Engine Ops with No exhaust system. facts wanted.

[Ultralights]

A question was posted somewhere else about operating an aircraft engine less the exhaust system, or just straight pipes, out and down, for each individual cylinder, and how would this effect the engines performance?
I found the question interesting, as i am unsure of the answers even though i have done some well respected courses on engine ops and management, but of course, a lot of responses dont make logical sense, and the number of OWT's soon swamped discussion, so i decided to post here, knowing the background and expertise of some contributors here in engine ops, LOP, etc etc, in the hope some will answer using Scientific fact and repeatable observations..

So i guess the questions are, will no pipes/straight out pipes, (no Muffler) result in damage to valves? is back pressure really required to ensure smooth operation? will it effect mixtures?

from my understanding, it will effect nothing other than ones ears.. nothing more, am i correct? (no references to 2 stroke engines, i know it matters in those engines)

[Blueskymine]

What does a drag racer have?

What did the wartime piston engines use?

There's your answer.

[Avgas172]

Could be wrong but I thought most Barons had no mufflers, just a straight through system.

[Guptar]

According to this, straight pipes are best on ultra high volume systems. (0 USG per minute.........sheesh!


Top Fuel and Funny Car

At the top levels of drag racing, in particular Top Fuel and Funny-Car, the exhaust systems might seem very simple. The header systems, known as "zoomies," consist of a single pipe on each cylinder, dumping straight into the atmosphere, with each tube bent so that it faces upward, rearward, and often outward. The outward angle of bend in Funny Cars is typically larger than would be seen in an unbodied Top Fuel car, in order to eliminate bodywork damage from both temperatures and exhaust concussion forces.

In addition to the noise, a notable feature of these exhaust systems is the large volume of open, whitish flame standing just off the ends of these pipes, as shown in Figure Eight. That flame-front is the byproduct of two intersecting parameters.
Secondary Combustion
Figure 8
Secondary Combustion

First, these highly-supercharged, nitromethane-nourished engines have fuel flow rates stated to be in the 80 to 90 gallon-per-minute range. With that amount of fuel being delivered, it is clear that there will be a certain amount of fuel puddling behind the intake valve. When the intake opens, some portion of that collected fuel will be either in liquid form or in a mixture which is too rich to burn (insufficient oxygen molecules). Further, these engines apparently use a large amount of overlap in order to assist in cooling. The combination of the excess fuel and the long overlap assures that a non-trivial quantity of raw fuel and fuel mixture is short-circuited directly down the exhaust pipe, and heated during its journey. When it exits the primary, it finds an abundance of oxygen and initiates an energetic secondary combustion. The combination of the large momentum-change of the mass flow through the engine, plus this secondary combustion has been calculated by at least one aerospace engineer to generate normal reaction forces in excess of 2500 pounds (1130 kg).

Given that the pipes are angled in both the lateral and longitudinal planes, that exhaust reaction force can dramatically affect the vehicle stability. The vertical component obviously provides downforce to the chassis. The rearward component will add propulsive thrust. If everything is in balance, the sideward components generated by the left and right sets of pipes should counterbalance and net to near zero. However, I was told that the loss of one cylinder on a Funny-Car can cause the driver to have real difficulty controlling the car. That is because the loss of a single cylinder unbalances the sideward-thrust and adds a yaw-moment from the now-asymmetric rearward thrust. That same (highly-credible) source told me that the loss of two cylinders on the same bank will amost certainly render the car uncontrollable.

As might be expected, the length of the primaries plays a critical role in the engine tune. I was told by a lead engineer on a prominent Funny-Car team that there was a considerable amount of development effort required just to get the gasses out from underneath the Funny-Car bodywork.

That source also said that when they tried collector-systems, the result was that the engines ran "horribly". The theory is that the huge amount of exhaust gas flow into a relatively-confined space raised the collector pressure enough to create a destructive blockage in the collector pipe.

As far as the pipes themselves are concerned, it is well known that "too sharp a bend" in the primary or "too much length" dramatically reduces engine performance. Apparently, in supercharged nitromethane engines, any tuning on the exhaust side (cam, ports, headers) requires a substantial alteration in the fuel delivery curves. After experimenting with various exhaust system changes, then working to get the fuel system back into line with the engine changes, the net change in performance was typically considered to be not worth the time and effort. After having determined a working combination, experience has shown that development efforts in areas other than the exhaust system will be more productive.

I was told that currently, there is not a large amount of development effort on the Funny Car exhaust system, as the result of several practical and economic factors. It is hard to imagine the level of difficulty involved in doing engine development on a system which is not well suited to a dyno cell, and therefore must be tested on the track in 5-second test sessions. Without taking into account salaries, logistics, transportation, food, lodging, and other "overhead" expenses, the out-of-pocket cost to make "one more test run" is uncomfortably close to ten thousand dollars.

Moto-GP

[waren9]

What does a drag racer have?

What did the wartime piston engines use?

There's your answer.

i guess it depends on the question being asked.

4 stroke exhausts are sensitive to tuning, like 2 strokes.

weight, ease of manufacture and cost are not the only variables

[cockney steve]

What does a drag racer have?

