At 300 km/h (188 mph), the air-box of an F1 car could never generate an inlet pressure of more than some 3-4% above the atmosphere, why I would be a little surprised if that was taken into account when deciding the length of inlet-trumpets.
"I spent most of my money on wine and women...I wasted the rest"
xpensive wrote:At 300 km/h (188 mph), the air-box of an F1 car could never generate an inlet pressure of more than some 3-4% above the atmosphere, why I would be a little surprised if that was taken into account when deciding the length of inlet-trumpets.
I think at this level, there is NOTHING that is not taken into account in any aspect of design...
Henning wrote:Check out the Mazda 787B that won Le Mans in 1991. It has variable length input trumpets. I don't have a video to hand to post, but it looks very cool to see the trumpets moving up and down.
One of my favorite cars of GT3/4!
And it is a rotary engine, so maybe the variable trumpets are of a different philosophy? I dont know it the trumpet design would be the same for direct injection and fuel injection...
I'm sure that someone here can explain the difference, if there is any...
Henning wrote:Check out the Mazda 787B that won Le Mans in 1991. It has variable length input trumpets. I don't have a video to hand to post, but it looks very cool to see the trumpets moving up and down.
First thing that came to my mind.
And because it is a Rotary the variable trumpets make an even larger effect than in a piston eater. Yes it would be the same for DI & FI... we are talking about the air as it moves thru the engine.
Optimum intake runner length is determined by the acoustic properties of the intake tract at any given operating condition. The objective of a tuned intake runner is to design the passage such that the negative pressure wave created by the opening of the intake valve passes up the intake runner and creates a positive pressure reversion wave going back down the inlet tract, due to the rapid change in flow passage cross section at the trumpet mouth. The pressure wave travels at sonic velocity, and the speed of sound varies with pressure, density or temperature. The trick is to select an intake runner length that makes the reversion positive pressure wave arrive back at the intake valve just as it is closing. Thus forcing more intake charge into the cylinder. The same process applies to tuned exhaust headers.
This all works fine at a fixed engine operating frequency, but at other engine speeds it can hinder performance. A variable length runner solves this problem by creating an acoustically tuned length for a wide range of engine operating frequencies (ie. speeds).
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
In the end, it's clear from the equation (I hope) that for faster traveling waves, a shorter pipe is needed. I wonder if you could construct a manifold with an "extra path". This extra-path could be controlled by a valve, which could be opened or closed depending on RPM. Has this ever been tried? Is it legal? Would it be useful?
To answer your question Ciro yes it has been tried, and yes it has been banned. This is useful because it would help make a maximum torque curve throughout the rev range, some production performance automobil's have these type of variable runner length systems. It think so BMW had one a few years ago that was not like what you described as a valve changing runner length, but that it was contiuosly variable, in other words it had a minimum and maximum runner length and anything in between, I'll see if I can find details....
That wouldn't be a Trumpet.... That would be a Trombone!
Your not wrong mate, indeed it would mimic a trombone, http://www.youtube.com/watch?v=Go3Fgd1wgic
This is a video of the Mazda 787B which used that type of "trombone" runner length system, unfourtunatley the commentary is in Japanese, but about 4:45 into the video the engine cover is taken off and you see the intake runners move in and out, demonstrating constantly variable runner length.
That just makes me sick to learn how important and innovative the CVT (constant variable trumpet) is to engine performance, and it is now banned in F1.
Riff-Raff,
Is it possible to amplify the rebound wave that comes back to the intake port? Almost like a passive super-charger? I can picture what I am trying to say, but let me think about it a bit more, and I'll update this post when I have something.
Internal acoustics are deffinately interesting, and I have been playing with the Waveguide designer in CATIA. Is it possible to passively amplify an acoustic wave?
This freeware application only models a single cylinder, but it is very sophisticated none the less. It allows you to model intake and exhaust effects. It's a great learning tool for studying the engine design process.
So, it may be possible to put piezeos in the exhaust header and have them drive piezeos in the intake manifold to amplify the acoustic pressure-charging effects of the reflection wave?
The timing would need to be computer controlled no doubt, but could it actually increase the efficiency on a NA ICE?
Riff-raff, thank you for that link! I just bought a collection of books and this type of software will really allow me to experiment. If you have any other links like that, please shoot me a PM, I would greatly appreciate it!
