Smoke after engine braking

[WAWalker]

As far as the two user created encyclopedia (Wikipedia) references go.

 

They contardict themselves.

 

The first says that the compression stroke provides compression braking.

 

The second says that increased vacuum provides compression braking.

 

As the name implies, compression braking is compression braking. Compare engine compression psi to intake vacuum in/hg, and see which one is going to use the most energy to produce.

[frag]

As far as the two user created encyclopedia (Wikipedia) references go.

 

They contardict themselves.

 

The first says that the compression stroke provides compression braking.

 

The second says that increased vacuum provides compression braking.

 

As the name implies, compression braking is compression braking. Compare engine compression psi to intake vacuum in/hg, and see which one is going to use the most energy to produce.

 

Thanks for the scientific experiment Wawalker. But I'm not sure I understand your last statement correctly. When an engine is idling and even more when it's running at part or full throttle there must be, like you say, a big difference between the energy needed to compress a full mixture of air and fuel and the force needed to suck that same mixture inside the cylinder while the thottle plate is open.

But on "compression" braking, the shutting of the throttle plate and the partial shutting (like you measured) of the injectors not only means the energy needed to move the piston on the intake stroke increases considerably, but the force needed on the compression stroke must become not only nil but probably negative : the vaccuum produced during the intake stroke is pulling the piston up on the "compression» stroke.

If I'm right - and I'm all willing to be shown where my logic is failing - the name "compression braking" is false and should be replaced by "vaccuum" braking.

What's your expert opinion on this?

[WAWalker]

If you look at the numbers I posted, then engine is "pumping" more air on decel. There for there is more air in the cylinder that needs to be compressed, and more energy used in compressing it.

 

There is not enough fuel being injected into the cylinders to produce power from combustion to aid in compressing the next cylinders air charge, there for you get a braking effect due to the need to compress more air without the help of power from combustion.

 

I'm not saying that the increase of vaccum has NO affect. But I think if you did the math it would be nill.

 

It takes much more energy to compress a cylinder full of air than it does to draw air from the intake into the cylinder.

[WAWalker]

I'm not that good at math, so I don't know if there is a way to convert in/hg (vacuum) to compair to psi (compression pressure), but............

 

Let's look at this in a unsientific way.

 

Take a car with a maunal transmission. Park it on a hill in gear, without the e-brake. Engine compression keeps it from rolling down the hill.

 

Now remove the intake valves from this engine. Without the intake valves, there will still be vacuum when the pistion travels downward in the cylinder, but there will be no compression.

Park it on a hill in gear, without the e-brake. It will roll down the hill like it was in nuetral.

 

Oh, and...................There is still fuel being injected into the cylinders, and the spark plugs are still firing during engine braking. Cumbustion has not ceased 100%, so no, there is not a vacuum being drawn during the cumbustion stroke.

[frag]

 

It takes much more energy to compress a cylinder full of air than it does to draw air from the intake into the cylinder.

 

Yes, but that's not what's happening during "compression braking". During the compression stroke on "compression" braking, there is not a "cylinder full of air" to be compressed. To make this (open to discussion) statement I rely on three things:

 

1) According to your own numbers there is only a third of the volume of air getting inside the cylinder on the intake stroke when on "compression" braking compared to what's happening at idle. The maff is showing 3.1 gs instead of 2.35 gs on engine deceleration, but the rpms are 4 times higher . For the same volume of air to get into the cylinders on "compression" mode at the stated rpms, the numbers would have to be around 9.4 gs.

2) On a normal intake stroke (at idle for example) the injectors are squirting a vaporized mist of fuel that must certainly occupy an additionnal volume inside the cylinder. This, according to your own numbers, is almost nil during "compression" braking contributing to the vac being produced at the intake stroke.

3) The last argument is part intuitive and part scientific. I think we all agree that a vaccuum is created inside the cyl at the intake stroke when "compression" braking (this is what explains the oil being sucked inside the cylinder). Vac means air is not completely filling the void produced by the piston moving away from the valves and that atmospheric pressure is pushing on the other side of the piston. In that situation it does'nt take much energy, if at all, to push the piston back up.