What did the wartime piston engines use?

There's your answer.

Ah, but it's NOT that simple.!

Cam-timing and profile have an influence...you're aware of the 2-stroke's tuned inlet and exhaust tracts?...-So what makes you think the same laws of physics are suspended for 4-strokes?

The above examples both totally ignore any ecomomics or longevity issues, indeed, most of a dragster's potential energy is "!wasted" on crowd-spectacle, as only the most concentrated part of the power is effectively used.

Apart from the fire and noise issues, aero-piston engines have the huge advantage that they are virtually "constant-speed" machines.....therefore, inlets, exhausts and cam-timings can all be optimised to be "in tune" atthe normal operating revs.
This gives you better fuel-efficiency, better power output and longer life. it can also have a marked effect on sound level.

No doubt one of the proper engineers will explain the nuances of resonant-frequencies, valve-overlap,gas-column inertia and the likes.

meanwhile, just ask yourself....if an exhaust wasn't needed, why would manufacturers waste money designing and fitting it?

It's well known that most commercial designs are a compromise of ease of manufacture v benifit obtained....the cynic says they get away with the cheapest and easiest, not the best solution.......hence you get the premium-quality aftermarket items.


Where an engine design indicates, a proper exhaust system CAN, as part of a design, allow the incoming charge to go straight through the "cly";) cooling the exhaust-valve, then, by virtue of pressure-waves, use the otherwise-wasted exhaust energy, to push that "spilled" charge back up into the combustion chamber...in effect, the cylinder is already pressurised before the piston starts compressing the charge.

Apart from watching the pretty flames and enjoying the crackles and bangs, there's no point omitting an exhaust system!

[Ultralights]

a good hypothetical would be, A IO540 in a RV10, would it run better with pipes straight out the side Mustang/spitfire style, or the longer run pipes into one under and behind the nosewheel and engine cooling exit?

[27/09]

Most FU24-950's had straight pipes, 4 out each side of the cowl. It's a sound that has diminished significantly with the introduction of turbines. :{

[sms777]

Pipes out the side will definitely make your engine more efficient and as a side effect...more noisy. If noise is not your concern why not fit a IO720 in the RV10. Imagine the looks of four pipes per side and the awsome sound. Top fuel in a homebuilt.....Yeah baby. :E

[Sunfish]

Changing the exhasut from what is standard will change the pressure pulse behaviour and hence cylinder pressure. the end result might be burned valves unless you do your research.

[Ultralights]

thats just the thing, i have been researching, and can find no difinitive answer on what the "pressure pulse" does, or what effect it has on an engine with no pipes, or short pipes? or how it will "burn valves" a lot of info about it doing these things, but no hard data on why.. remember, forget 2 strokes, i know they need the -ve pressure pulses to scavange and help drag through the next fuel/oil/air charge into the chamber, but nothing on a conventional 4 stroke engine other than, its just bad and will cause this and this etc.. if it reduces cyclinder pressures, it would produce less power, but how would it result in burnt valves? what changes are made to engines with straight short pipes, to stop this supposed change in cylinder pressures??

[oldpax]

interesting that the griffons as fitted to the shackeltons were origanly fitted with "fishtail exhausts"which went from a round pipe to a narrow long slit and then changed to a collector pipe system which I think was to alleviate some of the horrendous noise they produced !Maybe there are Griffon people out there who would know?

[Sunfish]

Ultralights, the pressure pulse is reflected back towards the valve from the end of the pipe and may assist in reducing or increasing gas flow as the exhaust valve closes. I dont quite know the dynamics, but what I do know is that changing exhaust systems meant complete rejetting of carburettors and a changed torque peak position. From memory a shorter stack moved the torqu peak up the RPm scale and a longer pipe did the reverse.

Be aware that there is no such thing as a free lunch and you are not going to suddenly find oodles more power by replacing a competently designed system with some snake oil salesmans product.

Having said that, Rotax has just announced changes to the 912 iS Intake system, along with ECU changes, that radicaly modify its torque curve for the better

[Old Akro]

thats just the thing, i have been researching, and can find no difinitive answer on what the "pressure pulse" does

43 years later, this book is still " the bible"

The scientific design of exhaust and intake systems - Philip Hubert Smith, John Cruickshank Morrison - Google Books (http://books.google.com.au/books/about/The_scientific_design_of_exhaust_and_int.html?id=oIZTAAAAMAA J)

[tdracer]

The principles behind a tuned exhaust are quite simple, although proper execution boarders on black magic. When an exhaust valve opens it sends a pressure wave down the exhaust - which will reflect back from internal features and the exhaust opening. If the exhaust is properly 'tuned', those pressure waves can be manipulated such that the engine is exhausting into less than ambient pressure. Depending on inlet/exhaust valve overlap, the exhaust can also be used to increase the back pressure when in inlet valve opens but before the exhaust valve closes. The down side is that the effect is RPM dependent - the more aggressive the exhaust tune, the more narrow the effective power band.
On two stroke engines, the effects can be huge - 30% power increase with the right tuned exhaust. When I was racing, exhaust tuning was critical - a difference in 1/4" in the header length could mean 1/2 second/lap.
With a four stroke, the effect is far more subtle, but is there none the less. Top fuel dragsters are an anomaly - look at most naturally aspirated racing engines and the exhaust headers look like a bundle of snakes.
An aircraft engine would be ideal for a tuned exhaust - as noted previously they work best over a limited rpm so constant rpm would allow for a highly optimized exhaust.

