Operational Information

The Medium Speed 4 Stroke Trunk Piston Engine

The Rocker Gear and Valves

 

 

The 4 stroke marine diesels used for main or auxiliary power on board ship will have multiple inlet and exhaust valves fitted to the cylinder heads. On the medium speed engines this normally takes the form of two inlet and two exhaust valves per unit. The reasons for this are as follows:

  • The area of the valve openings must be large enough to provide for an efficient gas exchange process. If the area is too small then not enough air will be pushed/drawn into the cylinder during the induction stroke, and on the exhaust stroke the engine will be doing work pushing the exhaust gas out of the cylinder.

  • The cylinder head must accommodate inlet and exhaust valves, so unlike a two stroke engine, one large central exhaust valve is not possible.

  • If the valves are too large, then the strength of the cylinder head will be compromised.

  • Keeping the exhaust valve temperature within acceptable limits is of paramount importance. It is easier to cool a smaller valve.

  • The moving parts and springs are of smaller proportions reducing the inertia of the parts and the power demand on the engine.

  • A symmetrically designed strong cylinder head  is achieved.

Exhaust valves are subject to arduous conditions, and require regular overhaul. To aid this, exhaust valves are often fitted in separate cages. This allows the exhaust valve to be changed and overhauled without removing the cylinder head. The cages have water cooling passages connected to the cylinder head cooling water.

The cage is of cast steel. The cooled seats are made from a heat resistant molybdenum steel which may be stellite faced. The exhaust valve may be of a similar material or of a nimonic alloy.

Inlet valves are subject to much less arduous conditions and are not usually fitted in separate cages.

Two different sized springs are fitted to aid positive closing of the valves. The reason for fitting two springs are that if one fails, the other will prevent the valve dropping down into the cylinder. The two springs have different vibration characteristics, so the incidence of resonance is reduced. (resonance is where two items vibrate at the same frequency thus the amplitude of the vibration is amplified.)

Exhaust valves are designed to rotate in service. The reasons for this are to prevent uneven temperatures  so it does not distort and leak by, and to help dislodge any build up of deposits on the valve and seat which may prevent the valve closing properly and lead to "hammering" of the seating faces. A mechanical method is generally used, and this is either the "rotocap" or the "turnomat". Winged rotators or spinners as used on the 2 stroke engine exhaust valves can also be used, but this entails using a ball bearing race between the spring carrier and the cover.

Burning Out of Exhaust Valves

Once an exhaust valve does not seat correctly, the high pressure burning gas will pass across the faces of the valve and seat during the power stroke. This will cause the temperature of the valve and seat to rise in this area, weakening the material and distorting the surfaces. The velocity of  the burning gas will erode the surface, allowing more gas to leak by. The temperature of the valve in this area will rise further, leading to further burning and greater distortion. The first indication of a valve burning out will be a rise in the exhaust temperature, which will rapidly increase together with a loss of power from the unit.

Vanadium slag deposits which occur at temperatures above 540 C cause corrosion of the valve surfaces which can lead to exhaust gas blow by. This is combated by effective cooling and  the use of suitable materials (stellite and nimonic alloys).

Rocker Gear

Master and Slave

Most medium speed four strokes use push rods and rocker gear to open and shut the valves at the correct time. Operated by cams, mechanically timed to the crankshaft, the pushrods transmit the motion to the rocker gear, which pushes the valves open at the correct time. Because there are two of each valve mounted in the cylinder head, the rocker gear must operate both valves simultaneously. Various methods are used including master and slave arrangements (Sulzer ZA40) and yoke (Fiat).

A hydraulic operating system was introduced in place of push rods and rocker gear on the Sulzer ZA50. This had the advantage of being able to adjust the timing of the closing of the valves to suit operating conditions.

 

Yoke

Rocker or Tappet Clearances

Rocker or Tappet clearances refer to the clearance between the top of the valve spindle and the rocker arm. It is to ensure that the valve closes properly when it expands as it gets to operating temperature. Clearances are set according to manufacturers instructions, but usually done with the engine cold, and with the push rod follower on the base circle of the cam. (one way of ensuring this is to turn the unit being adjusted to TDC on the power stroke.)

If the clearance is too small, then not only is there a chance that the valve will not close properly when it comes up to temperature, but it effectively will open early and close late.

Conversely if the clearance is too large, then the valve will open late and close early

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