Diesel return line?

[philthane]

I'm no expert on diesel installations (my previous boat was home built with a petrol engine modded to run on paraffin!) but now I've got one I'm trying to understand it. I believe the normal fuel line set up has a return from the common rail or high-pressure pump back to the filter and then to the tank, the reasoning being that the fuel injectors react more quickly than a pump can so it's better to have excess pump capacity and return the excess fuel than to have the pressure drop when the injectors want more. My boat doesn't have any of that, just a single fuel line via the filter to the pump. I guess that because it's an old engine (Perkins 100 series) without any fancy ECU set up the injectors just squirt what the pump sends. Could also be that it was originally a petrol engined cruiser and the diesel was fitted by a cowboy, so I'd like your opinions. Is it OK as it is?

[Tony Brooks]

1. Its not a common rail diesel.

2. It shoudl be perfectly possible for any form of lift pump to supply more than enough fuel for injection. After all each injection only delvers less than a pinhead full of fuel at full power.

3. On our diesels the injectors take what they are given, all the fuel metering and pressurisation is done in the injection pump.

4. The injectors will pass a little fuel past the needle in the nozzles. This helps cooling and lubrication of the needle. You need to get rid of that leak off fuel so it is returned to the tank.

5. Unless you can guarantee the fuel system is 100% suction tight AND no gasses will be dissolved in the fuel you are likely to get air building up in the top of the filter and/or the injector pump so it is normal practice to provide a small leak back from the filter head or from the injector pump body so any air is returned to the tank, this is often done by joining to the injector leak off pipe.

Some engines like the old Volvo MD range did not seem to have any leak backs apart form the injectors but I would say a leak back from the filter head or injector pump makes good sense form a resilience and reliability point of view. However your engine seems to be far eastern based using a normal 3 element inline injector pump so I would have expected more than the injector leak back but if its running well bets not tempt fate by altering it.

If you want more generic info on diesel fuel systems and how they work try my website here http://www.tb-training.co.uk/CIsys.htm

[philthane]

THanks (again) Tony. I'll have a look at your site.

[Chewbacka]

2 hours ago, philthane said:

I'm no expert on diesel installations (my previous boat was home built with a petrol engine modded to run on paraffin!) but now I've got one I'm trying to understand it. I believe the normal fuel line set up has a return from the common rail or high-pressure pump back to the filter and then to the tank, the reasoning being that the fuel injectors react more quickly than a pump can so it's better to have excess pump capacity and return the excess fuel than to have the pressure drop when the injectors want more. My boat doesn't have any of that, just a single fuel line via the filter to the pump. I guess that because it's an old engine (Perkins 100 series) without any fancy ECU set up the injectors just squirt what the pump sends. Could also be that it was originally a petrol engined cruiser and the diesel was fitted by a cowboy, so I'd like your opinions. Is it OK as it is?

You wont find many (probably none at present) canal boats with a diesel engine with common rail/ecu technology.  Boat engines are really quite crude compared to modern cars.