dynamic compression ratio

[Guest Anonymous]

I think I agree with Kevin, although it's hard to say; his posts are too long winded for my ADD to keep up with

 

The way I see it, to calculate the absolute cyl. pressure, it would be more accurate to figure absolute pressure at the point where the valve closes, then multiply that by the dynamic compression...

 

I mean, it just seems to go hand in hand...

[johnc]

As I said much earlier in this thread, there ain't not such thing as dynamic compression ratio. That's just another term for cylinder pressure. Its like Microsoft making up another marketing term for a technology that's been around for a long time.

[Guest Anonymous]

Well cyl. pressure is a number, whereas "DCR" is a ratio...

 

So it seems to me there's a difference.

[pparaska]

David Vizard writes about "Dynamic Compression Ratio" all through his book on camshafts and valvetrains for the SBC (although it's an excellent book on general camshaft stuff).

 

I think DCR is just as valid as SCR. It's the "compression ratio" the mixture sees, due to the intake valve closing part way throught the compression stroke. I'd also say it's alot more useful than static compression ratio, since it speaks to actual low rpm cylinder pressure, where SCR is just for a ficticous engine where the intake valve closes at BDC before the compression stroke.

 

I guess I'm saying I respectfully disagree - DCR is real, and at least one well known author of engine theory books uses the term. Good enough for me.

[johnc]

If I remember correctly, Vizard "usually" discusses compression ratio in the context of cylinder pressures. I had links to a few of his articles about high compression engines but with the demise of MotorTec Magazine those links be busted.

 

It's the "compression ratio" the mixture sees, due to the intake valve closing part way throught the compression stroke.

 

Which is cylinder pressure normally measured in two places: 1) at the point you mention above and 2) usually from 10 to 15 degrees after TDC on the [Edit: power] stroke when max cylinder pressures are reached. Its a measure of combustion efficiency and power. Its also rpm dependent because of port velocities. I guess you can express the differences as ratios, but the value lies in the absolute numbers reached.

 

BTW... Now that I think about it some more, DCR (if it exists) is less meaningful because the spark plug fires sometime before TDC so the actual cylinder pressures measured at TDC on the compression stroke (if anyone actually did that) are mostly a result of the burning of the air/fuel mixture.

 

BTW2... but what do I know, I'm a suspension guy. Engines to me are big hunks of metal in the way of good front suspension deisgn.

[Guest Anonymous]

Do I hear someone confusing the compression stroke with the power stroke???.....LOL

[pparaska]

Well, Vizard talks about both. But DCR is in his venacular. I read it several times within a few pages the other night in the book I mentioned.

 

Yes, DCR is the "compression ratio" at TDC for a slow moving mixture. Not an exact indication of the pressure at the time of spark. But that time can vary by 30-some degrees based on your ignition curve. So it wouldn't make any sense to try to figure that into a "compression ratio". Neither would it make sense to try to figure static compression ratio as a function of spark timing.

 

The point that Vizard and others here are making is that the intake valve closing point along with the static compression ratio must work together to have a dynamic compression ratio in a certain range or cranking pressure in a certain range. One is just a statement of the other. Pat Kelley likes to talk about a DCR of 8.2:1 or so for pump gas. Others like to talk about acceptably high cranking pressure. Too low of either and the engine is a dog at lower rpm.

 

~10:1 SCR or ~8:1 DCR or ~170psi cranking (gage) pressure. They're just rules of thumb for pump gas.

[johnc]

I fixed it (damn 4 stroke engines)...

[Guest Anonymous]

DELETE

[Kevin Shasteen]

.., They're just rules of thumb for pump gas..,

 

John,

 

I dont believe you can dismiss DCR nor SCR; afterall, every engine mfg that has ever existed has used them as tools to building their engines...THE ENGINEERS CREATED THE TOOLS out of neccessity for constructing their engines: it is just good-ole plain math.

 

As Datsun Dude illuded: DCR &/or SCR are both Ratio's that measure volume of two related chambers while Cyl. Press. is a measurement of Pressure within that same cylinder. Alter anyone of the three: Cyl.Press, DCR, or SCR & you will have successfully effected the velocity of the airflow into the cylinders: and we still have not taken into consideration the efficiency or inefficiency of the Cyl.Head's Intake Ports. We have ratio's within ratio's and I believe this is the point of confusion in addressing any HiPo engine build.

