Octane rating vs CR

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Re: Octane rating vs CR

Post by Adger Smith »

I read most of this, but might have missed it.. Did anyone touch on the affect of Charge Density?
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Re: Octane rating vs CR

Post by BLSTIC »

David Redszus wrote: Sat Jun 19, 2021 7:04 pm
BLSTIC wrote: Sat Jun 19, 2021 6:54 pm Can we determine true compression ratio by multiplying static compression by not-MAP-corrected VE?

But you can only determine that with sims or a completed engine on a dyno.
Static compression ratio, multiplied by any number, will never produce a measured pressure.

VE is the ratio of air masses which is even more difficult to obtain.
The angle I was going for was VE*SCR correlating strongly with cylinder pressure, which correlates to knocking for similar engine/combustion chamber designs.

I would have thought VE was pretty easy to measure with an airflow meter? (Never tried it, just kind of assumed this, feel free to correct me)
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Re: Octane rating vs CR

Post by naukkis79 »

BLSTIC wrote: Sun Jun 20, 2021 2:23 am I would have thought VE was pretty easy to measure with an airflow meter? (Never tried it, just kind of assumed this, feel free to correct me)
Yes, every modern engine with MAF measures true VE for engine for every operating point. And engine tuning maps in ECU are based on VE, without MAF those VE-values are calculated, roughly from rpm and intake manifold pressure but with closed-loop those can be fine-tuned to be accurate.
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Re: Octane rating vs CR

Post by My427stang »

David Redszus wrote: Sat Jun 19, 2021 3:17 pm
I do agree that you pick a piston based on required compression, but you pick a compression ratio based on use, fuel, chamber, and camshaft selection, not to mention all the other variables that affect controlled and uncontrolled burn.
Now that we have picked a piston based on some required compression (on whatever basis), how do we know what the compression actually is?

We can measure chamber volume and displacement to determine SCR. That tells us almost nothing with regard to combustion behavior; anecdotal folklore aside.

We can include IVC to obtain DCR. That still does not reveal the impact on combustion performance, since we do not know compression pressure or temperature.

But certainly, using SCR and octane index values are practically worthless; dozens of exceptions to our personal experiences will be easily found.
ON EDIT - I believe that you were discussing the comparison of compression ratios to heat and pressure and not countering my experiencet directly. I do agree with you, and please review what I said below, but I do have a serious and respectfully presented question. How would we take a given combo, and predict those temps and pressures?

To add to that I rarely care about the SCR, but, if it's more than a 2 point spread, it's likely a weird combo and you need to look close on a pump gas FE

WE would not pick a compression ratio and see what has to make it work. My "WE" would evaluate the use of the vehicle, choose the induction all the way through the exhaust primary pipe size and length, and ensure all of the parts match to include all valve events. AFTER that, I would ensure the block/piston/head gasket combo gave decent quench and movement in a chamber given the family of engine I am working, and only then would I pick the piston design, and as I have said from the beginning, when you look at DCR, in repeated builds on a family, you see the trends.

To be honest, if a single engine family (or in comparable families) I tend to work the combo and end up adjusting dish size slightly with the DCR number. It's more of a checksum.

Since post one I have pointed out that there are plenty of assumptions, but if building a flat top or dished piston FE, they are pretty well proven.

Now, yet again, I am not disagreeing, but since the initial post, I think guys like me and a few others who have done healthy pump gas FEs have the empirical data to be able to offer some proven combos and options along that line. I wish I had the ability to forecast combustion temp and pressure at various loads and RPM and would likely get happily drunk in all the information, but so far, we are having some very good success over an engine family

If I am correct, the OP is building a TFS headed FE, which in 500-600 HP combos, peaking under 6000 RPM (depending on displacement and exact performance goals) like about 10-10.4:1 static, 8:1 dynamic, .039-.046 quench, a dished piston versus D-cup, and 36 degrees lead for max power, with rate unfortunately tied to the vehicle for daily use. Those are in order of clarity, and are not rules of thumb,just observed successes over multiple builds

If it is a standard FE iron chamber, any FE chamber, they all are horrible.... I generally like to see the same quench distance, typically look for a reverse D-cup due to the abysmal quench pad on the plug side of an FE chamber, but adjust static to limit dynamic to be about 7.8.

Both are for pump premium which is all over the map in mix and octane in the field, assume temp and mixture is under control and the curve is correct for pump premium. I cannot forecast what would run on 87 at higher load and rpm levels because I haven't built enough of them and my hunch is the power trade off is greater than the cost of premium fuel
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Re: Octane rating vs CR

Post by David Redszus »

How would we take a given combo, and predict those temps and pressures?

