Exhaust pressure under load

[dustyEd]

Greetings

pardon my ignorance, I am an electrical engineer, though I probably should have known something as basic as this

Ok here it goes

Suppose we have a mechanically controlled naturally aspirated diesel engine (to make things simpler).
When it is working under heavy load, there seems to be more exhaust noise, suggesting more pressure in the pipe.
This is more subtle in modern vehicles due to better mufflers, turbochargers EGR etc. , but you could very clearly hear it on older naturally aspirated engines , specially if they were unmuffled od the exahust was breached. Not to mention that some of them also threw flames when under heavy load.


So, the question would be:
Under SAME RPM, why is there more exhaust pressure if the engine is working under higher load than if it were under lighter load.

The only thing that comes to mind is that under heavy load, the diesel engine runs rich, which explains the smoke. But
why is there more noise when the engine is under high load?

thanks

 

[MikeHalloran]

For a naturally aspirated Diesel, the intake air flow doesn't change much with increasing load, but the fuel flow does. Which makes the exhaust gas hotter, which makes it expand, so the exhaust gas volume flow increases.

Also, with heavier load and more fuel, when the exhaust valve opens,
- the charge is still burning, which explains the smoke.
- the charge is at higher pressure, which explains the noise.

If a particular engine is making more smoke and more noise under load than it used to, it may be time for a valve job.




Mike Halloran
Pembroke Pines, FL, USA

 

[dustyEd]

Thanks for the quick response

So, you mean, a rich mixture (more fuel injected) produces hotter exhaust gas?

Also, you've got me interested, in the valve thing you mentioned. This is unrelated to the thread, but what did you mean by valve job? I assume you meant that valves are opening too early, but how does load affect this? How does load affect timing of valves?

 

[MikeHalloran]

Yes. Theoretically, the air mass flow through a NA Diesel is constant at a given speed. When you add more fuel, the EGT rises, and the exhaust volume flow with it.

A valve job comprises disassembly, inspection and possible replacement of the engine valves. Exhaust valves normally fail by failing to stay closed, so a little hot gas leaks between the valve face and the valve seat. The hot gas erodes the valve and the seat in a localized area. Every combustion event then produces a jet of hot gas into the exhaust manifold long before the valve is supposed to open. The chamber pressure goes up with load and fuel, as does the leakage flow.

I.e., load doesn't affect the timing of a healthy valve, but the timing of a dying valve is uncontrolled.



Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

The noise is related to the amplitude of the volumetric flow rate pulsations at or near the exhaust valves and how the resulting pressure fluctuations react with the downstream geometry. It's a complicated interaction, but you can be sure that if the volume flow rate fluctuations increase, so will the tailpipe noise. By burning more fuel the in-cylinder pressures and temperatures increase. This causes a higher amplitude of fluctuations at the valves. Think of fluctuation amplitudes rather than absolute values.

 

[dustyEd]

SomptingGuy, thanks, I know what you mean, I didn't actually mean that there is a constant pressure in the pile, I meant that pulses of pressure get stronger.

Thank Mike, that really explains everything I wanted to know

 

[dustyEd]

Hi again

I've been thinking about this question some more, and I have another question about this issue.

Now it's clear why there is more gas going out through the exhaust under load, but what is still unclear to me is this:

Again, we have a naturally aspirated , mechanically controlled diesel engine with unit injection

The engine is in idle, you press the accelerator pedal,
giving it a certain increase in fuel that is being injected in each cylinder, so the engine starts reving up, and just when it starts to do that there seems to be a burs of smoke and noise in the exhaust, which clears up a bit (and quiets down) when the engine reaches stability (when it revs up to a certain RPM).

Now what I don't understand is. If exhaust flow and temperature depends on how rich the mixture is (amound of fuel injected), how come the exhaust quiets down after the engine had reached stable RPM?
I mean the amound of air in cylinder is constant in this type of engine, and so is the amount of fuel after you have pressed the pedal, and the torque generated is increasing engine speed, yet, DURING that process there seems to be more noise for the same amount of fuel injected than AFTER it. (while the fuel amount remained constant, or did it?)

Does that have something to do with the governor?
Does the fuel being injected actually decrese after the engine has reached stable RPM, even though you kept the pedal in the same postion?

I know this post is a mess, so I hope you understand my question.

