Modern Valve Timing Introduction engine has a lower volumetric efficiency in high speed. As the engine speed increases, intake valve open a shorter time. Less time for air fuel mixture enter the cylinders. If the intake valve opened earlier at high speed, the air fuel mixture would enter earlier so more would get in One way to improve volumetric efficiency by using variable valve timing system such as vtec system or VTC system VTC (variable timing camshaft) has a variable connection between the camshaft sprocket and the camshaft. Function of variable timing camshaft to advance and retard the engine valve timing (camshaft position) automatically depends on engine speed, engine load and driver request 2. to advance and retard engine valve timing automatically for emission control – to reduce carbon monoxide, hydro carbon & NOx

Modern Valve Timing 2 ways how the camshaft sprocket and camshaft are connected: hydraulic piston operated by engine oil pressure and a solenoid operated oil control valve When the electronic control module (ECM) signals the solenoid to close the valve, the oil pressure forces the piston forward. As the piston moves, the internal teeth of the piston slide up the inclined teeth on the helical drive gear. This moves the camshaft ahead and advances camshaft timing and intake valves open earlier. hydraulic vane operated by engine oil pressure and solenoid operated oil control valve - There are two oil passages (inlet & outlet) that go to a different chamber inside the VTC unit. - Each vane inside VTC unit divides the chamber into two sections that filled with engine oil. High pressure engine oil is required to damp the camshaft vibration. - When electronic control modules (ECM) send signals, solenoid open the valve, the oil pressure go to one side of the chamber to push the camshaft advance or retard the valve timing. - Other side of the chamber acts as a damper.

Modern Valve Timing

Modern Valve Timing Variable Valve Timing and Lift System change both valve timing and valve lift to gives the engine that smooth idle of a passenger car engine and high speed performance of a racing engine. designed to open the valves earlier, wider, and a longer duration of time, thus allowing more air fuel mixture to enter the cylinder Used three cam lobes. Each rocker arm has a built in hydraulic piston to engage or disengages that rocker arm. 1. The two lobes for low RPM function – for 1st & 2nd intake valve. 2. One lobe in the middle for high RPM function. two low RPM lobes actuate the valve rockers & pushes the valves open. The high RPM lobe actuates a follower. ECM sends a voltage signal to a solenoid and its opens and closes. Engine oil pressure is sent to the pistons in the selected rocker arms to engage or disengage depend on engine speed, engine load, vehicle speed, and coolant temperature.

Modern Valve Timing

Modern Valve Timing Basic operation Low RPM operations The two outer cam lobes directly actuate the two valve rockers. These low PRM lobes are optimized for smooth operation and low fuel consumption. The high RPM lobe actuates the follower. But since the follower isn't connected to anything, it doesn't cause anything to happen.

Modern Valve Timing High RPM operations - Oil pressures push a metal pin through the valve rockers and the follower, effectively binding the three pieces into one high RPM lobe pushes out further than the low RPM lobes, so the two valve rockers now follow the profile of the high RPM lobe. improved breathing allows the engine to sustain its torque output as RPM rises, thus resulting in higher power output

Modern Valve Timing 3 stage Vtec Operation Stage 1 - Both intake valve rockers operate independently. At this time only one intake valve opens and closes since the other intake valve follows an almost round cam profile. - The almost round cam profile is designed to open the valve just tall enough to avoid pooling of fuel above the valve. - This mechanism same as the low-RPM operation of the VTEC-E mechanism, resulting in great low RPM fuel economy.

Modern Valve Timing Stage 2 - Starting at about 2500 RPM, the first oil pressure is applied, pushing a pin to lock the two intake valve rocker arms together. - Both valves now follow the same low RPM cam profile in their operation. Thus far the operation has just been like a normal VTEC-E mechanism.

Modern Valve Timing Stage 3 - Second oil pressure is applied at about 4500 RPM. The second oil pressure pushes another pin through the valve rocker arms and a cam follower that is between the two valve rocker arms. - The cam follower operates from the high RPM cam lobe so now both intake valves follow the high RPM cam profile.