Why so few planes that fired thorugh the propeller hub?

[Shortround6]

Gun bore evacuator as you posted link to.

You don't have a 'reservoir' of high pressure gas in an engine cylinder with ports orientated in the proper direction to govern the gas flow. Most aircraft engines didn't use a manifold that had high speed moving gas flowing past the exhaust ports of other cylinders.

I don't have a problem with contributors for whom english is not their first language getting something wrong in translation. Me trying to post anything on a French language board (or any other language) would be a disaster.
I do have a problem with a new poster claiming stuff most of us have known(or think we know) for years is all wrong, then give no sources, botched technical descriptions (translation problems?) and when we ask for sources or clarification get told we are a bunch of stuck up know-it-alls.
If you are going to join a forum and challenge conventional wisdom you better have all your facts lined up and sources ready. Just saying you read it someplace is't going to go very far.
I have read a bunch of stuff in books and on the internet on a variety of subjects that was just wrong. I had one book on aircraft armament that had over 20 typos or mistakes in captions before I gave up counting (stuff like calling a B-25 an A-20 in a picture caption).

Every real engine would have a slightly different graph and trying to grab them off the internet in hurry means compromises. Obviously a supercharged engine might not ever have a vacuum in the cylinder as the positive pressure in the intake manifold would pressurize the cylinder even as the piston moves down on the intake stroke.

 

[Deleted member 68059]

will the DB 601C/D get any coverage in your book?

I have about six DB test and development reports on the 601C/D, so yes you will find out some things about it from the book, although
I do not have more than about two photos, which is dissapointing.

 

[ThomasP]

Hey Shortround6,

Although the moving exhaust gases from one cylinder passing by the second cylinder as they move down an exhaust manifold would increase the effect, it is not necessary for the scavenging/suction/vacuum that modelwiz was talking about.

I know you are familiar with the Bernoulli effect and the lift/drag equations but I am going reiterate them below for (hopefully) increased clarity of what I am saying.

Any time you have gases moving at a higher speed than other proximal gases, you have a difference in pressure in the area between, resulting in 'suction'/'vacuum'/lift/etc.

It does not matter if the different velocity gases are on opposite sides of an airfoil or in front/behind of each other in open air, a relative pressure differential is established and the gases will try to move from the higher pressure area to the lower pressure area (this plus 'weight' of air due to gravity equals atmospheric phenomena such as wind, tornadoes, hurricanes, etc.). If there is an object (airfoil, leaf, newspaper, building, mass of air, etc.) in between the high and low pressure area, the movement of the gases will try to 'draw'/'suck'/'push' the object with it. The higher the velocity differential, the higher the pressure differential (hence the v^2 in the lift and drag equations), the greater the lift/'suction'/'vacuum'/etc. Because of the extremely high velocity differential between the leading edge/bow wave of the 'pulse' of the exhaust gases and the 'stationary' gases inside the cylinder, you get an extremely high 'suction' effect immediately behind the 'pulse'.

If you are familiar with the movement of a child's toy called the Slinky (TM) after some one gives it a push, you can use the movement to visualize the behavior of the exhaust gases as they exit the cylinder. The end of the Slinky pushed in the direction of movement can be considered the high velocity leading edge/bow wave of the exhaust 'pulse', the center section can be considered the low pressure area, and the remaining gases in the cylinder and be considered to be the object to be 'dragged'/'drawn'/'sucked'/lifted. The kinetic energy of the pushed end of the Slinky effectively 'drags' the rest of the slinky along in its wake.


The UK did actually move from 6 individual ejector exhausts per bank to 3 two-cylinder type, because of the increase in efficiency. The effect of the front cylinder's exhaust gases traveling down the manifold past the rear cylinder's exhaust ports increased the scavenging effect, but the rear cylinder also effected the suction at the front cylinder's exhaust port, which also increased efficiency. The enclosed manifold sort of averaged the effect.


And yes, the bore evacuator example uses a reservoir, but all that does is allow a prolonging (or second 'pulse'?) of the effect we are talking about, resulting in a more efficient scavenging of the gases in the barrel.

I think the graph you posted is quite appropriate to our discussion.

