VVT-i (Variable Valve Timing- Intelligent)

Expectation needs to fulfill for an engine:

· Good gas mileage.

· More power.

· Runs smoothly with less noise.

· Environmentally friendly.

ABSTRACT

The VVT-i is an all-comprehensive engine control system that has achieved high levels of seemingly opposing performance of “higher power output”, “lower fuel consumption”, and “cleaner exhaust gases”.

BASIC OPERATION OF FOUR-STROKE ENGINE

· The basic operation of four stroke engine must be reviewed before we learn how VVT-i operates in order to get better understanding.

PROCESS OF DRAWING AIR-FUEL MIXTURE INTO CYLINDER

· Intake lag

o As the piston begins its descent from its top-dead-center (TDC) , a vacuum is created in the cylinder, causing the pressure in the cylinder to become lower than that of the intake port. The air-fuel mixture that was standing by in the intake port does not move immediately when the vacuum that is created in the cylinder is ready to draw the air-fuel mixture into the cylinder.

o The drawing of the air-fuel mixture into the cylinder is slightly delayed in relation to the descent of the piston, a phenomenon known as “intake lag”.

o Due to the “intake lag”, even after the piston turns around at its bottom-dead-center and begins its descent, the pressure in the cylinder for sometime remains lower than that of the intake port.

o Until this difference in pressure is eliminated by further ascension of the piston, the air-fuel mixture continues to be drawn into the cylinder.

o Engines are normally designed with these factors in mind.

o Consequently, in order to draw in as much air-fuel mixture as possible during the intake stroke, it is important to take the “intake lag” into consideration when determining the timing of the opening and closing of the intake valve.

ENGINE RPM AND VALVE TIMING

· During low engine rpm

o The intake lag varies in accordance with the engine rpm because the piston's descent speed is proportionate to the engine rpm.

o Because the piston descends at a slow speed and the air-fuel mixture also flows at a slow speed, only a slight resistance is created against the drawing of the air-fuel mixture into the cylinder.

o Therefore, the air-fuel mixture in the intake port is drawn into the cylinder without lagging too far behind the movement of the piston.

o Because the "intake lag" is short, the difference in pressure that is created in the cylinder and in the intake port at the piston's bottom-dead-center is only slight.

o Therefore, the pressure in the cylinder becomes higher than the pressure in the intake port as soon as the piston starts to ascend.

o Therefore, a larger amount of air-fuel mixture can be drawn in if the intake valve is opened near the piston's top-dead-center and closed near the bottom-dead-center.

· During high engine rpm

o Because the piston descends at a fast speed and the air-fuel mixture also flows at a fast speed, a considerable amount of resistance is created against the drawing of the air-fuel mixture into the cylinder.

o Therefore, the air-fuel mixture in the intake port is drawn into the cylinder lagging considerably behind, without being able to catch up with the movement of the piston.

o When the “intake lag” becomes long, even after the piston turns around at its bottom-dead-center and begins its ascent, the pressure in the cylinder for some time remains lower than that of the intake port.

o Thus, the air-fuel mixture continues to be drawn into the cylinder for quite some time after the piston has begun its ascent.

o Therefore, during high rpm operation, a larger amount of air-fuel mixture can be drawn into the cylinder when the intake valve is opened earlier than the top-dead-canter, and closed later than the bottom-dead-center.