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CATEGORIES (articles) > Engine, Gearbox > Technical > VTEC - Variable valve Timing and lift Electronic Control

VTEC - Variable valve Timing and lift Electronic Control


VTEC (standing for Variable valve Timing and lift Electronic Control) is a system developed by Honda Motor Co., Ltd. to improve the combustion efficiency of its internal combustion engines throughout the RPM range. This was the first system of its kind and eventually lead to different types of variable valve timing and lift control systems that were later designed by other manufacturers (ie. VVT-i from Toyota or VANOS from BMW).

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 when a valve is opened or closed with respect to the combustion cycle. 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 gases (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 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 & stay open just the right amount of time for the engine speed in use.

In practice, such a perfectly adjustable system is complex and expensive to implement and is therefore found only in costly experimental and limited production engines. The vast majority of modern automobile engines operate with a fixed camshaft profile that represents a compromise between low RPM smoothness and high RPM power output. And since the average automobile engine spends most of its time running in the low RPM region, there is typically more emphasis on low RPM smoothness at the expense of high RPM output. Performance-tuned engines have cam profiles that are optimized more towards high RPM operation, where the greatest power can be obtained. But this means that low speed operation is compromised. Anyone who has heard a racing car or a highly-tuned hot rod sitting at idle will note that the engine sounds like it is barely capable of running at that speed.

DOHC VTEC

Honda's VTEC system is a simple and fairly elegant method of endowing the engine with multiple camshaft profiles optimized for low and high RPM operations. Instead of only one cam lobe actuating each valve, there are two - one optimized for low RPM smoothness & fuel efficiency and one to maximize high RPM power output. Switching between the two cam lobes is controlled by the engine's management computer. As engine RPM increases, a locking pin is pushed by oil pressure to bind the high RPM cam follower for operation. From this point on, the valve opens and closes according to the high-speed profile, which opens the valve further and for a longer time. The VTEC system was originally introduced as a DOHC system in the 1989 Honda Integra sold in Japan, which used a 160 hp (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 V6. The DOHC VTEC system has high and low RPM cam lobe profiles on both the intake and exhaust valve camshafts. This resulted in the most power gain at high RPMs and DOHC VTEC engines were thus used in the highest performance Honda automobiles. In contrast to the SOHC implementation which switches between cam profiles seamlessly, when the DOHC version switches cams there is a definite change in the engine note.

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 motor, 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 RPMs, it was used to increase efficiency at low RPMs. At low RPMs, only one of the two intake valves is allowed to open, increasing the fuel/air mixture's swirl in the cylinder and thus allowing a very lean mixture to be used. As the engine's speed increases, both valves are needed to supply sufficient mixture, and thus a sliding pin as in the regular VTEC is used to connect both valves together and start the second one moving too.

In North American markets, VTEC-E can be found in Honda's most fuel efficient cars, including the 1992-1995 Civic VX and 1996-2000 Civic HX.

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.

i-VTEC

i-VTEC 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 2002. 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 rpms. The effect is further optimization of torque output, especially at low and midrange RPMs.

In 2004, Honda introduced an i-VTEC V6 (an update of the venerable J-series), but in this case, i-VTEC had nothing do to 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 mph) operation. The technology was originally introduced to the US on the Honda Odyssey, and can now be found on the Honda Accord Hybrid and the 2006 Honda Pilot. An additional version of i-VTEC was introduced on the 2006 Honda Civic's R-series four cylinder engine. This implementation uses very small valve lifts at low rpm and light loads, in combination with large throttle openings (modulated by a drive-by-wire throttle system), to improve fuel economy by reducing pumping losses.

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.

VTEC in motorcycles

Apart from the Japanese market-only Honda CB400 Super Four Hyper VTEC, introduced in 1999, 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, upsetting the stability and throttle response of the bike.




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