Variable Valve Timing System 气门调节的认识，图片+video

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Variable valve timing
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In internal combustion engines, variable valve timing, often abbreviated to VVT, is a generic term for an automobile piston engine technology. VVT allows the lift, 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.

Contents[hide] 1 Overview 2 History 2.1 Steam engines 2.2 Aircraft 2.3 Automotive use 3 VVT Implementations 4 See also 5 References 6 External links

[edit] Overview
The i-VTEC system found in the Honda K20Z3


Piston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) by cams 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 belts, gears or chains.
The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine revolutions per minute (RPM), and this tradeoff normally limits low-end torque, or high-end power. VVT allows the cam profile to change, which results in greater efficiency and power, over a wider rev-range.
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, 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. For this reason, pure racing engines cannot idle at the low speeds (around 800rpm) expected of a road car, and idle speeds of 2000 rpm are not unusual.
Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Furthermore Honda's I-VTEC can alter intake valve timing continuously.

[ 本帖最后由 3-SGTE 于 2009-8-10 11:52 PM 编辑 ]

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History

[edit] Steam engines
The first variable valve timing systems came into existence in the nineteenth century on steam engines. 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, 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. [1].


[edit] Aircraft
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.[1] 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.


[edit] Automotive use
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). The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.

n September 1975, General Motors (GM) patented a system intended to vary valve lift. GM was 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.

Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330). The 1980 Alfa Romeo Spider 2.0 L had a mechanical VVT system in SPICA fuel injected cars sold in the United States. Later this was also used in the 1983 Alfetta 2.0 Quadrifoglio Oro models as well as other cars. The system was engineered by Ing Giampaolo Garcea in the 1970s.[2]

Honda's REV motorcycle engine employed on the Japanese market-only Honda CBR400F in 1983 provided a technology base for VTEC.

In 1987,[citation needed] Nissan developed their own form of VVT with the VG30DE(TT) engine for their Mid-4 Concept. Nissan chose to focus their NVCS (Nissan Valve-Timing Control System) mainly on torque production at low to medium engine speeds, because, the vast majority of the time, automobile engines will not be operated at extremely high speeds. The NVCS system can produce a smooth idle and high amounts of torque at low to medium engine speeds. The VG30DE engine was first used in the 300ZX (Z31) 300ZR model in 1987. It was the first production car to use electronically controlled VVT technology.

The next step was taken in 1989 by Honda with the VTEC system. Honda had started production of a system that gives an engine the ability to operate on two completely different cam profiles, eliminating a major compromise in engine design. One profile designed to operate the valves at low engine speeds provides good road manners, low fuel consumption and low emissions output. The second is a high lift, long duration profile and comes into operation at high engine speeds to provide an increase in power output. The VTEC system was also further developed to provide other functions in engines designed primarily for low fuel consumption. The first VTEC engine Honda produced was the B16A which was installed in the Integra, CRX, and Civic hatchback available in Japan and Europe. In 1991 the Acura NSX powered by the C30A became the first VTEC equipped vehicle available in the US. VTEC can be considered the first "cam switching" system and is also one of only a few currently in production.

In 1991, Clemson University researchers patented the Clemson Camshaft which was designed to provide continuously variable valve timing independently for both the intake and exhaust valves on a single camshaft assembly. This ability makes it suitable for both pushrod and overhead cam engine applications.[3]

In 1992 BMW introduced the VANOS system. Like the Nissan NVCS system it could provide timing variation for the intake cam in steps (or phases), the VANOS system differed in that it could provide one additional step for a total of three. Then in 1996 the Double Vanos system was introduced which significantly enhances emission management, increases output and torque, and offers better idling quality and fuel economy. Double Vanos was the first system which could provide electronically controlled, continuous timing variation for both the intake and exhaust valves. In 2001 BMW introduced the Valvetronic system. The Valvetronic system is unique in that it can continuously vary intake valve lift, in addition to timing for both the intake and exhaust valves. The precise control the system has over the intake valves allows for the intake charge to be controlled entirely by the intake valves, eliminating the need for a throttle valve and greatly reducing pumping loss. The reduction of pumping loss accounts for more than a 10% increase in power output and fuel economy.

Ford began using Variable Cam Timing in 1998 for Ford Sigma engine. Ford became the first manufacturer to use variable valve timing in a pickup-truck, with the top-selling Ford F-series in the 2004 model year. The engine used was the 5.4L 3-valve Triton.

