SFOC: intake temperature sensitivity

Formula, assumptions, limits

Formula

Where is the change in specific fuel oil consumption (g/kWh) for a deviation in intake or charge-air temperature (°C) from the engine’s reference test condition. The sensitivity coefficient ranges from 0.3 to 0.5 g/kWh/°C across modern marine diesel engines; this calculator uses 0.4 as a representative mid-band value.

Derivation

ISO 3046-1 (Reciprocating internal combustion engines: declarations of power, fuel and lubricating oil consumptions, and test methods) defines a reference ambient state and provides correction factors to translate measured fuel consumption back to the reference. The dominant first-order term in the correction is intake-air temperature: hotter air is less dense, so the engine ingests less oxygen per stroke, combustion efficiency drops, and SFOC rises. The 0.3 to 0.5 g/kWh/°C coefficient comes from OEM shop-test correlations with the ISO 3046 correction equations, valid in the linear region near the reference.

Assumptions

Engine is operating at or near MCR (Maximum Continuous Rating). Off-design operation has its own SFOC curve and the temperature sensitivity differs. Charge-air after-cooler is functioning normally. A fouled cooler raises the intake temperature without the engine “knowing”, and the SFOC penalty exceeds what this formula predicts. Fuel quality (LCV, sulphur, viscosity) is at the reference specification. Off-spec fuel introduces a separate SFOC delta that this formula does not capture. The temperature deviation is within ±20 °C of reference. Beyond that the relationship becomes non-linear and a full ISO 3046 correction is needed. Humidity and barometric pressure are at reference. Tropical environments combine high temperature with high humidity; the humidity term adds 0.05 to 0.15 g/kWh per 10% relative humidity above reference, not captured in this simplified formula.

Worked example

A vessel operating in the Persian Gulf has an intake-air temperature of 45 °C against an ISO reference of 25 °C, so °C. With g/kWh/°C, the predicted SFOC penalty is g/kWh. If the engine’s reference SFOC is 175 g/kWh, the actual at-condition SFOC is approximately 183 g/kWh, a 4.6% increase. Over a 10,000 t fuel-budget voyage, that is roughly 460 t of additional consumption, which is material at any fuel price level.

Edge cases and limits

Slow steaming: at 50% MCR or below, the SFOC sensitivity to intake-T can be lower (closer to 0.3 g/kWh/°C) because the engine has more thermal margin and the after-cooler is over-sized for the actual airflow. Use this formula’s mid-band value as a conservative upper bound. Dual-fuel engines on gas mode: methane number sensitivity to intake-T differs from diesel-cycle sensitivity. For X-DF, ME-GI, ME-LGI engines on gas, consult the OEM’s gas-mode correction curve; the diesel-mode formula is not directly applicable. Exhaust gas recirculation (EGR) engines: when EGR is active, intake-T effects are confounded with EGR-rate effects. Run the formula only with EGR disabled or fixed-rate. SCR / methanol / ammonia: alternative-fuel engines have additional terms that dominate over the simple intake-T sensitivity. This formula is for conventional 2-stroke and 4-stroke marine diesels in diesel-mode. Reference condition definition: ISO 3046 reference is 25 °C / 60% RH / 100 kPa / 25 °C charge-air. Some OEMs publish “tropical” reference conditions (45 °C / 90% RH / 100 kPa) that pre-bake the temperature delta. Confirm which reference the published SFOC was measured against.

Regulatory basis

ISO 3046-1:2002: declarations of power, fuel and lubricating oil consumptions, and test methods for reciprocating internal combustion engines. Defines the reference ambient state and the correction methodology. IMO MEPC.245(66): 2014 Guidelines on the method of calculation of the EEDI for new ships. EEDI calculations require SFOC at reference conditions; intake-T correction is therefore implicit when computing attained EEDI. NOx Technical Code 2008: NOx measurements during certification are also corrected for ambient conditions; the same temperature-sensitivity principle applies.

Common errors

Mixing up reference temperatures: applying the correction to an SFOC value that was already corrected by the OEM. Always confirm whether the published baseline is “as measured” or “ISO-corrected”. Forgetting the after-cooler: a 5 °C rise in scavenge-air temperature due to a fouled after-cooler is not the same as a 5 °C rise in ambient. The after-cooler effect is on top of the ambient effect. Linear extrapolation beyond ±20 °C: at extreme temperature deviations the relationship is not linear; consult the OEM’s full SFOC vs ambient-T curve. Ignoring humidity: in tropical conditions humidity adds another 0.5 to 1.5 g/kWh on top of the temperature effect. Use the full ISO 3046 correction (or OEM correction curve) when both deviations are large. Using engine-room temperature: the relevant temperature is at the turbocharger inlet, not the engine-room ambient. Engine-room can be 10 to 15 °C hotter than ambient due to radiated heat.

Symbol legend

SymbolMeaningUnitSource

Change in specific fuel oil consumptiong/kWhderived Intake-air temperature deviation from reference°Cintake RTD Sensitivity coefficient (0.3-0.5; default 0.4)g/kWh/°COEM correlation Baseline SFOC at ISO reference conditiong/kWhshop-test or sea-trial