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VTEC

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VTEC is an acronym for Variable Valve Timing and lift Electronic Control. VTEC is a valve train system developed by Honda to improve the volumetric efficiency of a 4-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. This was the first system of its kind. Different types of variable valve timing and lift control systems have also been produced by other manufacturers (MIVEC from Mitsubishi, VVTL-i from Toyota, VarioCam Plus from Porsche, VVL from Nissan and so on). 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]

Contents

Introduction to VTEC

In the regular four-stroke automobile engine, the intake and exhaust valves are actuated by lobes on a camshaft. The shape of the lobes determines the timing, lift and duration of each valve. Timing refers to an angle measurement of when a valve is opened or closed with respect to the piston position (TDC or BDC). Lift refers to how much the valve is opened. Duration refers to how long the valve is kept open. Due to the behavior of the working fluid (air and fuel mixture) before and after combustion, which have physical limitations on their flow, as well as their interaction with the ignition spark, the optimal valve timing, lift and duration settings under low RPM engine operations are very different from those under high RPM. Optimal low RPM valve timing, lift and duration settings would result in insufficient filling of the cylinder with fuel and air at high RPM, thus greatly limiting engine power output. Conversely, optimal high RPM valve timing, lift and duration settings would result in very rough low RPM operation and difficult idling. The ideal engine would have fully variable valve timing, lift and duration, in which the valves would always open at exactly the right point, lift high enough and stay open just the right amount of time for the engine speed in use.

VTEC was initially designed to increase the power output of an engine to 100 ps/liter or more while maintaining practicality for use in mass production vehicles. Some later variations of the system were designed solely to provide improvements in fuel efficiency, or increased power output as well as improved fuel efficiency.

In practice, a fully variable valve timing engine is difficult to design and implement.

The opposite approach to variable timing is to produce a camshaft which is better suited to high RPM operation. This approach means that the vehicle will run very poorly at low RPM (where most automobiles spend much of their time) and much better at high RPM. VTEC is the result of an effort to marry high RPM performance with low RPM stability.

Additionally, Japan has a tax on engine displacement, requiring Japanese auto manufacturers to make higher-performing engines with lower displacement. In cars such as the Toyota Supra and Nissan 300ZX, this was accomplished with a turbocharger. In the case of the Mazda RX-7 (turbo) and RX-8, a rotary engine was used. VTEC serves as yet another method to derive very high specific output from lower displacement motors.

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 surging if the engine is asked to operate continuously at or around the switch-over point. The DOHC VTEC system has high and low lift cam lobe profiles on both the intake and exhaust valve camshafts.

The VTEC system was originally 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. DOHC VTEC engines soon appeared in other vehicles, such as the 1992 Acura Integra GS-R (B17 1.7 liter engine). And later in the 1994 Honda Prelude VTEC (H22 2.2 liter engine) 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 engines, which shares a common camshaft for both intake and exhaust valves. The trade-off is that SOHC engines only benefit from the VTEC mechanism on the intake valves. This is because in the SOHC engine, the spark plugs need to be inserted at an angle to clear the camshaft, and in the SOHC engine, the spark plug tubes are situated between the two exhaust valves, making VTEC on the exhaust impossible.

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 at 6200 rpm and 133 Nm at 5000 rpm.[3]

3-Stage VTEC

Honda also introduced a 3-stage VTEC system in select markets, which combines the features of both SOHC VTEC and SOHC VTEC-E. At low speeds, only one intake valve is used. At medium speeds, two are used. At high speeds, the engine switches to a high-speed cam profile as in regular VTEC. Thus, both low-speed economy and high-speed efficiency and power are improved. This engine has the motorcode D15B-vtec and it produces about 130bhp.

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.). 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 configuration) 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 maximum advance at full throttle and low rpm. The effect is further optimization of torque output, especially at low and midrange RPM.

For the K-Series motors there are 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 distiguishable by the factory rated power output: the performance motors make around 200 hp or 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 1,8lt and an output of 140Ps. Recently, another variant was released, the 2-litre 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

Honda’s i-VTEC I Engine is a variant of the K-series DOHC engine family featuring gasoline direct injection. It made it’s debut in the previous generation 2004 Honda Stream 7-seater MPV in Japan, but the current Stream does not use this engine anymore, instead using a 2.0 liter version of the R-series i-VTEC SOHC engine.

The engine featured the ability to use ultra-lean air-fuel mixtures of about 65:1, much leaner compared to the the usual direct injection engine 40:1 ratio, and of course so much more leaner than the stoichiometric air-fuel mixture of 14.7:1 - this saves fuel. Fuel consumption dropped to 15km per liter. Power ratings remain the same at about 155 horsepower[5].

Advanced VTEC

A September 25, 2006 press release announced the launch of the Advanced VTEC engine by Honda. The new engine combines continuously variable valve lift and timing control with the continuously variable phase control of VTC (Variable Timing Control). This new system permits optimum control over intake valve lift and phase in response to driving conditions, achieving improved charging efficiency for a significant increase in torque at all engine speeds. Under low to medium load levels, the valves are set for low lift and early closure to reduce pumping losses and improve fuel economy. In comparison to the 2.4L i-VTEC these advancements claim to increase fuel efficiency by 13%. Honda also claims that new engine also meets exhaust emission standards compliant with U.S. EPA - LEV2-ULEV regulations and Japanese Ministry of Land, Infrastructure and Transport requirements for Low-Emission Vehicles, with emission levels 75% lower than those required by the 2005 standards. The Advanced VTEC goes into production models in 3 years. [6] & [7]

February 9, 2007: Temple of VTEC announced the discovery and analysis of an entry from the U.S. Patent Office which they believe to be the new Honda A-VTEC system (http://www.vtec.net/news/news-item?news_item_id=659664).

VTEC in motorcycles

Apart from the Japanese market-only Honda CBR400F Super Four HYPER VTEC>, introduced in 1983, the first worldwide implementation of VTEC technology in a motorcycle occurred with the introduction of Honda's VFR800 sportbike in 2002. Similar to the SOHC VTEC-E style, one intake valve remains closed until a threshold of 7000 rpm is reached, then the second valve is opened by an oil-pressure actuated pin. The dwell of the valves remains unchanged, as in the automobile VTEC-E, and little extra power is produced but with a smoothing-out of the torque curve. Critics maintain that VTEC adds little to the VFR experience while increasing the engine's complexity. Drivability is a concern for some who are wary of the fact that the VTEC may activate in the middle of an aggressive corner, potentially upsetting the stability and throttle response of the bike.

References

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Notes

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External links

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