Answer
The MAN B&W ME-GI and ME-GA are electronically controlled, dual-fuel, two-stroke marine engines designed for LNG (methane) operation.
ME-GI: Marine Engine – Gas Injection (High Pressure)
ME-GA: Marine Engine – Gas Amission (Low Pressure, Otto Cycle)
1. ME-GI Process (Modified Diesel Cycle)
Gas Admission & Combustion: Compression takes place using pure air. Near Top Dead Centre (TDC), high-pressure gas fuel is injected directly into the combustion chamber at approximately 300 bar, alongside a small pilot oil injection (approx. 1.5–3%).
Combustion: Ignition is initiated by the pilot diesel fuel. The high-pressure gas burns via micro-diffusion flames, operating strictly on a Diesel Cycle.
Control: Knocking/detonation is non-existent because gas is not premixed with air prior to ignition.
2. ME-GA Process (Pre-mixed Otto Cycle)
Gas Admission & Combustion: During the scavenge and compression strokes (early in the cycle), low-pressure gas at approximately 5 to 6 bar is admitted into the cylinder through gas admission valves (located in the lower/mid cylinder liner).
Combustion: Gas mixes thoroughly with scavenge air during compression. Near TDC, a small pilot injection of fuel oil ignites the lean air-gas mixture, burning on a Pre-mixed Otto Cycle.
Control: Susceptible to engine knocking/pre-ignition; engine load, air-fuel ratio, and exhaust recycling (EGR/iCER) are actively monitored to prevent knocking.
| Feature / Parameter | MAN B&W ME-GI | MAN B&W ME-GA |
|---|---|---|
| Thermodynamic Cycle | Diesel Cycle (Diffusive combustion) | Otto Cycle (Pre-mixed lean burn) |
| Gas Supply Pressure | High Pressure: 300 bar | Low Pressure: 5 to 6 bar |
| Gas Admission Point | Injector in Cylinder Cover (near TDC) | Admission Valve in Liner/Scavenge space (during compression) |
| Methane Slip | Negligible: <0.2 g/kWh (no unburnt gas escapes) | Higher: Requires Exhaust Gas Recirculation (EGR / EcoEGR) to suppress slip and knock |
| Thermal Efficiency | Higher: Equivalent to standard 2-stroke diesel engines | Slightly lower than ME-GI due to compression ratio limitations |
| Engine Knocking Risk | None | Present; mitigated via ignition timing and EGR |
| Tier III Compliance | Requires EGR or SCR in Gas & Fuel Mode | Complies with Tier III in Gas Mode via EGR (MAN EcoEGR) |
| Fuel Gas Supply System (FGSS) | High-pressure compressor or High-Pressure Cryogenic Pump + Vaporiser (PVU) | Simple low-pressure gas compressor/heater (Lower capital expenditure for FGSS) |
Difference between Diesel cycle & Otto cycle
The fundamental difference between the Diesel cycle and the Otto cycle lies in how heat is added during the combustion process: the Otto cycle adds heat at constant volume, whereas the Diesel cycle adds heat at constant pressure.
Key Technical Differences| Parameter / Feature | Otto Cycle | Diesel Cycle |
|---|---|---|
| Heat Addition Process | Isochoric (Constant Volume) | Isobaric (Constant Pressure) |
| Ignition Method | Spark Ignition (SI) via external spark plug | Compression Ignition (CI) via high air compression temperature |
| Fuel-Air Mixing | Pre-mixed before compression stroke (homogeneous mixture) | Heterogeneous mixing; fuel injected into compressed air near TDC |
| Compression Ratio (r) | Lower range: 6:1 to 12:1 (limited by fuel auto-ignition / knocking) | Higher range: 14:1 to 24:1 (up to ~30:1 in large 2-stroke marine diesels) |
| Thermal Efficiency (η) | Higher theoretical efficiency for the same compression ratio | Higher real-world thermal efficiency due to operating at much higher compression ratios |
| Combustion Speed | Rapid, almost instantaneous flame front propagation | Slower, diffusion-controlled combustion sustained during piston movement |
| Cut-off Ratio (rc) | Not applicable (heat added at fixed volume) | Applicable; defined as the ratio of cylinder volumes after and before combustion |
| Engine Weight & Construction | Lighter build due to lower peak combustion pressures | Heavier, robust construction to withstand high peak pressures (Pmax) |