Working principles
Governing: MCA SV Marine Diesel Engineering written examination syllabus, item 1. Builds on: AEC 1 and AEC 2.
Why this comes first
Everything else in the module hangs off the cycle. The syllabus asks for the four-stroke cycle, its timing diagram, its PV diagram and how the two relate, the scavenge process, valve overlap and starting-air timing. It also asks for the basic difference between compression ignition and spark ignition. You will be drawing these in the exam, so practise sketching them freehand.
The four-stroke cycle
| Stroke | Piston | Valves | What happens |
|---|---|---|---|
| Induction | TDC to BDC | Inlet open | Charge air drawn or pushed in |
| Compression | BDC to TDC | Both shut | Air compressed and heated above fuel self-ignition temperature |
| Power | TDC to BDC | Both shut | Fuel injected just before TDC, burns, gas expands |
| Exhaust | BDC to TDC | Exhaust open | Burnt gas pushed out |
One cycle takes 720 degrees, so each cylinder fires once every two revolutions. At 1,800 rpm that is 900 power strokes per cylinder per minute. An even-firing engine spaces its firings at 720 divided by cylinders: 120 degrees for an in-line six, 60 for a V12, 45 for a V16.
A diesel is compression ignition: it compresses air alone and injects fuel into it. A petrol engine compresses a mixture and lights it with a spark, so it needs a lower compression ratio to avoid detonation.
The timing diagram
Draw a circle, TDC at the top. Events never happen exactly at the dead centres:
- Inlet opens before TDC and closes after BDC, using the inertia of the moving air and charge-air pressure to fill the cylinder.
- Exhaust opens before BDC so pressure is falling before the piston starts pushing, and closes after TDC.
- Injection starts before TDC to allow for ignition delay.
- The arc where inlet and exhaust are both open around TDC is valve overlap.
Take the actual angles from the engine manual. Any figures you have seen in a textbook are illustrations, not rules.
The PV diagram and its link to timing
On the PV diagram, compression is the rising curve from BDC, combustion the steep rise and peak near TDC, expansion the falling curve, and the low pumping loop the exhaust and induction strokes. Each corner on the PV diagram maps to an event on the timing circle: exhaust opening is where the expansion line drops sharply before BDC. The enclosed area is work per cycle, which is the basis of the next-but-one lesson.
Scavenging and overlap
On a pressure-charged four-stroke, charge-air pressure is higher than exhaust manifold pressure during overlap. Air sweeps through, clearing residual gas and cooling the exhaust valve, seat and head. Too little overlap leaves hot residuals; too much wastes air.
Starting-air timing
In a direct air start engine the air start valve opens just after TDC on the power stroke and closes before the exhaust valve opens. With enough cylinders, at least one is always in the window, so the engine starts from any crank position.
The two-stroke
A two-stroke completes its cycle in 360 degrees, so each cylinder fires every revolution: 1,800 power strokes per minute at 1,800 rpm. Exhaust and scavenge happen together near BDC, through ports, or ports plus an exhaust valve (uniflow). It needs a scavenge air supply because there is no induction stroke. On yachts you meet it in outboards and older auxiliary engines; propulsion and generation are almost all four-stroke high-speed.
TIP — Draw, then label In the written paper, a clean labelled sketch earns more than a paragraph. Put TDC, BDC, IVO, IVC, EVO, EVC, injection and overlap on every timing diagram you draw.