Diesel question

On an older diesel like my '85 GM 6.2L, what exactly happens when you press
down on the accelerator pedal? I understand that it controls fuel in some
way, but how? As I understand it, the injector pump controls the specific
charge of fuel to each cylinder for each firing. If this is changed then,
all you are doing is changing the mixture. It doesn't seem like you want
the accelerator pedal to be a rich/lean controller. Does it somehow control
the total flow rate of the fuel, and then distribute it evenly among the
firings? Maybe the engine would naturally adjust its RPM until the mixture
was correct?

Any insight would be helpful. I've searched all over the web and can't find
the answer to this.

Charles wrote:
>
> On an older diesel like my '85 GM 6.2L, what exactly happens when you press
> down on the accelerator pedal? I understand that it controls fuel in some
> way, but how? As I understand it, the injector pump controls the specific
> charge of fuel to each cylinder for each firing. If this is changed then,
> all you are doing is changing the mixture. It doesn't seem like you want
> the accelerator pedal to be a rich/lean controller. Does it somehow control
> the total flow rate of the fuel, and then distribute it evenly among the
> firings? Maybe the engine would naturally adjust its RPM until the mixture
> was correct?
>
> Any insight would be helpful. I've searched all over the web and can't find
> the answer to this.

The diesel cycle is completely different than gasoline engines.
Diesel fuel ignites at a very, very wide range of mixtures.
Basically, the only limit to how much diesel will combust is
the available oxygen. At part throttle the mixture would be
considered very lean if it were a gas engine, but gasoline
needs to be at a very specific mixture ratio to combust
properly. Mixture is irrelevant to a large extent in diesels.
So, you vary power output by varying the amount of fuel
injected, and the air intake is unthrottled, or wide open.
With the intake being basically an open hole with an air filter
on top, diesel engines have much lower pumping losses because
they're not using energy to suck air past a partially closed
throttle plate. This is part of the reason they get better fuel
efficiency.

Oh, and engine oil burns just fine in a diesel engine, so
if you have a diesel with enough wear to allow large amounts
of oil to be sucked past the rings or some other leak source
the engine can "run away", that's a bad thing because you can't
shut off a diesel by turning off the ignition, being that
there's not an ignition. The only way to stop a runaway is
to block off the air going into the motor.

JazzMan
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"JazzMan" > wrote in message
...

> ... Mixture is irrelevant to a large extent in diesels.
> So, you vary power output by varying the amount of fuel
> injected, and the air intake is unthrottled, or wide open.

Are you saying that when I step on the gas, I'm simply increasing the amount
of fuel per cylinder firing? If mixture is largely irrelevant, then how
would changing the mixture in such a way have an effect on, for example, RPM
under no load (in neutral)?

Or to put it another way, if I keep fuel per cylinder firing constant with a
certain accelerator position, what keeps the engine from running away? It
seems that I must be controlling fuel _flow_ with the accelerator, fuel per
unit time. Otherwise the motor would just fly apart. Or so it seems to me.

The duration that the injector is open is controlled by the accelerator
pedal.
So the "gas pedal" really does control the amount of fuel.
On an old mechanically controlled diesel, when you mash the pedal from idle
the engine goes way rich and you get a big cloud of black smoke. Watch old
heavy equipment like a bulldozer for a graphic example.

Erich

"Charles" > wrote in message
news:xNhJd.27330$OF5.3072@attbi_s52...
> On an older diesel like my '85 GM 6.2L, what exactly happens when you
press
> down on the accelerator pedal? I understand that it controls fuel in some
> way, but how? As I understand it, the injector pump controls the specific
> charge of fuel to each cylinder for each firing. If this is changed then,
> all you are doing is changing the mixture. It doesn't seem like you want
> the accelerator pedal to be a rich/lean controller. Does it somehow
control
> the total flow rate of the fuel, and then distribute it evenly among the
> firings? Maybe the engine would naturally adjust its RPM until the
mixture
> was correct?
>
> Any insight would be helpful. I've searched all over the web and can't
find
> the answer to this.
>
>

"Kathy and Erich Coiner" > wrote in message
news:XojJd.24433$Os6.11356@trnddc08...
> The duration that the injector is open is controlled by the accelerator
> pedal.
> So the "gas pedal" really does control the amount of fuel.
> On an old mechanically controlled diesel, when you mash the pedal from
> idle
> the engine goes way rich and you get a big cloud of black smoke. Watch
> old
> heavy equipment like a bulldozer for a graphic example.
>
> Erich
>

Why does the black smoke ever stop then? If you increase the fuel per
injection with the gas pedal and it gets over-rich, won't it stay over-rich
forever until you let off the gas? The air per injection is essentially
constant I believe.

