Static Compression VS Dynamic compression

[rebelrouser]

I understand that intake valve closing point determines trapped or dynamic compression.

You "understand" wrong.
In a running engine, the cylinder continues to be filled, long past BDC. As long as the intake valve is open, and the pressure above the valve is greater then the pressure below the valve, you will have positive flow into the cylinder. The air flow doesn't care where the piston is, it just flows towards the lower pressure. As the piston starts uo from BDC, it starts to increase the pressure in the cylinder, but if the cam is correct, the reduction in valve lift of the closing intake valve will also increase the pressure above the valve, and keep it above the pressure in the cylinder for an extended amount of time. This is truly the dynamic compression, because it changes thru-out the rpm range. Unfortunately, it's not something you can calculate with a simple formula.

I agree its very hard to calculate accurate Dynamic Compression Ratio. But if you are doing work with a computer program on an engine combination, lets say for a pump gas combination, and the program gives a calculation using intake valve closing, I use that as an indicator of how the engine will run when built. not perfect but its just one factor when considering lots of other factors, and better than a wild ass guess. years ago I built a couple pump gas class pulling truck engines, they had 13 to 1 static compression, used the calculated dynamic compression numbers to help pick out a cam, engines were successful, won a lot of hooks, competitors tore us down twice, but all was legal.

[CamKing]

years ago I built a couple pump gas class pulling truck engines, they had 13 to 1 static compression, used the calculated dynamic compression numbers to help pick out a cam, engines were successful, won a lot of hooks, competitors tore us down twice, but all was legal.

You probably would have been just as successful, using a dart board to pick the cam.
DCR is just a "Rule of Thumb", pretending to be an equation.

[David Vizard]

I understand that intake valve closing point determines trapped or dynamic compression.

You "understand" wrong.
In a running engine, the cylinder continues to be filled, long past BDC. As long as the intake valve is open, and the pressure above the valve is greater then the pressure below the valve, you will have positive flow into the cylinder. The air flow doesn't care where the piston is, it just flows towards the lower pressure. As the piston starts uo from BDC, it starts to increase the pressure in the cylinder, but if the cam is correct, the reduction in valve lift of the closing intake valve will also increase the pressure above the valve, and keep it above the pressure in the cylinder for an extended amount of time. This is truly the dynamic compression, because it changes thru-out the rpm range. Unfortunately, it's not something you can calculate with a simple formula.

I agree its very hard to calculate accurate Dynamic Compression Ratio. But if you are doing work with a computer program on an engine combination, lets say for a pump gas combination, and the program gives a calculation using intake valve closing, I use that as an indicator of how the engine will run when built. not perfect but its just one factor when considering lots of other factors, and better than a wild ass guess. years ago I built a couple pump gas class pulling truck engines, they had 13 to 1 static compression, used the calculated dynamic compression numbers to help pick out a cam, engines were successful, won a lot of hooks, competitors tore us down twice, but all was legal.

Mike,
You are correct in you analysis of the dynamic CR. However knowing what the CR is using the intake valve closure point is a good half way house toward getting a better match for the cam duration and CR as rebelrouser suggests. My point here Mike is, don't dismiss this easily determined factor out of hand. It has a function in the it can better steer the home engine builder than nothing at all.

DV

[CamKing]

My point here Mike is, don't dismiss this easily determined factor out of hand. It has a function in the it can better steer the home engine builder than nothing at all.

I'm sure it can be helpful in some cases.
My issue is, it can also give the user bad information. Due to all the other factors that the DCR calculation doesn't take into account, you can have a cam with a later intake closing point, that pulls more mass into the cylinder, then one with an earlier closing point. In this case, the effective compression would be higher with the later closing cam. This could cause real issues for the engine builder that's using DCR calculations, and assumes the later closing cam, can run more static compression, or lower octane fuel.

That's why I call DCR, a rule of thumb, pretending to be an equation.
I don't use, or recommend using something that can easily give you false information, and cause major problems.
Just because something is better then nothing at all, doesn't make it good. The Comp Cams catalog is better then nothing at all, for picking the correct cam for an application, but I sure as hell wouldn't recommend relying on it.

[David Redszus]

Dynamic compression is the target, but it can be elusive since it depends on IVC and pressure above the valve as Mike Jones indicated. Static is merely used to compare one engine to another but provides no useful info.

Actually, pressure ratios, either kind, don't mean much. We need to know actual compression pressure and temperature. And they are affected by inlet air temperature and pressure.

Using Stan's engine dimensions, and an IVC of 98 deg BTC, a static CR of 18 becomes a trapped CR of 12.90.

Since that is merely a ratio of volumes, it does not tell us much of value.

But, suppose Stan's engine inlet pressure at IVC was atmospheric (14.7 psi) and the ambient air temperature
was 100 deg F, then the maximum compression would be 441 psia with a temperature of 1006 deg F, at TDC.

Do not confuse compression ratio with compression pressure. The compression ratio is only a comparison of volumes. As Mike Jones indicated, compression pressure will change with dynamic operating conditions, such as, rpm, barometric pressure, inlet pressure tuning, throttle position, etc. Now we need to know the inlet air pressure (and temperature) upstream of the closing inlet valve.

The auto ignition point of gasoline is well below 1006 deg F. This could be a problem.
Fortunately, the spark plug will fire before TDC, at a lower temperature and pressure.