In 2005 General Motors offered the first Variable Valve timing system for pushrod V6 engines, LZE and LZ4.

In 2007 DaimlerChrysler became the first manufacturer to produce a cam-in-block engine with independent control of exhaust cam timing relative to the intake. The 2008 Dodge Viper uses Mechadyne's concentric camshaft assembly to help boost power output to 600 bhp (450 kW).

In 2009 Fiat Powertrain Technologies introduced the Multiair system in Geneva Motor Show. The Multiair is a hydraulically-actuated variable valve timing system, which gives full control over valve lift and timing. The new technology will be available in Alfa Romeo MiTo starting from September 2009.[4]

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[edit] VVT Implementations
Aftermarket Modifications - Conventional hydraulic tappet can be engineered to rapidly bleed-down for variable reduction of valve opening and duration.
Alfa Romeo

Twin Cam - some versions are equipped with Variable Valve Timing technology.
Twin Spark - is equipped with Variable Valve Timing technology.
JTS - is equipped with Variable Valve Timing technology, both intake and exhaust.
Multiair continuously varies the timing of the inlet valve by changing oil pressure.
BMW

Valvetronic - Provides continuously variable lift for the intake valves; used in conjunction with Double VANOS.
VANOS - Varies intake timing by rotating the camshaft in relation to the gear.
Double VANOS - Continuously varies the timing of the intake and exhaust valves.
Fiat

"StarJet" FIRE-based engine.

Ford

VCT Variable Cam Timing - Varies valve timing by rotating the camshaft.
Ti-VCT Twin Independent Variable Camshaft with two fully variable camshafts used in Ford Sigma engine and Ford Duratec engine.
Chrysler - Varies valve timing through the use of concentric camshafts developed by Mechadyne enabling dual-independent inlet/exhaust valve adjustment on the 2008 Dodge Viper.
General Motors Corporation (GM)
VVT - Varies valve timing continuously throughout the RPM range for both intake and exhaust for improved performance in both overhead valve and overhead cam engine applications.(See also Northstar System).
DCVCP (Double Continuous Variable Cam Phasing) - Varies intake and exhaust camshaft timing continuously with hydraulic vane type phaser (see also Ecotec LE5).
Alloytec - Continuously variable camshaft phasing for inlet cams. Continuously variable camshaft phasing for inlet cams and exhaust cams (High Output Alloytec).

Honda

VTEC - Varies duration, timing and lift by switching between two different sets of cam lobes.
VTEC-E - This system is designed solely for the purpose of improving fuel economy. A variation of the VTEC mechanism is used to create an offset of lift between the two intake valves, one valve opening only slightly to prevent accumulation of fuel in the intake port. The asymmetrical opening of the intake valves creates a powerful swirl in the combustion chamber and allows for a very lean intake charge to be used under certain conditions. Under normal operation the two intake valve rocker arms are locked together and both valves follow the normal lift cam profile.
i-VTEC - In high-output DOHC 4 cylinder engines the i-VTEC system adds continuous intake cam phasing (timing) to traditional VTEC. In economy oriented SOHC and DOHC 4 cylinder engines the i-VTEC system increases engine efficiency by delaying the closure of the intake valves under certain conditions and by using an electronically controlled throttle valve to reduce pumping loss. In SOHC V6 engines the i-VTEC system is used to provide Variable Cylinder Management which deactivates one bank of 3 cylinders during low demand operation.
Advanced VTEC - This is the latest Honda VVT system and is the most unique of all the VTEC systems. Rather than switching between cam lobes the Advanced VTEC system uses intermediate rocker arms with a variable fulcrum to continuously vary intake valve timing, duration and lift.
Hyundai

MPI CVVT - Varies power, torque, exhaust system, and engine response.
Kawasaki
- Varies position of cam by changing oil pressure thereby advancing and retarding the valve timing, 2008 Concours 14.
Lexus

VVT-iE - Continuously varies the intake camshaft timing using an electric actuator.
Mazda

S-VT - Continually varies intake timing and crank angle using an oil control valve actuated by the ECU to control oil pressure.
Mitsubishi

MIVEC - Varies valve timing, duration and lift by switching between two different sets of cam lobes. The 4B1 engine series uses a different variant of MIVEC which varies timing (phase) of both intake and exhaust camshafts continuously.

