Engines and thrusters

The engine page — revs, oil pressure, coolant temperature, fuel rate, hours, and the warning lamps — comes from the engines themselves. On a modern boat the engine or its gateway broadcasts all of that onto the NMEA 2000 bus continuously, one set of readings per engine.

muxen-nmea2000 collects those readings, keys them by engine instance — which is what makes one of them "port" and the other "starboard" — converts them out of the standard's units into the ones a display wants, and publishes one message describing every engine on the boat.

Bow and stern thrusters work the same way and are covered at the end of this chapter.

Two things are worth knowing before reading a number off the page:

• The daemon reports what the engine says. It does not calculate, correct or second-guess. A wrong reading is an engine or gateway problem.
• A value that stops arriving becomes zero or null, not the last value. An engine that goes quiet drops out of the report entirely after five minutes. Nothing on this page is ever a frozen leftover.

The engine report

Enable it with --report-motor, or ReportMotor in the deploy configuration.

• Topic nmea/motor
• Rate 1 Hz, or 10 Hz with --motor-high-speed / MotorHighSpeed
• QoS 0, not retained
• expireAfterSec 5

Not retained is deliberate and differs from most topics here: engine data is only meaningful live. A screen that subscribes after the daemon has stopped gets nothing rather than a plausible-looking snapshot of a stopped engine.

Nothing is published at all while no engine is active. An empty engine page on a boat with the module enabled usually means the engines are off, not that the module is broken.

Shape

data
├── motors
│   ├── "0"            ← engine instance, as a string key
│   │   ├── engine
│   │   ├── transmission
│   │   ├── trip
│   │   └── trimTab
│   └── "1"
└── vessel
metadata

motors is an object keyed by the engine instance number as a string, not an array — so a single-engine boat with the engine at instance 1 has motors."1" and no motors."0".

engine field	Unit
rpm	revolutions per minute
boostPressureKpa	kPa
tiltTrimPercent	−100 … +100
oilPressureKpa, fuelPressureKpa, coolantPressureKpa	kPa
oilTemperatureC, coolantTemperatureC	°C, converted from kelvin
fuelRateLph	litres per hour
totalEngineHours	hours
loadPercent, torquePercent	percent
warnings	object of booleans, below

transmission field	Meaning
gear	forward, neutral, reverse or unknown
oilPressureKpa, oilTemperatureC	as above
warnings	overTemperature, lowOilPressure, lowOilLevel

trip field	Unit
fuelUsedL	litres this trip
averageFuelRateLph	litres per hour
instantFuelEconomyLpNm, averageFuelEconomyLpNm	litres per nautical mile

trimTab carries portPercent and starboardPercent, −100 … +100.

vessel is per-boat rather than per-engine, and appears only once data for it has arrived: timeToEmptySec, distanceToEmptyNm, fuelRemainingL, tripRunTimeSec.

metadata carries rxDate, rxTimestamp, expireAfterSec and motorCount.

Warning flags

engine.warnings mirrors the two discrete-status bytes the engine broadcasts. Each is a plain boolean:

checkEngine, overTemperature, lowOilPressure, lowOilLevel, lowFuelPressure, lowSystemVoltage, lowCoolantLevel, waterInFuel, chargeIndicator, preheatIndicator, engineShutdown, emergencyStop.

These are the engine's own warnings, passed through unchanged. They are not NMEA 2000 alerts and do not go through the alarm subsystem (Alarms) unless something else raises an alarm for them.

Two-tier ageing

Engine data ages out in two stages, because "the revs stopped arriving" and "the engine is gone" are different events:

Tier	Timeout	Effect
dynamic values — revs, temperatures, pressures, fuel rate, load, torque	5 s	set to 0 or null
the engine itself	5 min	removed from motors entirely

Values that describe the engine rather than its current state — total hours, the warning flags, the selected gear — are kept for the full five minutes. An engine that has just been shut down therefore still shows its hour meter and its last gear while its revs read zero, which is what an operator expects.

Which PGNs feed it

PGN	Name	Feeds
127488	Engine Parameters, Rapid Update	rpm, boostPressureKpa, tiltTrimPercent
127489	Engine Parameters, Dynamic	pressures, temperatures, fuel rate, hours, load, torque, warnings
127493	Transmission Parameters, Dynamic	transmission
127496	Trip Parameters, Vessel	vessel
127497	Trip Parameters, Engine	trip
130576	Trim Tab Status	trimTab

Other engine PGNs — 127490, 127494, 127498 — are decoded and published on the per-device raw topics, but do not feed this report.

The thruster report

Enable it with --report-thruster, or ReportThruster.

• Topic nmea/thruster
• Rate 1 Hz
• QoS 0, not retained
• expireAfterSec 5
• Data older than 10 s is dropped from the report

data
└── thrusters
    └── "<identifier>"
        ├── control
        ├── info
        └── motor
metadata

Group	Fields
control	direction, speedPercent, powerEnabled, retractControl, azimuthDeg, commandTimeoutSec, controlEvents
info	motorType, powerRatingW, maxRotationalSpeedRpm, maxTemperatureC
motor	currentA, temperatureC, operatingTimeMin, motorEvents

metadata carries thrusterCount alongside the usual timestamps.

PGN	Name	Feeds
128006	Thruster Control Status	control
128007	Thruster Information	info
128008	Thruster Motor Status	motor

The daemon reports the thruster; it never commands one. There is no path in this daemon that transmits a thruster control message.

Diagnostics

mosquitto_sub -h 127.0.0.1 -t 'nmea/motor' -v
mosquitto_sub -h 127.0.0.1 -t 'nmea/thruster' -v

--verbose-motor logs each engine as it appears, each instance change and each expiry, prefixed Motor:.

If nothing is published, work through:

1. Is the module on? Look for Motor in the startup summary line, or read nmea/config.
2. Is anything transmitting? nmea/devices lists the bus, and nmea/device/<addr>/pgn/127488 shows raw engine frames when raw PGN publishing is enabled with --mqtt-publish-pgn.
3. Are both engines on the same instance? Two engines shipped at instance 0 collapse into one entry. Fix it with the I key in muxen-nmea2000-config — Navigation.

More in Troubleshooting.