Air Intake Volume.

[jonD6B 0]

Taking different types of air filters out of the equation. What forces are involved in getting air volume into an engine? If a car is travelling at 100mph will the air intake be more than a stationary vehicle given that in theory the air is being forced in rather than being taken in at atmospheric pressure or is it simply down to how much air any given engine can 'suck' in unrestricted?

[M3Evo 0]

Had this arguement at work yesterday as it happens (No answers I'm afraid though)

 

I reckon that the inlet area to the engine is the defining factor in the pressure generated by forward movement, the other guy reckons having a big forward facing air inlet will generate masses of inlet pressure.

 

It occurs to me that the pressure accross any frontal area will be exactly the same and that only the force on that area will vary.

 

That's probably really badly explained, but I'd be really interested to hear what the score really is!

[Dom9 2]

A lot of the touring cars used to engineer their engines so that the heads had their inlets facing forward (like the Mi16 is normally) to make full use of the air rushing into the trumpets.

 

I believe it actually takes some serious forward speed to actually get positive pressure in the inlet manifold, I want to say it is something of the order of 150mph... However, any help the inlets can get is better than nothing and I presume that, even if you don't actually get positive pressure, by 'helping' the air flow down the inlets you are actually reducing the pumping losses in the engine, therefore giving you more power.

[Guest Michael MRK]

well, as far as i know, and as far as I studied in that Area, the Volume of air entering the Intake Manifold is Constant, from after the Air filter to the intake, but the Pressure is Variable, that's why even if u don't have a T/C, and installed a boost gauge, it'll show u how much pressure/vacuum change with the throttle input, Applying a Mathematical Equation of Fluid Dynamics (i guess) PV/T = PV/T, at a constant Volume, then the Pressure and Temperature totally has the most effect (neglecting friction and stuff like that cos they're really small)

Forces Affecting the Air Entering, is only Vacuum, by the Piston motion, that motion creating pressure difference, between the Atmospheric pressure and the pressure inside the engine, that leads the air to enter, and as the speed of the piston increase, the pressure difference increase leading to more air enters..!!

[jonD6B 0]

So is it right then that the amount of air drawn in is the same at any speed (at constant revs) or even stand still and it's only the pressure created from the different rpm that varies but this pressure isn't enough to affect anything like the burn?

Edited April 28, 2006 by jonD6B

[Guest Michael MRK]

theoretical, that true, that at constant rev in different shift the pressure should be the same, but practical, it differs cos there is another things takes some power from the engine as the flywheel, or the water pump, etc.., which affect in turn the piston speed at the same rev.

pressure always affect burn, cos if ait goin at 1 bar, won't be equal to air goin at 1.2 bar, etc.., the c/r itself with the pressure differs, as u know, that C/R are Constant, but the charge itself is variable, to the dynamic C/R changes, but Static remains the same, let's take an example as the 1.9 GTi Engine, its Static C/R is 9.6:1, but that doesn't mean that is always compress 9.6:1, it compress it static, but not dynamic, 9.6:1 only occurs when its at a full throttle, and rpm at peak HP, so the charge (a/f) itself differs, so dynamic C/R changes in turn, yea the piston sweeps with a constant C/R 9.6:1, but the charge is small (in case of low rev.), so it lowers the pressure...

 

Hope that explains..!!

[Rally Slag 0]

on my old ax gt i fitted a renult 19 air vent on top of the car carb and ducting down to the air filter, it made a difference on cold days when you had really cold air going through the vent straight into the carb

[jonD6B 0]

theoretical, that true, that at constant rev in different shift the pressure should be the same, but practical, it differs cos there is another things takes some power from the engine as the flywheel, or the water pump, etc.., which affect in turn the piston speed at the same rev.

pressure always affect burn, cos if ait goin at 1 bar, won't be equal to air goin at 1.2 bar, etc.., the c/r itself with the pressure differs, as u know, that C/R are Constant, but the charge itself is variable, to the dynamic C/R changes, but Static remains the same, let's take an example as the 1.9 GTi Engine, its Static C/R is 9.6:1, but that doesn't mean that is always compress 9.6:1, it compress it static, but not dynamic, 9.6:1 only occurs when its at a full throttle, and rpm at peak HP, so the charge (a/f) itself differs, so dynamic C/R changes in turn, yea the piston sweeps with a constant C/R 9.6:1, but the charge is small (in case of low rev.), so it lowers the pressure...

 

Hope that explains..!!

 

Well explained. The thing I'm still not 100 % about though is; if a cars engine is set up whilst static by altering fuel and air mixtures to gain a particular given result i.e CO, CO2 and H/C, would these figures then alter whilst the car is at speed?

[petert 586]

I think you need 150mph to get 1psi.

[Guest Michael MRK]

about the emissions figure u get, or published for any car in the world, that emissions are always tested on idle speed as the main reference for the car, cos even in Injection Engines, its usin a feedback control type, which measure the o/p, then enters a new i/p, and that i/p also brings o/p, that o/p is taken and modify the new i/p, so at any rpm, u can't get an exact value of emissions at any given time, except on idle, where there is no load on the engine.

 

as an example (true one), the air mass sensor sense the air entering (by any mean as temp. volume, pressure, etc..), then gives the signal to the ecu, where the ecu take other signals as engine temp., rpm, crank position, etc.., make some equations, and send a signal to the injectors for the required quantity of fuel according to the engine status, spark plug ignite and an exhaust comes out, measure by a lambda sensor, that lambda sensor sends a signal to the ecu telling it whether the mixture is lean or rich, and then the compare it with the newer data for the air mass, and again, returns to the ecu and etc.., so its a feedback system, that need to measure first the o/p, to adjust the i/p, so its always in constant change, and that's why lamda sensor is always fluctuating above and lower the stoichometric value which is on 1, so u never get that 1 value for lambda, it can only get it when ther engine speed is constant, where the o/p is equally proportion with the i/p (theoretically also, practically fluctuating but with like +/- 1% in modern cars....!!, sometimes lesser, depends on the car and which brand, bimmers are like +/- 0.2% or something, highly accurate)

 

1 psi ??? u mean 1 bar, cos at a fully opened throttle, u won't get more than 0.8~0.9 bar, where 1 bar is equal to 14.5 psi, so u get like 7 psi mot of the time, at t/c condition, if ur car is fully prepared for a t/c, u get like 1.2, if its highly tuned for t/c, u get like 1.6~1.8 bars, never heard of more, except in F1 Cars, which is Naturally Aspirated and get like 2 bar, cos the air iduction system is above the car, so the faster the car go, the faster air enters, and colder air..!!

 

for Chris, that for sure, Colder air means more denser air, so when it enters, its pressure increase more than hot air, that's why there is a cold air intakes, also cold air, contains more oxygen molecules, which is better for the ignition...!!

Edited April 29, 2006 by Michael MRK

[M3Evo 0]

Think Peter means 1psi over atmospheric pressure, so 15.7psi at 150mph.