THEORETICAL HEAT CYCLE
Marine diesel engine works on dual combustion cycle.
DUAL COMBUSTION CYCLE : The behaviour of many slow-speed engines is more correctly represented by a mixed-cycle or dual-cycle. This was achieved by the “blast injection principle”. Today, diesel (HFO for marine engines) is universally used for reciprocating engines in which heat as induced by the compressed air in the cylinder, ignites a finely atomized spray of fuel. Thus, the theoretical cycle on which modern diesel engine works is presented by the dual cycle illustrated in the diagram below:
FIG.: DUAL COMBUSTION CYCLE
- Starting from point 1, the air is compressed adiabatically to point 2.
- Fuel injection begins at 2, and heat is added to the cycle partly at constant volume 2-3, and partly at constant pressure 3-4.
- At 3, expansion begins and continues adiabatically till point 5.
- Then the heat is rejected to the exhaust at constant volume 5-1.
Note: Otto cycle has highest regards to fuel consumption as it rejects minimum heat but highest pressure is high. Dual cycle has least economy in fuel consumption but maximum pressure is low. Dual cycle falls between these two cycles i.e. fuel consumption is not very high as well as peak pressure is within limits.
The actual/ practical cycle is slightly different from the theoretical cycle in the following:
Reference: Marine diesel engines by Deven Aranha