Amount of fuel returned to tank

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pcatterall

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Just reading some blurb from MarShip about modern diesel fuel and modern engines. Fuel quality getting worse but modern engines really requiring better ( so a double whamy situation !!)
One comment was that 'Marine diesel engines are unique' in that they return 60% of the fuel to the tank. I understood that most 'marine' engines are just derivatives of vehicle/plant engines so wonder if the statement is correct?
They also talk about the need for very fine filtering on modern engines.
Perhaps I will stick with our old 4108 !
On that subject will my 4108 use a lot more fuel than a modern engine would?
(We cruise at 5.5kn at 1800 revs and use around 0.45 galls per hour.)
 
Not sure what they mean about 'marine' diesels. Are these ours or ships? When I looked at this a while ago it was evident that some installations use a weir system returning fuel from the injector pump to the tank whereas others do not. My Yanmar only returns fuel from the injectors, very small amounts that are overspill from the metered amounts being injected. I measured mine at 2000 rpm, less than 1 ml per minute. Cars with common rail systems circulate large volumes back to the tank.
 
Absolute nonsense, all conventionally injected diesel engines will have fuel return. The amount of fuel returned will vary with throttle setting. I don't know if there are returns on modern Common rail and/or electronically injected engines though.
 
Only time I've ever measured it, a 3GM30 did not return anything like 60% to the tank.
A modern car returns more than 60% to the tank most of the time, the fuel is fed by a constant volume pump and what's not used is returned by the pressure regulator.
My bike circulates petrol at something like 200 litres per hour, even at idle.
An old-skool diesel like the 3GM30, the return is just what leaks by the injector pintles and lubricates them in the injector body. If they were leaking 60% of the fuel, then the injectors would never open and the engine wouldn't run?
 
pcatterall said:
Just reading some blurb from MarShip about modern diesel fuel and modern engines. Fuel quality getting worse but modern engines really requiring better ( so a double whamy situation !!)
One comment was that 'Marine diesel engines are unique' in that they return 60% of the fuel to the tank. I understood that most 'marine' engines are just derivatives of vehicle/plant engines so wonder if the statement is correct?
They also talk about the need for very fine filtering on modern engines.
Perhaps I will stick with our old 4108 !
On that subject will my 4108 use a lot more fuel than a modern engine would?
(We cruise at 5.5kn at 1800 revs and use around 0.45 galls per hour.)
Click to expand...

The rate of fuel return to the tank depends entirely on the engine design; some return lots, some return little. Doesn't really matter as the returned fuel has already been filtered and will then get filtered again (possibly many times).

Your 4.108 will use a similar amount of fuel to a modern engine, maybe a bit more, but not massively more.
 
Common rail engines require extremely clean fuel, due to the very high pressures involve, much higher than in "normal" engines. The pressures are high enough that minute traces of contamination can score the injectors, which in turn can lead to catastrophic engine failure. I look after a large mobo with 900hp common rail engines, each engine has 5 filters !
 
A properly old school engine like a Mercedes OM636 returns well over ten times as much fuel to the tank as it uses.

With my two tank installation, if I feed off one tank and return to the other (there is, deliberately, no balance pipe) I can transfer fuel from one to the other at well over 50 litres an hour. Even flat out the engine doesn't burn much over 4 litres an hour

(I believe this is because there is no injector pump(s) as such. The injector manifold is pressurised by the fuel pump and the injectors simply inject the thus pressurised fuel. I may be wrong in this belief!)
 
A mate was away in his boat and had fuel issues so he put in a new tank alongside the old tank just to get home. Feed off the new one and return to the old one. This was about 18-20nm in a Moody 346 with a newish Beta 30. He asked me to go back to base with him. He said he'd put 20 litres in the new tank. I said that won't be enough so he took a can as well. We motored and ran out of fuel at the marina entrance. We reckoned it was in excess of 50% return.
 
Where I've had to run the engine from a can, a 22l container doesn't last much above half an hour. Of that, probably only 1l, or not much more, is burnt.

Yanmar engine. I presume most of it comes back from the secondary filter take off where there is self-bleeding of air down the return pipe.

Plus side, I can start the engine with a primary filter full of air and by the time the secondary filter has been drained, new fuel is arriving via the lift pump. None of this twiddling my finger to bleed it.
 
My Volvo 2003 returns a steady trickle back to the tank; you can hear it running when the tank is part full. It must be litres per minute. I reckon that most of the fuel goes through the filters several time before being burnt.
 
MoodySabre said:
A mate was away in his boat and had fuel issues so he put in a new tank alongside the old tank just to get home. Feed off the new one and return to the old one. This was about 18-20nm in a Moody 346 with a newish Beta 30. He asked me to go back to base with him. He said he'd put 20 litres in the new tank. I said that won't be enough so he took a can as well. We motored and ran out of fuel at the marina entrance. We reckoned it was in excess of 50% return.
Click to expand...

I would confirm that quite possible. I have a Volvo 2020MD & i fitted another tank which returned to the old tank & the new tank emptied too quick to make it viable. I reckoned at least 40% & very likely 50%
 
There are some misconceptions in this thread. Generally the amount of fuel returned to the tank is fairly small something less than a cup a minute. It is the leakage from the injectors and the piston elements in the inline injector pump.

Generally engines with inline injector pumps feeding individual injectors don't return significant amounts.

But there are exceptions anyone who has an engine with an old fashioned CAV rotary pump the one that looks like a car distributor will have much more leaking from the pumping pistons inside the pump and returned to the tank.

Note it does not have to be returned to the tank some setups return it to the tank side of the lift pump.

Some modern common rail systems return significant amounts to the tank.
 
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Couple of comments.
I now run two cars with common rail diesels. The book says the leak past is partly to cool the bits as the injection pressure is so high. They also knock out about the same HP as my last 'old skool' diesel, but use quite a lot less fuel to do the same job. This might be applicable to your 4-108. Except, I would not want the electronics on a boat. Well, at least on the size I can afford:o)

Looking in the equipement tent at the La Rochelle boat show. Some very nice engines. A few based on the usual suspects with conventional jerk pumps (so expect a noticable return flow) but lots based on VW group car engines with all the ecu bits. Aimed at the mobo market..
 
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