Good question.
Let us start with the calculation for DCR. We need the following input data:
Bore
Stroke
ConRod length
IVC angle (BTC)
Chamber volume (TDC)

From these inputs, we can calculate the SCR, DCR, and Thermal Efficiency for any engine.
Compared to DCR, using SCR will overstate TE. TE is very useful in order to predict the
energy extraction possible from a mass of fuel, and is necessary to predict engine power.
DCR....TE
10.2....60.5%
8.3.....57.2
7.1.....54.4
6.3.....51.9
5.6.....49.8
Each DCR compression point is worth about 3% TE and therefore 3% in power.

By incorporating Inlet Air Temperature, Inlet Air Pressure, and the ratio of specific heats, (for the
air composition), we can calculate the compression temperature and pressure at any crank angle.

If we know the distillation temperature curve of the fuel and auto-ignition temperature, we can
predict, when or if, the fuel will ignite (possible pre-ignition) and, when or if, the air/fuel end gas will
self-ignite (detonation). Fuel octane will determine when the end gas will ignite, not if it will ignite.
Octane buys us a small bit of time.

We still do not know VE since we have no measure of air (and fuel vapor) mass.
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Re: Octane rating vs CR

Post by My427stang »

Very interesting, and makes sense.

Given changes in fuel blends on a street vehicle and changes in atmospheric pressure, air quality and temp, can we get to a workable output for planning?

I would add that I think being conservative on planning the resulting compression ratio for a street vehicle is probably in an owner's best interest though. If it is 3% increase for 1 point

Given an engine I recently did, 461 inch, pure street motor, 10.4:1 SCR, hyd roller, 7.9 DCR using .006 cam events, good chamber and dished piston, .040 quench, and a 231/235 @ .050 cam, we made 568 HP at 5750. A quarter point of DCR would be worth 4 HP. I'd likely trade that for some buffer in environmental conditions. However it certainly shows me that if you can narrow the range of the environmental and fuel variables, every bit you can get is worth it for power
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Re: Octane rating vs CR

Post by frnkeore »

David Redszus wrote: Mon Jun 21, 2021 7:10 pm
How would we take a given combo, and predict those temps and pressures?

Good question.
Let us start with the calculation for DCR. We need the following input data:
Bore
Stroke
ConRod length
IVC angle (BTC)
Chamber volume (TDC)

From these inputs, we can calculate the SCR, DCR, and Thermal Efficiency for any engine.
Compared to DCR, using SCR will overstate TE. TE is very useful in order to predict the
energy extraction possible from a mass of fuel, and is necessary to predict engine power.
DCR....TE
10.2....60.5%
8.3.....57.2
7.1.....54.4
6.3.....51.9
5.6.....49.8
Each DCR compression point is worth about 3% TE and therefore 3% in power.

By incorporating Inlet Air Temperature, Inlet Air Pressure, and the ratio of specific heats, (for the
air composition), we can calculate the compression temperature and pressure at any crank angle.

If we know the distillation temperature curve of the fuel and auto-ignition temperature, we can
predict, when or if, the fuel will ignite (possible pre-ignition) and, when or if, the air/fuel end gas will
self-ignite (detonation). Fuel octane will determine when the end gas will ignite, not if it will ignite.
Octane buys us a small bit of time.

We still do not know VE since we have no measure of air (and fuel vapor) mass.
I've been MIA on this thread, for the last week, because I had to help put on and help run, a tractor show and pull. Just got unloaded about a hr ago.

Thank you, David. That's the best input yet, on this subject!

Are there any calculations on octane # needed per actual cyl pressure created?
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Re: Octane rating vs CR

Post by David Redszus »

Are there any calculations on octane # needed per actual cyl pressure created?
Cylinder pressure does not cause detonation; temperature does.

However, increases in DCR will increase compression temperature which has an effect on combustion.

Pressure, while not causing detonation does determine the heat transfer rate from the flame front to the un-burned
end gas. Higher pressures, whether due to compression or combustion, reduces the time before the end gas detonates.

What fuel octane does is to delay the onset of end gas ignition.

The source location of detonation is typically not constant; it will move to the most ignitable location and condition.
Often, detonation is quite intermittent, moving from cylinder to cylinder, and cycle to cycle. Also varying in frequency and amplitude.

For a given engine type and configuration, it is possible to empirically derive a correlation between DCR, octane, and detonation, provided that inlet temperature, fuel ratio and preparation, chamber turbulence and timing remain unchanged or accounted for. Not an easy task unless we only work on a single type of engine. The empirical correlation may or may not apply to other almost identical engines.

Fitting a knock sensor with a signal fed into a data logger along with other engine parameters would provide real insight .
Thirty years ago I thought we knew everything there was to know about knock. Way wrong.
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Re: Octane rating vs CR

Post by frnkeore »

I have to disagree with this:
"Cylinder pressure does not cause detonation; temperature does."

