phân phối khí thông minh VTEC

New VTECAs in the SOHC VTEC, high-speed and low-speed cams with differing profiles are provided on the intake camshaft, the former taking control of the high-speed range whereas the form

VTEC System Construction and Function

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Table of Contents

„ Introduction

„ Types of VTECs

„ SOHC VTEC Construction

„ DOHC VTEC Construction

„ New VTEC Construction

„ New VTEC Operation

„ 3-stage VTEC Construction

„ VTEC-E Construction

„ Introduction

The Variable Valve Timing and Lift Electronic Control (VTEC) system is engineering features

to change valve timing and lift parameters in response to engine speed characteristics Itsaction allows combustion properties to match as perfectly as possible to the demands ofcurrent engine operating conditions, hence delivering both high performance and high

efficiency

Put simply, each valve in an engine employing the VTEC system is assigned a number ofindividually shaped cam lobes These are all formed on the same camshaft and, through theimplementation of electronic control, are switched in response to engine conditions usinghydraulic pressure, thus enabling the above mentioned features to be realized

SOHC VTEC

High-speed and low-speed cams with differing profiles are provided on the intake camshaft inthe SOHC VTEC engine As is the case with the DOHC implementation, the low-speed camoperates valves in the low- and mid-speed range, and the high-speed cam operates in thehigh-speed range – although this is only true for the intake valve in this case This techniqueallows the engine to offer the best possible combination of ease-of-driving in the practicalspeed range, high power output, and fuel efficiency

New VTEC

As in the SOHC VTEC, high-speed and low-speed cams with differing profiles are provided

on the intake camshaft, the former taking control of the high-speed range whereas the former

is active at low- and mid-range speeds In this implementation, secondary intake valves arekept almost stationary at low vehicle speeds while the primary intake valves allow air to besupplied to the cylinders In combination with refinement of the shapes of combustion

chambers and ports, this action creates a swirl in each chamber to ensure that combustion iscarried out more efficiently The new VTEC engine can deliver substantial power and torquewhile still boasting excellent fuel economy

3-stage VTEC

The three different stages of this VTEC unit correspond to low-speeds (one valve operated

by the low-speed cam), mid-range speeds (both valves operated by the low-speed cam), andhigh-speeds (both valves operated by the high-speed cam) This design allows the

realization of an engine with excellent fuel efficiency in the low-speed range, excellent torqueoutput at mid-range speeds, and excellent power output in the high-speed range

VTEC-E

The intake valve camshaft is provided with independently profiled low-speed and mid-speedcams At low-speeds, the secondary valve is operated by the low-speed cam (although inreality it is almost motionless); both valves are operated by the mid-speed cam at mid-rangespeeds As a result, this engine delivers exceptional fuel efficiency while at the same timemaintaining a high-level of driveability

Each of these implementations of VTEC is described in detail below

„ SOHC VTEC Construction

This is the most basic type of VTEC System and comprises the following components:

4 Mid rocker arm

5 Primary rocker arm

6 Secondary rocker arm

7 8

The intake camshaft for the SOHC VTEC engine has three types of cams, namely primary,mid, and secondary These cams have independent profiles to provide different valve timingand lift

1 Secondary rocker arm

2 Primary rocker arm

3 Mid rocker arm

4 Camshaft

5 Stopper piston

6 Secondary rocker arm

7 Mid rocker arm

8 Primary rocker arm

9 Synchronizing piston B

10 Synchronizing piston A

A: Secondary cam B: Mid cam

C: Primary cam

1

2 3

4

5 6 7

8 9 10

Lost Motion Mechanism

The lost motion assembly includes a lost motion piston, a lost motion guide, and lost motionsprings A and B It is in constant contact with the mid rocker-arm

At low speeds, the lost motion mechanism suppresses unnecessary movement of this rockerarm; it functions as an auxiliary spring at high speeds to ensure smooth valve operation

1 Lost motion assembly

2 Mid rocker arm

3 Lost motion spring A

4 Lost motion guide

5 Lost motion piston

6 Lost motion spring B

7

Spool Valve

A spool valve assembly is mounted at the side of cylinder head It consists of a screen, asolenoid, and the spool valve

The function of this valve is to control the oil passage between the oil pump and the

synchronizing pistons When the solenoid is activated, the spool valve opens the oil

passage and hydraulic pressure is applied to the synchronizing pistons, thus activating theVTEC system

A pressure switch is located at the rear of the spool valve It senses the pressure in thesynchronizing piston’s oil passage and provides feedback to the ECM should rocker armswitching not occur as intended

Control System (ECM)

The VTEC system is controlled by the PGM-FI ECM Using a lot of sensors, the ECMmonitors engine speed, the degree of engine loading, vehicle speed, engine coolant

temperature, and many other factors Then, in reference to this data, the ECM determinesthe current engine operating condition and activates the solenoid valve accordingly (Thesolenoid valve in turn controls the hydraulic pressure supplied to the spool valve.)

