phân phối khí thông minh VTEC

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VTEC System C & F

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VTEC System Construction and Function
Table of Contents
„ Introduction
„ Types of VTECs
„ SOHC VTEC Construction
„ SOHC VTEC Operation
„ DOHC VTEC Construction
„ DOHC VTEC Operation
„ New VTEC Construction
„ New VTEC Operation
„ 3-stage VTEC Construction
„ VTEC-E Construction

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„ 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. Its
action allows combustion properties to match as perfectly as possible to the demands of
current 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 of
individually shaped cam lobes. These are all formed on the same camshaft and, through the
implementation of electronic control, are switched in response to engine conditions using
hydraulic pressure, thus enabling the above mentioned features to be realized.

„ Types of VTECs
Currently there are five types of VTEC systems as follows:

DOHC VTEC
This application of VTEC technology sees high-speed and low-speed cams with differing
profiles provided on both the intake and exhaust camshafts. In the low- and mid-range
engine speeds, the intake and exhaust valves are operated by the low-speed cams. Above
this range, the high-speed cams take over. The combination of these actions allows the
engine to offer ample torque and flexibility at moderate speeds and deliver both the sharp
response and high power output at higher operating speeds.

SOHC VTEC
High-speed and low-speed cams with differing profiles are provided on the intake camshaft in
the SOHC VTEC engine. As is the case with the DOHC implementation, the low-speed cam
operates valves in the low- and mid-speed range, and the high-speed cam operates in the

high-speed range – although this is only true for the intake valve in this case. This technique
allows the engine to offer the best possible combination of ease-of-driving in the practical
speed range, high power output, and fuel efficiency.


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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 are
kept almost stationary at low vehicle speeds while the primary intake valves allow air to be
supplied 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 is
carried out more efficiently. The new VTEC engine can deliver substantial power and torque
while 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), and
high-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 torque

output 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-speed
cams. At low-speeds, the secondary valve is operated by the low-speed cam (although in
reality it is almost motionless); both valves are operated by the mid-speed cam at mid-range
speeds. As a result, this engine delivers exceptional fuel efficiency while at the same time
maintaining a high-level of driveability.
Each of these implementations of VTEC is described in detail below.


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„ SOHC VTEC Construction
This is the most basic type of VTEC System and comprises the following components:
•
•
•
•
•

Camshaft

Rocker arms
Lost motion mechanisms
Spool valve
Control system (ECM)

1

2
3
4
5
6

8

7
1 Synchronizing piston A
2 Lost motion assembly
3 Synchronizing piston B
4

Mid rocker arm

5 Primary rocker arm
6 Secondary rocker arm
7 Intake valve
8 Camshaft


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Camshaft
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 timing
and lift.

A: Secondary cam
B: Mid cam
C: Primary cam

Rocker Arms
The primary, mid, and secondary rocker arms are incorporated into one mechanism. The
primary and secondary rocker arms make contact with the valves. Each rocker arm consists
of synchronizing pistons, a stopper piston, and a spring. It is through the action of these
components that the motion of the individual rocker arms can be linked or unlinked during
operation of the engine.
4
2
3

10
1

5

9
8

7

6

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


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Lost Motion Mechanism
The lost motion assembly includes a lost motion piston, a lost motion guide, and lost motion
springs 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 rocker
arm; it functions as an auxiliary spring at high speeds to ensure smooth valve operation.
1

2
4

3

6
7

5
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 Camshaft


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Spool Valve
A spool valve assembly is mounted at the side of cylinder head. It consists of a screen, a
solenoid, 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 the
VTEC system.
A pressure switch is located at the rear of the spool valve. It senses the pressure in the
synchronizing piston’s oil passage and provides feedback to the ECM should rocker arm
switching not occur as intended.

2

3

1

1 Screen

5

2 Solenoid

4

3 Pressure switch
4 Spool valve
5 Cylinder head


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Control System (ECM)
The VTEC system is controlled by the PGM-FI ECM. Using a lot of sensors, the ECM
monitors engine speed, the degree of engine loading, vehicle speed, engine coolant
temperature, and many other factors. Then, in reference to this data, the ECM determines
the current engine operating condition and activates the solenoid valve accordingly. (The
solenoid valve in turn controls the hydraulic pressure supplied to the spool valve.)