Nissan

N-VCT - Varies the rotation of the cam(s) only, does not alter lift or duration of the valves.
VVL - Varies timing, duration, and lift of the intake and exhaust valves by using two different sets of cam lobes.
CVTC introduced with the HR15DE, HR16DE, MR18DE and MR20DE new engines in September 2004 on the Nissan Tiida and North American version named Nissan Versa (in 2007); and finally the Nissan Sentra (in 2007).
VVEL introduced with the VQ37VHR Nissan VQ engine engine in 2007 on the Infiniti G37.

Porsche

VarioCam - Varies intake timing by adjusting tension of a cam chain.
VarioCam Plus - Varies intake valve timing by rotating the cam in relation to the cam sprocket as well as duration, timing and lift of the intake and exhaust valves by switching between two different sets of cam lobes.

Proton Campro CPS

- Varies intake valve timing and lift by switching between 2 sets of cam lobes without using rocker arms as in most variable valve timing systems. Debuted in the 2008 Proton Gen-2 CPS[5][6] and the 2008 Proton Waja CPS.
PSA Peugeot Citroën
CVVT - Continuous variable valve timing.

Renault
Clio 182, Clio Cup and Clio V6 Mk2 VVT - variable valve timing.

Rover
VVC - Varies timing with an eccentric disc.

Suzuki
- VVT - Suzuki M engine

Subaru
AVCS
- Varies timing (phase) with hydraulic pressure, used on turbocharged and six-cylinder Subaru engines.
AVLS - Varies duration, timing and lift by switching between two different sets of cam lobes (similar to Honda VTEC). Used by non-turbocharged Subaru engines.

Toyota

VVT - Toyota 4A-GE 20-Valve engine introduced VVT in the 1992 Corolla GT-versions.
VVT-i - Continuously varies the timing of the intake camshaft, or both the intake and exhaust camshafts (depending on application).
VVTL-i - Continuously varies the timing of the intake valves. Varies duration, timing and lift of the intake and exhaust valves by switching between two different sets of cam lobes.

Volkswagen

Group - VVT introduced with later revisions of the 1.8t engine, and the 30v 2.8l V6. Similar to VarioCam, the intake timing intentionally runs advanced and a retard point is calculated by the ECU. A hydraulic tensioner retards the intake timing. Most modern VW Group petrol engines now include VVT on either the inlet cam, or both inlet and exhaust cams, as in their V6, V8 and V10 engines.

Volvo

- CVVT

Yamaha

- VCT (Variable Cam Timing) Varies position of cam thereby advancing and retarding the valve timing.

Proton

- VVT introduced in the Waja 1.8's F4P renault engine (toyota supplies the VVT to renault)

[ 本帖最后由 3-SGTE 于 2009-7-6 11:05 PM 编辑 ]

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An Imagery of the i-VTEC Implementation

EGR Effect: By swirling back some of the exhaust air back into the combustion chamber, combustion temperature is lowered, and NOx output is reduced.

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[ 本帖最后由 3-SGTE 于 2009-7-7 10:50 PM 编辑 ]

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Information

Valvetronic engines use a combination of hardware and software to eliminate the need for a conventional throttle mechanism.

Valvetronic varies the timing and the lift of the intake valves. The Valvetronic system has a conventional intake cam, but it also uses a secondary eccentric shaft with a series of levers and roller followers, activated by a stepper motor. Based on signals formerly taken mechanically from the accelerator pedal, the stepper motor changes the phase of the eccentric cam, modifying the action of the intake valves.

Valvetronic was introduced on the all-alloy 1.8-liter, 4-cylinder engine for the E46 316ti Compact, and it will subsequently be applied to most eight and 12-cylinder engines within a few years. The new E65 7 Series has Valvetronic engines.
The Valvetronic engine replaces the function of the throttle butterfly by using an infinitely variable intake valve lift. The Valvetronic engine does not require a timing belt or chain. Valvetronic has its own computer housed in a separate unit away from the engine management system, networked with the digital engine management system incorporating a 40-megahertz, 32-bit computer.

A unique set of images from BMW showing how Valvetronic was designed. Click for a larger image

Valvetronic reduces maintenance costs, improves cold start behavior, lowers exhaust emissions, and provides a  smoother running engine. Valvetronic does not need specific fuel grades or fuel qualities because of its fine atomization of fuel.
The entire Valvetronic system is pre-assembled and inserted as a module into its position in the cylinder head. Valvetronic engines are built at BMW's brand new engine plant at Hams Hall near Coventry, England.
Because Valvetronic allows the engine to breathe more freely, fuel consumption is reduced by 10%. The fuel savings are greatest at lower engine revs. Valvetronic is an important element in BMW's aim of meeting the 2008 carbon dioxide fleet requirements of 140 gm/km.
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Here's how it works:

Fuel injection systems monitor the volume of air passing through the throttle butterfly and determine the corresponding amount of fuel required by the engine. The larger the throttle butterfly opening, the more air enters the combustion chamber.  