VTEC pressure switch

VTEC solenoid valve

From

oil pump

Engine control module (ECM)

Engine speed

Engine load

Vehicle speed

Engine coolant temperature

„SOHC VTEC Operation

At Low Engine Speeds (System Not Activated)

The VTEC system is not active at low engine speeds (Actually, many different factors areinvolved in determining whether the system operates To keep the explanation simple, theywill be overlooked here.) The spool valve is closed and no hydraulic pressure is applied tothe synchronizing pistons inside the rocker arms Accordingly, each of the rocker arms isfree to move independently and are operated by the primary, mid, and secondary camsrespectively In this condition, the primary and secondary valves open and close followingthe timing and lift determined by the profiles of the primary and secondary cams Naturally,the mid rocker-arm is being operated by the mid cam at this time, but it causes no furtheroperation and is suppressed by the lost motion assembly to prevent rattling

1 Synchronizing piston A

2 Synchronizing piston B

3 Stopper piston

4 Secondary rocker arm

5 Mid rocker arm

6 Primary rocker arm

3

4 5

6

At High Engine Speeds (System Activated)

Once the engine speed exceeds a predetermined limit, the ECM outputs a signal to thespool valve solenoid causing it to open Hydraulic pressure from the oil pump can now passthrough the oil passage inside the camshaft to the rocker arms, where it acts on the

synchronizing pistons pushing them sideways If, however, any of the rocker arms are incontact with cams at the moment, all of the pistons will not be lined up together

Consequently, the rocker arms will continue to move even though the hydraulic pressure isacting on the pistons When all three rocker arms left the cam simultaneously, the pistonswill slide and the arms will be secured together In this condition, both primary and

secondary valves will be operated by the mid cam – profiled for high-speeds – through theaction of the mid rocker arm

Hydraulic pressure

When engine speed subsequently drops, the spool valve will close and the hydraulic

pressure will drop The stopper piston spring will try to push the pistons back to their originalpositions As before, this will be achieved when all of the pistons are lined up The rockerarms are disengaged from each other by this action and start to operate independently

„ DOHC VTEC Construction

Whereas in the SOHC VTEC system the intake camshaft alone was fitted with VTEC

components, the DOHC VTEC sees this technology applied to both the intake and exhaustcamshafts This enables both intake and exhaust characteristics to be controlled in response

to engine speed

1 Camshaft

2 Low speed cam

3 High speed cam

4 Primary rocker arm

5 Mid rocker arm

6 Secondary rocker arm

9 10

11

12

2

4 6

1

2 3 2

5

„ DOHC VTEC Operation

Apart from the fact that DOHC VTEC has two independent VTEC systems as opposed toone in SOHC VTEC, the modes of operation of these two systems are essentially the same

Primary + secondary cams

„ New VTEC Construction

The New VTEC system was realized through further development of SOHC VTEC Thisdevelopment added the following components

• Timing plate

• Timing piston

1 Timing plate

2 Mid rocker arm

3 Secondary rocker arm

8 9

Secondary cam

Timing Plate and Timing Piston

A timing plate and timing piston are mounted on the primary rocker arm in the New VTECsystem

The timing plate is positioned on the outside of the rocker arm and both of these componentsmove in unison The timing piston is mounted in line with the synchronizing piston A Asection of the timing plate passes through an opening in the primary rocker arm and engageswith a channel in the timing piston

1 Timing plate

2 Synchronizing piston A

3 Timing piston

4 Primary rocker arm

5 Mid rocker arm

6 Secondary rocker arm

7 8

9 10

„ New VTEC Operation

Although the operating principles of the New and SOHC implementations of VTEC

technology are essentially the same, they do exhibit differences with respect to the followingtwo items:

• Secondary valve opening

• Timing mechanism

Valve Opening

At low engine speeds, the primary and secondary valves in SOHC VTEC exhibit almost thesame amount of lift However, the cam profile employed in New VTEC ensures that thesecondary valve opens only slightly when the primary valve is opened

This ensures that a swirl* is created in the combustion chamber due to supply of the air/fuelmixture via one valve only Flame propagation speed is, therefore, increased and the

burning of lean mixtures is stabilized

If the secondary valve were to be completely closed at this time, a certain amount of fuelwould accumulate at the intake port It is to prevent this situation that the valve is openedslightly

At high engine speeds, both valves are activated by the high speed cam (i.e., the mid cam)

