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Yofy 02-23-2008 01:10 PM

Fule overflow rate
Does anybody know what the fuel overflow rate is for a diesel engine?

billyruffn 02-23-2008 01:45 PM

I'm no expert, but I think you're referring the the difference between the amount of fuel pumped to the injectors vs the amount actually consumed (i.e. sprayed into the cylinders and burned). The "overflow" (fuel not burned) is then returned to the fuel system, sometimes to the tank, sometimes it loops back to the engine fuel filter or some other point in the engine's fuel lines. The specific overflow rate will vary by engine type, HP, etc.

Yofy 02-23-2008 04:35 PM

Yes that's what I'm talking about. I know the rate will vary some but I would like a rough idea... in percentage let's say.

Does anybody know?


Yofy 02-23-2008 04:36 PM

Yes that's what I'm talking about. I know the rate will vary some but I would like a rough idea... in percentage.

Does anybody know?


Omatako 02-23-2008 05:47 PM

Of what importance is it? All the fuel the engine doesn't need goes back.:)

As long as you valving system ensures that the overflow goes back to the same tank it came from, no problem. Some systems (mine is one of them) has two valves and if forgetful it is possible to return fuel to another tank and that's not clever, especially if that tank is already full.:(

If you have a smaller volume "day tank", use it always because returning fuel to a tank where it will live for a while will lift the temperature of that tank and promote condensation and the growth of bugs and other uninvited guests.


billyruffn 02-23-2008 08:29 PM

Omatako wrote:
"Of what importance is it? All the fuel the engine doesn't need goes back"

In general I agree, but it would be neat to know exactly how much fuel the engine was buring at a given moment, rather than the average consumption like I get when I measure what's pumped into my day tank after running the the engine for a few hours.

The other day I inquired about a 'fuel flow gauge' that claims to show the current rate of fuel consumption in GPH as well as a cumulative number of gallons consumed. Apparently, this system uses the difference between the flow rate of the fuel lift pump and overflow ratio of the engine's injector pump -- at least that's what I presume given the questions their website wanted answered before they would give me a model number and price.

They've got such fuel flow gauges in aircraft and I guess this company has tried to adapt them to diesel engines. The measurement of fuel flow in a gasoline engine is much simplier because everything that goes down the fuel line and into the carburator is consumed (or wasted). Calculating instantaneous fuel consumption of a diesel engine is, I guess, a tad more complicated because of this overflow issue.


Manny, I have no idea what the ratio is, but it's not insignificant. I got a leak in the fuel overflow line once and it made a real mess.

Omatako 02-23-2008 08:52 PM

How badly do you want to know?

Originally Posted by billyruffn (Post 270922)
it would be neat to know exactly how much fuel the engine was buring at a given moment,

How neat? How much money is it worth to know this? :)

The gear on this link is not likely to be cheap but it will very accurately measure effectively anything that flows through a pipe.

Manny, I reckon the flow back to the tank is way, way more than the engine burns because, looking at the volume that flows back on my system, if my engine used as much, my tank would be empty in no time.


billyruffn 02-23-2008 11:32 PM

Omatko wrote:
How neat? How much money is it worth to know this? :)

There's only been one time in the last 7 years when it might have been worth it -- When we sailed from Costa Rica to Hawaii we burned 70 percent of the fuel in the first 1200 nm (of 4800) trying to find wind. We used the engine manufacturer's graphs of RPM vs fuel consumption to help estimate how to get of max distance / gallon, but it wasn't precise by any means. We were also forced to estimate the fuel remaining by measuring what we were pumping from each of the tanks -- which works fine in most circumstances, but in this situation, having a more accurate estimate of what remained in the tanks would have been valuable information. Winds on that route were light that year and as the hurricane/cyclone season was rapidly approaching we wanted to keep the boat moving down the rhumb line.

It it was a few hundred dollars, I might consider for the next round of capital improvements. If it's much more than that, we'll wait for the next time we have to motor to Hawaii.

The product I was looking at is made by Floscan

I think the issue w/ diesel engines is which "pipe" do you put the sensor on? You really need two measurements: one of fuel flowing into the injection pump and one for the fuel flowing down the overflow line after the injectors. Then you need a small 'black box' to calculate the difference. I guess that's what makes it expensive.

Yofy 02-24-2008 02:49 AM

Well I thought it just might be a simple question... but to make myself clearer I'll go into a little background on why I want to know.

Reason #1: Simular to billyruffn's experience, we are getting ready to cruise up to the Med soon. As we are currently lying in the Red Sea this passage will require a long beat into strong headwinds up the Gulf of Suez. Several days of motor sailing into the wind. Then we will have two days of motoring through the Suez canal. We are a small boat (Fortune 30) and while we do have what I consider to be good tankage (150 litres), we will have to be careful. Until Port Suez, we will be without options to refuel. (Yes we will also carry jerry cans of fuel as emergency backup, but I want to be able to estimate my fuel consumption.)

Reason #2: I have reasons to question if enough fuel is reaching my injector pumps/ feed pump. (I am not sure if there is a free flow of fuel in the line).

I don't need to know the exact rate, I'm just looking for a way to gauge here... a rough idea... an estimated percentage to help me figure out both issues. I have heard of 70 % but it doesn't make sense to me.

So, now that you know why... does anybody have a rough idea?


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