At light throttle, the throttle butterfly partially or even nearly closes. The pistons are still running, taking air from the partially closed intake manifold. The intake manifold between the throttle and the combustion chamber has a partial vacuum, resisting the [词语过滤-#17]ing and pumping action of the pistons, wasting energy. Automotive engineers refer to this phenomenon as "pumping loss". The slower the engine runs, the more the throttle butterfly closes, and the more energy is lost.

Valvetronic minimizes pumping loss by reducing valve lift and the amount of air entering the combustion chambers.

Compared with conventional twin-cam engines with finger followers, Valvetronic employs an additional eccentric shaft, an electric motor and several intermediate rocker arms, which in turn activates the opening and closing of valves. If the rocker arms push deeper, the intake valves will have a higher lift, and vice-versa. Thus, Valvetronic has the ability to get deep, long ventilation (large valve lift) and flat, short ventilation (short valve lift), depending on the demands placed on the engine.

Operating Parameters:

• Valve lift is variable between 0 and 9.7 mm.  
• Adjustment of the worm gear from one extreme to the other takes 300 milliseconds.
• Combined with double-Vanos valve timing technology, the camshaft angle relative to the crankshaft can be adjusted by up to 60?
• The intermediate arm is finished to a tolerance of 0.008 mm.
• The cams controlling the eccentric shaft are machined to tolerances of a few hundredths of a millimeter.

Additional Benefits:

• In Valvetronic engines coolant flows across the head, resulting in a temperature reduction of 60%.
• The water pump size is cut in half, reducing power consumption by 60%.
• The power steering fluid is warmed quickly, reducing the power used by the hydraulic pump.
• Mounting the water and power pump on the same shaft and a heat exchanger between coolant and engine oil reduces oil temperature by 30%.

The efficiency of Valvetronic engines drop rapidly at over 6,000 rpm since stronger valve springs are required. The stronger springs create higher friction losses. Don't expect to see Valvetronic in the "M" series engines any time soon.
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[ 本帖最后由 3-SGTE 于 2009-7-6 11:50 PM 编辑 ]

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What is MIVEC? In the early ‘90s, Mitsubishi Japan introduced a valve control system to combat Honda’s VTEC design. This system is labelled MIVEC (Mitsubishi Innovative Valve and Lift Electronic Control System). In its simplest form, MIVEC switches between two different intake and exhaust cam lobes depending on engine rpm. At low rpm, the valves receive relatively modest lift and opening duration. At high rpm, the secondary cam lobe is engaged and the valves receive greater lift and duration (which results in increased overlap). The purpose of the secondary cam lobe is to deliver greater engine breathing and the ability to maintain torque at very high rpm (which means greater power). The MIVEC system achieves its high power without the driveabililty, fuel consumption and emissions trade-offs typical in a conventional engine. In addition to the base MIVEC principle, Mitsubishi also released a sophisticated MIVEC-MD (Modular Displacement) system in the ‘90s. The MD system is an early form of cylinder deactivation which involves closing the intake and exhaust valves at light engine load. This means the driver must open the throttle further to maintain power and, as a result, pumping losses are reduced and active cylinder pressures are increased. This results in greater efficiency and fuel economy. Depending on conditions, the MIVEC-MD system can reduce fuel consumption by 10 – 20 percent.

[ 本帖最后由 3-SGTE 于 2009-7-11 10:23 AM 编辑 ]

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HOME > Corporate Info > About Us > Technology > Driving Fun > MIVEC

MIVECRemarkable MIVEC Engine makes power performance and environmental protection compatible MIVEC = Mitsubishi Innovative Valve timing Electronic Control system The dual-intake valve camshaft enables changing between low-speed and high-speed modes, resulting in easy operation from low to high rpms, improving the driving experience when starting from a stop light, merging onto the freeway, or accelerating to overtake another car. In the pursuit of pure driving enjoyment, potentially incompatible goals like fuel economy, environment-friendliness, and clean driving have all been achieved. [Low-speed Mode] The difference in the dual-intake valve lift (low lift and medium lift) and enhanced in-cylinder streaming further stabilize combustion without compromising fuel economy, emissions, and torque. [High-speed Mode] Extending the injection valve opening time and expanding the valve lift range increases intake air mass and achieves output close to best in class. The Grandis is equipped with the 2.4L MIVEC and the Colt is equipped with with the 1.3 and 1.5L MIVEC. 4G69 MIVEC (2.4L SOHC 16-valve, 4-cylinder) (Grandis) MIVEC Switching Mechanism Grandis MIVEC Engine Performance Curve (4G69)

