What is a DOHC VTEC engine
VTEC = Variable valve Timing and lift E.lectronic C.ontrol
The system was developed a variable valve timing from Honda 1983 for motorcycles. since 1989 This system is built into cars with the aim of providing a high-revving sports engine with high torque at low speeds in order to achieve everyday usability despite the sporty design. The system is now also used to provide low-emission and economical engines with sufficient power (VTEC-Economy, i[intelligent]-VTEC).
The functional principle of the variable valve timing is based on the fact that the shape and size of the cams on the camshaft the power output and the torque of an engine (via corresponding Valve opening times and Valve lifts) influence. Basically, the faster the valves open and close and the higher the valve lift, the more power can be achieved, but the less torque is available and the more fuel is consumed.
The combination of a high-performance engine with high torque at low speeds is achieved by basically integrating two camshafts in one.
Valve train of a dohc VTEC engine
The photo clearly shows that the two valves are not controlled by a cam each, but that three Cams each control two valves. It is also clear that VTEC does not work with directly controlled valves, but that (depending on the engine design) tilt or (as in the photo) Rocker arm are needed.
At low speeds [engl. low revs] operate the cams for low speeds Cam profiles for low revs] the outer rocker arm [engl. primary and secondary rocker], while the cam freewheels for higher speeds because the spring [engl. return spring] has opened the connection (= pin A and pin B) between the two rocker arms and therefore the middle rocker arm [engl. middle rocker] is not operated.
At high speeds [engl. high revs] a solenoid valve opens and allows the engine oil. oil flow] to exert pressure on the gate valve (= pin A and pin B) oil pressure], so that there is a solid connection between the three rocker arms. Since the middle rocker arm has a "sharper" cam for high speeds. Cam profile for high revs] is moved lower than the two outer rocker arms, the two outer cams run free in this situation and only the middle cam determines the valve movement.
When the solenoid valve closes again, the spring pushes the locking slide back and the connection between the three rocker arms is broken: the middle cam runs free again.
Roller rocker arm of the F20C
In principle, they all work VTEC- Variants according to the previous description. However, dohc motors are mechanically easier to design than sohc motors. For the latter, rocker arms are used instead of rocker arms. VTEC-E-Motors are usually only variably controlled on the intake side. In multi-valve engines, different control times and strokes of the two inlet valves are often implemented using complex lever and cam constructions, up to and including the complete shutdown of an inlet valve at low speeds. In addition, the switching speed can be changed as required via the electronic engine management and could in principle even be changed depending on the power requirement etc. while driving.
With the further developed i-VTEC an inlet valve is also shut down at low speeds. In addition, there is a continuous adjustment of the opening times of the valves on the inlet side (VTC = Variable Timing C.ontrol) the system to enable optimal utilization of the ignitable mixture through better cylinder filling and thus an improvement in combustion, which is also reflected in an increase in torque. This is achieved by a hydraulically implemented rotation of the intake camshaft, which enables the overlap with the exhaust camshaft to be varied.
The interaction of VTEC and VTC optimizes the system.
A lot of power is required when accelerating from low speeds. This is made possible by a slight overlap, which also takes advantage of the inertia of the sucked-in mixture. This represents VTEC-System at low speeds a high torque through the "tame" cams available. After exceeding the switching speed, the switches VTEC-System on the "sharp" cams and that VTC keeps his attitude. That reinforces the power output again.
When driving at high revs at constant speeds, on the one hand power is required, but on the other hand the desire for the lowest possible consumption is loud. Larger overlaps in valve opening times are a good compromise in this case. They reduce the number of gas changes, which leads to lower consumption. In addition, more exhaust gases are fed back into the combustion chamber and burned again during the next combustion, which leads to fewer pollutant emissions.
A slight overlap is desired when idling or at very low speeds, because this creates eddies in the intake port that produce a better, more homogeneous mixture that burns better. This means that less fuel is required again and the combustion is more complete, with fewer pollutants.
Due to the variable design of the intake duct, the effects of the VTC can still be increased. If the air is sucked in via a long intake manifold at low speeds, the flow speed increases, which leads to better filling of the cylinder. At high speeds, on the other hand, the intake duct is shortened. This means that more air flows into the cylinder and more power is delivered.
Honda used that on the Civic Hybrid and other models VTEC-Principle for Cylinder deactivation (VCM = Variable C.ylinder M.anagement) for the purpose of further reducing fuel consumption. The i-DSI-Motor LDA as a gasoline-consuming part of the hybrid drive has a device on three of the four cylinders to shut down both the intake and exhaust valves. This reduces the frictional resistance in the deactivated cylinders, which leads to higher feedback energy for charging the IMA battery. Honda successfully carried over this system to the V6 IMA units and other engines. Honda developed this system to such an extent that, depending on the required output, a complete row of cylinders or different cylinders can be switched off.
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