And if the plug happens to misfire, the mixture will ignite anyway.

[Stan Weiss]

Dynamic compression is the target, but it can be elusive since it depends on IVC and pressure above the valve as Mike Jones indicated. Static is merely used to compare one engine to another but provides no useful info.

Actually, pressure ratios, either kind, don't mean much. We need to know actual compression pressure and temperature. And they are affected by inlet air temperature and pressure.

Using Stan's engine dimensions, and an IVC of 98 deg BTC, a static CR of 18 becomes a trapped CR of 12.90.

Since that is merely a ratio of volumes, it does not tell us much of value.

But, suppose Stan's engine inlet pressure at IVC was atmospheric (14.7 psi) and the ambient air temperature
was 100 deg F, then the maximum compression would be 441 psia with a temperature of 1006 deg F, at TDC.

Do not confuse compression ratio with compression pressure. The compression ratio is only a comparison of volumes. As Mike Jones indicated, compression pressure will change with dynamic operating conditions, such as, rpm, barometric pressure, inlet pressure tuning, throttle position, etc. Now we need to know the inlet air pressure (and temperature) upstream of the closing inlet valve.

The auto ignition point of gasoline is well below 1006 deg F. This could be a problem.
Fortunately, the spark plug will fire before TDC, at a lower temperature and pressure.

And if the plug happens to misfire, the mixture will ignite anyway.

David,
While I can adjust them I normally use BP = 29.92 and Temp = 60.

Stan

[CamKing]

Do not confuse compression ratio with compression pressure. The compression ratio is only a comparison of volumes.

This is a good point.
I think most engine builders would agree that you can't run the same compression ratio with 30lbs of boost, as you can with an N/A engine.
Why not ? they're both the same compression ratio. If the cams are the same, thay'd have the same DCR.
The answer is found by looking at the mass in the cylinder. The more mass, the more pressure at any given compression ratio.
The fuel requirements for a boosted engine with 9:1 compression, is completely different from the fuel requirements for an N/A engine with 9:1. Both are different from the requirements of a restricted inlet engine with 9:1.

[johnef]

If memory serves me rite, Mike Jones made mention of trapped mass in the cylinder during intake duration is actual cylinder volume. Meaning if engine is running 130% VE X static compression= dynamic compression at peak torque RPM. Mike is this what you meant?

[Stan Weiss]

What is your definition of Dynamic Compression?

If I have a 4 cylinder engine

Bore = 2.89"

Stroke = 2.323"

Rod Length = 2.323"

CR = 18:1

If my intake valve closes @ BDC I will have about 600 psi

If my intake valve closes 90 ABDC I will have about 290 psi

Now if I start and run the engine on 3 cylinders with a gauge in the 4 cylinder, and at some RPM I see 600 psi or more.

Stan

First let me say that the rod length above should be 4.65"

Second an easy example of the difference between dynamic compression ratio and cylinder pressure is do a cranking compression test at sea level an in Denver on the same engine.

Third in the above example to have about 600 psi with the intake valve closes 90 ABDC you would need about bp = 61 in the cylinder at valve close.

Stan

[MadBill]

Is 18:1 even remotely practical from a valve shrouding standpoint with the likely huge piston dome?

Also, the law of diminishing returns sets in as CR rises. David Redzus probably has the formulae at his fingertips, but thermodynamically, going from 15:1 to 18:1 is likely worth ~ 1% HP, assuming no loss of flow or excessive spark advance requirement.

[naukkis79]

Is 18:1 even remotely practical from a valve shrouding standpoint with the likely huge piston dome?

Valve shrouding isn't such a big problem, problem is that it's incredible difficult to have high compression with combination of long duration cams and short stroke big valve 4V head. Combustion chamber is mostly valve reliefs in piston so when increasing valve lift in tdc there is need to increase valve reliefs and lose compression ratio.

-> compression ratio is usually just as high as what could be achieved without compromising valve timing or valve sizes.

Detonation isn't usually problem at all with high-rpm high tumble low valve angle 4V chambers.

[hoffman900]

Looking at compression ratio only is not seeing the forest for the trees, especially in regards to flame propagation / burn efficiency.

[David Redszus]

Is 18:1 even remotely practical from a valve shrouding standpoint with the likely huge piston dome?

Also, the law of diminishing returns sets in as CR rises. David Redzus probably has the formulae at his fingertips, but thermodynamically, going from 15:1 to 18:1 is likely worth ~ 1% HP, assuming no loss of flow or excessive spark advance requirement.

Thermal efficiency is based on trapped compression ratio, not static.
However, if we go from 16: to 18:1, and leave the resulting TCR values as they come, we would raise the thermal efficiency from 59.67 to 61.31%. However, raising the compression ratio alters the surface area to volume relationship of the chamber, resulting in greater heat loss. There is point of diminishing returns.

Studies by Porsche have indicated that a static compression ratio of 16:1 is the maximum effective CR.
I have their white paper but it does not show the TCR numbers.

[Enigma]

Seems like a lot of you guys are confusing "dcr", which is a fixed, calculable ratio.........with dynamic cylinder pressure, which is something vastly different.

[CamKing]

"dcr", which is a fixed, calculable ratio........

No, it's not.
It assumes the cylinder stops filling at BDC, and compression doesn't start increasing until the intake valve closes.
This is 100% false. That's not how a running engine works.