[ 本帖最后由 3-SGTE 于 2009-7-11 10:06 AM 编辑 ]

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Nissan VVL engines
Nissan SR20VE engineDisplacement2.0LPower:187 hp (139 kW)Torque:145 lb·ft (197 N·m)Production:1997-2001Vehicle:Nissan Primera
Nissan Bluebird
Nissan Wingroad
Nissan SR16VE engineDisplacement1.6LPower:173 hp (129 kW)Torque:119 lb·ft (161 N·m)Production:1997-2001Vehicle:Nissan Sunny VZ-R
Nissan Lucino VZ-R
Nissan Pulsar VZ-R
Nissan SR20VE '20V' engineDisplacement2.0LPower:204 hp (152 kW)Torque:152 lb·ft (206 N·m)Production:2001-PresentVehicle:Nissan Primera 20V
Nissan SR20VET engineDisplacement2.0LPower:276 hp (206 kW)Torque:228 lb·ft (309 N·m)Production:2001-presentVehicle:Nissan X-Trail GTNissan Ecology Oriented Variable Valve Lift and Timing (commonly known as VVL) is an automobile variable valve timing technology developed by Nissan. VVL varies the timing, duration, and lift of valves by using hydraulic pressure switch between two different sets of camshaft lobes. It functions similarly to Honda's VTEC system.
The SR20VE is the most common engine with NEO VVL. There have been two main versions of this engine. The first version made 187 hp (139 kW) and 145 lb·ft (197 N·m) torque. This engine was used by Nissan from 1997 to 2001. It is found in the Nissan Primera, Nissan Bluebird, and the Nissan Wingroad.
The second variant of the SR20VE is found only in the 2001 and up, P12 Nissan Primera. This version of the SR20VE makes 204 hp (152 kW), and 152 lb/ft torque. This engine is commonly known as the SR20VE '20V'. Although, in automotive terms, '20V' would normally be interpreted as having twenty valves, this is incorrect. The name '20V' is the name of the trim level of the Nissan Primera that it is found in. It is also a shortened version of the name SR20VE. This engine has 16 valves like the rest of the SR20 engines. This newer '20V' engine, is the only SR20 engine to get a restyled valve cover. It also has an upgraded intake manifold, which has longer runners and a larger 70 mm (2.8 in) throttle body (earlier SR20VE has 60 mm).
Another version of the VVL SR engines, is the 1.6L SR16VE. The engine block for the SR16VE is the same as the SR20VE, it also has the same cylinder bore. The crankshaft has a shorter stroke, which lowers the displacement, but allows the engine to safely rev to higher RPM. Although this engine has 1.6L of displacement, it has more aggressive camshaft specifications. It manages to make 173 hp (129 kW). The camshafts from this engine are considered to be an upgrade for SR20VE owners.
From 1997 to 1998, Nissan produced 500 limited edition SR16VE N1 engines. These engines made 197 hp (147 kW). They had further upgraded camshafts, upgraded intake manifold untilising 8 injectors and a larger 70 mm (2.8 in) throttle body. These engines were found in the limited edition Nissan Pulsar VZ-R N1. They were only sold in Japan.
The most powerful VVL engine so far, is the SR20VET. The SR20VET is a turbocharged '20V' SR20VE. It uses a Garrett GT2560R turbo charger, and makes 280 PS (206 kW; 276 hp) . Nissan's technical information about this engine states that it is 9:1 compression ratio, but it really adds up to 8.8:1.[citation needed] Compaired to the SR20DET (used in the Nissan Silvia, and Bluebird), the SR20VET (aside from having VVL technology) has improved airflow in the cylinder head, higher compression, and also improved coolant passages.
One difference from Honda's VTEC system, is that NEO VVL engages the change of intake and exhaust cams independently for a flatter, more consistent power band. On the SR20VE the intake camshaft is switched at 5000 rpm, and the exhaust at 6500 rpm. However this trait was not included on the newer '20V' version, as both camshafts

