A diesel engine relies on a controlled explosion in four stages: intake, compression, explosion and output (see the first video on the right, above; Schwenke, 2013). Unlike gas, diesel ignites on compression alone in the cylinder: no spark plug is needed. The explosion is made possible by high compression of a mixture of air, admitted by intake valves, and diesel, atomized by injectors. The explosion pushes the piston downwards, turning the camshaft, but also generates a lot of heat. The residual gas outlet valve is opened as the piston descends. The camshaft is connected to the transmission, which slows shaft rotation and turns the propeller (if the transmission is engaged).
The explosion itself technically requires only compression, air and diesel. The diesel is filtered and fed to the injectors, which spray it in fine droplets into the cylinder (intake). Compression requires sturdy metal parts (block and cylinder) of almost equal dimensions.
It is the friction of the metal that requires oil to reduce friction and lower the temperature to prevent the engine from melting (!). The oil circulates throughout the engine to be cooled by the cooling system and then falls into the oil sump. The cooling system passes seawater, which is then injected into the combustion residue (and directed to the exhaust).
The rotation of the camshaft not only drives the propeller (at the rear), but also the belt and the alternator, which recharges the batteries.
Identifying Components on an Actual Engine
Engine Summary from an Input/Output Perspective
In short, a diesel engine requires the following inputs: diesel fuel, air, sea water, coolant and oil. As outputs, a diesel engine produces hot water, hot oil, hot air, carbon dioxide and rotary motion.
The block and cylinders used for combustion are very robust and most certainly, cannot be repaired at sea. It’s everything to do with inputs and outputs that are generally problematic if poorly maintained. These are also the only elements over which we have control at sea.
- Diesel should be clean (filtered) and without water.
- Air should have little to no particles.
- The engine must be able to cool off (water, cooling liquid and oil).
Remembering the inputs and the outputs is useful, as most diagnostics procedures are based on their examination.
Want to Know More?
Have a look at the « boatwork » section of this website for more practical tutorials and techniques.
References
Jollands S. (s.d.). How diesel engines work, webpage retrieved in November 2023 from this address.
Schwenke T. (2013). How diesel engines work, YouTube video retrieved in November 2023 from this address.
Veritassium (2012). Fire Syringe, YouTube video retrieved in November 2023 from this address.
Yanmar (s.d.). 3YM30AE, webpage retrieved in November 2023 from this address.