Answer EKM Question 3

Q3. With reference to the 4-stroke medium speed engines:

A. Define the cause and effect of thermal stressing in cylinder Heads, liners and pistons.

B. Explain why thermal stressing is aggravated with increased cylinder bore.

C. Explain how stress concentration and its effects are relieved by maintenance and operational practices.

Answer:

A. Define the cause and effect of thermal stressing in cylinder Heads, liners and pistons.

Cooling presents problems as it also induces thermal stress due to different rates of expansion across the section. Consider a liner wall section with temperature $\displaystyle \small \mathrm{T_H}$  on the combustion chamber side and $\displaystyle \small \mathrm{I_C}$ on the cooling water side. With a thin section most of the temperature change on the hot side from the combustion gases to the actual material takes place in the fluid whilst on the cold side most of the change takes place in the cooling water at the interface with the metal. For a thick section there is little temperature change in the fluid at these interfaces so the actual temperature change in the material is higher. The larger temperature difference in the material means a greater differential expansion, if the hot side was allowed to expand freely. This is not the case in an engine component such as a cylinder liner, cover or piston as thermal expansion is restricted. The hot side which wanted to expand cannot expand as much as it wishes and is subject to a compressive stress, while the cold side is subject to a tensile stress in order to balance the compressive stress induced in the hot section. Tensile stress is a problem because when added to the tensile stress induced due to cylinder pressure it can put the component at stress levels which will cause fatigue cracks to develop and propagate. Ideally the cooling medium should be placed as close as possible to the source of the heat as that allows heat to be removed without causing large temperature gradients and thermal stress. The problem is that high working pressures require thick section material in order to resist the mechanical loading, which again makes bore cooling a solution as it allows for thick sections and a cooling arrangement close to the heat source..

B. Explain why thermal stressing is aggravated with increased cylinder bore:

Thermal stresses are aggravated with increase in cylinder bore:
- Hoop stress in cylinder liners can be expressed by tie formula =$\displaystyle \small \mathrm{\frac{PD}{2T}}$ .
Therefore if the pressure (P) in the cylinder remains the same, but the Diameter (D) increased then to keep the hoop stresses the same, the thickness of the material (T) must be increased. Increased thickness leads, as described above, to increase thermal stressing.
- In the case of pistons, increased cylinder diameter will lead to a larger unsupported area of the crown because this must be strong enough to withstand the thickness of the material must be increased.
- In the case of cylinder heads, the thickness of the flame plate must be increased for similar reasons.

C. Explain how stress concentration and its effects are relieved by maintenance and operational practices.

- Modern engines employ bore cooling in the cylinder liner to bring the cooling water as, close as possible to the liner surface to reduce thermal stresses to a minimum without compromising the strength. Some cylinder heads also employ bore cooling and sulzer employ cooling bores combined with oil jet sprayers on their two stroke engines.
- Reducing the thermal stressing during operation is achieved by the engine is thoroughly warmed through prior to starting. When ramping up to MCR this should be done in accordance with engine operation instructions, and again when the slowing down in preparation for standby. Because a build-up of deposit externally or internally will have an insulting effect , increasing the likelihood of increased stressing cooling water should be adequately dosed with nitrite/borate treatment to prevent scale formation and corrosion, and cooling water outlets limited to 80 deg cel.
- oil cooled piston returns on two stroke engines should be monitored to ensure flow is adequate and that the temperature does not rise above 55 deg cel or else carbon deposit are likely to form on the underside of piston crowns.
- Fuel should be correctly timed and injection equipment maintained in good order to prevent deposit building up on crowns, and cylinder oil, whilst Maintained in good order to prevent deposit building up crowns, and cylinder oil, whilst maintained at an adequate level commensurate with minimal liner wear , should not be increased beyond that level because of its ask forming tendencies.
- When withdrawing cylinder liners, ensure the cooling space is clean and free from scale when opening up pistons crowns again ensure the cooling space is clean. If the pistons are oil cooled (as the majority are) and they have a layer of burnt carbon, then this is the best removed using a shot blasting technique.