How a Battery Functions

VTEC System Introduction

Today’s auto industry faces the ever important challenge of producing engines that minimize both fuel consumption and pollution while also providing higher levels of power output. Honda’s Variable Valve Timing and Lift Electronic Control (VTEC) system constitutes one of the technological responses to these conflicting demands.

At the time of writing, Honda vehicles currently use the following six types of VTEC system:

• DOHC VTEC
• SOHC VTEC
• VTEC-E
• New VTEC
• 3-stage VTEC
• i-VTEC

Although the basic operation of each of these systems is identical, they vary with respect to their principle purpose – that is, some will be more suited to dynamic, sporty driving while others support increased fuel economy. Accordingly, different components will be employed in the various systems. This article will present an overview of the VTEC system using the most basic type of system – the SOHC VTEC – as an example.

Principle of Operation

A VTEC system is fitted to the intake valves of a 4-valve-per-cylinder engine. (Note that some DOHC engine has a VTEC mechanism on both the intake and the exhaust valves.)

In a VTEC system, two cam lobes with different cam profiles – i.e., for low-speed and highspeed operation – are provided for each intake valve. At a certain predetermined condition, an electronically-controlled hydraulic system operates, thus changing the cam which operates each of the valves. In this way, the VTEC system provides optimum valve timing and lift in all speed ranges, thereby maximizing combustion efficiency. As a result, the VTEC system reduces fuel consumption and increases power.

VTEC System Major Components

The SOHC VTEC system consists of the following main components:

• Camshaft
• Rocker arms
• Lost motion mechanism
• VTEC solenoid valve
• VTEC control system 

1. Synchronizing piston A
2. Lost motion assembly
3. Synchronizing piston B
4. Mid rocker arm
5. Secondary rocker arm
6. Primary rocker arm
7. Camshaft

Camshaft

In a conventional engine, each of the valves will be operated by its own dedicated cam. However, an SOHC VTEC engine has three individual cams – namely, the primary, secondary and mid cams – for the two intake valves which are fitted to each cylinder.

At low and mid speeds, the two valves are operated by the primary and secondary valves respectively. These two cams have slightly different cam heights and can consequently create an optimal turbulence condition within the combustion chamber.

At high speeds, both valves are operated by the mid cam.

Rocker Arms

The two intake valves which are fitted in the cylinders of an SOHC VTEC engine are provided with a set of three rocker arms – as above in the case of the cams, these are referred to as the primary, mid, and secondary rocker arms. The primary and secondary rocker arms make actual contact with the valves. The rocker arm assembly incorporates synchronizing pistons, a stopper piston, and a spring so that the rocker arms can be linked and unlinked during engine operation.

No hydraulic pressure is applied to the synchronizing pistons at low and mid speeds. Referring to the following drawing, the synchronizing pistons and the stopper piston are pushed leftward, and the three rocker arms operate independently at these speeds. In this condition, the primary and secondary rocker arms transmit the motion of primary and secondary cams to the primary and secondary valves respectively. The mid rocker arm is operated by the mid cam, but the arm moves freely without affecting the valves. Furthermore, the lost motion mechanism suppresses chattering of this arm. 

1. Camshaft
2. Stopper piston
3. Secondary rocker arm
4. Mid rocker arm
5. Primary rocker arm
6. Synchronizing piston B
7. Synchronizing piston A
8. Rocker shaft

At high speeds, hydraulic pressure is applied to the synchronizing pistons. The pistons move sideways against the return spring force and combine the three rocker arms as one integrated unit. In this condition, the mid rocker arm transmits the motion of mid cam to both valves via the primary and secondary arms.

VTEC Valve Lost Motion Mechanism

 At low and mid speeds, the lost motion mechanism suppresses the chattering of the mid rocker arm. At high speeds, it functions as an auxiliary spring 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. Camshaft

VTEC Solenoid Valve

The VTEC solenoid valve controls the hydraulic pressure which acts on the synchronizing pistons and the stopper piston. When the VTEC control system activates the solenoid valve, it opens an oil passage between the oil pump and synchronizing pistons. These pistons will then move to lock the rocker arms together. When the control system subsequently deactivates the solenoid valve, the hydraulic pressure is released, the synchronizing pistons are pushed back to their original positions by the return spring, and the rocker arms are disconnected from each other.

This drawing shows a DOHC VTEC system.

1. Rocker arms
2. Solenoid
3. Pressure switch
4. Spool valve
5. Filter

VTEC Control System

The VTEC system is electronically-controlled by the ECM/PCM. Using a variety of different sensors, the ECM/PCM monitors the engine speed, the extent of engine loading, the vehicle speed, the engine coolant temperature, and other similar factors. Using this data, the ECM/ PCM can determine the engine’s operating condition and then activates the VTEC solenoid valve accordingly. A hydraulic switch senses the pressure in the synchronizing pistons’ oil passage and provides feedback to the ECM/PCM when rocker arm switch-over is performed.

1. To rocker arms
2. Hydraulic pressure switch
3. VTEC control solenoid valve
4. Engine speed (Crank angle sensor)
5. Load (MAP sensor)
6. Vehicle speed (Vehicle speed sensor)
7. Engine coolant temp. (ECT sensor)