DOHC I VTEC

The DOHC i-VTEC system utilizes Honda's proprietary VTEC system and adds (VTC) Variable Timing Controller, which allows for dynamic and continuous intake valve timing and overlap control. The demanding aspects of fuel economy, ample torque, and clean emissions can all be controlled and provided at a higher level with VTEC (intake valve timing and lift control) and VTC (valve overlap control) combined.

Since this variant of VTEC still used the same VTEC operation, which is having the ability to drive single valve with multiple cam profile with the addition of VTC, VTEC does not need to be explain.

As Honda clearly stated in this K20A article, iVTEC is VTEC with VTC added. K20A uses a combination of VTEC-E and power-VTEC that switches between 12-valve economy mode and a 16-valve mode with a separate wild intake cam. VTC complements this VTEC variation. For the K20A engine however, outright power is not the sole objective of iVTEC, the other important parameters of fuel economy and emissions has been given equally important footing, a direction that Honda has always promoted in the past. This impossible balance of power and economy has previously only been realizable in Honda's D15B 3-stage VTEC first used on the previous generation EK3 (and the D17A on the current Civic).

They identify the VTC actuator as well as the oil pressure solenoid valve, both attached to the intake camshaft's sprocket. The VTC cam sensor is required by the ECU to determine the current timing of the intake camshaft. The VTEC mechanism on the intake cam remains essentially the same as those in the current DOHC VTEC engines except for an implementation of VTEC-E for the 'mild' cam.

VTC (Variable Timing Control mechanism)

The VTC actuator, installed on the intake camshaft, is controlled by oil pressure, to allow for dynamic and continuous valve timing to fit the load of the engine.

Honda's pictorial representation of the VTC mechanism illustrates very clearly the operating principle of generic VVT - ie turning the camshaft in relation to the driving sprocket. Readers should bear in mind that the generic VVT mechanism described on the iVTEC article is just that - generic. Different manufacturers will implement the mechanism using different details.

However, the important part is the introduction of a mechanism to isolate the camshaft from its driving sprocket and which also allows the relative rotational alignment between the two to be accurately controlled and continuously changes.

The iVTEC operating principle

There are four major cam variations for the K20A's iVTEC implementation. Of these, variations 1 to 3 are based on the so-called low cams and only variation 4 uses the high-cams. Note that the diagrams shows that VTEC is implemented only on the intake cam.

Now, note that there is an annotation indicating a 'mostly resting (intake) cam' in variations 1 to 3. This is the 'approximately 1-valve' operating principle of VTEC-E. Ie, one intake valve is hardly driven while the other opens in its full glory. This instills a swirl effect on the air-flow which helps in air-fuel mixture and allows the use of the crazy 20+ to 1 air-to-fuel ratio in lean-burn or economy mode during idle running conditions.

On first acquaintance, variations 1 and 3 seems identical. However, in reality they represent two different engine configurations - electronic-wise. Variation 1 is lean burn mode, the state in which the ECU uses >20:1 air-fuel ratio. VTC closes the intake/exhaust valve overlap to a minimal. Note that lean-burn mode or variation 1 is used only for very light throttle operations as identified by the full load Torque curve overlaid on the VTC/RPM graph. During heavy throttle runs, the ECU goes into variation 3.

Lean-burn mode is contained within variation-2 as a dotted area probably for the reason that the ECU bounces between the two modes depending on engine rpm, throttle pressure and engine load, just like the 3-stage VTEC D15B and D17A. In variation-2, the ECU pops out of lean-burn mode, goes back to 14.7 or 12 to 1 air-fuel ratios and brings the intake/exhaust overlap right up to maximum. This as Honda explains will induce the EGR effect, which makes use of exhaust gases to reduce emissions.

Note also that variations 1 to 3 are used in what Honda loosely terms the idle rpm. For 3-stage VTEC engines, idle rpms takes on a much broader meaning. It is no longer the steady 750rpm or so for an engine at rest. For 3-stage VTEC, idle rpm also means low running rpm during ideal operating conditions, ie closed or very narrow throttle positions, flat even roads, steady speed, etc. It is an idle rpm range. The K20A engine implements this as well.

Variation-4 is activated whenever rpm rises and throttle pressure increases, indicating a sense of urgency as conveyed by the driver's right foot. This mode sees the wild(er) cams of the intake camshaft being activated, the engine goes into 16-valve mode now and VTC dynamically varies the intake camshaft to provide optimum intake/exhaust valve overlap for power.