NoOne

I just sat down to start watching Qualifying for the German Grand Prix.

They were talking with each team about how the new V8's were working out.

The next engineering question they are going to be working on is the inlet speed of air into the cylinder.

They are approaching the speed of sound on the inlet size and aren't sure really whats going to happen when they push past it. They aren't too far away, I believe they said they were already around 750mph, so only 10 or so more to go.

Slalom4me

It appears to involve a bit more than just boosting inlet speed another
11 MPH. Temperature and the makeup of the gas are two additional
variables that will affect things, for instance.
Nevertheless, I wonder whether the sonic boom will help or hinder
cylinder filling. Also, will the F1 crews need to line the passageways
with heat resistant tiles when velocities eventually reach high
hypersonic speeds?

.

Solofast

Well, sorry to say neither will happen.

What they are doing is approaching what is called "choked flow" That is, the flow velocity at the maximum restriction reaches the speed of sound and no matter how much the pressure is reduced on the back side of the restriction, the flow can't go any faster. The answer is, if the flow is choked in a restriction, there won't be any more flow and they are done... There is no way to increase the flow thru the restriction unless you increase the pressure or density on the upstream side. You could chill the inlet some and increase the density a bit, but that isn't going to help much. High temperatures at hypersonic speeds result from the heating across the shock, which won't occur here since the flow is just getting to Mach 1, and you need really high velocities (like Mach 3+) to get things really hot.

When this happens across the board, everybody pretty much gets the same horsepower, and they have to figure out other things to do outside of cramming more flow in to get an advantage. This means reducing friction, windage losses , improving handling, reducing drag, and other things to try to find an advantage....

mlongo99

Since crossing the speed of sound makes nozzles become diffusers and vice versa, you cannot use the same shaped paths for both types of flow with good results.

Also, the reason sonic speed flows become choked is that the pressure difference, which is what drives the flow, can never be seen upstream of the flow. The pressure waves travel at the speed of sound (since technically they are sound) and since that is the same speed of the flow, this "information" doesn't travel upstream. Therefore the upstream flow doesn't "see" the pressure drop at the other end and it doesn't speed up. So to increase speed you need to diffuse the flow. This fixes the point where the flow can cross the sound barrier. Unfortunately, whenever you want a slower flow rate, the diffuser will cause the flow to slow down since is it sub-sonic at that point.

That's gonna be a tough obstacle to overcome.

CentralCoaster

Couldn't have said it better myself.

xsiveone

I saw that and thought that it was cool.

I also learned something when they were talking about the vibration of the different engine configurations. They were saying that a V12 is inherently balances out the vibrations, the V10 had vibration up to about 14K rpm and then smoothed out and that the V8 gets more vibration the more that you rev it.

mlongo99

Thanks. I remember lots of stuff from my fluids and thermodynamics classes. Those were my favorites.

63Corvette

This is known as a "sonic orfice", which is what the FIA uses instead of a restrictor plate to "govern" the speed/hp of racing engines to provide "equality" or "parity" in race cars. good luck

AU N EGL

I wonder if this is partly why GMPP changed the angle of the injectors on the heads from 11* to 15*. Stating that the 15* was quicker in filling the cyclinder and allowed for equal cumbustion arcoss the pistion vs the 11* did not quite as good ??

Back in the 70-80s there were veriable hight velcoty stacks to change the air intake speeds and pressures.

On that that thought what about the intake air speeds on our LSx engines?? larger to smaller intakes into the TB? Double intake vs single intake into the TB? Narrow to wide back to narrow intakes to change the pressure gradiants i.e. speed which air flows into the tb.

Rifling of the intake, phesudo turbonator? Rifeling helps bullets travel faster out of a gun barrel. Rifeling helps water travel faster through high pressure pipes too.

smooth vs golf ball dimpes on the inside of the intake .

Solofast

The idea behind velocity stacks is to increase the pressure at restriction by creating a pressure pulse in the inlet that gets there when the valve is opened. The reason for the variable length is to get that effect at different speeds, longer stacks at low speeds, shorter ones at higher speeds. If it isn't outlawed, I am sure they are doing it already...

Pretty sure the flow velocity at the throttle body of most modern cars is relatively low. After velocities in ducts get much over .3 mach you start seeing pressure losses, and all engine designers don't want much pressure loss, so they try to keep mach numbers in a reasonable range. Mach numbers at the throttle plate are generally high when the throttle is a low openings, but as you open it up the mach numbers drop down to reduce the losses.

In the olden days you needed some restriction in the venturis of carburetors to introduce the fuel and get good atomization. With the advent of fuel injection, you don't want the losses, and other than making sure you get a good measurement of the inlet airflow you don't want any restriction. That is why just putting on a bigger throttle body pretty much does nothing for most engines. There isn't much restriction in the throttle body anyway, so hanging on a bigger throttle body doesn't gain measurable performance until you add a cam and ported heads that let the overall flow increase and start to create higher losses in the TB...

I thought the rifiling of the bore of a gun is what causes the bullet to spin and makes it stable, I don't think it is responsible directly for the higher higher muzzle velocity, maybe a gun guy can chime in on that one... And with refrence to the turbonator, if you have a restriction, and increase the angle of the flow going thru it, you actually loose total flow by the sine of the angle, since your relative area is reduced by the trig function. This is why swirl is a bad thing in turbine engine ducts and in places where you are flow restricted by high mach numbers. In high pressure pipes you are at low mach numbers since the flow is incompressible, and what you are doing with the rifiling is to decrease the boundary layer effects. Inlet passages are relatively short (in terms of reynolds numbers, and there isn't a huge amount of boundary layer buildup in the inlet passage, so rifiling effects on boundary layer have to be weighed against pressure loss, and my guess is that isn't a good trade.