[FokkerInYour12]

Powerflow Systems make some tuned exhaust system retrofits for various singles - mainly the IO/O360 range. I've heard first hand from some who have fitted them of impressive performance (or efficiency at same % power settings) gains.

[stevef]

A slight drift but I remember reading somewhere that the Merlin ejector exhaust stubs added about 7% thrust. There was apparently a small gain from the radiator outlet/s too, which I'd be interested to know more about.

[Andy_RR]

with an aero engine with fixed ignition timing and a big red knob, the only two things that are used by engine developers to control temperatures are both in the hands of the gods - self-proclaimed in the case of mixture.

On this basis, you could run virtually any exhaust you like, provided you didn't exceed cylinder head temperature limits.

[catseye]

Did someone say fletcher with open exhausts.... you'll have to speak up.

[yr right]

A few things here to take note of.
Nitro methane produces its own oxygen, if you hit a drop of it with a hammer it will explode.
TF FC engine are making around the 10000hp mark that approx. 1250hp per cly of 62ci.
An engine will turn approx. 440 time per 1/4 mile run
You cant lift and trip out of the buckets the fuel require in a run faster than it uses it.
Next overlap is use to get the maximum charge into the cly and has nothing at all to do with cooling,
They generally run 3 fuel nozzles on of which is directly sprayed into the combustion chamber the other two into the intake port,
The angle of the headers are up and layed back they also plant the car to the track and if you loose a cly it make the car turn that's how much engry is being lost.


As for cam timming if you advance your timing you will get quicker ET
Retard your timing you get faster speed as a rule of thumb




Now a blown engine dose not need the exhaust to be particular great as the incoming charge is being forced into the chamber,
A NA engines looses HP in the exhaust if its not right, the exhaust flow helps the scavenge of the cly spent gasses to be removed from the other cly.
If you look at a NA drag car normally a 4 into 1 system at around primary tube length of approx 30 inches give or take.
Now remember the roots type blower was before being hot-rodded onto the top of an engine was a GMC scavenge pump use to remove the exhaust from GMC diesel engines.
Short stacks where used on blown fighters as there is no looses and remember they where super charged.


Also even thought an aircraft engine is relatively not a high rpm engine its cly area is large and as such the gas is slow to be removed. Most aircraft exhaust system's are built for cost and resections of space is a limiting factor.


Yes there also was a power advance from the spitfire but more from the mustang heat exchangers due to the heated air and the velocity increase sorry I cant remember exactly how much it was but it was an increase.


Cheers

[OZBUSDRIVER]

Meredith Effect
Meredith effect - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Meredith_effect)

google is your friend. Agree with NA engines and tuned length exhausts. Lots of power to be had.

[cockney steve]

There seems to be some confusion over pipe-length......Remember, we're dealing with SOUND...Take a guitar, pluck the string, it will vibrate with maximum amplitude at the centre (skipping-rope)
Now lightly touch the centre of thestring and pluck in the normal place, the string will now have a s-shape...2 opposite-phase vibrations at twice the "free" frequency.......repeat at the quarter-node and again,the string will vibrate, snake-like an 4 sections.

The same principles apply to brass instruments didgeridoos and aircraft exhausts!....the shorter pipe has a much tighter-defined frequency-range from "into" and "out of" resonance, than a longer pipe.....all to do with Fundamental Frequency and Harmonics.
Also ties in with vibration, resonance and stuff vibrating itself to bits!

Regarding the "burnt-valve" issue.....draw a circle and plot yopur cam timings on it.....you'll see a large number of degrees where both inlet and exhaust are open simultaneously.....Why doesn't the "new"charge get ignited by the previous one, and why doesn't it all go straight downthe flue-pipe?- Indeed, why doesn't the flame-front go back up the open inlet valve and set fire to the whole inlet-manifold and carburettor?

lots of questions, a complex answer of interacting physics.

[porch monkey]

In essence tho, there isn't a huge difference between the nitro funny car engine and the larger aircraft engines, large capacity, superchargers etc, but more importantly limited rpm range. Funny car engines are actually rather limited with their rpm range, the clutch does all the work of loading the engine actually. Which is one reason why the exhaust systems are rather similar - open pipes. As far as burning valves, that happens because of the uncorrected leanness of mixture that will generally occur when you take the exhaust system off. A properly tuned engine won't burn valves just because it has no exhaust to speak of. But if it was designed and tuned for one, and you remove it, that's another story.

[sms777]

That's exactly right.....If in doubt read Andy RR's post #18 he has got it spot on. It is all in the hand's of the God's.

[yr right]

I think you missed the point. Any supercharge engine is not reliant on the exhaust system. N/A engines are that's the point fuel engines are the extreme example that's all as they are in your face and you can see the result when it all goes wrong.
Cheers