[ 本帖最后由 3-SGTE 于 2009-7-12 12:44 AM 编辑 ]

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SUBARU (AVCS)

A balance between increasing engine performance, improving fuel economy, reducing emissions and stabilizing idle is difficult to achieve. In its 3.0-liter 6-cylinder and turbocharged 2.5-liter 4-cylinder engines, Subaru strikes that balance using the Active Valve Control System (AVCS). What AVCS Does and Its Effects Pulling the connecting rod, the crankshaft pulls the piston toward the center of the engine, drawing in air and fuel from the intake system. The air and fuel enters the cylinder through the intake ports opened by the intake valves. This is like a person taking a breath – inhaling. AVCS adjusts exactly when intake valves begin to open. With both valves closed, the turning crankshaft forces up the piston, compressing the air/fuel mixture. The spark plug ignites this compressed mixture, causing an explosion that forces down the piston and connecting rod, which, in turn, rotates the crankshaft. These explosions within the cylinders provide the engine’s power. The crankshaft forces the piston to the top of the cylinder again, this time pushing leftover gases out of the combustion chamber past the opened exhaust valves and into the exhaust system. This is similar to a person exhaling. The camshaft is a very precise engine component, with lobes that open and close the intake and exhaust valves with the critical timing required for the 4-stroke cycle. AVCS changes the timing of the intake valves by adjusting the positions of the camshafts based on inputs from various sensors in the powertrain. The system varies when the camshaft lobes open and close the intake valves during the 4-stroke cycle. The effects of variable valve control include greater power through a wider range of engine speeds, improved fuel economy and reduced emissions. But to better understand how it works, let’s start with engine basics – the 4-stroke engine cycle. The 4-Stroke Cycle Most of today’s automotive gasoline engines function via a 4-stroke cycle. Engine components continuously cycle through four strokes, named for their functions of intake, compression, power and exhaust. AVCS affects the roles of the camshafts in this process. Actuation is mechanical, by direct contact or through a combination of lifters, tappets and/or pushrods, depending on engine design. How the camshafts are designed essentially gives engines their personalities. Camshafts in Subaru engines are belt-driven (4-cylinder) or chain-driven (6-cylinder) by the crankshaft. Intake valves open to let the air into the combustion chamber, and exhaust valves open to let out the exhaust gases. AVCS operation affects the intake valve timing or at exactly what point each valve opens and closes. Overall, intake- and exhaust-valve operation during the 4-stroke cycle follows this pattern: However, there are nuances in operation, and that’s where AVCS plays a part. In the 4-stroke sequence, the exhaust cycle immediately precedes the intake cycle. Overlapping the timing of the closing of the exhaust valves and the opening of the intake valves can help the engine perform better under heavy loads, but not under light loads. AVCS continuously varies this overlap through an infinite number of positions. Overlap ranges between a slight overlap (“retard” position) through as much as 35 degrees of the crankshaft rotation (“advance” position). AVCS Components Variable valve timing is controlled through a hydraulic system that takes instruction from a system of electronic controls. Engine management computer: Electronic control is by the engine management computer, which uses input from a number of engine sensors to determine the ideal position for the camshafts. The sensors include those that measure airflow into the intake system, coolant temperature, throttle position and camshaft position. Oil control valve: The control unit then actuates changes through an oil control valve positioned at each intake camshaft sprocket. The oil control valve uses oil pressure from the engine to advance and retard the intake camshafts via the AVCS actuator. Actuator: Mounted in the chain- or belt-driven drive sprocket, the actuator is fitted directly to the camshaft. Chambers in the actuator allow oil pressure to advance or retard it within the timing-belt sprocket. The oil fills the chambers and pushes against three lobes to turn the actuator and the camshaft on its axis. AVCS – Bringing It All Together At idle: The intake valves open just after the piston reaches the top of the cylinder (called “top dead center” or TDC; BDC refers to “bottom dead center”) at the end of the exhaust stroke, as the piston begins the intake stroke. The exhaust stroke creates negative pressure within the chamber, and intake air enters the cylinder with positive pressure “to fill the void.” There is very little or no overlap between the exhaust and the intake strokes. Retarding valve timing improves the smoothness of engine operation at idle, which tends to be a problem area in high-performance engines without variable valve control. (If you remember the muscle cars of the 1960s and 1970s, you may recall how roughly they idled.)

[ 本帖最后由 3-SGTE 于 2009-7-12 12:57 AM 编辑 ]