I ordered:
Chassis Engineering/Chassis Design, Building & Tuning for High Performance Handling
Author: Herb Adams
Competition Car Suspension
By Allan Staniforth
Competition Car Aerodynamics
By Simon McBeath
Competition Car Composites
By Simon McBeath
The Haynes Car Electrical Systems Manual
By Martynn Randall
Car Builder's Manual: Designing and Constructing Your Own Car from Scratch
By Lionel Baxter
If there are any more that you would suggest, please reply!
It is basically an electronically controlled one-way valve in the inlet runner that prevents the positive reversion wave from escaping. It is very effective at increasing VE for very little power input.
Regards,
riff_raff
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
Before the regulations banned them a few years ago (I can't remember which year) F1 engines did have variable length intake trumpets. Not two stage systems, but hydraulic systems.
The speed of sound is proportional to the ratio of specific heats, gas constant and the gas temperature and the first two do not change much regardles of conditions so temp is the most influencial.
In regards to the helmholtz resonator topic, I think you have slightly mixed up the applications of such a device, but they are widely used on engines. The acoustic tuning of an engine using trumpet lengths is considered purely from the traveling time of an acousic wave. As the piston moves down the bore, its trying to draw air in through and orifice so a negative pressure wave is generated, starting at the piston crown. The amplitude of this wave is a function of piston velocity and the effective flow area past the intake valves. For this we will consider the peak amplitude to be when the piston is moving fastest down the bore but in reality there is a time delay for the air to start moving. This negative wave then travels up the inlet track until it reaches a sufficiently large change in cross-sectinal area and air volume for a positive pressure wave to be refracted back down the inlet. This positive wave travels to the inlet valve thus increasing the inlet pressure which improves cylinder filling since air flow is proportinal to flow area and velocity, and velocity is proportional to the pressure difference which in our case is the pressure difference between the cylinder and inlet system. Most times you want this positive pressure wave to return just before the inlet valve closes because the piston will be starting to come up the bore and pressurise the cylinder so you want inlet pressure>cylinder pressure or you will get flow from the cylinder in to the inlet.
When the positive pressure wave has traveled back to the cylinder, a negative pressure wave is refracted from within the cylinder up the inlet system, this again travels up the inlet track where another positive wave is refracted back in to the cylinder. These waves continum to travel up and down the system as long as the valves are open. They are known as the harmonic orders, in general on a production car, you can not make an inlet system short enough to use the first harmonic. The amplitude of the waves reduce with each harmonic as they are dampened.
Exhaust is fairly similar but the aim is slightly different and the speed of sound is a lot quicker so lengths are different. You can do different things with exhaust since you are trying to get a negative pressure wave and what you can do is make Inlet pressure>cylinder pressure>exhaust pressure so you can have a rush of air through to clear out residuals and actually use the exhaust pressures to get more air in the cylinder, not just use it to clear spent gases.
I worked on the mentioned Mahle project. I do other acoustic projects but most can not be mentioned, one was vibration of the throttle butterfly to generate desired inlet frequencies. I mentioned previously that tuned lengths to use the first order harmonics on a system are incredibly short but I worked on an idea where the exhaust primaries are stepped individualy so each cylinder has its own pressure wave refracted from within the primarie rather than wait untill the wave travells to the collector, this allows a much shorter track untill refraction allowing the engine to use the first order harmonic which is much bigger in amplitude. I used this very sucessfuly to keep exhaust valves choked for a longer period of time on initial opening. This is being used in F1 now.
The basis behind the helmholtz resonator idea is like any type of spring going in to resonance. When a volume of air hits its resonant frequency is can cause the pressure inside to increase but they can also hit anti-resonance which is the opposite. The frequency of a spring is given by (1/2pi)*(k/m)^0.5 where K is the stiffness and M is the mass. We can apply this to a gas like air since it is effectively a bundle of masses separated by springs.
When we apply it to air the stiffness becomes the air pressure and the mass becomes the air density, you can imagine a higher pressure chamber of air has more stiffness.
I appologise for any spelling but I am not a writer plus I dont know how to input proper formulas. If anyone wishes to ask any further questions, you are welcome.
The thing that always "bothers" me about this is that the 2nd, 3rd, 4th orders, etc, are all pretty close to each other (about 1000rpm between them?), therefore just 500rpm after the point at which the pressure wave helps there's a point when the pressure wave reduces cylinder filling.... then 500rpm later a point which helps again! Most race cars must operate over at least 3000rpm (i.e. between gear changes), so that means you get a couple of "good" points, and a couple of "bad" points within your operating range.... so why bother tuning for any particular speed.....??? I guess you could aim to get a "good" point just at the rpm at which you change into the gear and then change gear just before you get to a negative point??? You've probably already gone through a couple of good and bad points though....