 

Yes, the Cylinder Pressure is our end objective but it is not an Island unto itself! There is a whole lot going on in an engine that leads up to Cyl.Press. Taking your argument that DCR & SCR doesnt exist, while we are at it, why dont we dismiss Cyl. Press & instead only embrace "Atmospheric Pressure"? Your answer should be, "That would be crazy-you cant dismiss Cyl.Pressure and only accept Atmoshperic Press." Likewise, you can not dismiss the DCR nor the SCR in designing performance into any engine.

 

If your confusion lies in your thinking that DCR's & SCR's dont exist as pressures...then yes you are correct-because they are not pressures.

 

Neither DCR nor SCR are pressures...they are only ratio's and because they are not pressures you can not measure them as such. They are a tool for measuring the relationship between the Cylinder Volume in the engine & the Combustion Chamber Volume in the cyl.heads. Both these Relationships (ratio's) coupled with the Cyl.Head's Intake Port's efficiency to flow air also have to be considered. These are tools for calculating the characteristics of the airflow velocity. Yet if you are merely rejecting DCR & SCR because you dont understand them...then you are making an incorrect assumption.

 

Better yet, if we are going to dismiss how we measure the relationship between Cyl. Volume and Combustion Chamber Volume...then why dont we also dismiss Bore x Stroke-which yeilds the very Displacement of our engines?

 

The "RATIO" of the relationship between the Cyl.Volume and Combustion Chamber of the Cyl.Heads CAN NOT be rejected if you plan on building any engine with any kind of expected output-they are requirements.

 

Cyl. Press is a function of Atmoshperic Pressure acting hand in hand with your DCR. Your engine will take this relationship & then multiply it times the amount of cylinders in the very block we are going to use: this results in our total combined cylinder output: measured as Torque at the Flywheel. DCR is a function of the IVC (Intake Valve Closing) and is determined by the profile of the cam acting in conjuction with the Cyl.Volume in the engine block + Total Combustion Chamber of the Cyl.Heads on top of the engine. The intent of the engine determines what SCR is required for whatever level of performance you wish...AKA: Cyl.Press at a designed Engine Speed (RPM) coupled thru Gearing = Veh.Speed.

 

Example:

HP = .257 x CFM x #of Cyl's

MEP = ((HP x 792,000) / (Displacement x RPM))

MEP = CRE^1.2 x AP

DriveLine TQ = FWTQ x Trans.Gear x Diff.Gear x VE%

..from here you can play w/the RPM & MPH forulas to generate Veh.Speed.

 

You can not dismiss any tools/principles/phenomenon that occurs in the entire process.

 

You cant dismiss any of the steps between the first and last: all steps must be taken into consideration or all you have is a "HUGE GUESS". Guesses dont win races, unless everyone else around you is guessing also-then it is basically a crap shoot.

 

One last thing: I also believe a lot of confusion about DCR is that there is this "Perfect DCR" for an engine. In reality there is no perfect IVC for any one engine, rather, there is only a perfect IVC for a particular expected "level of performance" at a given rpm. If you find that your Veh.Speed is mostly below this Peak Power RPM-then your IVC is by all means late, on the other hand-if you find your Veh.Spped mostly above this Peak Power RPM then your IVC is early. Either scenario equates to a mismatch in your engine components.

 

So once again we are basically addressing, "What is the intent of the engine, where do you want your Peak RPM Power combined with your choince of Veh.Speed when that Peak RPM Power surfaces: and more importantly...how are you going to plan on taking that level of expected performance from paper to the machine & finally to the street/track?

 

Cyl.Pressures:

1) 130 - 145psi = Typical Passenger Car

2) 150 - 180psi = High Perf. and or Sports Car

3) 160 - 210psi = Dedicated Race Car

4) 100 - 130psi = Turbo/Supercharged prior to boost

 

Sorry for being so long winded...again

 

Kevin,

(Yea,Still an Inliner)

[Guest Anonymous]

DELETE

[Guest Anonymous]

Cyl. Press is a function of Atmoshperic Pressure acting hand in hand with your DCR.