With dynamic pressure (the process of creating it), air rises 2 deg F, for every 1 psi that is created. I don't know how much the fuel vapor may cool it, in that process but, it will self ignite at about 500F or less than 250 psi, depending on ambient and engine temp.

Since there is "tons" of anecdotal evidence, that ~8/1 DCR is usually good, with 91-93 oct gas, I was hoping that there was the same type evidence for 87 oct gas but, there doesn't seem to be. I had thought that ~7.5 DCR (all things equally managed) might be a good approximation but, I REALLY DON"T KNOW.

My reasoning for asking this, is that as things get "greener" that is all that may be available, at some point. I'm a product of '70's racing and I would have NEVER thought that 102 octane pump gas would be gone :( It happened pretty fast though!!!

So, I think it is important to research what can be done with 87 oct gas.
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Re: Octane rating vs CR

Post by mt-engines »

You can disagree about temperature not being an influencer. But it most certainly is.

2 similar fuels can have completely different reid vapor pressures. Yet same octane. One detonate at 120 degree iat, and the other not at all at 120 it? Same engine, same load, same temp?

Why can an engine with lower cylinder head temperatures tolerate less octane?

Why do people use intercooler or meth injection?
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Re: Octane rating vs CR

Post by frnkeore »

I'm not disagreeing about temps.

What I said is that if you create pressure, dynamically, it increases temp. I other words, you can not create pressure in a engine, w/o creating temp.
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Re: Octane rating vs CR

Post by mt-engines »

Gotcha.. I still think DCR is a waste of time.

Different driving style can impact the octane requirements.. thats not an input to the formula
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Re: Octane rating vs CR

Post by David Redszus »

frnkeore wrote: Tue Jun 22, 2021 5:35 pm I have to disagree with this:
"Cylinder pressure does not cause detonation; temperature does."
What exactly do you disagree with?
With dynamic pressure (the process of creating it), air rises 2 deg F, for every 1 psi that is created.
We can be more specific. Starting with a DCR of 7.0-1, IAT = 80F and IAP = 14.7 psi, we find the following at TDC.
DCR....Deg F....PSI
7.00..707.37...224.08
7.01..708.02...224.52
7.02..708.62...224.96
7.03..709.32...225.41
for pure air. But....

DCR....Deg F....PSI
7.00..663.23...195.55
7.01..663.38...195.91
7.02..664.41...196.28
7.03..665.01...199.65
for air/fuel vapor.
The difference is not due to fuel evaporation, but to the ratio of specific heats of the air vapor.
But of course, we ignite the fuel well before TDC where temperature and pressure are much lower.
Since there is "tons" of anecdotal evidence, that ~8/1 DCR is usually good, with 91-93 oct gas, I was hoping that there was the same type evidence for 87 oct gas but, there doesn't seem to be. I had thought that ~7.5 DCR (all things equally managed) might be a good approximation but,
Once upon a time, oil companies such as Union 76 and Phillip 66, performed extensive engine testing comparing compression ratios and conditions to various octane levels.
The reason there is no predictive formula available is a direct result of their findings. There is no causation between DCR and fuel octane due to the large number of engine types and operating variables.
I'm a product of '70's racing and I would have NEVER thought that 102 octane pump gas would be gone .
There never was a 102 octane pump gas. The advertised octane numbers were RON values; a number of limited utility. The actual MON number was 92 which is not far from the 88 MON values of premium (93 index) pump gas available today. The actual MON values will depend on brand, season and geographical location.
So, I think it is important to research what can be done with 87 oct gas.
If you mean 87 index, you are actually asking to build high performance race engines using 82 MON fuel. The engines that you visualize are being built today using turbos and electronic controls. If we insist on using old design, big bore stuff, we need to make it really, really big. Just like the land speed record cars of the 1930s.
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Re: Octane rating vs CR

Post by hoodeng »

I was of the understanding that the biggest thing these days in efficiency gain is in the control of the combustion process, I was told that in the modern production engine arena that more research is put into combustion efficiency than induction control, although VVT has contributed in that area considerably.

Direct injection has opened up a new approach in the said process, no longer are the constraints of a pre mix being admitted to the chamber then trying to wrangle the best combustion efficiency outcome from that.

In diesel, ongoing research into piezo control, with multiple independent orifices in common rail injection has been cleaning up emissions for a lot of years now, not to mention the power per capacity gains. This research was always going to have filter down application in spark ignition engines, as has happened for a number of years now.

Unfortunately, this has no retro fit on a lot of what we do,,,,,,unless someone wants to do DI research and manufacture for our projects? Hands up anyone?
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Re: Octane rating vs CR

Post by Truckedup »

David Redszus mentioned not to long ago that combustion burn rate is mostly controlled by turbulence created by chamber design. Since the faster the burn, the less time there is for detonation to occur, this should be quite important when considering CR and fuel octane..
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