 

[modelwiz]

Hey guys,

For anyone who is interested, modelwiz is correct in his statement:

"And BTW, when the exhaust valves open the pressure differential between the burnt fuel air mixture and outside air also create what you would call a vacuum as the piston moves upward in the exhaust stroke. In all kinds of internal combustion engines."

modelwiz used the words "as the piston moves upward in the exhaust stroke" instead of 'during the exhaust stroke' which may have contributed to a misunderstanding, but what he meant is correct.

In the Merlin engine at +18 lbs of boost the piston rising only pushes about 5% of the combustion gases out of the cylinder, the exhausting high pressure/high speed gases draw the other ~95% out of the cylinder. In a naturally aspirated diesel with a 22:1 compression ratio the piston would only push about 3% of the combustion gases out, with the other 97% being drawn out by the exhaust gases.

The reason one can conceptually know this applies to an internal combustion engine is due to the piston velocity vs the exhaust gas velocity. The piston velocity in the Merlin is only 50 ft/sec at 3000 rpm. If the exhaust gases were being pushed out by the piston the maximum velocity of the gases would be about 570 ft/sec and there would be a loud harsh hiss (for lack of a better descriptive) instead of the bangs that we hear. The reason we hear the bangs (of course) is that the gases are expanding/traveling at more than the speed of sound (typically around 1700-2000 ft/sec). The pulses of high velocity expanding/traveling gas in effect act as short-lived virtual pistons being thrown away from the exhaust ports, creating a low pressure area in their wake while traveling down the exhaust manifold (and to a lesser degree after they leave the manifold), drawing the gases out of the cylinder. In the Merlin, by the time the piston has traveled ~1/2 of the way up on the exhaust stroke over 90% of the exhaust gases have already left the cylinder.

This is all due to the same principle under which a wing generates lift, i.e. the Bernoulli effect.

The above principle is the majority of the reason for putting 2 exhaust ports in a cylinder, the minority of the reason being a more uniform effect within the cylinder. If it were practical to increase the single exhaust port to the same area as the cylinder area (say by using the entire top of the cylinder as an exhaust port) it would be more efficient to do so.

Thank you for giving me the benefit of the doubt. Strictly speaking, the intended correct term would have been pressure differential. And yet, you would never call out "pressure differential-assisted brakes". In technology vacuum tends to describe a negative pressure differential actuated system or mechanism. Technically, vacuum is a space devoid or nearly devoid of matter (e.g. outer space) which does not occur anywhere in the mechanical systems of atmospheric aircraft (or vehicles, or ships, or....). I was being colloquial rather than rigidly technical. Sorry for the many misunderstandings.

 

[Shortround6]

I can certainly understand the difference between absolute vacuum and a pressure differential and I have driven vehicles with both "pressure differential-assisted brakes" and " pressure differential-assisted windshield wipers".

However I have bit of trouble swallowing the pressure differential explanation in a cylinder with a rising piston. On a Kharkiv model V-2 diesel at 1800rpm the piston is rising at an average speed of 17.75ft per second or 1065fpm. This is average speed with the piston starting at a dead stop and ending at a dead stop every 7.1 inches of travel and this speed is only for the piston moving in one direction. The piston completes this one way trip in 1/60th of a second (0.0166 seconds) and we are to believe that the out rushing gases leave with such velocity that they create a lower than atmospheric pressure in the cylinder even as the piston rises and reduces the volume in the cylinder 14-15 times in that 17/1000 of second. Or in other words, even if the the pressure in the cylinder had been magicly reduced to just a bit over 1lbsq/in at the bottom of the stroke the rising piston would have raised the pressure to just about normal atmospheric pressure (14.7psi) by the time it reached the top if the exhaust valves were closed.
There is no doubt that the pressure in the cylinder is much higher than the pressure outside (normal atmospheric pressure) when the exhaust valves open but in order for there to be less than 14.7psi (our colloquial vacuum) the out rush of gas has to suck the remaining gasses out of the cylinder faster than the piston is rising.

 

[tomo pauk]

...
I do have a problem with a new poster claiming stuff most of us have known(or think we know) for years is all wrong, then give no sources, botched technical descriptions (translation problems?) and when we ask for sources or clarification get told we are a bunch of stuck up know-it-alls.
If you are going to join a forum and challenge conventional wisdom you better have all your facts lined up and sources ready. Just saying you read it someplace is't going to go very far.
...

