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Old 09-12-2018, 07:31 AM   #81
Art's Avatar
City: SF Bay Area
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 11,251
Originally Posted by sunchaser View Post
Here is a two part question-

Do any of us have an engine on our boat that has a 5X or more fuel variation at constant RPM? I can think of at least two logical "yes" answers.

In both these cases how is the variable fuel flow managed with zero input from the skipper?
"...5X or more fuel variation... is a huge #; at constant RPM!

- 1 gal to 5 gal"
- 5 gal to 25 gal
- 25 gal to 125 gal

"... or more fuel variation at constant RPM?"

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Old 09-12-2018, 09:49 AM   #82
City: San Francisco
Join Date: Apr 2018
Posts: 1,665
5x is possible. If you set the "throttle" at 1800 rpm with the transmission out of gear, the only loads on it are friction and pumping losses. It will not use much fuel. The same engine loaded to max torque will use a lot of fuel.

Remember, diesels aren't like gas engines, there is no throttle. There is a lever we call the throttle which chooses the set point of the rpm governor. The engine's injection system then tries to add fuel until the set point is reached. You don't control the fuel, only the rpm - the injection system controls the fuel. Once the lever is set, the injection system will attempt to maintain rpm regardless of load.

It would be harder to reach 5x on a gas engine. It has a real throttle and consequently large pumping losses at high rpm and low throttle settings, requiring more minimum fuel to keep it going. On a gas engine as the load is varied the throttle must be adjusted to maintain the desired rpm, as you are controlling the fuel delivery with the throttle.

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