VTEC System Description
The VTEC improves fuel efficiency and reduces exhaust emissions at all levels of engine speed, vehicle speed, and engine load. The VTEC mechanism changes the valve lift and timing by using more than one cam profile.
The i-VTEC provided in this engine is a system to switch the valve control depending on the running status to two stages of a normal valve control and an all-cylinder valve stop control. In the normal valve control, the intake and the exhaust valve are allowed to operate and the vehicle is allowed to run based on the engine torque or the engine torque + IMA motor assist depending on the running conditions. This is for the purpose of providing an improved running performance and an improved fuel consumption. In the all-cylinder valve stop control, the operations of the intake and the exhaust valve are stopped and the vehicle is allowed to run based on the IMA motor torque. By stopping all cylinders, the effect of reducing emission gas is improved and a reduced friction loss and an increased regenerative energy by the IMA motor are realized. Thus, lower consumption is realized.
Construction
VTEC is composed of the VTEC camshaft, the VTEC rocker arm, and the rocker arm oil control valve.
VTEC Camshaft
The camshaft is structured so that each cylinder has an intake cam and an exhaust cam. The same cam nose is used both in the normal valve control and the all-cylinder valve stop control.
VTEC Rocker Arm
The rocker arm is structured so that the intake side is divided to two parts and the exhaust side is divided to two parts. The rocker arm includes a VTEC switching piston and a return spring.
Rocker Arm Oil Control Valve
The rocker arm oil pressure sensor A and the rocker arm oil pressure sensor B are provided in the rocker arm oil control valve and monitor the hydraulic status and the statuses of the rocker arm oil control solenoid and the valve.
Control
Based on the vehicle speed signal, engine speed signal, engine load signal, throttle aperture signal, and engine coolant signal sent from various sensors, the PCM controls the ON/OFF of the rocker arm oil control solenoid.
Operation
At Start
The valve is controlled by the valve lift mode 1 and the intake and the exhaust valve are allowed to operate normally.
At Abrupt Acceleration at Low Vehicle Speed
As in the start, the valve is continuously controlled by the valve lift mode 1 and the intake and the exhaust valve are allowed to operate normally.
At Cruise Running at Low Vehicle Speed
The valve is switched from the valve lift mode 1 to the all-cylinder stop mode. Thus, the operations of the intake and exhaust valves are stopped. The vehicle is allowed to run by the IMA motor torque.
At Abrupt Acceleration at High Vehicle Speed
The valve is switched from the all-cylinder stop mode to the valve lift mode 2. Thus, the intake and the exhaust valve are allowed to operate normally. In the valve lift mode 2, the VTEC system check is performed for about 2 seconds. Then, the valve lift mode 2 is switched to the valve lift mode 1. Then, the intake and the exhaust valve are allowed to operate normally.
At Cruise Running at High Vehicle Speed
The valve is continuously controlled by the valve lift mode 1 and the intake and the exhaust valve are allowed to operate normally.
At Deceleration
The valve is switched from the valve lift mode 1 to the all-cylinder stop mode. Thus, the operations of the intake and the exhaust valve are stopped. Then, just before the timing at which the vehicle is stopped, the all-cylinder stop mode is switched to the valve lift mode 1. Then, preparation for the engine start is performed.
At Vehicle Stopping
The valves prepares for the engine start while still being in the valve lift mode 1. In the valve lift mode 1, the rocker arm oil control solenoid A and the rocker arm oil control solenoid B are both at the OFF position. A hydraulic pressure from the oil pump flows into a normal operation control oil path. The hydraulic pressure is provided by connecting the primary side and the secondary side of the intake and exhaust rocker arm by a piston so that the intake and the exhaust valve are operated normally. In the all-cylinder stop mode, the rocker arm oil control solenoid A and the rocker arm oil control solenoid B are both at the ON position. The valve A and the valve B are in the operating status. The hydraulic pressure from the oil pump flows into the cylinder stop operation control oil path. The hydraulic pressure is applied to the respective pistons of the intake and exhaust rocker arm. By separating the primary side and the secondary side of the rocker arm, the intake and the exhaust valve are stopped. In the valve lift mode 2, the rocker arm oil control solenoid A is at the OFF position and the rocker arm oil control solenoid B is at the ON position. A hydraulic pressure from the oil pump flows into a normal operation control oil path. By turning the rocker arm oil control solenoid B ON, the valve B is allowed to operate quickly in order to cope with a delayed response from the stop operation control to the normal operation control. The hydraulic pressure is applied to the respective pistons of the intake and exhaust rocker arm. By connecting the primary side and the secondary side of the rocker arm, the intake and the exhaust valve are operated normally. After the switching to the valve lift mode 2, the VTEC system check is performed for about 2 seconds.