Variable valve timing
The cams open (lift) the valves for a certain amount of time (duration) during each intake and exhaust cycle.
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.
[2][3] The simplest form of VVT is cam-phasing, whereby the phase angle of the 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 solely with a cam-phasing system.
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%.
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.
[3] Studies have shown early intake valve closing reduces pumping losses by 40%, and increases fuel economy by 7%.
A possible downside to early intake valve closing is that it significantly lowers the temperature of the combustion chamber, which can increase hydrocarbon emissions.
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.
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.
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.
[5] An early experimental 200 hp Clerget V-8 from the 1910s used a sliding camshaft to change the valve timing[citation needed].
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.
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).
[13] While the earlier 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.
Variable valve timing has been applied to motorcycle engines but was considered a non-useful "technological showpiece" as late as 2004 due to the system's weight penalty.
Volvo Penta's VVT marine engine uses a cam phaser, controlled by the ECM, which continuously varies advancement or retardation of the camshaft timing.
[15] In 2007, Caterpillar developed the C13 and C15 Acert engines which used VVT technology to reduce NOx emissions, to avoid the use of EGR after 2002 EPA requirements.
[16][17] In 2010, Mitsubishi developed and started mass production of its 4N13 1.8 L DOHC I4, the world's first passenger car diesel engine that features a variable valve timing system.
These designs use an oscillating or rocking motion in a part cam lobe,[clarification needed] which acts on a follower.
The BMW (valvetronic),[20] Nissan (VVEL), and Toyota (valvematic) oscillating cam systems act on the intake valves only.
[citation needed] This system consists of a cam lobe that varies along its length[22] (similar to a cone shape).
This is achieved by moving the camshaft axially (sliding it across the engine) so a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration.
In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.
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.
During the downward movement of the cam, at a particular instant, the return passage opens and the oil pressure gets released to the engine sump.
