[Alberg30] RE: Bukh 10 Startup

[J Bergquist]

Okay I know this is a sailboat list and we shouldn't spend too much time 
talking motors...

but the marine engineer in me really wanted to remember why diesels are 
set up to pump excess fuel from the tank to the injectors and then back 
into the tank. I knew there was a good reason and I didn't buy that it 
was cooling (though an interesting theory...cooling the engine using the 
fuel?) So I decided to posit my own explanation, which follows and is a 
bit lengthy (conciseness not being my long suit).

If you're interested in hearing more, read on, and if not, then hit delete.

Turns out that the fuel injectors in a diesel engine each have their own 
individual injection pump, usually of the 'jerk-piston' type. These 
pumps deliver fuel at the extremely high pressure necessary for proper 
atomization and timing during the top part of the power stroke. Since 
high pressure fuel lines are dangerous, they are kept short, which is 
why each cylinder has its own injection pump. It is also a requirement 
that the fuel delivered to each cylinder be very precisely measured in 
terms of both quantity and pressure. Remember that a fuel-injected 
diesel engine is controlled by throttling its fuel, where a carbureted 
gas engine is controlled by throttling its air. I am pretty sure that 
ALL diesels are fuel-injected.

Now all these injection pumps have their intakes on a common manifold, 
which is also pressurized (but much lower pressure) by the main engine 
fuel pump. Thus, the fuel system on a diesel is really a 2-stage pumping 
system. One low pressure pump supplying the manifold, and one high 
pressure piston pump supplying each cylinder. Each time a cylinder comes 
up for the power stroke (some might say 'up for the down stroke' ;-) its 
injector pump piston begins to descend, thereby drawing the precisely 
measured quantity of fuel from the manifold and pumping it via the 
injector into the cylinder at the high pressure necessary for proper 
atomization. If there were not excess fuel in the manifold, the manifold 
pressure would drop significantly when this happens.

As a result, the engine is set up to have a return line so that roughly 
3 times the MCR fuel flow requirement for the engine will be supplied to 
the manifold during normal operation. This ensures that each time an 
injector pump draws fuel from the manifold, the pressure variation in 
the manifold is much smaller than it would be if only 1x the total fuel 
requirement at MCR were being supplied. This reduction of the pressure 
variations in the manifold ensures good fuel flow into the individual 
injector pumps and therefore proper atomization and consequent smooth 
running.

The reason that gas engines are not setup like this falls into 2 categories:

1. In the case of our beloved carbuereted atomic-4's, fuel is atomized 
on demand by the jets in the carburetor. There is no 2-stage pumping 
mechanism. The fuel system must simply have sufficient fuel to not run 
dry. Therefore, there is no need for a return line and our old-reliable 
low pressure diaphragm pumps work fine to keep the carb float bowl full.

2. In the case of more sophisticated fuel injected gas engines, most of 
the time the fuel is injected into the intake manifold's throttle body. 
This means that its measurement and pressure need not be so tightly 
controlled (because the throttle body pressure is pretty low compared to 
the pressure in the cylinder at the top of the compression stroke on a 
diesel). So I think this is why there's no return line on a fuel 
injected gas engines. I believe these engines must control engine output 
by controlling both fuel and air? I'm not sure about this. Heck, for all 
I know, there may BE a return line on fuel injected gas engines. I'd 
like to know if there is...

anyway, that's all I've got. I thought somebody might find it 
interesting. FYI, my reference was the SNAME book Marine Engineering, 
edited by Harrington. There is a chapter on marine diesel engines 
written by Alan Rowen.

Enough engine talk. I'd prefer going sailing!

J Bergquist
Calliope #287
 +---------------------------------------------------------------+
 |                This Old Boat by Don Casey                     |
 | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
 +---------------------------------------------------------------+

Public-list mailing list -- Public-list at alberg30.org
http://www.alberg30.org/mailman/listinfo/public-list
To unsubscribe: email to Public-list-request at alberg30.org
Include command "unsubscribe <password>" in subject or body.
Use command "help" for more options.

 1053622269.0