digplanet beta 1: Athena
Share digplanet:

Agriculture

Applied sciences

Arts

Belief

Business

Chronology

Culture

Education

Environment

Geography

Health

History

Humanities

Language

Law

Life

Mathematics

Nature

People

Politics

Science

Society

Technology

Cylinder head of Honda K20Z3. This engine uses continuously variable timing for the inlet valves

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[citation needed] many automotive manufacturers to use VVT systems.

Two-stroke engines use a power valve system to get similar results to VVT.

Background theory[edit]

The valves within an internal combustion engine 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. Without variable valve timing or variable valve lift, the valve timing must be the same for all engine speeds and conditions, therefore compromises are necessary.[1] An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range.

Piston engines normally use valves which are driven by camshafts. 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. The camshaft is driven by the crankshaft through timing belts, gears or chains.

An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions.

Continuous versus discrete[edit]

Early variable valve timing systems used discrete (stepped adjustment). For example, one timing would be used below 3500 rpm and another used above 3500 rpm.

More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of the valve timing. Therefore the timing can be optimized to suit all engine speeds and conditions.[1][2]

Cam Phasing versus variable duration[edit]

The simplest form of VVT is cam-phasing, where the angle of a camshaft is rotated forwards or backwards (relative to the crankshaft). Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered with a cam-phasing system.

Achieving variable duration on a VVT system requires a more complex system, such as multiple cam profiles or oscillating cams.

Typical effect of timing adjustments[edit]

Late intake valve closing The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke. The air which is expelled fills the manifold with higher pressure, and on subsequent intake strokes the air which is taken in is at a higher pressure. Late intake valve closing has been shown to reduce pumping losses by 40% during partial load conditions, and to decrease nitric oxide (NOx) emissions by 24%. Peak engine torque showed only a 1% decline, and hydrocarbon emissions were unchanged.[2]

Early intake valve closing Another way to decrease the pumping losses associated with low engine speed, high vacuum conditions is by closing the intake valve earlier than normal. This involves closing the intake valve midway through the intake stroke. Air/fuel demands are so low at low-load conditions and the work required to fill the cylinder is relatively high, so Early intake valve closing greatly reduces pumping losses.[2] Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%. It also reduced nitric oxide emissions by 24% at partial load conditions. A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.[2]

Early intake valve opening Early intake valve opening is another variation that has significant potential to reduce emissions. In a traditional engine, a process called valve overlap is used to aid in controlling the cylinder temperature. By opening the intake valve early, some of the inert/combusted exhaust gas will back flow out of the cylinder, via the intake valve, where it cools momentarily in the intake manifold. This inert gas then fills the cylinder in the subsequent intake stroke, which aids in controlling the temperature of the cylinder and nitric oxide emissions. It also improves volumetric efficiency, because there is less exhaust gas to be expelled on the exhaust stroke.[2]

Early/late exhaust valve closing Early and late exhaust valve closing can also reduce emissions. Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions.

Challenges[edit]

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.[citation needed] An engine operating at 3000 revolutions per minute will rotate the camshaft 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, in 2013, are not cost effective for production vehicles.

History[edit]

Steam engines[edit]

The history of the search for a method of variable valve opening duration goes back to the age of steam engines when the valve opening duration was referred to as “steam cut-off”. The Stephenson valve gear, as used on early steam locomotives, supported variable cutoff, that is, changes to the time at which the admission of steam to the cylinders is cut off during the power stroke.

Early approaches to variable cutoff coupled variations in admission cutoff with variations in exhaust cutoff. Admission and exhaust cutoff were decoupled with the development of the Corliss valve. These were widely used in constant speed variable load stationary engines, with admission cutoff, and therefore torque, mechanically controlled by a centrifugal governor and trip valves.

