Variable valve timing and Especifismo: Difference between pages

Revision as of 04:02, 30 December 2005

Revision as of 01:47, 4 July 2007

Line 1:

{{anarchism}}

'''Variable valve timing''', or '''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 [[2-stroke power valve system|Power valve system]] to get similar results to VVT.

==Overview==

[[Image:FordtaunusV4front.jpg|thumb|right|250px|[[Poppet valve|Valve]] timing [[gear]]s on a [[Ford Taunus V4 engine]] — the small gear is on the [[crankshaft]], the larger gear is on the [[camshaft]]. Since the camshaft gear is twice the diameter of the crankshaft gear, it runs at half the crankshaft RPM. See [[gear ratio]]. (The small gear left is on the [[balance shaft]])]]

[[Piston engine]]s normally use [[poppet valve]]s for intake and exhaust. These are driven (directly or indirectly) by [[cam]]s on a [[camshaft]]. 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 belt]]s, [[gear]]s or [[chain]]s.

<b>Especifismo</b> is an [[anarchist]] [[praxis]] which originates in [[South America]]. Some [[argue]] that it is a [[natural]] [[evolution]] of the [[ideas]] of [[Platformism]] and [[Sintetismo]]. It [[involves]] "1) The [[need]] for a [[specifically]] anarchist [[organization]] [[built]] around a [[unity]] of ideas and praxis. 2) The [[use]] of the specifically anarchist organization to [[theorize]] and [[develop]] [[strategic]] [[political]] and [[organizing]] work. 3) [[Active]] [[involvement]] and building of [[autonomous]] and [[popular]] [[social movements]], called [['social insertion]].' The [[praxis]] [[implies]] the [[existence]] of an orginization that [[manifests]] an [[open]] and specifically anarchist [[adhesion]]. This would be done in order to [[disseminate]] anarchist ideas in a [[form]] that can be visible to the [[militant]] anarchist [[element]] of [[society]]. Especifismo is a praxis that [[expresses]] itself in the [[national]] and [[regional]] anarchist organizations of the world.

The profile, or position of the cam lobes on the shaft, is optimized for a certain engine [[rpm]], and this tradeoff normally limits low-end [[torque]] or high-end [[Horsepower|power]]. VVT allows the cam profile to change, which results in greater efficiency and power.

{{anarchism-stub}}

At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance.

On the other hand, if the cam keeps the valves open for longer periods of time, like 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.

Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems (like [[Mazda]]'s S-VT) advance or retard the timing of the intake or exhaust valves. Others (like [[Honda]]'s VTEC) switch between two sets of cams at a certain engine rpm. Still others can alter duration and lift continuously.

==History==

The first experimentation with variable valve timing and lift was performed by [[General Motors]]. GM was actually interested in throttling the intake valves in order to reduce emissions. This was done by minimizing the amount of lift at low load to keep the intake velocity higher, thereby atomizing the intake charge. GM encountered problems running at very low lift, and abandoned the project.

The first functional variable valve timing system, including variable lift, was developed at [[Fiat]]. Developed by [[Giovanni Torazza]] in the [[1970s]], the system used hydraulic pressure to vary the fulcrum of the cam followers. The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.

The next big step was taken by [[Honda]] in the late [[1980s]] and 90s, where Honda began by experimenting with variable valve lift. Pleased with the results, engineers took the knowledge and applied it to the [[Honda_Engines_%28B-series%29#B16A|B16A]] engine, fitted to the [[1989]] EF9 [[Honda Civic]]. From there it has been used in a variety of applications, from sport to utility, by many different auto makers.

In the year [[1992]], [[BMW]] introduced [[VANOS]], their version of a variable valve timing system, on the [[BMW]] M50 engine used in the [[3 Series]]. [[VANOS]] significantly enhances emission management, increases output and torque, and offers better idling quality and fuel economy. The latest version of [[VANOS]] is [[double-VANOS]], used in the new M3. [[Double-VANOS]] adds an adjustment of the intake and outlet camshafts.

Variable valve timing was the sole ___domain of overhead cam engines until [[2005]], when [[General Motors]] began offering the [[GM High Value engine#3.5|''LZE'' and ''LZ4'']], [[pushrod engine|pushrod]] V6 engines with VVT. For the [[2006]] [[model year]], [[General Motors]] will introduce the ''[[GM Vortec engine#6200|Vortec 6200]]'', the first mass-produced pushrod engine with variable valve timing.

==VVT Implementations==

* [[BMW]] [[VANOS]] - Varies intake and exhaust timing by moving the [[fulcrum]] of the [[camshaft]]

* [[Ford Motor Company|Ford]] [[Variable Cam Timing]] - Varies valve timing by rotating the camshaft

* [[General Motors|GM]] [[GM Family 1 engine#DCVCP|DCVCP]] (Double Continuous Variable Cam Phasing) - Varies timing with [[hydraulic]] vane type phaser (see also [[GM Family II engine#LE5|Ecotec LE5]]).

* [[Honda]] [[VTEC]] - Varies intake, duration, and lift by using two different sets of cam [[lobe]]s

* [[Honda]] [[i-VTEC]] - Adds cam phasing (timing) to traditional VTEC

* [[Hyundai Motor Group|Hyundai]]/[[Kia Motors|Kia]] [[CVTT]]

* [[Mazda]] [[S-VT]] - Varies timing by rotating the [[camshaft]]

* [[Mitsubishi Motors Corporation|Mitsubishi]] [[MIVEC]] - Varies valve lift

* [[Nissan]] [[N-VCT]] - Varies the rotation of the cam(s) only, does not alter lift or duration of the valves.

* [[Nissan]] [[VVL]] - Varies intake, duration, and lift by using two different sets of cam [[lobe]]s

* [[Porsche]] [[VarioCam]] - Varies intake timing by adjusting tension of a cam chain

* [[Porsche]] [[VarioCam Plus]] - Varies intake timing by adjusting tension of a cam chain as well as valve lift by different cam profiles

* [[MG Rover Group|Rover]] [[VVC]] - Varies timing with an eccentric disc

* [[Suzuki]] [[VVT]]

* [[Subaru]] [[AVCS]] - Varies timing (phase) with [[hydraulic]] pressure

* [[Toyota]] [[VVT-i]] - Varies intake timing by advancing the cam chain

* [[Toyota]] [[VVTL-i]] - Varies timing by advancing the cam chain and switching between two sets of cam lobes

==See also ==

*[[Camless|Camless engine technology]]

*[[Variable Length Intake Manifold]]

[[Category:Engine valves]]

[[Category:Engine technology]]

[[Category:Motorcycle_technology]]