For CI engine compression ratio is between 12.5 to 13.5. For life boat engine, compression ratio is 18 to 20.
Why the compression ratio of lifeboat engine is more?
Compression ratio improves the starting ability of the engine. So, in order to start the lifeboat at any temperature, compression ratio of lifeboat engine is kept more so that lifeboat engine can start even at low temperature.
SOLAS requirement for lifeboat engine states that any starting method should be capable of starting lifeboat engines at -15˚C of ambient temperature within two minutes of commencing of starting procedure.
Compression ratio of main engine can not exceed a certain limit because of following reasons:
- If compression ratio is increased, pressure inside the cylinder increases.
- High compression ratio increases temperature which results in increase in thermal stresse in the engine. Temperature of other parts also goes up.
- High compression ratio also results in increased load on bearing and hence weight and cost of engine will increase.
Advantages of higher compression ratio:
- Much better performance
- Higher relative fuel economy
- Higher usable power throughout
- Better emissions.
Advantage of lower compression ratio:
- Cheaper to build, run and repair.
- Lasts much longer
- Can be air cooled.
- Less noise and vibration.
- Can be run on relatively lower quality fuel.
- Do not generate as much heat