As poppet valves came into use, a simplified valve gear using a camshaft came into use. With such engines, variable cutoff could be achieved with variable profile cams that were shifted along the camshaft by the governor.[3]

Aircraft[edit]

Some versions of the Bristol Jupiter radial engine of the early 1920s incorporated variable valve timing gear, mainly to vary the inlet valve timing in connection with higher compression ratios.[4] The Lycoming R-7755 engine had a Variable Valve Timing system consisting of two cams that can be selected by the pilot. One for take off, pursuit and escape, the other for economical cruising.

Automotive[edit]

The desirability of being able to vary the valve opening duration to match an engine’s rotational speed first became apparent in the 1920s when maximum allowable RPM limits were generally starting to rise. Until about this time an engine’s idle RPM and its operating RPM were very similar, meaning that there was little need for variable valve duration. It was in the 1920s that the first patents for variable duration valve opening started appearing – for example United States patent U.S. Patent 1,527,456.

In 1958 Porsche made application for a German Patent, also applied for and published as British Patent GB861369 in 1959. The Porsche patent used an oscillating cam to increase the valve lift and duration. The desmodromic cam driven via a push/pull rod from an eccentric shaft or swashplate. It is unknown if any working prototype was ever made.

Fiat was the first auto manufacturer to patent a functional automotive variable valve timing system which included variable lift. Developed by Giovanni Torazza in the late 1960s, the system used hydraulic pressure to vary the fulcrum of the cam followers (US Patent 3,641,988).[5] The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.

Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330).[5] The fuel injected models of the 1980 Alfa Romeo Spider 2000 had a mechanical VVT system. The system was engineered by Ing Giampaolo Garcea in the 1970s.[6]

In 1987, Nissan was the first manufacturer to release an electronically-controlled VVT engine.[7] The NVCS system fitted to the 300ZX (Z31) 300ZR model alters cam phasing, to improve idle quality and low-rev torque.

In 1989, Honda released the VTEC system.[8] Where the Nissan NVCS alters the phasing of the camshaft, VTEC switches to a separate cam profile at high engine speeds to improve peak power. The first VTEC engine Honda produced was the B16A which was installed in the Integra, CRX, and Civic hatchback available in Japan and Europe.[citation needed]

In 1992, Porsche first introduced VarioCam, which was the first system to provide continuous adjustment (all previous systems used discrete adjustment). The system was released in the Porsche 968 and operated on the intake valves only.

The BMW Double VANOS was the first system which could provide electronically controlled, continuous timing variation for both the intake and exhaust valves.[citation needed]

Marine[edit]

Variable valve timing has begun to trickle down to marine engines. Volvo Penta's VVT marine engine uses a cam phaser, controlled by the ECM, continuously varies advance or retardation of camshaft timing. [9]

Diesel[edit]

In 2010, Mitsubishi developed and started mass production of its 4N13 1.8 L DOHC I4 world's first passenger car diesel engine that features a variable valve timing system.[10][11]

Automotive nomenclature[edit]

Hydraulic vane-type phasers on a cut-out model of Hyundai T-GDI engine

Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include:

  • AVCS (Subaru)
  • AVLS (Subaru)
  • CPS (Proton, Volvo)
  • CVTCS (Nissan, Infiniti)
  • CVVT (Alfa Romeo, Citroën, Geely, Hyundai, Iran Khodro, Kia, Peugeot, Renault, Volvo)
  • DCVCP - dual continuous variable cam phasing (General Motors)
  • DVVT (Daihatsu)
  • MIVEC (Mitsubishi)
  • N-VCT (Nissan)
  • S-VT (Mazda)
  • VANOS (BMW)
  • VarioCam (Porsche)
  • VCT (Ford, Yamaha)
  • i-VTEC (Honda)
  • VVC (MG Rover)
  • VVL (Nissan)
  • Valvelift (Audi)
  • VVEL (Nissan, Infiniti)
  • VVT (Chrysler, General Motors, Proton, Suzuki, Volkswagen Group)
  • VVT-i (Toyota, Lexus)

Methods for implementing Variable Valve Control (VVC)[edit]

Cam switching[edit]

This method uses two cam profiles, with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however the adjustment is discrete rather than continuous.

