[Alberg30] RE: Bukh 10 Startup

[Taylor Echlin]

Thank you for taking the time to provide this excellent explanation.

My boat has a Bukh 10 diesel.  I have no aptitude for mechanical stuff - and
I found your comments quite easy to follow.  My fuel injector was replaced
just 2 years ago - and thanks to you I now know how it works :)

The Bukh 10 is a single cylinder - and it will put-put around forever like a
singer sewing machine.  It's a relatively simple engine and drives the boat
effortlessly.

Taylor
-- 
Taylor Echlin
le papillon  KC 612

> From: J Bergquist <j at ship.saic.com>
> Reply-To: Alberg 30 public list <public-list at alberg30.org>
> Date: Thu, 22 May 2003 12:51:09 -0400
> To: Alberg 30 public list <public-list at alberg30.org>
> Subject: Re: [Alberg30] RE: Bukh 10 Startup
> 
> 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
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