Answer EKM Question 34

Q34. (A) What is “virtual tappet” in the hydraulically actuated air spring return exhaust valves, and how is it set.

(B) Explain why the damage occurs to the seats of the exhaust valves due to furrowing and cutting.

(C) How an incident of “valve drop” leading to extensive damage to running gear can occur.

Answer: A. Virtual Tappet (or Hydraulic tappet):

The definition of Tappet is that a mechanical part that reciprocates to receive or transmit intermittent motion. The part of an internal-combustion engine that transmits motion from the camshaft to the push rods or valves.

Exhaust Valves actuated by means of a push rod and rocker unit face a problem of valve expansion during service. This require a clearance to be kept between rocker arm and valve stem called tappet clearance.

In Hydraulically operated valves there is no such mechanical means to push the exhaust valve down to open it but the oil pressure is used to carry out this job. Thus it is termed as virtual tappet or hydraulic tappet.

We know the tappet is to be provided with a clearance to compensate the thermal expansion of valve stem. The diagram below shows the arrangement of Hydraulic oil cylinder.

At the top the valve spindle is in the form of a piston. A damper piston is mounted at the top of the spindle with a compressed spring. At the end of upward motion of the spindle, first damper piston enters the channel, which slows down its upward motion and prevent hammering.

Orifice plug (or bleed valve) provided at the top of the hydraulic cylinder drains off the excessive oil. This bleed valve allows a controlled amount of oil to pass to air cylinder located  below the hydraulic cylinder for lubrication.

Setting: The virtual tappet setting means the setting of the timing of exhaust valve opening and closing. This is checked and set during maintenance of valve actuator by the following methods:

(a) Check the cam position: For checking the postion of the exhaust cam, turn the engine Ahead direction until the roller guide has been lifted 10mm. This lifting can be measured on the activator plunger.
If the cam has been correctly adjusted, the crankthrow for the cylinder concerned will then be in a position of $\displaystyle \mathrm{A^o}$  past TDC.
To check the lead angle, continue turning in Ahead direction until the roller guide has been lifted 10mm, corresponding to turn of $\displaystyle \mathrm{B^o}$  of the crankshft.
The lead angle is then
$\displaystyle \mathrm{180^o - \frac{A^o+B^o}{2}}$
The correct position of the cam must be within $\displaystyle \mathrm{+/- 0.5^o}$ .

(b). Roller lift measurement:

Place the roller guide vertically upside down on a couple of planks with the roller hanging freely. Place a dial gauge against the roller, then lift the roller as much as the clearance permits, which makes it possible to read the clearance on the dial.
Also slide bearing clearance can be measured.

B. Explain why the damage occurs to the seats of the exhaust valves due to furrowing and cutting.

Valve not closing completely, through the incorrect tappet or roller clearance, or build-up of deposit on exhaust valve seat from fuel impurities or excess  lubricating oil, or valve spindle becoming sluggish due to dirty oil and carbon build-up in valve spindle guide. The valve may remain slightly open and start leaking. The passage of hot gas over a small area of seat causes distortion which then increases the amount of leakage. The temperature of the valve and seat in the area of the leak rises considerably, the strength of the metal is impared, and its resistance to the erosive effect of the hot gases is seriously reduced. During the combustion period gases at high temperature in the cylinder expand across the location of the leakage at very velocities. At this stage across the valve seat increases very rapidly and large burned areas occur in the seat and valve.

C. How an incident of “valve drop” leading to extensive damage to running gear can occur.

Absence of closing air under the air piston will cause the valve to remain in open state. The upward motion of the piston will hit the valve and this impact will cause the mechanical damage to both piston and valve.

During this period the air charge which needed to be compressed will not be present and atomized fuel will pass with scavenge air to the exhaust pipe and burning will take place outside the engine.