i-VTEC
It is a combination of the VTEC and VVT-i into a more powerful VVT mechanism. It is called the Honda i-VTEC. Like Toyota's VVTL-i, it provides:
 Continuous cam-phasing variable valve timing
 2-stage variable valve lift plus valve-opening duration
 Can be applied to both intake and exhaust valves.

Basically, the camshaft is purely VTEC - with different cam lobes for implementing 2-stage variable lift and timing. On the other hand, the camshaft can be phase-shifted by a hydraulic actuator at the end of the camshaft, so valve timing can be varied continuously according to need.
Advantage: Continuous VVT improves torque delivery across the whole rev range; Variable lift and duration lift high rev power.
Disadvantage: More complex and expensive
3.5 VTEC (Variable Valve Timing and Lift Electronic Control)
VTEC (Variable Valve Timing and Lift Electronic Control) is an electronic and mechanical system in some Honda engines that allows the engine to have multiple camshafts. VTEC engines have an extra intake cam with its own rocker, which follows this cam. As the engine moves into different r.p.m. ranges, the engine's computer can activate alternate lobes on the camshaft and change the cam's timing. The profile on this cam keeps the intake valve open longer than the other cam profile. At low engine speeds, this rocker is not connected to any valves. At high engine speeds, a piston locks the extra rocker to the two rockers that control the two intake valves. In this way, the engine gets the best features of low-speed and high-speed camshafts in the same engine.
3.6 Rover's unique VVC system
Rover introduced its own system calls VVC (Variable Valve Control) in MGF in 1995. Many experts regard it as the best VVT considering its all-round ability - unlike cam-changing VVT, it provides continuously variable timing, thus improve low to medium rev torque delivery; and unlike cam-phasing VVT, it can lengthen the duration of valves opening (and continuously), thus boost power.
Basically, VVC employs an eccentric rotating disc to drive the inlet valves of every two cylinder. Since eccentric shape creates non-linear rotation, valves opening period can be varied.
VVC has one draw back: since every individual mechanism serves 2 adjacent cylinders, a V6 engine needs 4 such mechanisms, and that's not cheap. V8 also needs 4 such mechanism. V12 is impossible to be fitted, since there is insufficient space to fit the eccentric disc and drive gears between cylinders.

Fig 3-4: Lift v angle (High speed ROVER VVC)
Advantages: Continuously variable timing and duration of opening achieve both drivability and high speed power.
Disadvantages: Not ultimately as powerful as cam-changing VVT, because of the lack of variable lift; Expensive for V6 and V8; impossible for V12.
Applications:
 Rover 1.8 VVC engine serving MGF Caterham
 Lotus Elise 111S.

CHAPTER 4
CONCLUSION

Unlike conventional fixed valve timing, VVT continuously adjusts intake valve timing based on R.P.M. and other engine conditions. This allows the timing to be automatically advanced at high r.p.m. and then relaxed at low r.p.m. so that full thrust of the burn cycle can be optimized at all engine speeds. The benefit, in short, is the ability to achieve higher peak horsepower without sacrificing low-end torque, all of which makes the engine a more satisfying companion in all driving situations.
VVT devices have been used in IC engines to improve engine performance over a wide range of speeds by utilising variable valve timing and lift. It can also be used to give fully variable stroke in other reciprocating devices. This can be used in a wide range of applications where a fully variable displacement-pumping device is required. Examples include the aerospace, processing, automotive, and medical industries.
VVT's benefit to fuel consumption and emission
EGR (Exhaust gas recirculation) is a commonly adopted technique to reduce emission and improve fuel efficiency. However, it is VVT that really exploit the full potential of EGR.
In theory, maximum overlap is needed between intake valves and exhaust valves’ opening whenever the engine is running at high speed. However, when the car is running at medium speed in highway, in other words, the engine is running at light load, maximum overlapping may be useful as a mean to reduce fuel consumption and emission. Since the exhaust valves do not close until the intake valves have been open for a while, some of the exhaust gases are recirculated back into the cylinder at the same time as the new fuel-air mix is injected. As part of the fuel-air mix is replaced by exhaust gases, less fuel is needed. Because the exhaust gas comprise of mostly non-combustible gas, such as CO2, the engine runs properly at the leaner fuel-air mixture without failing to combust.


REFERENCES
1. ANGELUCCI, ENZO; BELLUCCI, ALBERTO: The Automobile: From Steam to Gasoline. (McGraw-Hill)
2. GILL, PAUL W.; SMITH Jr., JAMES H.; ZIURYS, EUGENE J.: Fundamentals of Internal Combustion Engines. (Oxford)
3. Auto India Magazine (Various issues; 1999-2004)
4. Overdrive Magazine (Various issues; 2000-2004)
5. Business Standard Motoring Magazine Monthly (Various issues; 1999-2003)
6. howstuffworks.com
7. autozine.org
8. meaa-mea.com (Mitsubishi Electric Automotive America)
9. mce-5.com


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21-11-2012, 11:17 AM

to get information about the topic "variable valve timing" full report ppt and related topic refer the link bellow

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