The first production use of this system was Honda's VTEC, which uses a locking pin to engage the "high rpm" cam follower.

Cam phasing[edit]

Many production VVT systems are the cam phasing type, using a device known as a variator. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however the duration and lift cannot be adjusted.

Oscillating Cam[edit]

These designs use an oscillating or rocking motion in a part cam lobe,[clarification needed] which acts on a follower . This follower then opens and closes the valve. Some oscillating cam systems use a conventional cam lobe, while others use an eccentric cam lobe and a connecting rod. The principle is similar to steam engines, where the amount of steam entering the cylinder was regulated by the steam “cut-off” point.

The advantage of this design is that adjustment of lift and duration is continuous.[12] However in these systems, lift is proportional to duration, so lift and duration cannot be separately adjusted.

The BMW (valvetronic),[13] Nissan (VVEL), and Toyota (valvematic) oscillating cam systems act on the intake valves only.

Eccentric Cam Drive[edit]

Eccentric cam drive systems operates through an eccentric disc mechanism which slows and speeds up the angular speed of the cam lobe during its rotation. Arranging the lobe to slow during its open period is equivalent to lengthening its duration.

The advantage of this system is that duration can be varied independent of lift[14][15] (however this system does not vary lift). The drawback is two eccentric drives and controllers are needed for each cylinder (one for the intake valves and one for the exhaust valves), which increases complexity and cost.

MG Rover is the only manufacturer that has released engines using this system.[citation needed]

Three-Dimensional Cam Lobe[edit]

This system consists of a cam lobe that varies along its length[16] (similar to a cone shape). One end of the cam lobe has a short duration/reduced lift profile, and the other end has a longer duration/greater lift profile. In between, the lobe provides a smooth transition between these two profiles. By shifting area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered. This is achieved by moving the camshaft axially (sliding it across the engine)a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration. The downside to this arrangement is that the cam and follower profiles must be carefully designed to minimise contact stress (due to the varying profile).

Ferrari is commonly associated with this system,[17][18] however it is unknown whether any production models to date have used this system.

Two Shaft Combined Cam Lobe Profile[edit]

This system is not known to be used in any production engines.

It consists of two (closely spaced) parallel camshafts, with a pivoting follower that spans both camshafts and is acted on by two lobes simultaneously. Each camshaft has a phasing mechanism which allows its angular position relative to the engine’s crankshaft to be adjusted. One lobe controls the opening of a valve and the other controls the closing of the same valve, therefore variable duration is achieved through the spacing of these two events.

The drawbacks to this design include:

  • At long duration settings, one lobe may be starting to reduce its lift as the other is still increasing. This has the effect of lessening the overall lift and possibly causing dynamic problems. One company claims to have solved the uneven rate of opening of the valve problem to some extent thus allowing long duration at full lift.[19][20][21]
  • Size of the system, due to the parallel shafts, the larger followers etc.

Coaxial Two Shaft Combined Cam Lobe Profile[edit]

This system is not known to be used in any production engines.

The operating principle is that the one follower spans the pair of closely spaced lobes. Up to the angular limit of the nose radius the follower “sees” the combined surface of the two lobes as a continuous, smooth surface. When the lobes are exactly aligned the duration is at a minimum (and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.

This is the principle behind what seems to be the very first variable cam suggestion appearing in the USPTO patent files in 1925 (1527456). The “Clemson Camshaft” is of this type.[22]

Helical Camshaft[edit]

Main article: Helical camshaft

Also known as "Combined Two Shaft Coaxial Combined Profile with Helical Movement", this system is not known to be used in any production engines.[23][24][25][26]

It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes the restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations.

The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly.

Camless engines[edit]

Main article: camless engine

Engine designs which do not rely on a camshaft to operate the valves have greater flexibility in achieving variable valve timing and variable valve lift. However, there has not been a production camless engine released for road vehicles as yet.