This is what it's all about here, and in another 2-3 recent threads. Where we can read a comment, for example: 'all of the participants were misinformed' in the ww2 ammo thread - really??

 

[ThomasP]

Hey ShortRound6,

Once more into the breach? Sorry it took so long to get back to you on this.

First, because I am an AR type and because I figure you would like to know when you are mistaken, I should point out that you made a math error concerning the average piston speed of the V-2. Using a simplified formula to calculate the piston speed we get:

1800 rpm x ((7.1" up + 7.1" down) / 12") = 2130 ft/min

2130 ft/min / 60 sec = 35.5 ft/sec

Not a big deal. You figured the time available for the combustion gases to exit the cylinder correctly, so it does not effect your argument.

(7.1" / 12") = .5817 ft traveled one way from BDC to TDC.

.5817 ft / 35.5 ft/sec = .0164 sec travel time from BDC to TDC.

The link below is a website you can use to calculate the actual velocity of the piston at any point in its movement from TDC to BDC, with 0º being TDC:

(L&M Engines, Inc. - Piston Velocity Calculator)

If you do not have the particulars for the V-2, the necessary numbers are:

Bore: 5.9055"
Stroke: 7.087" (master rod bank)
Rod Length: 12.598" (master rod bank)
RPM: 1800 per your example


Second, please read my and modelwiz's posts again. Nowhere in them did we say that the pressure in the cylinder would be, or had to be, reduced to less than ambient pressure.

What I said (and what modelwiz tried to say if I understand him correctly) is that a relative pressure differential occurs between the front of the high speed exhaust wave and the remaining exhaust gases in the cylinder.

This creates a suction (per the Bernoulli effect and other named principals) which helps draw out the exhaust gases faster than the piston can push them out. (If you disagree with this I would suggest that you have someone help you figure out just how fast the 0-71 ft/sec piston would push the gases out by itself. Then try to explain why the gases at the end of the exhaust manifolds/stubs are traveling at supersonic speeds.)

This suction will continue to take place until the velocity of the exhausting gases equals the velocity of the gases being pushed out of the cylinder by the piston/incoming charge.

Again, nowhere did we say that the pressure in the cylinder would be, or had to be, reduced to less than ambient pressure.


Third, although modelwiz and I did not say that the pressure inside the cylinder would go below ambient due to this effect, it can and often does. It will occur in one regime or another, sometimes all regimes, in any naturally aspirated 4-stroke engine if it is reasonably efficient depending on the rpm, size/# of exhaust/intake ports, valve overlap, timing, etc.

See the link below:

(https://www.enginelabs.com/engine-tech/exhaust/performance-exhaust-system-design-and-theory/)

I think you will find the entire article interesting as it is very informative about the entire exhaust system, but the most important section relative to what we are discussing is about mid-way down the page under "A Little Theory First: Backpressure And Scavenging" and includes the graph, the next 2 paragraphs, the photo of the inside of the cylinder during valve overlap, and the 3 paragraphs below the photo.

In supercharged 4-stroke aeroengines this suction effect occurs also, but the pressure inside the cylinder will usually only go below ambient in certain regimes. It is difficult to say exactly what regimes as ambient pressure, boost, and what type of exhaust manifolds/ejector stubs are used all have an effect, in addition to the factors mentioned above for the naturally aspirated engine. In general, the lower the boost the more likely you are to have cylinder pressures below ambient. But if you are using ejector exhausts you may never have cylinder pressures below ambient.

In 2-stroke diesels the suction effect also occurs, but I am unaware of any conventional 2-stroke diesel where the pressure inside the cylinder would drop below ambient during normal running.


Hope this helps.

 

[DerAdlerIstGelandet]

But this is not how a DIESEL operates. And I was referring to the V-2, which was a diesel with four exhaust valves per cylinder operated by two separate camshafts. But of course you knew that. You know everything. You know me so very well. Thank you for the lecture and condescension.

Kettle, pot, black?

I didn't feel there was any condescending tone in his response. This is a forum, used to exchange ideas and facts. Most of the time there are various views, interpretations and opinions. You have to provide facts and sources to back up posts. That goes for everyone.

So how about you relax a lil?

