Intake closing

[hoffman900]

....What effects the REAL dynamic compression is HOW MUCH air and gas is in the cylinder when the intake valve closes, not at what degree ABDC the valve closes....

UDHarold

So considering Harold’s statement above and this from another thread:

A volume of air can exist at various pressures, which will determine density, and therefore MASS.

An internal combustion engine depends entirely on AIR and FUEL MASS, never volume.

For volume to have any meaning, we must know its density. Volume is a static metric, a space.

Is an attempt to calculate dynamic compression an exercise in futility if using swept volume without knowledge of density and mass?

I think it's an exercise in futility because ultimately cylinder pressure and burn rate / quality dictate what geometric compression ratio works with kind of fuel. Intake valve closing does as well, especially in regards to Miller Cycle engines where IVC is before BDC (this is what modern F1 engines are).

I guess my point is Dynamic Compress Ratio (as most people know it from magazines and hot rodding books) only works for these old type engines where there are again, "rules of thumb" based on nothing but experience. If you look at pressure traces on a bent crank V8 with a single carb, some cylinders make more pressure than others and some make more pressure at a different crank degree than others. And what cylinder changes with rpm as maldistribution and pressure wave interference in the manifold changes as the cylinders come in and out of tune due to varying geometries of the intake length (and this is not adding g forces to it). Furthermore, on a pushrod engine, the valve events are going to get shorter with rpm due to stiffness issues. Billy Godbold has shared good data on this for all to see, so IVO, IVC, etc. are all variable. Billy has talked about making everything stiffer and fixing things on the Spintron, and the engine builder has to relearn what events work because what they thought worked, was really just what they found to work without realizing it was just a compensating value to manage flex they didn't know / understand they had. Furthermore, when you look at today's modern high tumble designs, with 12.5-13:1 compression on bores as large as 4" or more (and turn 13,000rpm in the case of Ducati) on pump gas, you realize they're not using IVC to bleed pressure, but they are doing it with better engine management and good in cylinder motion. Obviously 2 valve engines would struggle with this as they are swirl base (except a hemi type chamber, but that has other problems), but 2 valve engines can have compact combustion chambers which help (take a look at a modern NASCAR chamber for example). Then add better cooling systems, piston oil squirters, etc. and things get "it depends" really fast.

But consider above, and then think about what DCR tells you where you just consider swept volume and IVC to tell you, vs all the things that effect cylinder pressure in a running engine above, and I'm not sure what it tells you other than people have correlated it through testing to some octane threshold for a very specific type of engine, which makes it a rule of thumb for only a certain type of engine that author / builder has experience with.

[skinny z]

DCR... which makes it a rule of thumb for only a certain type of engine that author / builder has experience with.

Exactly this.
Once I learned what the SCR and DCR thresholds for a given engine were, that being an iron headed Gen 1 SBC, I've managed to use that "rule of thumb" for, at the very least, a jumping off point for the next iteration of the same.

That said, what I meant to get to earlier with my question regarding LSA, was really more about overlap and what it's value has. Let's say as determined on a seat to seat valve event basis.
Is it something tangible or again, like the dynamics of a running engine, something that has to be learned from one build to the next (all else being equal as in similar engine architecture)?

[Truckedup]

most of this is beyond what I understand.......But the two cylinder vintage OHV bike engines I mess with are very responsive to changes in lobe centers. Hemi chamber with a squish area and separate intake and exhaust cams....Moving the intake from 103 degrees to 105 and the exhaust from 105 to 107 made a noticable increase in power for land speed racing..

[naukkis79]

LSA = (Intake Centerline + Exhaust Centerline)/2 . 100 ICL and 112ECL is 106 LSA as is 104 ICL and 108 ECL, and so would 106 ICL and 106 ECL. Those cams would run completely different. If the valve lift profiles are asymmetrical, they would all be different still.

You are describing just one cam, that's what can be done with timing with single cam, changing it's timing affect both intake and exhaust lobes similarly so they cancel themselves. Changing LSA need separate intake and exhaust cams or in case cam operates both different cam with different LSA.

[hoffman900]

LSA = (Intake Centerline + Exhaust Centerline)/2 . 100 ICL and 112ECL is 106 LSA as is 104 ICL and 108 ECL, and so would 106 ICL and 106 ECL. Those cams would run completely different. If the valve lift profiles are asymmetrical, they would all be different still.

