Blowdown cycle duration

[L79]

What is the duration in crank degrees of the blowdown period? I realize it must vary with load and RPM, and am interested primarily in 4000-6000RPM, WOT, on a Chevy small-block. Also, I would expect blowdown to be faster with a roller cam, so if someone could compare blowdown duration for flat tappet cams versus rollers that would be helpful. Please assume aggressive ramps like a high performance cam.

I've seen a few exhaust pressure graphs that suggest something in the 45-60 degree range, but those had no cam profile or type information.

 

[ivymike]

I wouldn't expect the type of cam-follower interface to have a big effect. I'd look at the lift profile, and the valve size & geometry for the biggest effects.

If you have a particular use in mind for this information, perhaps someone could tailor a response to your needs...

 

[L79]

It wasn't really the roller follower that I expected to make a difference, but the more radical ramp rate it allows.

I'm trying to spec out valve timing for a cam designed specifically to work with a resonant exhaust system. On a V8, if you have the two banks joined by equal length pipes, you can get a resonant effect. You will see peak pressure from a given cylinder at the peak of blowdown, which seems to be 45-60 degrees after exhaust valve opening. If it was 45 degrees after EVO, then you would see pressure minimums at 90, 180, and 270 degrees after EVO. Then you set things up so you are in a pressure minimum near top dead center during valve overlap, and it should give you excellent scavenging. You also want to close the exhaust valve before the next pressure peak comes.

So, given all that, it is important to know where the peak pressure in the exhaust port occurs after EVO.

 

[ivymike]

Your statement about the roller follower sounds reasonable enough.

In regard to the crank angle numbers you mentioned, it sounds to me like you're referring to resonance effects at a particular engine speed. I'm not sure that you're right about wanting to close the exhaust valve before the arrival of the next pressure pulse. I was under the impression (perhaps mistaken) that if you had really good scavenging going on, you would suck some unburned charge straight through the cyl and into the exhaust, and a high-pressure pulse just before EVC could shove some (or all) of that charge back into the cylinder.

I don't think I can really contribute anything useful to the discussion, so I'm going to shut up now. There are some other folks in here who might be able to give some good advice re: pipe lengths, valve timing, etc.

 

[L79]

The valve phasing can be made correct for the resonant case without concern for what RPM the resonance will occur at. This is because when the system resonates the pressure peaks (and valleys) are spaced 90 degrees of crank rotation apart, regardless of what frequency the resonance occurs at.

So, with this long intro of "why I care" about blowdown duration, does anyone have some measured or simulated numbers to share?

 

[L79]

Actually, when resonating the system produces a pressure cycle every 90 degrees of crank rotation regardless of what RPM the resonance occurs at. So, you can set the valve phasing independent of the resonant frequency, as long as you know how long blowdown takes so you can predict the position of pressure peaks and valleys relative to exhaust valve opening.

So, does anyone have some measured or simulated data on blowdown duration?

 

[MaxRaceSoftware]

L79,

if you post ;
1-What RPM you want Peak HP to occur at ?
2-Intake + Exhaust Valve Diameters for both
3-Engine CID size
4-Rod Length C-C inches
5-Compression Ratio

i can calculate cam specs and other things ??
Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[FireLover]

I have measured blowdown on a production V6 engine, in the neighborhood of 100 crank angle degrees at 1200 rpm. But this will not help you. The way you measure it is to measure cylinder pressure, and with some calculations you can calculate the mass flow out of the cylinder during the exhaust event. Cylinder pressure is measured with a piezo-electric pressure transducer through either a machining in the head or a probe in the spark plug. Then you need a high speed data acquisition system. All of this equipment is costly, over $1k for the sensor (

http://www.kistler.com)

, > $30K for the acquisition (

http://www.hi-techniques.com),

and more for additional equipment. However, Measuring blowdown is the least benefit to be had by this equipment.

However, to ballpark it is not very complicated. The begining of blow down is defined by when the exhaust valve opens. Because the cylinder pressure is higher than that in the exhaust, you get a high mass flow out of the cylinder. The end of blow down happens when the pressure in the cylinder drops below that in the exhaust and instead of gasses flowing out of the cylinder, they flow back into the cylinder from the exhaust system. This is a short time after bottom dead center, at 1200 rpm between 0 and 20 cad after.

However, the end of blowdown is irrelavent. What you are describing is the reflection of the initial pressure wave from blow down. When this positive pressure wave reaches a transition in the exhaust manifold to a bigger diameter (ie. collector plenum), a negative wave will be reflected back towards the cylinder. If this negative pressure wave reaches the exhaust valve right at closing, you will get a benefit in scavanging. The speed that this wave travels is dependent on exhaust gas temperature and back pressure. It will travel at the speed of sound for the exhaust gas at these conditions. So you want to set your exhaust manifold to the appropriate length to "catch" this reflected negative pressure wave.

The problem is that the length will depend on engine speed. So, once the exhaust manifold length is set, it is tuned for a single speed (or a very narrow range of speeds).

This same concept is applied to the intake as well. Checkout the new variable intake runner length systems by Mercedes. Anyway, hope this helps.