Variable Valve Timing explained
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 Power valve system to get
similar results to VVT.
Overview
Valve timing gears 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
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 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 power. VVT allows
the cam profile to change, which results in greater efficiency and
power.
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
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.
One
thing that separates the BMW variable timing from what Honda developed
is that VANOS provides stepless adjustments to the valve timing
throughout a rpm regime to provide optimum timing at all engine speeds
within that regime. Honda's VTEC triggers a single step change at in
the timing when the engine speed reaches a preset rpm (this rpm occurs
near high rpm ranges).
Variable valve timing was the sole domain of
overhead cam engines until 2005, when General Motors began offering the
LZE and LZ4, pushrod V6 engines with VVT. For the 2006 model year,
General Motors will introduce the Vortec 6200, the first mass-produced
pushrod engine with variable valve timing.
VVT Implementations
• BMW VANOS - Varies intake and exhaust timing by rotating the camshaft in relation to the gear.
• Ford Variable Cam Timing - Varies valve timing by rotating the camshaft
• GM DCVCP (Double Continuous Variable Cam Phasing) - Varies timing with hydraulic vane type phaser (see also Ecotec LE5).
• Honda VTEC - Varies intake, duration, and lift by using two different sets of cam lobes
• Honda i-VTEC - Adds cam phasing (timing) to traditional VTEC
• Hyundai/Kia CVTT
• Mazda S-VT - Varies timing by rotating the camshaft
• 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 lobes
• 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
• 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
