VTEC
From Free net encyclopedia
VTEC (standing for Variable valve Timing and lift Electronic Control) is a system developed by Honda to improve the combustion efficiency of its internal combustion engines throughout the RPM range. This was the first system of its kind and eventually led to different types of variable valve timing and lift control systems that were later designed by other manufacturers (VVT-i from Toyota, VANOS from BMW, and so on).
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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, a fully variable valve timing engine is difficult to design and implement. Attempts have been made, using solenoids to control valves instead of the typical springs-and-cams setup, however these designs have not made it into production automobiles as they are very complicated and costly.
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 (think: older high performance American cars such as Camaros, Mustangs which idle loudly at stoplights) will run very poorly at low rpm (where most automobiles spend the majority 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 Supra and 300ZX, this was accomplished with a turbocharger. In the case of the RX-7, a wankel 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, with the other designed to maximize high RPM power output. Switching between the two cam lobes is determined by engine oil pressure. 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. In contrast to the SOHC implementation which switches between cam profiles seamlessly, the DOHC VTEC configuration has a pronouncedly different exhaust note when the switch is made.
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 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, as in the regular VTEC, is used to connect both valves together and allows opening of the second valve.
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 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 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.
Turbocharged VTEC
For 2007 models, Honda's Acura luxury division announced the RDX crossover SUV which will feature a new turbocharged 2.3 litre inline 4 cylinder i-VTEC engine. While Honda has worked with turbo engines for race applications (most notably in Formula 1 during the 1980's Turbo Era) this is one of only a handful of times Honda has applied turbocharging to a road car engine (previous examples incldue the Honda City Turbo and City Turbo II) and the first time to an engine with i-VTEC.
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.
The VTEC experience
Driving vehicles with a VTEC engine that targets performance-minded consumers is a rush for the driving enthusiast. For example, the Honda (Civic CR-X) del Sol VTEC car -- with the B16A DOHC VTEC engine -- is hailed as one of the most fun cars to drive by those who have owned one. In these cars, the driver and passenger are aware when the VTEC "kicks in" due to the sudden increased rushing sound of air being "sucked up" by the intake. While this experience is exciting to those who enjoy driving hard, it isn't for everyone.
Driving a vehicle with VTEC often requires significant additional driver input to operate at very high power outputs. In particular, the Honda S2000 has received complaints from both owners and detractors due to its very high (9,000 rpm) redline in the early models. While the engine produces very high output (120hp per litre), some see the continual need to "row the gears" as a distraction from driving. For contrast, another approach at making high power output is to increase the displacement of a given engine (the 7 litre LS7 motor being an extreme example). Increasing displacement generally harms engine efficiency, but dramatically improves engine output.
Further argument against VTEC is that it is primarily used to control emissions. While there can be no argument that VTEC motors produce substantially higher output, it is true that Honda seems to apply the VTEC label to many engines, some of which are simply "cleaner."
To date Honda have never provided a visual indicator of VTEC engagement eg. on the instrument cluster. Several tuning companies have produced an aftermarket part to perform this function. eg. http://www.icemode.com or http://www.do-luck.co.uk/content/products/index.php
References
Honda Motor Co., Ltd. (2004). Technology Close-up. Retrieved Sep. 16, 2004.