VARIABLE VALVE TIMING

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VARIABLE VALVE
TIMING

• Submitted by
• DIVYA CHAUDHARY
• 09ITMG1128ECE

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VARIABLE VALVE TIMING

• 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 (in
  various combinations) of the intake and/or
  exhaust valves to be changed while the engine is
  in operation. Two-stroke engines use a power
  valve system to get similar results to VVT.

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HISTORY

• First used as a technology through experimental
  aircraft of the 1940s, It wasn't until 1982 when
  the first automotive use of VVT was found inside
  the engine of an Alfa Romeo Spider. Becoming the
  launch pad for the technology, Honda introduced
  the VTEC system in Japan and Europe in 1989. It
  was the 1990 Acura NSX that brought variable
  valve control to the United States, eventually
  sparking a flourishing interest in VVT to the
  mainstream auto market of today

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WORKING OF VVT

• 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 timing to
  change, which results in greater efficiency and
  power, over a wider range of engine RPMs.

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• 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.

2007 Honda Civic The VTEC system equipped on the Honda Civic has been adding excitement to 4- cylinder engines.
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ADVANTAGES OF VVT

• Variable valve timing is not a horsepower booster
  like a turbocharger. Instead, VVT is designed to
  make use of the engine's existing power in the
  most effective way possible. Assisting right from
  engine start up, timing would configure the
  valves for a shorter opening period aiding the
  initial warming up of the engine. When the engine
  reaches higher performance, valve staging is
  adjusted for a longer opening time allowing more
  air/fuel to flow into the combustion chambers.
  Ultimately, engine efficiency is greatly improved
  which also means less fuel is needed to run a VVT
  engine. Vehicles including VVT engines would
  expect fuel saving benefits of 5-10.

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ADVANTAGES (contd.)

• VVT systems are also an effective component for
  emission control. Since the 1970s, automobile
  engines would reuse exhaust gases through the
  next combustion cycle to reduce NOx, an major
  smog-creating pollutants. For this, an EGR
  (Exhaust Gas Recirculation) valve was developed.
  VVT technology uses a process called internal EGR
  where a small amount of exhaust gas leaks back
  into the combustion chamber.

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Well into the future, valve timing may be
accomplished more easily as engine's camshaft may
be completely discarded and replaced with
electrical solenoids controlling each valve.
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THANK YOU