Variable Valve Timing & Lift Electronic Control (VTEC) is a valve 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. VTEC 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.
Context and description
Japan levies a tax based on engine displacement, and Japanese auto manufacturers have correspondingly focused their research and development efforts on 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’s VTEC system was the first production engine to use cam‑switching technology to vary both valve timing and lift.
The VTEC system provides the engine with valve timing optimized for both low and high speed 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 back from high- to low-rpm cams is set to occur at a lower engine speed than the switch up (representing a hysteretic 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-speed operation. The improvements in low-speed 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. VTEC attempts to combine low-rpm fuel efficiency and stability with high-rpm performance.
History
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 angular 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. Owing to the behavior of the working fluid (the air and fuel mixture) before and after combustion which imposes physical limitations on the flow, as well as the interaction with the ignition spark the optimal valve timing, lift and duration settings under low speed engine operations are very different from those under high speed. Optimal low speed valve timing, lift and duration settings would result in insufficient filling of the cylinder with fuel and air at high speed, thus greatly limiting engine power output. Conversely, optimal high speed valve timing, lift and duration settings would result in very rough low speed 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.
DOHC VTEC
VTEC was introduced as a DOHC (dual overhead camshaft) system in Japan in the 1989 Honda Integra XSi, which used the B16A engine. The same year, Europe saw the arrival of VTEC in the Honda Civic and Honda CRX 1.6i-VT, using a B16A1 variant. The United States market saw its first VTEC system with the introduction of the 1991 Acura NSX, which used a 3-litre DOHC C30A V6 with . DOHC VTEC engines soon appeared in other vehicles, such as the 1992 Acura Integra GS-R (B17A1), and later in the 1993 Honda Prelude VTEC ( H22A) and Honda del Sol VTEC ( B16A3). The Integra Type R (1995–2000) available in the Japanese market produces 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.
SOHC VTEC valvetrain
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 J37A2 3.7L SOHC V6 engine introduced on all 2009–2012 Acura RL 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.
The primary exhaust rocker arm contacts a low-profile camshaft lobe during low-speed 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, thereby locking both exhaust rocker arms together. The high-profile camshaft lobe which normally contacts the secondary exhaust rocker arm alone during low-speed engine operation is able to move both exhaust rocker arms together which are locked as a unit. The same occurs for the intake rocker shaft, except that the high-profile camshaft lobe operates the primary rocker arm.
The J37A2 is able to use both intake and exhaust VTEC by use of a novel design of the intake rocker arm. Each exhaust valve on the J37A2 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.
VTEC-E
The earliest VTEC-E implementation is a variation of SOHC VTEC which is used to increase combustion efficiency at low rpm while maintaining the mid range performance of non-vtec engines. VTEC-E is the first version of VTEC to employ the use of roller rocker arms and because of that, it forgoes the need for having 3 intake lobes for actuating the two valves—two lobes for non-VTEC operation (one small and one medium-sized lobe) and one lobe for VTEC operation (the biggest lobe). Instead, there are two different intake cam profiles per cylinder: a very mild cam lobe with little lift and a normal cam lobe with moderate lift. Because of this, at low rpm, when VTEC is not engaged, one of the two intake valves is allowed to open only a very small amount due to the mild cam lobe, forcing most of the intake charge through the other open intake valve with the normal cam lobe. This induces swirl of the intake charge which improves air/fuel atomization in the cylinder and allows for a leaner fuel mixture to be used. As the engine's speed and load increase, both valves are needed to supply a sufficient mixture. When engaging VTEC mode, a pre-defined threshold of speed (must be moving), rpm and load must be met before the computer actuates a solenoid which directs pressurized oil into a sliding pin, just like with the original VTEC. This sliding pin connects the intake rocker arm followers together so that, now, both intake valves are following the "normal" camshaft lobe instead of just one of them. When in VTEC, since the "normal" cam lobe has the same timing and lift as the intake cam lobes of the SOHC non-VTEC engines, both engines have identical performance in the upper powerband assuming everything else is the same. This variant of the VTEC-E is used in some D-series engines.
With the later VTEC-E implementations, the only difference it has with the earlier VTEC-E is that the second normal cam profile has been replaced with a more aggressive cam profile which is identical to the original VTEC high-speed cam profile. This in essence supersedes VTEC and the earlier VTEC-E implementations since the fuel and low speed torque benefits of the earlier VTEC-E are combined with the high performance of the original VTEC. There are three intake cam lobes: two for the low-rpm mode (one for almost closed valve, one for normally opened) and one for the powerful mode when the VTEC solenoid is activated. The lowest rpm for activating the VTEC is 2500, or it may be higher if the load is weak - ECM dependant. With the VTEC solenoid is on the third biggest lobe begins to push all the intake valves with the more aggressively profile. This variant of the VTEC-E is used in the F23A, F22B and JDM F20B SOHC VTEC engines.
Three-stage VTEC