You are describing just one cam, that's what can be done with timing with single cam, changing it's timing affect both intake and exhaust lobes similarly so they cancel themselves. Changing LSA need separate intake and exhaust cams or in case cam operates both different cam with different LSA.

It doesn’t cancel themselves because advancing the intake by a couple degrees doesn’t mean the exhaust cam wants to either. Variable timing DOHC cam strategies show this to be true.

[juuhanaa]

Left to right or right to left? FYI Tech Talk Tuesday.

IMG_20220508_195604__01.jpg

[David Redszus]

But consider above, and then think about what DCR tells you where you just consider swept volume and IVC to tell you, vs all the things that effect cylinder pressure in a running engine above, and I'm not sure what it tells you other than people have correlated it through testing to some octane threshold for a very specific type of engine, which makes it a rule of thumb for only a certain type of engine that author / builder has experience with.

Static compression ratio is calculated using bore, stroke, rod length and chamber volume. It does not change with rpm, cam
events, or inlet pressure. It only produces a ratio based on unchanged operating parameters.

Dynamic compression ratio also uses the above factors, but additionally considers IVC. While the static IVC is fixed, in a
running engine we often experience dynamic variances due to valve float and/or valve bounce.
Assuming we have determined dynamic valve events correctly, the dynamic compression ratio does not change with
rpm or ambient conditions.

However, DCR is a ratio and only a ratio; of what? It will indicate thermal efficiency but little else.

Until we add parameters of inlet temperature and inlet pressure. Then, along with the Ratio of Specific Heats
of the inlet charge, we can calculate both compression temperature and pressure at any crank angle. Including air density
and resulting mass.

A change in static compression ratio, without a resulting change in DCR, will have no affect on chamber compression
pressure or temperature or thermal efficiency. From a combustion perspective, intial chamber temperature is far
more important than pressure.

[naukkis79]

It doesn’t cancel themselves because advancing the intake by a couple degrees doesn’t mean the exhaust cam wants to either. Variable timing DOHC cam strategies show this to be true.

You missed my point. In your example you only have one camsaft timed differently. It's a 106 LSA cam with installed at zero degrees both intake and exhaust lobe centers are 106 degrees from tdc. Advance cam 2 degrees and lobe centers change to 104 intake lobe center and 108 exhaust. Of course different cam timing results different engine operation.

[David Redszus]

It doesn’t cancel themselves because advancing the intake by a couple degrees doesn’t mean the exhaust cam wants to either. Variable timing DOHC cam strategies show this to be true.

You missed my point. In your example you only have one camsaft timed differently. It's a 106 LSA cam with installed at zero degrees both intake and exhaust lobe centers are 106 degrees from tdc. Advance cam 2 degrees and lobe centers change to 104 intake lobe center and 108 exhaust. Of course different cam timing results different engine operation.

What you say is absolutely true. But the lobe centers have no contribution to performance; it is the actual valve events
that do the work.

Many different events can be used to present the same lobe centers.

A much more insightful approach is that demonstrated by Dr. Blair involving Specific Time Areas for each camshaft period: overlap, blowdown, induction pumping, exhaust pumping, ramming, etc. The time area technique allows a better understanding of how a camshaft
actually works and permits quantification of event effects as well.

[naukkis79]

What you say is absolutely true. But the lobe centers have no contribution to performance; it is the actual valve events
that do the work.

Many different events can be used to present the same lobe centers.

For normal symmetrical cam profiles you need to know duration and LSA to calculate all possible valve events for given duration measure point.

[juuhanaa]

What you say is absolutely true. But the lobe centers have no contribution to performance; it is the actual valve events
that do the work.

Many different events can be used to present the same lobe centers.

For normal symmetrical cam profiles you need to know duration and LSA to calculate all possible valve events for given duration measure point.

When we know valve events as per crank degrees, then we can calculate duration, lobe centers and LSA. I was trying to explain this using my drawing.. Im not sure what you mean by saying

to calculate all possible valve events for given duration measure point.