VTEC pressure switch

Oil flow

VTEC solenoid valve

From
oil pump

Engine speed

Engine
control
module
(ECM)

Engine load

Vehicle speed
Engine coolant
temperature


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„ SOHC VTEC Operation
At Low Engine Speeds (System Not Activated)
The VTEC system is not active at low engine speeds. (Actually, many different factors are

involved in determining whether the system operates. To keep the explanation simple, they
will be overlooked here.) The spool valve is closed and no hydraulic pressure is applied to
the synchronizing pistons inside the rocker arms. Accordingly, each of the rocker arms is
free to move independently and are operated by the primary, mid, and secondary cams
respectively. In this condition, the primary and secondary valves open and close following
the 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 further
operation and is suppressed by the lost motion assembly to prevent rattling.

2

1

3

1 Synchronizing piston A
2 Synchronizing piston B
3 Stopper piston
4 Secondary rocker arm

6

5

4

5 Mid rocker arm
6 Primary rocker arm

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At High Engine Speeds (System Activated)
Once the engine speed exceeds a predetermined limit, the ECM outputs a signal to the
spool valve solenoid causing it to open. Hydraulic pressure from the oil pump can now pass
through 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 in
contact 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 is
acting on the pistons. When all three rocker arms left the cam simultaneously, the pistons
will 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 the
action of the mid rocker arm.

Hydraulic pressure


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(Cont’d.)
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 original
positions. As before, this will be achieved when all of the pistons are lined up. The rocker
arms are disengaged from each other by this action and start to operate independently.

Exhaust valve

Intake valve

Camshaft

Mid cam
Primary + secondary cams

Exhaust

Intake


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„ 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 exhaust
camshafts. This enables both intake and exhaust characteristics to be controlled in response
to engine speed.
1
3

2

1
2

2
3

4
2
5

9

6
10

8
4

11

6
5
7
1 Camshaft
2 Low speed cam
3 High speed cam
4 Primary rocker arm

12

5 Mid rocker arm
6 Secondary rocker arm
7 Synchronizing piston A
8 Synchronizing piston B
9 Stopper piston
10 Lost motion spring
11 Exhaust valve
12 Intake valve


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„ DOHC VTEC Operation
Apart from the fact that DOHC VTEC has two independent VTEC systems as opposed to
one in SOHC VTEC, the modes of operation of these two systems are essentially the same.

Camshaft

Exhaust valve

Intake valve

Mid cam

Exhaust

Valve timing

Primary + secondary cams

Valve lift

Primary +
secondary cams

Time
Intake

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„ New VTEC Construction
The New VTEC system was realized through further development of SOHC VTEC. This
development added the following components.
• Timing plate
• Timing piston
1
2

3

4
1 Timing plate

5
6

2 Mid rocker arm

7

3 Secondary rocker arm
4 Synchronizing piston B

9

5 Synchronizing piston A
6 Timing piston
7 Intake valve
8 Primary rocker arm

8

9 Camshaft

Primary cam

Exhaust

Mid cam

Intake

Secondary cam


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Timing Plate and Timing Piston
A timing plate and timing piston are mounted on the primary rocker arm in the New VTEC
system.
The timing plate is positioned on the outside of the rocker arm and both of these components
move in unison. The timing piston is mounted in line with the synchronizing piston A. A
section of the timing plate passes through an opening in the primary rocker arm and engages
with a channel in the timing piston.
4

1

2

6

5

10

3

9

8

1 Timing plate
2 Synchronizing piston A
3 Timing piston
4 Primary rocker arm
5 Mid rocker arm
6 Secondary rocker arm
7 Synchronizing piston B
8 Synchronizing piston A
9 Timing plate
10 Timing piston

7


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„ 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 following
two items:

• Secondary valve opening
• Timing mechanism

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

Exhaust

Mid cam

Intake
Secondary cam

This ensures that a swirl* is created in the combustion chamber due to supply of the air/fuel
mixture 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 fuel
would accumulate at the intake port. It is to prevent this situation that the valve is opened
slightly.
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 also
been modified to improve swirl characteristics.


