Answer
For NOx, ME-GA is better. For methane slip and overall GHG performance, ME-GI is better.
| Parameter | Better engine | Reason |
|---|---|---|
| NOx emissions | ME-GA | Otto-cycle, lean premixed combustion gives lower peak combustion temperature → lower NOx |
| Methane slip | ME-GI | High-pressure direct gas injection near TDC gives much more complete combustion; very little unburned methane escapes |
| CO₂ / fuel efficiency | ME-GI | Higher compression ratio and Diesel-cycle combustion give better thermal efficiency and lower fuel consumption |
| Overall GHG emissions | ME-GI | Very low methane slip + lower fuel consumption generally outweigh ME-GA's NOx advantage |
MAN currently quotes ME-GI methane slip at about 0.20–0.28 g/kWh over its operating load range. Otto-cycle engines such as ME-GA inherently have higher methane slip, although EGR is used to reduce it.
The trade-off is important: ME-GI's Diesel-cycle combustion has higher combustion temperature, so untreated NOx is roughly comparable with liquid-fuel Diesel operation; therefore EGR or SCR may be required for IMO Tier III. ME-GA's lean premixed Otto-cycle combustion is inherently more favorable for NOx.
Why ME-GI is better for GHGGHG is not just CO₂. Methane itself is a very powerful greenhouse gas, so methane slip can significantly reduce the climate advantage of LNG. ME-GI combines very low methane slip with higher combustion efficiency and lower fuel consumption than premixed Otto-cycle gas engines. MAN specifically states that these characteristics give ME-GI lower CO₂ emissions than Otto-cycle premixed gas engines.