Ditching the dipstick – diesel fuel monitoring goes electronic

Electronic monitoring of boat systems is nothing new, but the gauging of a boat’s performance has taken another step forward with the recent development of diesel fuel monitors that can accurately determine the amount of fuel used, the rate of fuel use and, with input from a GPS receiver (fast becoming the one sensor that every system on the boat needs to access — watch out when the galley stove needs access to GPS data!), users can see a projection of vessel range.

Diesel fuel monitors have been around for several years, but the news for sailors is the move toward units for smaller engines. FloScan Instruments, for example, has announced its new Series K fuel-flow monitoring system. This unit can be used on diesels as small as 25 hp, making it viable for voyaging sailboats. Another instrument manufacturer, Navman, is now offering a diesel-fuel monitoring system called the Diesel 3200. This fuel-monitoring package is designed for diesels of 100 hp or larger. However, it wouldn’t be surprising to see a future unit from Navman that will function with smaller-size engines.

The key for FloScan in making fuel monitors for smaller engines was in the sensors. FloScan’s engineers designed a low-volume rotor and placed it on low-friction sapphire bearings. “Our sensor is accurate down to one-third gallon an hour,” said Joe Dydasco, sales manager for FloScan. “The difference is in the rotor.” The FloScan Series K unit uses a turbine-type rotor. As the fuel spins the rotor, the blades interrupt a light beam to a photo transistor. The unit counts these pulses of light and translates this into rate of flow.

The Navman Diesel 3200 unit, on the other hand, uses a different approach. “We use a positive-displacement rotary piston,” said Bill Boudreau, product marketing manager for Navman. “The strokes on the piston indicate volume displaced by the fuel.” The movement of the piston is sensed using the Hall effect. A magnet on the piston is picked up by a sensor every time the piston sweeps by. This is similar to the way the Hall effect is used in a paddle-wheel speedometer; a count of paddle passes is converted into boat speed.

Most marine diesels are “extra fuel” engines. That is, not all the fuel that goes to the injectors is used in the cylinders; some flows back to the tank via return piping. Thus, in order to know how much fuel a diesel engine is actually burning, it’s necessary to measure not only the fuel going to the injection pump from the diesel tank, but also that fuel returning to the tank in the return piping. The Series K unit, for example, has two units measuring fuel flow, the forward sensor assembly and the return sensor assembly. The forward sensor measures all the fuel flowing out of the tank, while the return unit measures fuel headed back to the tank. The difference between the two readings is the fuel consumption.

One tricky aspect of measuring diesel fuel flow is properly accounting for the change in temperature in the fuel going back to the engine. Cool, dense fuel from the tank flows past the first sensor, through the injection pump and then into the injector piping. Some of the fuel returns to the tank, but while it is in the vicinity of the injector, it soaks up heat from the engine and expands. If the unit doesn’t compensate for the change in volume of the fuel, the return sensor reading will be incorrect. The FloScan units have built-in temperature sensors that allow the unit’s microprocessor to make the needed correction. The Navman 3200 also has a temperature sensor to compensate for the change in volume due to heating of the return fuel.

Another challenge involved in monitoring diesels versus gasoline-fueled engines is the engine vibration often transmitted by the fuel piping. These fuel pulsations mean that sensors have to be built to withstand a regular level of vibration.

When interfaced with a GPS, the Series K unit and the Diesel 3200 can compare a vessel’s boat speed with the fuel use to obtain a fuel efficiency value in nautical miles per gallon. The system can also be used to monitor engine and drive train performance. If the gallons per hour/nautical miles per gallon reading is different from a previously determined optimum value, then the engine may have fouled injectors; there could be damage to the propeller, or there may be some other type of problem. With this type of fuel monitoring, users should be able to get a better sense of what is happening with their engines.

FloScan has also made these units easier to install. “In the past we had height and distance requirements,” Dydasco said. Those limitations have been eliminated, and FloScan claims the Series K unit can be installed by “a moderately skilled owner with simple tools.”

While sailors with larger boats will find these fuel-monitoring units a helpful (though not strictly essential) addition to their engine instruments, there is another group of mariners that should be very interested in this type of information about their engines: power voyagers. When your engine or engines are your lifeline, fuel information becomes a big issue. “You’re managing your fuel on a daily basis, because that’s your prime mover,” said Roger Hellyar-Brook, manager of the systems program at the Landing School in Kennebunk, Maine. So having this high level of data certainly helps in range calculations and in general engine monitoring.

By Ocean Navigator