LNG: The Incumbent Alternative

7 min read

What you'll take away Weigh LNG's genuine maturity advantages against the methane slip problem, and judge an LNG claim by engine cycle and measurement rather than by averages.

If the fuel transition has an incumbent, it is LNG. More alternative-fuelled ships burn it than everything else combined, and the reasons are practical rather than romantic: the engines are proven across thousands of engine-years, class rules are settled, crews can be hired with experience, and, decisively, the fuel can actually be bought. DNV's analysis of the fleet and infrastructure counted 642 LNG-fuelled ships in service by the close of 2024, supported by a growing fleet of dedicated bunker vessels (the bunkering analysis is open). No other alternative fuel offers anything close to that maturity, and maturity is worth money: it is the absence of pioneering risk.

The physics are friendly too, up to a point. LNG is nearly pure methane, liquefied at around −162 °C, carrying roughly 50 MJ/kg, more energy per tonne than fuel oil. Burned cleanly, it emits on the order of a fifth less CO2 per unit of energy, with almost no sulphur oxides or particulates. The cryogenic temperature is the design price: vacuum- or foam-insulated tanks, cold-tolerant materials, and boil-off gas that must be burned, reliquefied or managed rather than wished away.

The methane problem

LNG's climate case has a leak in it, and the leak has a name: methane slip: fuel gas that passes through the engine unburned and leaves with the exhaust. It matters because methane is a ferociously effective greenhouse gas: roughly 28–30 times CO2 per tonne over a hundred years, and around 80 times over twenty. At those multipliers, a slip of one or two percent of the fuel claws back a large share of the CO2 advantage.

Slip is not one number; it is an engineering variable. High-pressure two-stroke engines working on the Diesel cycle burn the gas almost completely: slip is a small fraction of one percent. Low-pressure Otto-cycle engines, common in four-strokes and many two-strokes, have historically slipped several times more, though each engine generation has cut the figure. So the first question to ask of any LNG emissions claim is which engine cycle, and the second is measured how. Slip varies with engine load, so certified test-bed points and real operating profiles can tell different stories; onboard measurement campaigns exist precisely to close that gap, and well-to-wake regulation counts methane explicitly. ABS's whitepaper on LNG as marine fuel (open) treats the slip question alongside the design fundamentals, and Lloyd's Register's Fuel for thought LNG report (free after registration) carries it through to economics and compliance.

Methane slip by engine concept: indicative ranges High-pressure two-stroke, Diesel cycle: well under 0.5% Low-pressure Otto cycle (many four- and two-strokes): ≈1–2% 0 0.5% 1.0% 1.5% 2.0% share of fuel gas leaving the engine unburned Why a small slip matters: one tonne of methane warms like ≈28–30 tonnes of CO2 over 100 years: ≈80 over 20.
Slip is an engineering variable, not a property of the fuel: the engine cycle moves it by roughly an order of magnitude. Ranges are indicative teaching values, which is why the two questions to ask of any LNG claim are which cycle, and measured how.

Where LNG lands on the framework

Run lesson 2's axes. Energy density: good by mass, roughly double the tank volume per unit energy once containment is counted. Well-to-wake: better than fuel oil when slip is low and the upstream supply chain is tight; marginal when neither holds. Cost: a premium ship, but a liquid and competitive fuel market. Availability: the best of any alternative, by far. Safety: cryogenic hazards and flammability, but managed by mature rules and long practice. Readiness: complete.

One operational note deserves its own line: boil-off gas. Heat leaks into any cryogenic tank, and the methane that evaporates must go somewhere , to the engines as fuel, to a reliquefaction plant, or in the worst case to a combustion unit. A well-run LNG ship treats boil-off as part of the fuel plan, not a nuisance; a badly run one vents value and, if methane escapes uncombusted, greenhouse gas.

Two strategic notes finish the picture. First, LNG is widely described as a bridge, and the far end of the bridge is visible: liquefied bio-methane and e-methane are drop-in compatible with the same tanks and engines, so an LNG ship has a decarbonisation path that does not require a shipyard. Whether those molecules arrive at scale and sensible cost is an open question: lessons 8 and 9 take it up. Second, the slip problem is why LNG rewards diligence: between a well-chosen engine with measured slip and a carelessly specified one, the climate result, and under lifecycle regulation, the compliance bill, can differ by more than the choice between LNG and staying on fuel oil.

Check yourself

1. LNG's core advantage over the other alternative fuels is best described as…
2. Methane slip refers to…
3. Which engine concept generally shows the lowest methane slip?
4. Over a 100-year horizon, one tonne of methane warms the climate roughly how much more than one tonne of CO2?
5. Why does lifecycle regulation change the LNG business case rather than simply endorsing it?
6. At atmospheric pressure, LNG is stored as a liquid at approximately what temperature?
°C
7. Which of these are genuine parts of the LNG value proposition for an owner today?

Select all that apply.

8. Methane slip is a fixed property of LNG as a fuel, identical whichever engine burns it.