Four-Stroke Aux Engines — Layout to Lubrication

1. Speed Bands & Placement Logic

Each speed band has its duty:

Slow two-strokes

Turn propellers directly.

Medium-speed four-strokes

Generate power and gear down to shafts.

High-speeds

Drive road and yacht machinery.

Four-strokes win propulsion where headroom is tight — ferries, passenger ships — and where diesel-electric lets engines sit anywhere at steady efficient revs instead of in line with shafting.

Medium band runs 250–850 rpm with 2,000+ kW per cylinder at the top end, trading shorter maintenance intervals for power-to-weight no slow-speed can match.

• Slow/medium boundary: 300 rpm
• Medium/high boundary: 900 rpm
• Per cylinder, largest fours: 2000 kW

2. Layout — Fixed Frame, Moving Heart

Fixed: bedplate, block frame, liner, head, manifolds, covers. Moving: piston, rod, crankshaft, flywheel, gears, camshaft, pumps, valves. Learn the map below and every later component has an address:

3. MAN Vee — Residual Fuel by Design

Trunk pistons burning residual fuel to 380 cSt (180 cSt the economic optimum after heating, cleaning and maintenance are costed) at 105–118 kg/cm² peaks and 17.3–19.7 bmep for 900–1100 bhp per cylinder at 400–430 rpm. Inertia out-loads gas pressure on bearings — short stiff crankshafts on thin-shell lead-bronze mains and crankpins throughout.

Details that matter:

• Thin-plate heads shedding heat to water with ribbed backs taking the load.
• Caged exhaust valves lifting out whole with seat cooling that starves vanadium/sodium deposits — Nimonic heads on Stellite seats, split-collet spindles, roto-cap rotation; full story in Valve Gear & Fuel Injection.
• Tandem different-diameter plungers stretching slow-steaming to 1:4–1:5 without slip clutches.
• Alloy crown on light skirt with cocktail-shaker under-crown oil.
• Fuel, water and rocker oil routed so they can never meet (VV 52/55 separation).
• Cased needle valves draining leaks overside of the crankcase.

4. Trunk Lubrication Circuit

Dry sump, independent pumps, filters, cooler: common rail to mains → crankshaft drillings to crankpins → rod passages to gudgeons and piston cooling, with branches to gears, cams, detuner, rockers, governor, servos and both turbochargers on shared SAE 30 detergent oil. Cylinder oil stays a separate mechanical system — and any head fuel or water leak that reaches rocker drains shows why the circuits must never mix.