Question

How do roller finger followers work in a motor vehicle engine?

Answer 1

The Roller Finger Follower is used in the valve train of passenger cars as part of a cam follower system that is capable of shutting off a cylinder while the engine is running.

The valve train system is a common system used for internal combustion engines. This

mechanism has a very important role in the insurance of the economic and ecological

performances of the internal combustion engine. When the valve train system is functioning at

high speeds, the dynamic behavior of the mechanism can be influenced negatively by the

elastic links between the components, the mass distribution and the friction from the

mechanism joints. Also, because of the high acceleration, may occur early fatigue of the

components, and finally the destruction of the valve train mechanism.

This study is focused on the dynamic behavior of the finger follower valve train system, in

particular on the influence of the camshaft speed. The purpose of this paper, is to present a

virtual model which can be used in the dynamic analysis of a finger follower valve train

mechanism, for determination of the accelerations, forces and torques, which can be used later

for inquiry of the possible causes that can lead to the mechanism failure.

roller finger follower to implement the variable lift function. This finger follower consists of 2 parts - an inner roller finger follower which acts on the intake valve directly, and an outer roller finger follower. They can be detached or locked together by a lash adjuster, which is driven by oil pressure and controlled by ECU.

As in most other VVL designs, each of its intake valve is served with 3 cam profiles, i.e. 2 identical high-lift / long-duration "fast cams" sandwiching a low-lift / short-duration "slow cam". They activate the intake valve via the roller finger follower. The outer fast cams press on the outer finger follower. At low rpm, the lash is unlocked, thus the outer finger follower moves up and down freely without actually pressing on the valve. Meanwhile, the inner slow cam acts on the inner roller finger follower and activates the valve, therefore the engine runs with low valve lift.

At high rpm, where more air flow is demanded, the lash locks the outer and inner finger followers together, thus the fast cams can activate the valve via the locked finger followers.

Because of the 2-piece finger followers, I suppose iVLC could introduce more frictional loss than most other VVL systems, especially in low-lift mode. The additional moving mass could also limit its revvability a little. Judging from its output figures alone, the first 2.5-liter iVLC engine does not show any obvious advantages over the old engine.

DESCRIPTION OF THE FINGER FOLLOWER VALVE TRAIN SYSTEM

This design variant of the studied mechanism has double overhead camshaft, and 4 valves per

cylinder. In order to simplify the virtual model, a single valve actuation system was chosen

for analysis. A general view of the studied mechanism is presented in figure 1a. In figure 1b it

is shown the mechanism scheme, and its componentents.

The spring and the hydraulic lash adjuster have an important role in this mechanism. The

hydraulic pivot element adjusts the thermal gap between the finger follower and the engine

valve. The valve spring has the role, to return the valve on its seat.

A roller (incorporated in the finger follower) reduces the friction forces between the cam's

lobe and the finger follower. Also, this mechanism is wear resistant; due the fact the system

tunes the thermal gap automatically. Another role of the finger follower is to multiply the

displacement generated by the cam, acting as a 3th order lever.