I rest my case .

[frag]

I'm not that good at math, so I don't know if there is a way to convert in/hg (vacuum) to compair to psi (compression pressure), but............

 

Let's look at this in a unsientific way.

 

Take a car with a maunal transmission. Park it on a hill in gear, without the e-brake. Engine compression keeps it from rolling down the hill.

 

Now remove the intake valves from this engine. Without the intake valves, there will still be vacuum when the pistion travels downward in the cylinder, but there will be no compression.

Park it on a hill in gear, without the e-brake. It will roll down the hill like it was in nuetral.

 

Oh, and...................There is still fuel being injected into the cylinders, and the spark plugs are still firing during engine braking. Cumbustion has not ceased 100%, so no, there is not a vacuum being drawn during the cumbustion stroke.

 

I completely agree with the last part. But the fact that the engine is braking shows that almost no energy (pressure) is produced during the power stroke.

I like seat of the pants experiments but the only problem with the one you're describing is that's it's imaginary. If you could really put up such an experiment I would trust you with the results no problems.

I hope I'm not overextending the limits of your taste for discussions on technical points:burnout:

[WAWalker]

I hope I'm not overextending the limits of your taste for discussions on technical points:burnout:

 

No, but I am having a hard time understanding how it is that you had a perfectly clear understanding of engine braking in post #21, but are now saying the total oppisite.

[frag]

No, but I am having a hard time understanding how it is that you had a perfectly clear understanding of engine braking in post #21, but are now saying the total oppisite.

 

If you read post #24, you'll see where I finaly saw the light .

(Edit) In post #21, I was tinking "open intake valve" and forgetting "closed throttle plate".

Which shows I can admit it when I'm wrong.

Nothing I've said since that post has been proven wrong, in my opinion of course.

[frag]

No, but I am having a hard time understanding how it is that you had a perfectly clear understanding of engine braking in post #21, but are now saying the total oppisite.

 

And have you read post #30 ?

[WAWalker]

And have you read post #30 ?

 

I have read them all.

 

The second Wikipedia reference seems to have made you see the light?

 

Did you read the first one?

[WAWalker]

The intake system on any engine will not produce a tight enough seal to creat the vacuum that would be need to be to slow a car down when engine braking, sorry.

 

What you are saying is that the braking effect comes from a total reversal of what normally goes on inside an internal combustion engine.

 

That there is so much restriction due to a closed throttle and high RPM's that the pistion can not pull air into the cylinder, resulting in a vacuum inside the cyl once the valves are closed, and that this vacuum is actually pulling the pistion back up on the compression stroke. Sorry, I ain't buying it.

[frag]

I have read them all.

 

The second Wikipedia reference seems to have made you see the light?

 

Did you read the first one?

 

I just reread it and I think it's wrong.

I think I found something, also in Wikipedia, that strongly support my point. It's the explanation of the Jake brake functionning. Here goes :

 

"The Jake Brake or Jacobs Brake is a particular brand of engine brake manufactured and sold by Jacobs Vehicle Systems, Inc. While the term Jake Brake technically only describes Jake Brake brand engine brakes, it has become a genericized trademark and is often used to refer to engine brakes or compression release engine brakes in general, especially on large vehicles or heavy equipment. "Jake Brake" is a registered trademark of Jacobs Vehicle Systems, Inc.

An engine brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression release engine brakes.

When the accelerator is released on a truck, its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. Of equal importance, even with zero accelerator input, there will be some trace introduction of diesel fuel (make and model dependant) which will still undergo combustion. Any power created from this will hinder efforts to decelerate. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston and accordingly, the vehicle."

 

In fewer words, compression does not have a significant effect cause it acts like a spring giving back, on the power stroke, the energy it has absorbed during "compression". If diesel trucks needs the Jake brake or in other cases exhaust brakes, it's because compression braking in itself is nil and cause they are unable to use vac braking due to the lack of a throttle plate.

