Q. A. How is the power to weight ratio of an engine sought to be increased by continuous development? Discuss the limiting factors. What is the typical power to weight ratio of a slow speed marine diesel engine of current generation?
B. Discuss the importance of turbocharger compression ratio in increasing the MEP. Why is it necessary to introduce 2 stage turbocharging?
Answer:-
A) To improve power to weight ratio significant changes has been made over the years
1. Increase MEP for same bore engines at same speed.
2. Use thin walled shell bearings.
3. Crosshead pin structure with reduced size and weight
4. Introduction of reduced diameter and length of twin tie bolts.
5. Bottom end of tie rods threaded to the transverse girder of bed plate and hence reduction in material.
6. High piston top land. It ensures that the top piston ring is not exposed to high combustion chamber temperature. Along with high top land, CPR piston rings are used to reduce the pressure drop across the ring and hence prevent ring collapse.
7. Reduce height of the cylinder block and the cylinder cam box.
8. Holding down bolts in a sunken structure to make compact design.
9. Use of 2 stage turbocharger so as to have a higher charge air density. Hence more fuel can be burnt for same size of engine.
10. Reduce the spacing of fwd cylinders in MCC engines.
11. Use of single piece main bearing top half MCC instead of 2 (MCC)
12. Improve compression ratio.
13. Due to reduced mass of moving parts the second order moment decrease by about 8% improving the vibration condition.
14. Development of electronic engines such as MEC and RTFLEX
Typical specific weight ME engines = 13.3 Kg/KW
MC Engines = 16.2 Kg/KW
Limiting factors:
1. Part must have sufficient strength to withstand mechanical load.
2. Parts must have sufficient strength to withstand thermal load.
3. Increase in MPS too much will effect scavenging and combustion.
4. Pressure ratio of turbocharger.
5. Limitation of cylinder lubrication in case of higher thermal loads.
6. Limitation of Pmax for hydrodynamic oil film to exist.
7. Material of components to withstand high temperature corrosion and thermal loads.
B) Importance of turbocharger compression ratio in increasing MEP
Higher turbocharger compression ratio will mean a higher charge air density in the cylinder; hence a greater amount of fuel can be burnt. This would result in higher MEP (the average pressure that acts on the piston during the power stroke).
2 Stage turbocharger
1. There is a limit to the max pressure ratio that can be generated using a single stage turbocharger hence the gain in MEP.
2. It is well established that a air compressor uses intercooler to take heat of compression from first stage compressed air. By removing the heat of compression the temperature of air entry at next stage reduces and becomes denser, this improves the compression efficiency and the output pressure.
3. The above said principle is being employed in the case of the 2 stage turbocharger. The compressed from the first stage passes through the intercooler when the heat of compression is removed by the cooling water.
4. The compressed air temperature drops to near ambient temperature in the path of the intercooler, thereby increasing the charge air density.
5. Once again the compressed, cooled charged air enters the 2nd stage of turbocharger system where the pressure ratio are increased much beyond conventional turbocharger pressure.
6. The results are high power density and in conjunction with miller’s cycle, lower emission and lower SFOC.
7. Compression approaches near isothermal in multistage compression.
8. Work done In compressing the air is reduced thus power can be saved.
