VTEC (Variable Valve Timing and Lift Electronic Control)

Is a valvetrain system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. It was invented by Honda R&D engineer Ikuo Kajitani.[1] It can be said that VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VTEC.[2] VTEC was the first system of its kind, though other variable valve timing and lift control systems have been produced by other manufacturers (MIVEC from Mitsubishi, VVTL-i from Toyota, VarioCam Plus from Porsche, VANOS from BMW, VVL from Nissan, etc).

DOHC VTEC

Honda's VTEC system is a simple method of endowing the engine with multiple camshaft profiles optimized for low and high RPM operations. Instead of one cam lobe actuating each valve, there are two: one optimized for low-RPM stability & fuel efficiency; the other designed to maximize high-RPM power output. Switching between the two cam lobes is controlled by the ECU which takes account of engine oil pressure, engine temperature, vehicle speed, engine speed and throttle position. Using these inputs, the ECU is programmed to switch from the low lift to the high lift cam lobes when the conditions mean that engine output will be improved. At the switch point a solenoid is actuated which allows oil pressure from a spool valve to operate a locking pin which binds the high RPM cam follower to the low rpm ones. From this point on, the poppet valve opens and closes according to the high-lift profile, which opens the valve further and for a longer time. The switch-over point is variable, between a minimum and maximum point, and is determined by engine load. The switch back from high to low rpm cams is set to occur at a lower engine speed than the up-switch to avoid a situation in which the engine is asked to operate continuously at or around the switch-over point. Introduced as a DOHC system in the 1989 Honda Integra and Civic CRX SiR models sold in Japan and Europe, which used a 160 bhp (119 kW) variant of the B16A engine. The US market saw the first VTEC system with the introduction of the 1990 Acura NSX, which used a DOHC VTEC V6 with 270 hp (200 kW). DOHC VTEC engines soon appeared in other vehicles, such as the 1992 Acura Integra GS-R (B17A 1.7 liter engine). And later in the 1993 Honda Prelude VTEC (H22 2.2 liter engine with 195hp) and Honda Del Sol VTEC (B16 1.6 liter engine). Honda has also continued to develop other varieties and today offers several varieties of VTEC: iVTEC, iVTEC Hybrid and VTEC in the NSX and some Japanese domestic market cars.

SOHC VTEC

As popularity and marketing value of the VTEC system grew, Honda applied the system to SOHC (Single Over Head Cam) engines, which shares a common camshaft for both intake and exhaust valves. The trade-off was that Honda's SOHC engines only benefitted from the VTEC mechanism on the intake valves. This is because VTEC requires a third center rocker arm and cam lobe (for each intake and exhaust side), and in the SOHC engine, the spark plugs are situated between the two exhaust rocker arms, leaving no room for the VTEC rocker arm. Additionally, the center lobe on the camshaft can only be utilized by either the intake or the exhaust, limiting the VTEC feature to one side.

However, beginning with the J37A4 3.7L SOHC V6 engine introduced on all 2009 Acura TL SH-AWD models, SOHC VTEC was incorporated for use with intake and exhaust valves. The intake and exhaust rocker shafts contain primary and secondary intake and exhaust rocker arms, respectively. The primary rocker arm contains the VTEC switching piston, while the secondary rocker arm contains the return spring. The term "primary" does not refer to which rocker arm forces the valve down during low-RPM engine operation. Rather, it refers to the rocker arm which contains the VTEC switching piston and receives oil from the rocker shaft.

The primary exhaust rocker arm contacts a low-profile camshaft lobe during low-RPM engine operation. Once VTEC engagement occurs, the oil pressure flowing from the exhaust rocker shaft into the primary exhaust rocker arm forces the VTEC switching piston into the secondary exhaust rocker arm, thus locking both exhaust rocker arms together. The high-profile camshaft lobe which normally contacts the secondary exhaust rocker arm alone during low-RPM engine operation is able to move both exhaust rocker arms together which are locked as a unit.

The secondary intake rocker arm contacts a low-profile camshaft lobe during low-RPM engine operation. Once VTEC engagement occurs, the oil pressure flowing from the intake rocker shaft into the primary intake rocker arm forces the VTEC switching piston into the secondary exhaust rocker arm, thus locking both intake rocker arms together. The high-profile camshaft lobe which normally contacts the primary intake rocker alone during low-RPM engine operation is able to move both intake rocker arms together which are locked as a unit.

The problem which plagued previous SOHC VTEC systems from incorporating VTEC for both the intake and exhaust valves has been resolved on the J37A4 by a novel design of the intake rocker arm. Each exhaust valve on the J37A4 corresponds to one primary and one secondary exhaust rocker arm. Therefore, there are a total of twelve primary exhaust rocker arms and twelve secondary exhaust rocker arms. However, each secondary intake rocker arm is shaped similar to a "Y" which allows it to contact two intake valves at once. One primary intake rocker arm corresponds to each secondary intake rocker arm. As a result of this design, there are only six primary intake rocker arms and six secondary intake rocker arms.

