Can anyone explain it? with any necessary equations that come into play?

 

Put simply it is getting the ideal length of inlet or exhaust tract to get better power.
Every engine has an ideal length of these but this also varies dependant on rpm and load, so it is always a compromise.
That is why there are different length trumpets available for intakes on throttle bodies etc.
tabetha

 

An engine is basically an air pump.

But it is wrong to consider that air flows through the engine - instead it pulses i.e. a cylinder only draws air in once every 2 revolutions of the crank.

These pulses can be put to good use if you tune the length of the pipe the air is travelling through. An example of this is the beer bottle, as you blow over the top the air molecules inside vibrate - the more beer you drink the deeper the note (the lower the frequency at which the air is vibrating).

Enough?

Alex

 

Thats good, do you know of any equations that might be involved?

 

Im not sure you can really solve it all very well by hand. I think its gotta be done computationally really. lots of ittereations.

Not too sure of the equations but its gonna be bernuouli and momentum equations I'd gess.

Lotus have some cool software that I saw once, they might help you if you ask, but most likely would just offer to sell you the soloutions, which probably isnt a bad idea.

JAmes.

 

WHat are you trying to work out?

Alex

 

2 strokes don't fire draw air every 2 revolutions!!
tabetha

 

Pulse tuning is simple.

When the inlet valve opens all the charge down he inlet runners and ports is moving very quickly.

When the valve closes its compressed aginst the back of the valve head. Think of it like a spring.

It then pulses back up the port and through he inlet runners until it reaches atmospheric pressure (at you trumpet).

The pressure here is relatively high compared to the vacuum of the port, so the pressure wave bounces back down the inlet runners and port.

If you get it right, jut as the inlet valve opens, pushing in more air, thus increasing volumetric efficiency, which increases torque.

Thats why cars have varible intake lenghts.

Cars that need a low power bands have long runners as the wave has more time.

Cars the need high power bands like bike and F1 engines have almost no runners.


To calculate it correctly you need the inlet air temperature as the theory involves the speed of sound in air, which changes with temperature and the diameter of the port and cam shaft duration.

Lets say we have
270 degree cam
Speed of Sound = 340ms
Engine Speed = 5000rpm

each cycle has 2 rotations so

360 x 2 = 720 degrees

The cam duration from this is

720-270 = 500 degrees


@ 5000rpm a cycle take
500 degrees / 5000 rpm

0.1 mins

0.1 / 60 seconds = 0.0016666666667 seconds

So are pulse has to travel the lenght of the port twice in 0.00167secs!

Lets say the speed of sound is 340 ms

Speed = Distance / Time

Distance = Speed X Time

340 ms X 0.0016667 s = 0.5666 meters

Lengh yeah?

Well we can divide ths by two!

So this gives 0.283 meters, but still seem long.

This is from atmosphere to the valve so you have to take the port into consideration!

A zetec is about 100mm

So 283mm - 100mm = 183 mm which is about 11 inches

 

Exhausts are the simular but rely on the depression created by the exhaust valve closing and creating a vacuum. This pulls the air back to the exhaust valve. This wave should bonce off the exhaust vavle just before the exhaust valve opens suckin air out again increasing volumetric eff.