VTEC

VTEC (Variable Valve Timing & Lift Electronic Control) is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two (or occasionally three) camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT (variable valve timing) systems which change only the valve timings and do not change the camshaft profile or valve lift in any way. VTEC (Variable Valve Timing & Lift Electronic Control) is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two (or occasionally three) camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT (variable valve timing) systems which change only the valve timings and do not change the camshaft profile or valve lift in any way. Japan levies a tax based on engine displacement, and Japanese auto manufacturers have correspondingly focused their research and development efforts toward improving the performance of their smaller engine designs. One method for increasing performance into a static displacement includes forced induction, as with models such as the Toyota Supra and Nissan 300ZX which used turbocharger applications and the Toyota MR2 which used a supercharger for some model years. Another approach is the rotary engine used in the Mazda RX-7 and RX-8. A third option is to change the cam timing profile, of which Honda VTEC was the first successful commercial design for altering the profile in real-time. The VTEC system provides the engine with valve timing optimized for both low and high RPM operations. In basic form, the single cam lobe and follower/rocker arm of a conventional engine is replaced with a locking multi-part rocker arm and two cam profiles: one optimized for low-RPM stability and fuel efficiency, and the other designed to maximize high-RPM power output. The switching operation 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 certain conditions are met. At the switch point a solenoid is actuated that allows oil pressure from a spool valve to operate a locking pin which binds the high RPM rocker arm to the low RPM ones. From this point on, the valves open and close 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-down back from high to low RPM cams is set to occur at a lower engine speed than the switch-up (representing a hysteresis cycle) to avoid a situation in which the engine is asked to operate continuously at or around the switch-over point. The older approach to timing adjustments is to produce a camshaft with a valve timing profile that is better suited to low-RPM operation. The improvements in low-RPM performance, which is where most street-driven automobiles operate a majority of the time, occur in trade for a power and efficiency loss at higher RPM ranges. Correspondingly, VTEC attempts to combine low-RPM fuel efficiency and stability with high-RPM performance. 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. 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 (BTDC or ATDC). 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 and load in use. Introduced as a DOHC (Dual overhead camshaft) system in Japan in the 1989 Honda Integra XSi which used the 160 bhp (120 kW) B16A engine. The same year, Europe saw the arrival of VTEC in the Honda Civic and Honda CRX 1.6i-VT, using a 150 bhp (110 kW) B16A1 variant. The United States market saw the first VTEC system with the introduction of the 1991 Acura NSX, which used a 3-litre DOHC C30A V6 with 270 bhp (200 kW). DOHC VTEC engines soon appeared in other vehicles, such as the 1992 Acura Integra GS-R (160 bhp (120 kW)B17A1), and later in the 1993 Honda Prelude VTEC (195 bhp (145 kW) H22A) and Honda Del Sol VTEC (160 bhp (120 kW) B16A3). The Integra Type R (1995–2000) available in the Japanese market produces 197 bhp (147 kW; 200 PS) using a B18C 1.8-litre engine, producing more horsepower per litre than most super-cars at the time. Honda has also continued to develop other varieties and today offers several varieties of VTEC, such as i-VTEC and i-VTEC Hybrid. Honda also applied the system to SOHC (single overhead camshaft) engines such as the D-Series and J-Series Engines, which share a common camshaft for both intake and exhaust valves. The trade-off was that Honda's SOHC engines benefited from the VTEC mechanism only 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 cannot be utilized by both the intake and 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, using a total of six cam lobes and six rocker arms per cylinder. 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.