Types of camless engines include:

  • electro-mechanical (using electromagnets)
  • hydraulic
  • stepper motors
  • pneumatic

See also[edit]

References[edit]

  1. ^ a b Wu, B. (2007). A simulation-based approach for developing optimal calibrations for engines with variable valve actuation. Oil and Gas Science and Technology, 62(4), 539-553.
  2. ^ a b c d e Hong, H. (2004). Review and analysis of variable valve timing strategies - eight ways to approach. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 218(10), 1179-1200.
  3. ^ "Variable Valve Timing - 1886 - Practical Machinist". Practical Machinist. Retrieved 2010-04-04. 
  4. ^ Arthur W., Gardiner; William E. Whedon (25 February 1927). "REPORT No. 272: THE RELATIVE PERFORMANCE OBTAINED WITH SEVERAL METHODS OF CONTROL OF AN OVERCOMPRESSED ENGINE USING GASOLINE" (PDF). Langley Aeronautical Laboratory. 
  5. ^ a b "VALVE-ACTUATING MECHANISM FOR AN INTERNAL COMBUSTION ENGINE". freepatentsonline.com. Retrieved 2011-01-12. 
  6. ^ "Alfa Romeo Spider FAQ" (PDF). alfaspiderfaq.org. Retrieved 2008-11-29. 
  7. ^ http://www.autoweb.com.au/cms/newsarticle.html?&start=90&showall=&id=NIS&doc=nis9810153
  8. ^ http://asia.vtec.net/spfeature/vtecimpl/vtec1.html
  9. ^ "Volvo Penta Variable Valve Timing (VVT)". http://WWW.marineenginedigest.com/. Retrieved 2012-10-27. 
  10. ^ "Geneva 2010: Mitsubishi ASX (Outlander Sport) Debuts in Geneva", autoguide.com
  11. ^ Mitsubishi Motors UK Geneva motor show 2010 presskit
  12. ^ "Autozine VVT Article". Retrieved 17 January 2012. 
  13. ^ "Autospeed Valvetronic Article". Retrieved 17 January 2012. 
  14. ^ "Rover VVC Article". Retrieved 17 January 2012. 
  15. ^ "Autozine VVT Article - See Rover VVC Section.". Retrieved 17 January 2012. 
  16. ^ http://www.howstuffworks.com/camshaft2.htm
  17. ^ Lumley, John L. (1999). Engines - An Introduction. Cambridge UK: Cambridge University Press. pp. 63–64. ISBN 0-521-64277-9. 
  18. ^ "HowStuffWorks - Ferrari 3D cam article". Retrieved 17 January 2012. 
  19. ^ "USPTO 5052350". Retrieved 17 January 2012. 
  20. ^ "USPTO 5642692". Retrieved 17 January 2012. 
  21. ^ "Mechadyne VLD". Retrieved 17 January 2012. 
  22. ^ "USPTO 4771742". Retrieved 17 January 2012. 
  23. ^ "Performance Buildups" Magazine Vol.15 No.1 Pages 30-35 Author: Paul Tuzson
  24. ^ "Two Wheels" Magazine July 2008 pages 74-75 Author Jeremy Bowdler
  25. ^ "Fast Fours" Magazine July 2004 pages 100-108 Author: Paul Tuzson
  26. ^ "USPTO 6832586". Retrieved 17 January 2012. 

External links[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Variable_valve_timing — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
121878 videos foundNext > 

Car Tech 101: Variable valve timing explained

Few innovations under the hood have become as ubiquitous as variable valve timing. Automakers brag about it all the time, but seldom explain it. So Brian Coo...

Variable valve timing through cam phasing explained

Yes i have taken a break from my two stroke videos and dipped into the world of 4 stroke technology and theory. After stripping a brand new Nissan HR16DE eng...

Toyota VVT-iE, Variable Valve Timing

Toyota VVT-iE, Variable Valve Timing Get the book here: https://www.createspace.com/4321251 Describes the operation of the Toyota VVT-iE or variable valve ti...

