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Different types of VTEC explained.


jalesi2001

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Hi Guys, I have just found some old coursework notes on VTEC systems so I thought I would put them on here!

DOHC VTEC System

The main reasons why we have VTEC has already been explained in MBK89's topic "All about VTEC", and the basic construction of the DOHC has also been covered so I wont bore you all with it again, but heres a few notes on the basics!

1. Four valves per cylinder mechanism equipped with a third rocker arm.

2. Third cam lobe on intake AND exhaust sides.

3. Hydraulic pistons built into mid-rocker arm with secondary rocker arm to engage/disengage them.

4. Centre cam lobes for high RPM operation.

5. Primary and secondary cam lobes for low RPM operation.

6. Supplementary spring on mid rocker arm for reducing arm play in low speed range and smooth valve movement in high speed range.

SOHC VTEC System

This version of VTEC uses similar principles to adjust valve lift and timing but uses only one camshaft. The exhaust valves operate with constant lift and constant timing. The intake side of the SOHC-VTEC valve control system has three rocker arms for each cylinder. A seperate cam lobe operates each of these three rocker arms.

The cams for low engine speed control the primary and secondary rocker arms respectively, while the cam for high engine speeds acts on the mid rocker arm. A valve assembly is provided to lock the three rocker arms together mechanically and contains two hyraulic pistons. Engine oil pressure moves the pistons, thereby engaging the three rocker arms which then act as one unit. A lost motion spring, located above the mid rocker arm, pushes the mid rocker arm on to the mid cam lobe so that the free play on the mid rocker arm can be limited when the rocker arms are not locked together.

SOHC VTEC-E System

An improvement on the SOHC-VTEC is the SOHC 16 valve VTEC-E system, designed to improve combustion efficiency and reduce fuel consumption.

The VTEC-E system rests one of the intake valves at engine speeds below 2500 RPM. The opening of only one intake valve creates a swirl in the intake air/fuel mixture. This, combined with the special design of engine piston surfaces, ensures highly efficient combustion making possible the use of a very lean air/fuel mixture (up to 23.5 to 1).

Above 2500 RPM, known as the "cross over point", an ECM signal is sent to the solinoid valve which then lets oil pressure work on the upper side of the spool valve. The spool valve can then move against its return spring and open an oil passage to hydraulic pistons which lock the primary and secondary rocker arms together. Both valves then operate together with the same lift and timing, allowing the engine to develop its full power potential.

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