 

[MikeHalloran]

When the engine is no longer attempting to _accelerate_ the attached load, i.e., has reached a stable rpm at a stable load, yes, the governor is supposed to reduce the fuel flow, which will reduce the exhaust gas temperature, volume flow, and noise.



Mike Halloran
Pembroke Pines, FL, USA

 

[dustyEd]

What do you mean "supposed" to? Are the exceptions?

So, that would imply that the governor has effect on all RPM's (providing stabilisation, and making the pedal work more as speed control, than fuel control), and not just as a high-end RPM limit (for safety), right?

By the way. On some engines I've seen a "hose" connecting the "Bosch" unit pump with the the head of first cylinder in the row (just the first one). Now I assume this hose is taking the intake air from one of the intakes to control the governor.
Does that sound correct, or is the hose for something else?
(the engine is mechanically controlled, and N/A)

 

[MikeHalloran]

I don't know what the hose does.



Mike Halloran
Pembroke Pines, FL, USA

 

[dustyEd]

Ok thanks a lot

 

[franzh]

The hose I think DustyEd is referring to may be the map compensator on most turbo engines, I cant remember seeing one on a NA engine but it certainly could have been. It senses inlet pressure and compensates the injector pump. My old Cummins 5.9 had one and it connected to the intake plenum near #1 mostly due to convenience.

This hose/tube may also be called a smoke puff eliminator.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TheBlacksmith]

Diesel engine speed and/or power output is controlled by the amount of fuel injected. When you "press on the pedal", you increase the speed/power setpoint instantly, which the governor interprets as a large error signal and left to its own devices, increases the amount of fuel injected by a large amount, causing the black smoke. When the engine reaches the new setpoint, the error signal goes to zero and the governor now injects the amount of fuel necessary to maintian that speed/power level. As mentioned, a turbocahrged engine can increase its air supply with load as the turbo responds; the tube measures the air pressure, aka air available, and limits the response of the governor to large positive error signals, limiting the smoke. We used to say "air leads fuel" on an increase.

 

[dustyEd]

Blacksmith

thanks, I assume you are talking about electronically governed engines, I was just trying to figure out how the most basic and primitive mechanically governed engines would work. But the principle is the same I see.


Anyway, about that hose, franzh, here is the picture,
do you think it's the same thing that you were talking about?

I have highlighted the hose/pipe in red color, otherwise it is a freshly painted engine, so it was a little hard to see

http://free-os.t-com.hr/redmist/hose.jpg

As you can see, it connects just to one of the heads, and seems to be connected near the fuel line, just above the injector

 

[SomptingGuy]

Before the days of electronics there were two ways to govern a pump. Believe it or not the old rotary pumps actually had spinning weights, a bit like a steam engine. Alternatively there were hydraulic governers based on (supply??) pressure. Both were used in a mechanical feedback system to reduce the fuelling if the engine was going too fast (pressure too high or weights too wide).

As an engineer, this stuff is much more satisfying than electronic control. But I digress. Yes, noise is generally a function of load at a constant speed because fuelling may vary.

 

[dustyEd]

Yes, I checked later, and the engine on the picture I just posted, also has a centrifugal governor as you mentioned.

 

[TheBlacksmith]

As stated by others, I actually described a mechanical governor, the weights respond to engine speed and the "pedal" increases spring tension, the smoke limiter was a tapered pin that blocked fuel an increase in injection until a diaphram responding to manifold pressure moved it. Long before the first electronic computer was built, all manned of analog computing engines were devised using gears, levers, flyweights, air signals, etc.

 

[SomptingGuy]

Sorry TheBlackSmith, didn't mean to accuse you of being young. I was just reliving my joy at seeing those spinning weights during my training years. Plus all that light-load advance and cold-start retard stuff. Absolutely magic. Something you could see, touch and understand.

 

[TheBlacksmith]

SomptingGuy - went to the Virginia State Fair a few years ago, there was a live steam traction engine driving a swamill. Spent most of my time watching the flyball governor - amazingly responsive. My back was to the saw about 20 feet away and I could see the balls swing in before I heard the saw hit the wood and see the flys fly out right before I heard the saw stop cutting.

 

[Fabrico]

There's still a few million trucks on the road throughout the world with various forms of spinning weight (mechanical) governors.