 

[tyrodtom]

And then there's the problem with the V2 diesel, it does not have 4 exhaust valves.
It has 2 exhaust valves, and 2 intake valves, like any other 4 valve, 4 cycle engine.
Just a quick look at the cutaway pictures of a V2 diesel will tell you that, if you know what to look for.

 

[Shortround6]

First, thanks for the correction on the piston speed.
Next.

And BTW, when the exhaust valves open the pressure differential between the burnt fuel air mixture and outside air also create what you would call a vacuum as the piston moves upward in the exhaust stroke. In all kinds of internal combustion engines.

Second, please read my and modelwiz's posts again. Nowhere in them did we say that the pressure in the cylinder would be, or had to be, reduced to less than ambient pressure.

Again, nowhere did we say that the pressure in the cylinder would be, or had to be, reduced to less than ambient pressure.

Sorry for the confusion but to me when the pressure inside a container, engine cylinder, device, what have you is below the pressure of the outside air you have your pressure differential/"vacuum".

Outside air=ambient air. It doesn't have to equal seal level standard pressure.

Third, although modelwiz and I did not say that the pressure inside the cylinder would go below ambient due to this effect, it can and often does. It will occur in one regime or another, sometimes all regimes, in any naturally aspirated 4-stroke engine if it is reasonably efficient depending on the rpm, size/# of exhaust/intake ports, valve overlap, timing, etc.

It can but for how long?

graph from Grumpyvette. And this is for a gas engine in a racing state of tune?

Applying race engine theory/tricks to production engines is an area with several difficulties.

see;

for a one cylinder engine, but most aircraft/tank engines are not single cylinder

graph for a four cylinder engine. This article is concerned with the best exhaust flow pattern for a turbo-charger on a modern car. The cylinders interfere with each other and reduce the pressure of the exhaust pulse to the turbo, not something we are concerned with here but shows things are not simple.

Also note that these are pressures in the exhaust manifold and not necessarily pressures in the cylinder.

The engine (or close relative) that we are talking about. notice the cast iron "log" exhaust manifold. You are going to get a lot of interference between the cylinders and you can also kiss goodbye to any idea of exhaust pulse pressure waves bouncing back and forth from the exhaust outlet (on the rear of the tank deck) to the exhaust ports.

On most V-12 aircraft engines you can also kiss that idea goodbye because it depends on the speed of sound and the length of the exhaust pipe/duct/track vs the RPM and the aircraft engines don't run at anywhere near the rpm needed to get to get this effect to work well with the length of exhaust used. General rule of thumb for aircraft engines was no more than 3 cylinders per exhaust pipe. Two cylinders per exhaust outlet was the usual compromise.

In 2-stroke diesels the suction effect also occurs, but I am unaware of any conventional 2-stroke diesel where the pressure inside the cylinder would drop below ambient during normal running.

Considering that the intake ports are uncovered well before the Piston hits BDC and you have positive pressure from the supercharger in the intake manifold the chances of less than ambient pressure in the cylinder are indeed, pretty slim.

 

[modelwiz]

Kettle, pot, black?

I didn't feel there was any condescending tone in his response. This is a forum, used to exchange ideas and facts. Most of the time there are various views, interpretations and opinions. You have to provide facts and sources to back up posts. That goes for everyone.

So how about you relax a lil?

GrauGeist said:

I'm going out on a limb here, based on observation, and think that you're not all that familiar with how an internal combustion engine works.

What part of this statement does not seem condescending to you? Let me know so that I can clearly explain it to you.

 

[DerAdlerIstGelandet]

GrauGeist said:

I'm going out on a limb here, based on observation, and think that you're not all that familiar with how an internal combustion engine works.

What part of this statement does not seem condescending to you? Let me know so that I can clearly explain it to you.

I guess I just have thicker skin.

He was making an observation based off of what he read. It is a hell of a lot less condescending than this comment:

"I am sorry I wrote to such eminent authorities in my attempt to contribute to a dialogue. I will not try it again"

So I would suggest everyone takes a step back, relax a lil, and have some discussions like an adult.

 

[pbehn]

There is a huge difference between how a two stroke and four stroke engine we learn about at school work and how a high performance engine works. I think the problems start at the concept of "vacuum".