Fuel Injection and Combustion Timing
Large marine two-stroke engines are compression-ignition engines, so there is no spark
ignition system (no spark plug). Instead, fuel is injected directly into the cylinder near the end of
the compression stroke. The fuel atomizes, mixes with the hot compressed air, and auto-ignites.
Timing diagrams are used to show the start of injection and end of injection. Fuel injection
begins shortly before top dead center so that peak cylinder pressure occurs just after top
dead center during the power stroke; this allows the expanding combustion gasses to act on the
piston when the crankshaft geometry is most favorable for producing torque (a form of power).
If peak pressure occurs too early before top dead center, the piston will be forced upward against
the rising pressure; this creates mechanical stress, rough running, and reduced overall engine
efficiency.
Good to know - excessively early combustion can produce sharp pressure rises and a metallic knocking
sound, often referred to as 'knock' or 'diesel knock'.
Marine Slow Speed Two-Stroke Engine
Reading a Two-Stroke Timing Diagram
When reading a two-stroke timing diagram, focus on the sequence and duration of port and valve
openings rather than the separate strokes. Small changes in port height or piston geometry can
significantly affect timing, impacting power, efficiency, and emissions. Short durations with early
exhaust port closing favor low to mid RPM torque, while longer durations with late exhaust port
closing produce high RPM power. Comparing diagrams makes it possible to understand the
engine’s intended RPM range and application.
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Two-Stroke Engine Timing Diagrams Explained | pg 7