My bad: Is EDC fuel flow accurate?

Back in 2008 when I delved fairly deep into NMEA 2000 fuel management (1-Garmin, 2-FloScan, 3-Raymarine, 4-Maretron, 5-SmartCraft, and 6-Lowrance), I may have gotten a related concept wrong. While I was mostly experimenting with how fuel flow data gets integrated into an overall system, I noted a couple of times that if you have such data coming out of an electronically controlled engine, it should be more accurate than what can be measured by flow sensors. Well, as suggested by that "Assumed Fuel Consumption" label on that Steyr Motors display above, maybe not...

What is probably true is that the electronic fuel injection in a gasoline engine captures fuel flow precisely and with near instant response to rpm and speed changes. But while an Electronic Diesel Control (EDC) also accurately measures injected fuel, diesel architecture means that some of that fuel gets returned to the tanks. Now, I'm pretty ignorant about engines in general, but I'm being told that a typical EDC doesn't measure fuel returned and therefore can't accurately measure actual fuel used. Instead they purportedly make assumptions based on generalized fuel curves like the one for Gizmo's engine below. In other words, Steyr Motors is just being unusually honest about what's going on. But purportedly some other manufacturers have also downplayed their EDC-based fuel economy accuracy, due to complaints, and one big diesel maker regularly install FloScan mechanical flow sensors on generator sets that already have EDC flow info.
But, in fact, my main source on all this is the folks at FloScan! So I'm looking for some expertise from you all. Are you sure that the MPG readings on your snazzy electronic engine gauges is spot on? Do you understand more about the internal mechanics/electronics involved than I do (not hard)? My interest goes beyond curiosity (and not wanting to perpetuate myths). Gizmo is the sort of sweet power boat that runs well at many speeds, but according to that curve she using at least three times the fuel at 16 knots (2,200 rpms) that she is at 8 (about 1,300 rpms). I want to know where the sweet spots are, and I want to see how they change with different loads, sea conditions, amounts of bottom fouling, etc.. Please help if you can.

Written by: Ben Ellison

Ben Ellison is electronics editor for Bonnier Marine Group, specifically Yachting and Cruising World. He previously was electronics editor for Power & Motoryacht and SAIL, as well as a writer for Ocean Navigator. His blog posts appear courtesy of his website www.Panbo.com, which has 80,000 monthly readers worldwide.