trumpet length
trumpet length
(OP)
Hello
An engine with inlet system consisting of individual trumpets for each cylinder develops peak power at a speed of X rpm. The total length of each inlect duct (from valve seat to trumpet edge) is L.
Assuming a new target engine speed Y>X for peak power I would like to (approximately) calculate the corresponding (shorter) length of each duct
Do you have any suggestions on how to proceed to have a quick estimate?
Thank you
Gio1
An engine with inlet system consisting of individual trumpets for each cylinder develops peak power at a speed of X rpm. The total length of each inlect duct (from valve seat to trumpet edge) is L.
Assuming a new target engine speed Y>X for peak power I would like to (approximately) calculate the corresponding (shorter) length of each duct
Do you have any suggestions on how to proceed to have a quick estimate?
Thank you
Gio1





RE: trumpet length
RE: trumpet length
Title: "Design and Simulation of Four-Stroke Engines".
AUTHOR:Gordon P. Blair
ISBN Number: 0-7680-0440-3
RE: trumpet length
An excellent book on this subject is "Scientific Design of Intake and Exhaust Systems" by Smith(?) if it is still in print.
RE: trumpet length
One of the main problems that I had a hard time with is the measurments are taken from the opening of the "bell" to the CENTER of the intake valve...boys and girls, that center moves a bit! Also the Weber carbs I was using were not totally straight through with aux venturies, throttle plates, etc. in the airstream. Once I got something close on my first engine, all the engines since seem to be somewhat simpler to figure out.
Rod
RE: trumpet length
If this is so there is a thing (correct me if Iam wrong) called 1/2 wave reversion and it uses a shorter length to allow the wave to travel 2 times to an opening valve.
This will allow you to tune with some degradation, an acceptable inlet.
RE: trumpet length
OP, you might get some joy out of the free Lotus engine software at www.lesoft.co.uk
It is a wave dynamics model, therefore it should be sensitive to plenum lengths etc. That also makes it rather cpu intensive.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: trumpet length
There will be an optimum flow velocity that produces a torque peak somewhere in the Rpm range. Below peak torque velocity cylinder filling poor. Above the peak, pressure drop will reduce cylinder filling.
This peak can be moved around somewhat by also tuning the runner length, but the peak cannot be moved very far doing this. If the valves and ports are far too small, the engine is not going to make massive top end power, no matter how short the runner length is made!!
Pressure waves in the intake runner can be used to shift the torque curve around, and the intake length can resonate at more than one Rpm. It can be tunes to a different harmonic.
Second harmonic tuning is the strongest where length in inches = 108,000 divided by Rpm.
Third harmonic tuning = 97,000 divided by Rpm
Fourth harmonic tuning = 74,000 divided by Rpm
Fifth harmonic tuning = 54,000 divided by Rpm.
While second harmonic tuning is the strongest, you can only get one peak, and the runner length can be inconveniently long. It works best on very peaky high Rpm engines.
On a sports car something much shorter that uses fourth and fifth harmonic tuning is better because it will give a better spread of torque and be less peaky. For instance a ten inch runner will peak at 5,400 7,400, 9,700, and 10,800 Rpm. The peaks get higher at higher Rpm.
Having a double hump at both 5,400 and 7,400 might work rather well on a road car. It might also work well on a motorcycle engine peaking at 10,800 Rpm.
But tuning the runner length is only going to be effective if the induction flow area is sufficient to support enough flow at that Rpm.
A third tuning parameter is plenum volume, which can create a helmholtz resonance in the intake runner, usually at a fairly low Rpm. So there is plenty to think about apart from just runner length.
RE: trumpet length
Sure wish the calcs would come somewhere in the park for my five port Mini Cooper head...so far it's at peak torque around 6066/6250 and hp at 6945 with something like 15 inches(from memory, I don't have my notes handy) overall inlet with a Weber...arrived at by seeing what others were doing and then a lot of 'cut and try'. With the help of the engineers and techs at APT, I've picked up ~13hp with wave tuning and anti reversion work since I started this project last June. Not to shabby for a supposedly optimized 1300cc engine, hey?
Rod
RE: trumpet length
Acoustic end corrections depend on what kind of ambient the trumpet opens into. For an open-ended ambient, the correction is 0.6r, where r is the radius (note it's not 0.6 times the diameter as previously mentioned in this thread). And for a flanged ambient (one poking through a baffle board), it's 0.85r. So in real like, the correction is going to be somewhere between the two. Not important for long thin pipes (like exhausts), but VERY important for short fat ones (like intake trumpets)
RE: trumpet length
Too many variables and unknowns to work out an exact theoretical mechanical length. Final tuning really needs to be done on the dyno.
RE: trumpet length
On the topic of "end corrections", we have carried out tests in this very area using both tapered and straight pipes, with and without bellmouths, on a proven, validated single-pressure-pulse test rig and have seen the correction to be uniform over all the test specimens at approx 0.5D.
RE: trumpet length
RE: trumpet length
The resonant period or "round trip" time for the pressure waves are going to be influenced by piston movement and valve timing, and the actual pressure and velocity waves in the runner look nothing like the sinewaves that a simple harmonic resonance usually produces.
Some basis for an initial design is better than none, and the length formula given above will usually give a pretty good starting point. Only dyno results can tell for sure if it working as intended.
RE: trumpet length