 

I beg to differ--Although I agree with what you're saying, but I believe you ment to say "absolute pressure", not "atmospheric pressure"...

 

Or you meant to say what you said and you're just wrong

 

In any event, the way I see it, you would use absolute pressure to calculate your cyl. pressure using DCR, then subtract 14.7 from the sum.

 

It would be inaccurate to assume that the cyl. pressure is 14.7psi at the point where the intake valve closes. The air entering the chamber has but only a small passage expanding into the chamber where the piston acts as a plunger in a seringe. And, likewise, as the compression stroke begins, although the valve is not quite closed yet, the air has but only a small escape route. So even though the valve is not entirely closed, the air/fuel is already being compressed... And not compressed from its natural, atmospheric pressure, but from its absolute pressure at BDC.

 

By the way, can someone tell me, because I forgot :| , what the difference is between atmospheric and barametric(I hope I spelled that right) pressure? I think it's important since we're not all at sea level... (If you live in, say, Denver, your absolute pressure would be less than 14.7psi)

[Kevin Shasteen]

.., I beg to differ.., I believe you ment to say "absolute pressure", not "atmospheric pressure".., In any event, the way I see it, you would use absolute pressure to calculate your cyl. pressure using DCR, then subtract 14.7 from the sum.., By the way, can someone tell me, because I forgot :| , what the difference is between atmospheric and barametric? I think it's important since we're not all at sea level... (If you live in, say, Denver, your absolute pressure would be less than 14.7psi)

 

Datsun Dude,

 

I said what I meant to say: that doesnt mean I am above being incorrect(?) : if I am incorrect then I would ask that you please show me where I am wrong-as I hate being incorrect in calculating auto math stuff.

 

On one hand I believe you are correct in that what we read on a gauge is Absolute Pressure. Yet the engine doesnt "READ", so-in order for us to interpret what the engine sees we have to use a gauge. It is this gauge that is also effected by atmospheric pressure..so one must adjust their gauge reading in order to obtain the total absolute CRANKING pressure.

 

If we are calculating Cyl.Press on paper then we multiply the Atmospheric Pressure x DCR^1.2 to get your absolute pressure.

 

Correct me if I am wrong, but are you not confusing cranking pressure (absolute pressure), which is obtained by multiplying AP x DCR^1.2 paper calculation with Gauge Pressure taken from a gauge(?) and then confusing those two with Atmospheric Pressure.

 

Atmospheric Pressure doesnt care what altitude you live in, it is determined by whatever Altitude you find yourself at: and the reading will slightly rise or fall dependent upon the moisture content of the air molecule at that altitude.

 

The difference between Barometric Pressure & Atmospheric Pressure is in how they are obtained. They are both measuring the same phenomenon, which is air pressure, yet they are just going about it in a different manner.

 

Barometric Pressure obtains its reading by use of Mercury...thus the "in.Hg" symbols, while Atmospheric Pressure obtains its readings as an actual "psi".

 

It is your gauge reading where you should deduct your Atmospheric Pressure from your absolute pressure. I guess it depends on your approach in obtaining your psi reading. If you are calculating on paper the psi reading then you should use your Absolute Pressure psi reading obtained from your calculations. On the other hand, if your engine is already built and you are obtaining your psi readout from a gauge....then by all means you should deduct your 14.7 from your Cylinder Pressure calculations to match your gauge readings: or add your Atmospheric Pressure to your gauge to match your absolute pressure of your Cylinder Pressure calculations.

 

If you, or anyone, wishes to know their average air pressure then you should locate a list of those average pressure readings. Go to Corky Bell's book, "Maximum Boost" and you will find one of these charts on page 28.

Air pressure at 5000ft will be 24.90 in.Hg/2.036 = 12.23 psi Atmospheric Pressure.

 

If you have a process for calculating Cyl.Press w/out using Atmospheric Pressure, I would like to see it...as I'm always looking forward to learning something knew in our search for Hi/Perf. 8)

 

Kevin,

(Yea,Still an Inliner)