SOHC VTEC-E

Honda's next version of VTEC, VTEC-E, was used in a slightly different way; instead of optimising performance at high RPM, it was used to increase efficiency at low RPM. At low RPM, one of the two intake valves is only allowed to open a very small amount, increasing the fuel/air atomization in the cylinder and thus allowing a leaner mixture to be used. As the engine's speed increases, both valves are needed to supply sufficient mixture. A sliding pin, which is pressured by oil, as in the regular VTEC, is used to connect both valves together and allows the full opening of the second valve.

DOHC VTEC-DI

Honda also had a demonstration engine back in end 1999 where a 1.4 liter DOHC Honda engine was equipped with a VTEC-DI system. This was Honda’s first demonstration of direct injection to the public. The engine was installed in a Honda Logo (the predecessor to the Honda Fit/Jazz) and made power and torque outputs of 107 hp (80 kW) at 6200 rpm and 133 Nm at 5000 rpm

i-VTEC

i-VTEC (intelligent-VTEC)[4] introduced continuously variable camshaft phasing on the intake cam of DOHC VTEC engines. The technology first appeared on Honda's K-series four cylinder engine family in 2001 (2002 in the U.S.). In the United States, Honda first debut the technology on the 2002 Honda Civic Si EP3 with the economy version.

Valve lift and duration are still limited to distinct low- and high-RPM profiles, but the intake camshaft is now capable of advancing between 25 and 50 degrees (depending upon engine configuratio2n) during operation. Phase changes are implemented by a computer controlled, oil driven adjustable cam gear. Phasing is determined by a combination of engine load and rpm, ranging from fully retarded at idle to somewhat intelligent at full throttle and low rpm. The effect is further optimization of torque output, especially at low and midrange RPM.

The K-Series motors have two different types of i-VTEC systems implemented. The first is for the performance motors like in the RSX Type S or the TSX and the other is for economy motors found in the CR-V or Accord. The performance i-VTEC system is basically the same as the DOHC VTEC system of the B16A's, both intake and exhaust have 3 cam lobes per cylinder.

However the valvetrain has the added benefit of roller rockers and continuously variable intake cam timing. The economy i-VTEC is more like the SOHC VTEC-E in that the intake cam has only two lobes, one very small and one larger, as well as no VTEC on the exhaust cam. The two types of motor are easily distinguishable by the factory rated power output: the performance motors make around 200 hp (150 kW) or more in stock form and the economy motors do not make much more than 160 hp (120 kW) from the factory.

In 2004, Honda introduced an i-VTEC V6 (an update of the J-series), but in this case, i-VTEC had nothing to do with cam phasing. Instead, i-VTEC referred to Honda's cylinder deactivation technology which closes the valves on one bank of (3) cylinders during light load and low speed (below 80 km/h) operation. The technology was originally introduced to the US on the Honda Odyssey minivan, and can now be found on the Honda Accord Hybrid, the 2006 Honda Pilot, and the 2008 Honda Accord.

An additional version of i-VTEC was introduced on the 2006 Honda Civic's R-series four cylinder SOHC engines. This implementation uses the so-called "economy cams" on one of the two intake valves of each cylinder.

The "economy cams" are designed to delay the closure of the intake valve they act upon, and are activated at low rpms and under light loads. When the "economy cams" are activated, one of the two intake valves in each cylinder closes well after the piston has started moving upwards in the compression stroke. That way, a part of the mixture that has entered the combustion chamber is forced out again, into the intake manifold. That way, the engine "emulates" a lower displacement than its actual one (its operation is also similar to an Atkinson cycle engine, with uneven compression and combustion strokes), which reduces fuel consumption and increases its efficiency.

During the operation with the "economy cams", the (by-wire) throttle butterfly is kept fully open, in order to reduce pumping losses. According to Honda, this measure alone can reduce pumping losses by 16%. In higher rpms and under heavier loads, the engine switches back into its "normal cams", and it operates like a regular 4 stroke Otto cycle engine.

This implementation of i-VTEC was initially introduced in the R18A1 engine found under the bonnet of the 8th generation Civic, with a displacement of 1.8 L and an output of 140PS. Recently, another variant was released, the 2.0 L R20A2 with an output of 150PS, which powers the EUDM version of the all-new CRV With the continued introduction of vastly different i-VTEC systems, one may assume that the term is now a catch-all for creative valve control technologies from Honda.

i-VTEC I

Hondas new unreleased direct injection engines. Featuring a 65:1 air/fuel ratio. Honda boasts increased fuel economy, high performance, and excellent emissions. Honda has only now released that the 2.0l i-vtec i is in production. Honda uses the i-vtec engine as a base as well as hondas centering injection technology, which angles the fuel spray at the piston head for ultra-high air-fuel ratios of up to 65:1.