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Timing Mechanism
The main purpose of the timing mechanism is to secure the synchronizing piston in place
when the VTEC system is operational.
Operation of the VTEC system occurs at lower speeds in the New VTEC implementation
than in DOHC or SOHC. (This is also true for the other systems which employ timing
mechanisms such as the VTEC-E or 3-stage VTEC.) As a direct consequence of this, the
hydraulic pressure available for securing the synchronizing piston during operation of the
system is lower than in the cases mentioned previously. This low hydraulic pressure may
fluctuate and could conceivably result in unintentional motion of the synchronizing piston. In
order to prevent this from occurring, the timing piston is held securely in place by the timing
plate whenever the pistons are in a condition where sliding would be possible.

Timing plate

Synchronizing piston A

Stopper piston
Timing piston

* Synchronizing piston B is omitted

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 timing
piston, 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.


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(Cont’d.)
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 the
timing piston has moved a small distance from its original position, these two components
will 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 to
another 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 position
during 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.


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(Cont’d.)

When lift reaches zero, the synchronizing pistons are pushed back to their original positions
by a return spring, thus disengaging the rocker arms.

Valve Timing Change Condition
Engine speed:
Vehicle speed:
Engine coolant temperature:
Engine load:

2,300 to 3,200 min-1 (Depending on manifold pressure)
Over 10 km/h
Over 10 degrees Celsius
Determined from manifold vacuum

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„ 3-stage VTEC Construction
Further development of New VTEC technology has resulted in the 3-stage VTEC system
which controls the intake valves in three different stages. Although most components are the
same as those used in New VTEC, there are two switching pressure systems, and two spool
valves are also employed.

2

3

1
4

8
1
7

5
6

9

1 Synchronizing piston

6 Primary rocker arm

2 Lost motion assembly

7 Timing plate

3 Stopper piston

8 Low-/mid-range-speed switchover

4 Secondary rocker arm

9 Mid-/high-range-speed switchover

5 Mid rocker arm


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Rocker Arms
The rocker arms are connected to the two following independent switching systems – each
one 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, and
secondary rocker arms.
The timing plate engages with the low-speed to mid-range speed switching system’s timing
piston.

1
2
Engine speed

3

Throttle opening angle

Vehicle speed
Engine coolant
temperature

Hydraulic
circuit #1

4

Hydraulic circuit

#2

Solenoid valve

Hydraulic
pressure

5
Mid rocker arm
1 Secondary rocker arm
2 Mid rocker arm
3 Primary rocker arm
4 High speed cam
5 Low speed cam


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Operation
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 VTEC
low-speed operation).

Secondary rocker arm
Primary rocker arm

Mid rocker arm


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(Cont’d.)
At Mid-range Engine Speeds
One of the spool valves opens and hydraulic pressure is introduced into the low-speed to
mid-range speed switching system. This causes the timing piston to slide and connect the
primary and secondary rocker arms. Thus, both the primary and secondary valves are
subsequently activated by the primary cam.

Hydraulic pressure
for low-/mid-range

speed switchover


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At High Engine Speeds
The second spool valve opens and hydraulic pressure is introduced into the mid-range
speed to high-speed switching system. This causes the synchronizing pistons to slide and
connect the primary and secondary cams to the mid cam. This cam, profiled for high speed
situations, will subsequently activate the primary and secondary valves.
Timing piston

Synchronizing piston B
Synchronizing piston A

At mid-range speeds

At low speeds

At high speeds

Camshaft

Stationary

Crank angle (deg.)

2-valve operation with
high speed cam

Lift (mm)

2-valve operation
with low speed cam

Lift (mm)

Lift (mm)

1-valve operation

Crank angle (deg.)

Crank angle (deg.)


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Valve Timing Change Condition
Engine speed:

Low to mid range ... 3,000 min-1
Mid to high range ... 6,000 min-1
M/T ... Over 15 km/h
A/T ... Over 10 km/h
Low to mid range ... Over 40 degrees Celsius
Mid to high range ... Over 60 degrees Celsius
Determined from throttle opening angle

Vehicle speed:
Engine coolant temperature:
Engine load:

„ VTEC-E Construction
Although incorporating a timing plate in the same way as the New VTEC system, VTEC-E
does not have a mid cam or a mid rocker arm. Accordingly, there is no lost motion assembly
either.
The switching system is comprised of a timing piston, a synchronizing piston, and a stopper
piston.
2

3

1
2

3

5

4

8
4

5
7
6

1 Timing plate
2 Primary rocker arm
3 Secondary rocker arm
4 Synchronizing piston
5 Timing piston
6 Intake valves
7 Camshaft
8 Stopper piston