Variable Valve Timing flv

VTEC - Explained

What is VTEC? How does VTEC work? VTEC, or variable valve timing and lift electronic control is used to increase airflow within an engine cylinder at high rp...

SYSTEMS OF VARIABLE VALVE TIMING

το βίντεο αυτό περιγράφει τα συστήματα μεταβλητού χρονισμού που χρησιμοποιούν οι μεγαλύτερες κατασκευάστριες εταιρείες αυτοκινήτων, όπως TOYOTA, HONDA, AUDI,...

Variable Valve Timing

A visualization that shows how variable valve timing works.

2010 Subaru 2.5 Variable Valve Lift Solenoid and Switch Testing

This video is a great review of Section 2 "Switch Inputs" and Section 3 "Transistor drivers and Output Solenoids" in my eBook. On this system the intake valv...

Variable Valve Timing with intelligence (TOYOTA)

VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing technology developed by Toyota, similar in performance to the BMW's...

1998 Supra na Variable Valve Timing System

1998 Supra Variable Valve Timing System Variable valve timing or VVT-i has been incorporated on the 2JZ-GE to improve throttle response and low end power, wh...

121878 videos foundNext > 

4579 news items

 
Vox
Mon, 28 Jul 2014 09:33:14 -0700

"VVT" = variable valve timing. "Multi-valve" = multi-valve engines. "GDI" = gasoline direct injection. "Turbo" = turbocharged or supercharged technology. "CD" = cylinder deactivation. "CVT" = continuously variable transmission. EPA. In recent years, US ...

Economic Times

ASEAN Automotive News
Tue, 29 Jul 2014 01:28:10 -0700

All Ingenium engines will be turbocharged and feature central high-pressure direct injection, variable valve timing and stop-start systems, and will also support pairing with an electric motor for hybrid drive. The Jaguar XE will get its global ...
 
Digital Trends
Mon, 28 Jul 2014 12:04:16 -0700

The displacement has grown .4 liters, the engine is lighter, and thanks to variable valve timing, larger valves, and electronic throttle bodies, the V10 is much more powerful now, but it's still the same general layout. The Viper was Dodge's most ...

GTspirit

Automobile Magazine
Mon, 28 Jul 2014 11:33:45 -0700

Although it is better known for its four-wheeled off-road vehicles like ATVs, Polaris has been hard at work developing an on-road three-wheeler with a serious appetite for performance. Dubbed the 2015 Polaris Slingshot, this open-air performance ...

ZerCustoms

ZerCustoms
Tue, 29 Jul 2014 05:11:15 -0700

Toyota's 2015 Yaris is powered by a 1.5-liter, 16-valve, four-cylinder DOHC engine equipped with variable valve timing with intelligence (VVT-i) that develops 106 hp at 6,000 rpm and 103 lb.-ft. of torque at 4,200 rpm. Toyota is going to sell the 2015 ...

The Weekly Driver

The Weekly Driver
Sun, 27 Jul 2014 18:07:30 -0700

The 2015 Toyota Yaris has debuted with a new European-themed design, several new additional interior and exterior features and a manufacturer's suggested retail price range from $14,485 to $17,620. The price range extends from three-door L with a ...

3D Car Shows

3D Car Shows
Mon, 28 Jul 2014 09:07:30 -0700

Other technologies incorporated into Suzuki's advanced outboard engines include multipoint fuel injection, variable valve timing, and multi-stage induction, all of which contribute to enhanced performance and efficiency, as well as lower operating costs.

Kansas City Star

Kansas City Star
Fri, 25 Jul 2014 21:52:30 -0700

It has variable valve timing, direct fuel injection and cylinder deactivation. The change from V-8 to four-cylinder operation was barely perceptible on the highway. Full-size, nearly three-ton family haulers like the Yukon XL and Chevrolet Suburban are ...
Loading

Oops, we seem to be having trouble contacting Twitter

Talk About Variable valve timing

You can talk about Variable valve timing with people all over the world in our discussions.

Support Wikipedia

A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia. Please add your support for Wikipedia!