That strenghtens my point. Compression is not what brakes a "petrol" engines cause, like I said, there is minimal air in the cylinder to be compressed to begin with AND the compressed air, acting like a spring gives back the energy it has absorbed. Which leaves vaccuum braking.

Thanks for bearing with me WaWalker.

[WAWalker]

I agree with the statement about the volume of air at idle and while "engine braking" is not "proportional" went you consider the RPM differance.

 

If it was you would have an "eat the steering wheel" breaking effect.

 

But the fact is there is still sufficiant air to fill the void in the cyl. when the intake valves open. Not sufficiant volume to produce any amout of "power" on the cumbustion stroke, but enough that there will be significant energy to compress it.

 

The fact that the ECU is still injecting any fuel what so ever means that there is air enough to be compressed to provide combustion when the spark plug fires.

 

The fuel being injected would not ignite in a vacuum and would be pushed into the exhaust, causeing some massive destruction at the converter.

[frag]

I agree with the statement about the volume of air at idle and while "engine braking" is not "proportional" went you consider the RPM differance.

 

If it was you would have an "eat the steering wheel" breaking effect.

 

But the fact is there is still sufficiant air to fill the void in the cyl. when the intake valves open. Not sufficiant volume to produce any amout of "power" on the cumbustion stroke, but enough that there will be significant energy to compress it.

 

The fact that the ECU is still injecting any fuel what so ever means that there is air enough to be compressed to provide combustion when the spark plug fires.

 

The fuel being injected would not ignite in a vacuum and would be pushed into the exhaust, causeing some massive destruction at the converter.

 

Let's hope this discussion will have helped the first poster better understand the functionning of his engine and his problem. Otherwise, i could certainly be proven guilty of having criminaly hijacked this thread.

So this will be my last post tonight and we could open a new thread if we both have developed a taste for it.

The case for compression braking is, in my opinion, completely destroyed by the Jake brake explanation I've quoted above.

There are only two possibilities :

(1) If little or no air is compressed during the compression stroke then little or no energy is dissipated = little or no braking effect

(2) if enough air is present in the cylinder during the compression stroke it acts like a spring when comes the power stroke and gives back the energy it absorbed in the compression stroke. If you add to that a little fuel and a little combustion (like you yourself state) then the engine produces more energy than it has dissipated in the compression stroke and the result = no braking effect whatsoever.

Have a good night!

[WAWalker]

If you would like to start a new thread, I'm game.

 

We may be discussing a phenomonon that dosen't even exist in gasoline engines.

 

The more I think about it, the engine braking effect is only apparent when the transmission is manually shifted into a lower gear.

 

When simply decelarating, the car naturally slows due to no power being produced by the engine.

 

When shifting to a lower gear there is a significantly noticable amount of slowing due to increase in resistance due to gearing.

 

So..................if compression braking in a gasoline engine is negated, due to the "air spring" affect, and the small amount of combustion that takes place on the "power stroke". I can wrap my mind around that.

 

And if a closed throttle plate in the intake system can in fact produce enough resistance to provide engine braking.....................

 

Then if I jump in my '96 OB with manual trans., and run it down a nice long steep hill, turn off the ignition........................ coasting in gear, at sufficiant speed to maintaine engine RPM's around 3000......................with throttle closed there should be noticable engine braking, then with throttle open there should be a noticable lack of engine braking and increase of speed.

 

I'll give it a try.

[WAWalker]

Vac means air is not completely filling the void produced by the piston moving away from the valves and that atmospheric pressure is pushing on the other side of the piston. In that situation it does'nt take much energy, if at all, to push the piston back up.

I rest my case .

 

You have to remember that the PCV system is tied into the intake, on the piston side of the throttle plate.

 

All that increased vacuum in the intake is sucking on the crank case. There is vacuum in the crank case, not atmospheric pressure.

[frag]

I opened up a new thread.

I have difficulties having access to the Board since last night.

That explains the repeated new thread. I'll try to remove the first one (second one going down the list).