*: The shape of intake ports, combustion chambers, and other similar components have alsobeen modified to improve swirl characteristics

Primary cam

Exhaust

Secondary cam Intake Mid cam

The timing plate, mounted on the primary rocker arm, moves in unison with the rocker arm.However, the degree of this motion is limited by the stopper fitted to the camshaft holder.Thus, whenever the rocker arm is lifted, the timing plate slips out of the channel in the timingpiston, releasing the piston lock condition If switching pressure is acting on the timing piston

at this time, it will slide sideways by a small amount

Cam rotation will continue and when the amount of cam lift subsequently approaches zero,the timing plate will try to return to its original position However, due to the fact that thetiming piston has moved a small distance from its original position, these two componentswill not now engage When the lift reaches zero, the timing piston, and also the

synchronizing pistons will be slid by the hydraulic pressure, securing the rocker arms

together

When the timing piston reaches a certain position, the timing plate will once again engage toanother groove of timing piston and further sliding will be prevented

When hydraulic pressure drops as a result of the operation of the VTEC system being

terminated, a weak internal spring will push the timing piston back to its original positionduring the period of time in which the timing plate is pulled away by lifting of the rocker arm

The piston will then be secured in place once again by the timing plate

When lift reaches zero, the synchronizing pistons are pushed back to their original positions

by a return spring, thus disengaging the rocker arms

(Cont’d.)

Valve Timing Change Condition

Engine speed: 2,300 to 3,200 min-1 (Depending on manifold pressure)Vehicle speed: Over 10 km/h

Engine coolant temperature: Over 10 degrees Celsius

Engine load: Determined from manifold vacuum

„ 3-stage VTEC Construction

Further development of New VTEC technology has resulted in the 3-stage VTEC systemwhich controls the intake valves in three different stages Although most components are thesame as those used in New VTEC, there are two switching pressure systems, and two spoolvalves are also employed

1 Synchronizing piston

2 Lost motion assembly

3 Stopper piston

4 Secondary rocker arm

5 Mid rocker arm

6 Primary rocker arm

9

8

Rocker Arms

The rocker arms are connected to the two following independent switching systems – eachone of these hydraulic systems is controlled by one of the spool valves

• A low-speed to mid-range speed switching system which comprises a timing piston and a

stopper piston for connecting the primary and secondary rocker arms

• A mid-range speed to high-speed switching system which comprises a stopper piston,

synchronizing piston A, and synchronizing piston B for connecting the primary, mid, andsecondary rocker arms

The timing plate engages with the low-speed to mid-range speed switching system’s timingpiston

1 Secondary rocker arm

2 Mid rocker arm

3 Primary rocker arm

4 High speed cam

5 Low speed cam

Engine speed

Vehicle speed

Throttle opening angle

Engine coolant temperature

Hydraulic circuit #1 Hydraulic circuit

#2

Solenoid valve

Hydraulic pressure

Although the operation of all systems is very similar, three stage valve control is implemented

in the 3-stage VTEC

At Low Engine Speeds

All rocker arms operate independently

The primary valve is opened by the mid-range speed primary cam The secondary valve,following the secondary cam, is opened by a very small amount only (identical to New VTEClow-speed operation)

Mid rocker arm Secondary rocker arm

Primary rocker arm

At Mid-range Engine Speeds

One of the spool valves opens and hydraulic pressure is introduced into the low-speed tomid-range speed switching system This causes the timing piston to slide and connect theprimary and secondary rocker arms Thus, both the primary and secondary valves aresubsequently activated by the primary cam

Hydraulic pressure for low-/mid-range speed switchover

At High Engine Speeds

The second spool valve opens and hydraulic pressure is introduced into the mid-rangespeed to high-speed switching system This causes the synchronizing pistons to slide andconnect the primary and secondary cams to the mid cam This cam, profiled for high speedsituations, will subsequently activate the primary and secondary valves

Camshaft

At low speeds At mid-range speeds At high speeds

Stationary

high speed cam

Valve Timing Change Condition

Engine speed: Low to mid range 3,000 min-1

Mid to high range 6,000 min-1Vehicle speed: M/T Over 15 km/h

A/T Over 10 km/hEngine coolant temperature: Low to mid range Over 40 degrees Celsius

Mid to high range Over 60 degrees CelsiusEngine load: Determined from throttle opening angle

„ VTEC-E Construction

Although incorporating a timing plate in the same way as the New VTEC system, VTEC-Edoes not have a mid cam or a mid rocker arm Accordingly, there is no lost motion assemblyeither

The switching system is comprised of a timing piston, a synchronizing piston, and a stopperpiston

1 Timing plate

2 Primary rocker arm

3 Secondary rocker arm

2 3

4 5 6 7

2

8 4

5