[naukkis79]

When we know valve events as per crank degrees, then we can calculate duration, lobe centers and LSA. I was trying to explain this using my drawing.. Im not sure what you mean by saying

to calculate all possible valve events for given duration measure point.

You don't know valve events when you order cam, you are given cam specs. If you have say 260 degree cam 0.02 lift and 240 degree 0.05 lift which have lobe centers 106 degree apart that cam installed at 0 degree(both lobe centers @ 106 degree) has exhaust open point 0.02 (260/2 + 106) before tdc, 236 degrees or 56 degrees before bdc. And 0.05 46 degrees before. And for closing point (260/2-106) 24 degrees after tdc, or 14 @0.05. And same applies for intake, same crank degrees.

Now if you want keep intake valve events as they are but want advance exhaust 6 degrees you have to order cam at otherwise same specs but with 109 LSA and install it to 3 degrees advanced position( or 106 intake 112 exhaust lobe centers). Asymmetrical cam profiles does complicate things little but are they used nowadays? Finger follower cams have asymmetrical cam profiles but that's for compensating follower geometry to make actual valve lift symmetrical.

[juuhanaa]

When we know valve events as per crank degrees, then we can calculate duration, lobe centers and LSA. I was trying to explain this using my drawing.. Im not sure what you mean by saying

to calculate all possible valve events for given duration measure point.

You don't know valve events when you order cam, you are given cam specs. If you have say 260 degree cam 0.02 lift and 240 degree 0.05 lift which have lobe centers 106 degree apart that cam installed at 0 degree(both lobe centers @ 106 degree) has exhaust open point 0.02 (260/2 + 106) before tdc, 236 degrees or 56 degrees before bdc. And 0.05 46 degrees before. And for closing point (260/2-106) 24 degrees after tdc, or 14 @0.05. And same applies for intake, same crank degrees.

Now if you want keep intake valve events as they are but want advance exhaust 6 degrees you have to order cam at otherwise same specs but with 109 LSA and install it to 3 degrees advanced position( or 106 intake 112 exhaust lobe centers). Asymmetrical cam profiles does complicate things little but are they used nowadays? Finger follower cams have asymmetrical cam profiles but that's for compensating follower geometry to make actual valve lift symmetrical.

I appreciate if you want proceed left to right, but you are wrong saying i dont know the events when i order a cam, or when i spec it for some engine. I actually can put the cam in the engine and run it, if i think its good for the application. What matter is knowing the events and not the cam specs for a given engine, but the events are difficult to understand on a different engines.

I am not just saying this, im saying because i have run camshaft with oem markings in it, on a tarmac hillclimb event, and later undestood how the cams were working in that race winning engine. Now if i keep learning, the proceed right to left works for me better and better (assuming i dont forget the events, Intake closing for example..) Cheers,



-juhana

[juuhanaa]

Sorry i was talking BS, when i said i understood. I meant when i understood on a basic level.. Right now im trying to read books.

[Tom68]

Looking at inlet closing significance another way.

10 to 1 383, 950 Holley, RPM airgap, say 280cfm heads, Cam Saturday night special, 230 240 110 installed 4 deg advanced.

1. Add 5 degrees int duration closing int valve 5 degrees later.......More top end power, lose a bit off the bottom.

2. Reduce 5 degrees intake duration closing int valve 5 degrees earlier.....Reduced top end, increased bottom end power.

3. Add 5 degrees int duration opening int 5 degrees earlier.....Idle vacuum reduced, maybe pick up a couple of horsepower ?

4. Reduce 5 degrees int duration opening intake 5 degrees later....Idle vac increased, drop a couple of horsepower ?

5. Add 5 degrees exh duration closing exh valve 5 degrees later......reduced idle quality, maybe up a couple of horsepower ?

6. Reduce 5 degrees exh duration closing exh valve 5 degrees earlier....idle improved, maybe drop a couple of horsepower ?

7. Add 5 degrees exh duration opening exh valve 5 degrees earlier....Hotter exhaust pipe temp at cruise maybe ?

8. Reduce 5 degrees exh duration opening exh valve 5 degrees later.....No change I'd expect ? Unless the exhaust system was severly restricted or something odd.

If above assumptions are correct I'm back to inlet closing being the most important valve event in relation to power.

Now we just need to grind and test 9 camshafts.