Real-world tug emissions brief
Low load can be the dirtiest part of the workday
A tug can look clean on a spec sheet and still run ugly in the parts of the day nobody brags about: waiting, creeping, lining up, throttling in and out, then sitting again with big diesel machinery lightly loaded.
11.6-26.5
g/kWh NOx range reported in recent real-world tug maneuver testing.
3.4-5.5
g/kWh CO range reported, with CO rising in low-load and transient operation.
5-15%
Peak-power time for harbor-assist and escort tugs cited in MARAD guidance.
80-90%
Potential PM and NOx reduction range EPA highlights from newer marine engine standards when fully implemented.
The uncomfortable finding
Low load does not always mean low pollution per unit of useful work. Incomplete combustion, cool exhaust, inactive aftertreatment, repeated throttle changes and long idle periods can make a tug dirtier than expected even when fuel burn looks modest.
Research notes
Recent real-world harbor tug testing found CO rising under low-load and transient operation. A separate tug SCR study found meaningful NOx reduction with AdBlue, while flagging the challenge of low-temperature and transient operation. EPA harbor craft guidance points to shore power, engine upgrades, hybrid/electric propulsion, annual-hour tracking and engine-tier tracking.
Sources: Real-world harbor tug emission measurements, Real-world SCR impact on tugboats, EPA harbor craft best practices, MARAD energy efficiency guidance, Caterpillar hybrid tug operating profile.
Sources: Real-world harbor tug emission measurements, Real-world SCR impact on tugboats, EPA harbor craft best practices, MARAD energy efficiency guidance, Caterpillar hybrid tug operating profile.
The tug cycle that creates the problem
Standby Engines stay warm and ready, but useful work is low.
Slow transit The tug moves at low load, where big mains may be inefficient.
Assist setup Throttle changes and short maneuvers create transient emissions.
Hard push or pull Engines finally reach the load they were sized for.
Back to waiting The machinery drops back into the low-load zone again.
Low-load emissions map
| Operating condition | Main issue | Emission signal | Better control lever |
|---|---|---|---|
| Idle or standby | Large engines running for readiness instead of work. | Fuel is burned close to shore with limited useful output. | Shore power, auto shutdown, battery standby, auxiliary optimization. |
| Low-speed transit | Engines operate below their efficient load band. | CO and soot risk can rise from incomplete combustion. | Hybrid mode, smaller gensets, route timing, engine-load management. |
| Transient assist | Fast load changes during docking, braking and repositioning. | Standard steady-state factors can miss real maneuver behavior. | PEMS testing, load-profile data, smarter emissions inventory methods. |
| Cool exhaust | Aftertreatment may not stay in its strongest operating window. | SCR performance can be challenged at low temperature or transient load. | Temperature management, right-sized engines, hybrid controls, duty-cycle review. |
| Old engine hours | Older engines accumulate many local service hours near port communities. | NOx, PM, CO and diesel exhaust exposure remain a waterfront concern. | Repower, retrofit, Tier strategy, DPF/SCR review, electrification where practical. |
The false comfort of low fuel burn
Low load may burn less fuel per hour, but the tug may spend a huge share of the day there. If combustion is poor or aftertreatment is cool, the port can get more local pollution than the fuel ticket suggests.
Operator actions that matter first
Track load, not just gallons
Pull ECM data or portable testing data by mode: standby, transit, assist, full power and dockside.
Find cold aftertreatment hours
SCR and DPF systems need operating temperature. Low-load time can be a hidden control-system weakness.
Separate idle from working load
Do not average the whole day into one load factor. Tug emissions live in the operating modes.
Put hybrid where it fits
Hybrid makes the strongest case when standby and low-speed transit dominate the day.
Use shore power at the dock
EPA notes low-voltage shore power already serves tugs, fishing vessels and support craft in many U.S. ports.
Audit older high-hour engines first
EPA guidance points operators toward tracking annual service hours and engine tier performance.
Low-load emissions pressure tool
This simple model shows whether a tug’s daily profile is likely creating a low-load emissions problem worth measuring.
Use annual engine-on or operating hours for the tug.
Standby, loitering, slow transit and light maneuvering.
Use the real low-load average, not full-power burn.
Estimate possible reduction from shore power, hybrid mode, shutdown policy or smaller gensets.
Higher score means more emissions uncertainty during low-load operation.
Older or poorly monitored engines raise low-load uncertainty.
Low-load hours
1,650
Estimated annual hours spent in the zone most likely to hide emissions trouble.
Fuel at low load
74,250 gal
Estimated gallons burned in low-load operation before improvements.
Avoidable gallons
29,700 gal
Planning estimate from the selected reduction percentage.
Pressure signal
High
Measure the tug by operating mode before assuming the emissions file is clean.
Procurement takeaway
Do not buy a “clean tug” only from a rated engine tier. Ask for mode-by-mode emissions data, low-load aftertreatment performance, hybrid operating logic, shore-power readiness and proof from the actual duty cycle.