Dimples on a golf ball are there to excite the boundary layer at relatively moderate mach numbers and delay the separation of the flow around the ball, and therefore create a smaller wake and lower drag. At high mach numbers they wouldn't help a port pass more flow, they would only create more pressure loss in the duct upstream of the restriction.... Different mechanism at work here...

No easy solution, when you run up against the laws of physics...

AU N EGL

That is kinda what I thought. and yes varible height velocity stacks are illegal

CentralCoaster

I thought the dimples delay boundary layer seperation by reducing shearing forces at the surface. This decreases drag, and produces a smaller wake, which allows the rotation to give it more lift.

For this reason, I'd expect an intake on a street car to flow microscopically better with a rough surface than a polished one at low velocities, where the intake tract flows closer to 20 mph.

Solofast

Here is a good explanation of the effect of the dimples, and what they are doing to a golf ball.

http://www.aerospaceweb.org/question...cs/q0215.shtml

Basically what they are doing is "tripping" the boundary layer, which makes the flow turbulent in the boundary layer, which adds energy to the boundary layer and prevents early flow separation.

So it is the form drag from surface separation that the surface roughness is dealing with, and not the skin friction portion of the drag component.

In a smooth duct, where you are trying to basically pump flow at moderate mach numbers, you want it as smooth as possible, polished is even better. I would have to look at it, but the flow velocity in the duct is probably a lot higher than 30 ft per second. It could average that, but the maximum velocities are a lot higher than that. You have to look at the amount of flow that you are moving over the time period of the event. That is, you are flowing the air to fill any one cylinder in 90 degrees of crank angle every other revolution. The air isn't moving in the intake during the rest of the time. So if you calculate a volumetric efficiency of say 80%, times the volume of one cylinder, and then pass that through the passage in a sinusodial profile (flow starts low, peaks and then drops off), I figure that you are hitting pretty decent mach numbers in the duct at peak flow, and in the area near the valve they are pretty high, probably near choke. So, unless you want to induce some turbulence near the valve to prevent flow separation as you enter the cylinder, smooth ports are probably better.

MitchAlsup

The Wright book on the Ferrari F1 car of 2000 explains the moving velocity stacks and how this builds TQ over a large RPM range. This technology has been outlawed for the 07 or 08 season when they switch to 'standard' ECUs.

[QUOTE]Pretty sure the flow velocity at the throttle body of most modern cars is relatively low. After velocities in ducts get much over .3 mach you start seeing pressure losses, and all engine designers don't want much pressure loss, so they try to keep mach numbers in a reasonable range. Mach numbers at the throttle plate are generally high when the throttle is a low openings, but as you open it up the mach numbers drop down to reduce the losses.[QUOTE]

No modern street engine runs such that the vacuum at Wide Open throttle at the back side of the inlet valve is more than 1.5-2.0 PSI at peak TQ. This is the measure of all reastrictions through the whole inlet tracks from ambient (still) air. Race engines shoot for 0.5 PSI (or less--often at peak power) and are willing to take the low speed throttle modulation problems associated with widening the ports such that this lack of pressure drop is delivered.

This is why the inlet tracts are as short as they are--less than 6 inches from the back side of the valves to the top of the shortened velocity stack (including throttle/velocity stack).

Only when the engineers cannot either widen the tract to avoid creating a restriction, or when they cannot shorten the tract to avoid SoS issues will the SoS become a serious impediment. Right now it is but one of a miriad of problems with making HP at these kinds of RPMs. The real fun begins in the around 26K PRMs when the speed of buring gasoline becomes another power limitation.

VetteDrmr

Great read, but my brain is hurting!!!

Have a good one,
Mike

mlongo99

Yea, but it's a refreshing change of pace from the emblem/floormat/waxing discussions that flood the general sections.

mrgrimes

Here is a riddle - The cool thing about sonic flow is it can turn a non-linear flow orifice into a linear one, WTF am I talking about?

MrEracer

Answer to riddle:
When the flow through an orifice reaches SoS the flow becomes a direct function of flow area and nothing else. Thus the only way to increase the sonic flow through an orifice is to increase the area of the orifice... Therefore, sonic flow through an orifice is a linear function of its area... Something like that, right...
Shirl Dickey

ghoffman

Exactly, that is why fixed inlet jets (F-16, F/A-18) are limited to 1.8 mach or so. The airflow through the engine has to be subsonic for the reasons above. In order to go faster than that the shock wave must be kept outside the engine, so variable intake schemes are used by F-4, F-14, F-15, SR-71 among others so they can fly at 2.5 -3.0 or more. They all have a shock cone (-71) or a deflector of one way or another to change the inlet area and keep the shock wave outside the engine. If you don't. the engine can be infinitely more powerful but it won't matter because the compressor will stall.
I like the idea of having an absolute limit of supersonic flow and flame propagation for F1 engines, it will force the designers to focus on other things like power band, fuel economy, etc.
Good discussion!

RedHotBolt

You Guys Are Some Smart Mf'ers!!!


J