can you have a throttle body thats to big?

ed · 05-10-2006, 01:10 AM

huh?

were not talking about the propogation of sound or waves through the air, but the flow of the air itself...

and its hardly a technical argument even needing discussion of these matters.

**pilot** · 05-10-2006, 01:16 AM

That´s the definition os speed of sound. It varies with pressure, temperature and other factors. The Mach number is a given speed divided by the speed of sound in THAT medium.

As I said before, this wasn´t the key of what I wanted to explain. Anyone with enough interest to do this kind of calculations should have enough knowledge or be willing to study this to the extent of understanding this pretty well.

An engine works pretty much like a wave generator and it can be studied in that way.

The whole intake system (that includes valves, head runners, intake manifold runners, intake plenum, throttle, intake plumbing, intecooler, turbocharger, supercharger, etc) can be studied in the same manner. It is up to the designer/person in charge to decide whether the work will give a substantial gain or not.

**pilot** · 05-10-2006, 01:22 AM

For example: inf you have a forced induction engine and would like to maximize its output at a given RPM and WOT, you can make the calculations to achieve an air flow of around Mach 0,6 in as much of the intake system as possible (providing you can modify the intake system). The Mach number should be calculated for the air conditions that will be obtained in those conditions including its temperature and pressure.

ed · 05-10-2006, 04:06 AM

blagh... read the title of the topic eistein.

if you want to talk about harmonic tuning, find the appropriate thread. this thread is to discuss the size of the throttle body. this requires knowing the speed of sound as a measure of the speed of the fluid. the density of the fluid itself doing the flowing will not affect this consideration greatly - certainly not enough to start a nerd war over. yes it needs to be known for other far more onerous calculations, but not this one.

for this topic, 340m/s is the correct base number.

ed · 05-10-2006, 04:33 AM

http://www.eng-tips.com/viewthread.c...=111416&page=8

**pilot** · 05-10-2006, 05:09 AM

I don´t intend to argue with you, I just wanted to share what I got to know about the subject.

The article you linked doesn´t cover forced induction engines. A 50º difference in temperature already gives an important change is speed of sound, add to that a raise of pressure by 2 (1 kg/cm^2 of boost) and conditions change drastically.

ed · 05-10-2006, 05:16 AM

what changes exactly?

the way we measure how fast a hotter/denser fluid travels?

the volumetric flow rate remains unchanged
the mass flow rate increass with pressure

the local speed of sound remains the same

the speed of sound 'within' the fuid my change, but thats not what were measuring

**pilot** · 05-10-2006, 05:28 AM

The Mach 0,6 value works well because it is a good compromise between flow velocity and opposition to the flow. If opposition wouldn´t be a problem, we would aim to speeds very close to that of the sound, right? This opposition varies for a given physical speed depending on the pressure and temperature of the air. That´s why we have to calculate Mach number for each (or desired) condition of the fluid.

ed · 05-10-2006, 05:41 AM

"For convenience, the ratio of the typical velocity to the intake sonic velocity is called the inlet Mach index. From the science of fluid mechanics we know that the controlling velocity in a compressible flow system is usually the intake valve opening." (Ref 2) For a given cylinder and valve design, the inlet Mach index is proportional to the piston speed. This seems reasonable, that the fuel/air charge flows in faster when the piston moves down faster. Of course, at some point the constriction of the valve opening starts to limit this. When the inlet Mach index exceeds 0.5 (intake velocity equal to half the speed of sound), the volumetric efficiency falls rapidly with increasing speed. Therefore, engines typically are designed such that the inlet Mach index does not exceed 0.5 at the highest rated speed. (Ref 2)

2. Charles Taylor, The Internal Combustion Engine in Theory and Practice, 2nd ed

ed · 05-10-2006, 05:44 AM

The speed varies depending on atmospheric conditions; the most important factor is the temperature. Humidity has little effect on the speed of sound, nor does air pressure per se. Air pressure has no effect at all in an ideal gas approximation. This is because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature. Sound usually travels more slowly with greater altitude, due to reduced temperature. An approximate speed of sound in 0 % humidity (dry) air, in meters per second (m·s-1), at temperatures near 0 °C, can be calculated from:

so, how much different does a temp of 50degC have on mach index? not much

**pilot** · 05-10-2006, 05:56 AM

So you state that you can think and make calculations using Mach 0,6 = to a constant speed value. It is not right and that´s exactly why calculation are done in reference to Mach numbers and not absolute speed.

Speed of sound at atmospheric pressure can be calculated this way:
SS = (401.7 x T ) ^½

If you input T = 273 K you get SS = 331 m/s, if you input T = 323 K you get 360 m/s. that´s more than an 8% difference.

I won´t go on, I think it has no purpouse.

ed · 05-10-2006, 06:07 AM

no, the point was (originally) that this discussion didnt need to get this complex...

...which had been further hampered by your original estimation of the basic speed of sound being incorrect, and then your application of the 0.6mach index guide to the wrong part of the intake tract...

other than that, we're plain sailing

(yes i have oversimplified application of the mach number in this topic, but deliberately so. for practical and sensible purposes, as well as the role of this discussion, 340m/s is still the most desirable constant with which to work)