"Charles" > wrote in message
news:bzjJd.28131$IV5.24328@attbi_s54...
>
> "Kathy and Erich Coiner" > wrote in message
> news:XojJd.24433$Os6.11356@trnddc08...
> > The duration that the injector is open is controlled by the accelerator
> > pedal.
> > So the "gas pedal" really does control the amount of fuel.
> > On an old mechanically controlled diesel, when you mash the pedal from
> > idle
> > the engine goes way rich and you get a big cloud of black smoke. Watch
> > old
> > heavy equipment like a bulldozer for a graphic example.
> >
> > Erich
> >
>
> Why does the black smoke ever stop then? If you increase the fuel per
> injection with the gas pedal and it gets over-rich, won't it stay
over-rich
> forever until you let off the gas? The air per injection is essentially
> constant I believe.

Not on older diesels with a mechanical pump. The pump has an rpm/throttle
position based governor that controls the quantity of fuel injected.

Charles wrote:
>
> "JazzMan" > wrote in message
> ...
>
> > ... Mixture is irrelevant to a large extent in diesels.
> > So, you vary power output by varying the amount of fuel
> > injected, and the air intake is unthrottled, or wide open.
>
> Are you saying that when I step on the gas, I'm simply increasing the amount
> of fuel per cylinder firing? If mixture is largely irrelevant, then how
> would changing the mixture in such a way have an effect on, for example, RPM
> under no load (in neutral)?
>

Yep, that's it. Mixture is irrelevant in the context of combustibility.
In other words, diesel will combust without problems whether you inject
a little or a lot. Gasoline will only really combust if it's mixed in a
fairly narrow ratio with air centered on about 14.7/1. The more diesel
you inject per firing stroke the more power the engine puts out.

> Or to put it another way, if I keep fuel per cylinder firing constant with a
> certain accelerator position, what keeps the engine from running away? It
> seems that I must be controlling fuel _flow_ with the accelerator, fuel per
> unit time. Otherwise the motor would just fly apart. Or so it seems to me.

I don't understand what you're saying here. If you mash the
pedal to the floor on a diesel in neutral of course it's
going to rev up until something breaks, unless there is some
fuel flow control system to prevent it from doing that. Also,
don't forget that each cylinder that is firing is using some
of its power to move the other cylinders up and compressing
the air in them. That's why you can rev an engine up and hold
the revs at a certain RPM without running away. Well, that,
and internal engine friction, torque converter drag, etc.

JazzMan
--
************************************************** ********
Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
************************************************** ********
"Rats and roaches live by competition under the laws of
supply and demand. It is the privilege of human beings to
live under the laws of justice and mercy." - Wendell Berry
************************************************** ********

"Charles" wrote

> If you increase the fuel per injection with the gas pedal and
> it gets over-rich, won't it stay over-rich forever until you let
> off the gas?

Yes. If you watch an old diesel car climbing a mountain or pulling
a trailor (with accelerator floored), you may see a constant smoke cloud.

In modern cars the amount of fuel is controlled by a computer and the
position of the accelerator only one of the inputs. The electronic will
avoid over rich mixtures to save fuel and environment.

Thomas

"Charles" > wrote in
news:xNhJd.27330$OF5.3072@attbi_s52:

> On an older diesel like my '85 GM 6.2L, what exactly happens when you
> press down on the accelerator pedal? I understand that it controls
> fuel in some way, but how? As I understand it, the injector pump
> controls the specific charge of fuel to each cylinder for each firing.
> If this is changed then, all you are doing is changing the mixture.
> It doesn't seem like you want the accelerator pedal to be a rich/lean
> controller. Does it somehow control the total flow rate of the fuel,
> and then distribute it evenly among the firings? Maybe the engine
> would naturally adjust its RPM until the mixture was correct?
>
> Any insight would be helpful. I've searched all over the web and
> can't find the answer to this.
>
>

Keep in mind, along with what others have said, that the injector timing
plays a role. Sort of like the spark timing on a gas engine, the
injectors are timed to inject fuel btdc. (not much, as too much and the
engine will run backwards)



--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

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Diesels always run with excess air (lean using your terms).
Pressing the accelerator pedal increases the amount of fuel
injected. More fuel equals more energy = power production.
Unlike a gasoline engine, diesels don't depend on the proper
air fuel ratio to initiate combustion. With a gasoline
engine, if the air fuel ratio is not correct, the spark from
the plug may not ignite the mixture. In the case of a diesel
engine you are spraying fuel into a cylinder filled with air
that has been heated above the ignition point of the fuel by
compression. Combustion begins immediately as the fuel is
sprayed into the cylinder (hopefully in a fine mist of
droplets).

Ed

Charles wrote:
>
> On an older diesel like my '85 GM 6.2L, what exactly happens when you press
> down on the accelerator pedal? I understand that it controls fuel in some
> way, but how? As I understand it, the injector pump controls the specific
> charge of fuel to each cylinder for each firing. If this is changed then,
> all you are doing is changing the mixture. It doesn't seem like you want
> the accelerator pedal to be a rich/lean controller. Does it somehow control
> the total flow rate of the fuel, and then distribute it evenly among the
> firings? Maybe the engine would naturally adjust its RPM until the mixture
> was correct?
>
> Any insight would be helpful. I've searched all over the web and can't find
> the answer to this.