Compression Ratio vs AFR

[TAQuickness]

Search isn't helping me much here...


Anyone know if/how compression ratio effects air fuel ratio?

I pulled my heads, had them milled (raised compression about 0.5) put them back on and see no noticable difference in my logs. It was a small change in compression, so maybe the AFR change is small and I'm just not picking up on it. I dunno. I expected to see a change in the logs.

[caver]

I doubt that CR will change the AFR much airflow is still the same you are just squeezing it more and applying more pressure on the piston.
Timing might need some tweaking esp at low rpm.

[bink]

VE changes(increases) by a small amount as your cyl vol changed.

[joecar]

Increasing CR by 0.5 from 10.0 to 10.5 is a 5% reduction in volume;

Actual VE should now be 95% of previous VE (assumption...?),
so same mass of fuel for a smaller mass of air,
so I would have expected AFR to be richer by upto 5%.

EQ 1.0 -> EQ 1.05
14.63 -> 13.9

That's what I would have expected, but I assumed the VE would now be 5% less.

So somehow your actual VE is [almost] the same...
Increase in air velocity...?

Edit: I was wrong here... it's not 5% less total volume but 5% less combustion chamber volume which equates to about 0.5% total volume, so VE should now be 99.5% of the original.

[caver]

VE changes(increases) by a small amount as your cyl vol changed.


The bore and stroke is unchanged therefore the capacity is unchanged therefore the cylinder volume is unchanged, all that is changed is the combustion chamber volume.
Therefore same amount of air in, only increasing the pressure on the piston due to cramming the same amount of air into a smaller space and increasing the cylinder pressure when it goes bang.
Therefore requiring a change in the amount of advance required. Gasses are hotter therefore burning faster and requiring less advance.

[joecar]

Hmmm...

CR = vol1 / vol2

where (assuming piston top goes upto zero height (level with block gasket surface)):
vol1 = combustion chamber vol + gasket vol + cylinder vol
vol2 = combustion chamber vol + gasket vol

Combustion chamber vol is now decreased,
so vol1 and vol 2 are now both smaller,
so it can't be the same mass of air in, it has to be less...

unless the the higher CR increases intake/exhaust velocities causing actual VE to be higher than previously, and so pulling in the same mass of air into a smaller volume.

Edit: after a couple of edits I fixed my silly typos... :)

[joecar]

He he he, I'm all messed up :hihi:... let me try again...

Before head shave: CR = vol1/vol2 = 10
After head shave: CR = (vol1 - x)/(vol2 - x) = 10.5

where:
vol1 = combustion chamber vol + gasket vol + cylinder vol
vol2 = combustion chamber vol + gasket vol
x = volume removed from combustion chamber.

Combining and solving for x gives: x = vol2 / 19 = vol1 / 190

1/19 = 0.0526 and 1/190 = 0.00526

So vol2 is less by approx 5% and vol1 is less by only 0.5% which is quite small,
so VE is off by 0.5% as AFR would be.

EQ 1.000 -> EQ 1.005
AFR 14.63 -> AFR 14.56

TA, what is your new CR...?

[VetPet]

Just my 2 cents here, but VE shouldn't be affected as it is a measure of the engines efficiency in filling the cylinder, regardless of the volume of the cylinder. Whether the cylinder has a volume of 1 litre or 5 litres doesn't matter as VE is expressed as the % of the cylinder that can be filled with air. If a 1 litre cylinder can ingest 1 litre of air then it is 100% efficient. If a 5 litre cylinder can ingest 5 litres of air it is also 100% efficient. The only difference being the volume of air each cylinder is capable of holding.

Shaving your heads reduces the combustion chamber volume and the overall cylinder volume so if anything, your VE should get better because you've got less cylinder volume to fill. Compression gets higher because you squeeze the cylinder volume into a smaller area. If you were to change the bore and/or the stroke, the total cylinder volume would increase and then your VE would be affected (negatively) assuming all else on the engine remains the same. C.R. will change but if you're stroking the motor you'd be using a different piston with a different pin height, otherwise you'd have interference problems and/or extremely high C.R. requiring race gas.

Timing can actually be increased with a rise in C.R. providing that you're using fuel with sufficient octane rating. Race fuel is less prone to detonation because it's burn rate is usually slower based on the specific gravity of the fuel and this allows more timing. You don't want the fuel to burn quickly otherwise you'd get into detonation which is what will happen if you use a fuel with too low an octane rating for the C.R.

:cheers: :beer:

[TAQuickness]

Good food for thought here...


TA, what is your new CR...?

New CR is 11.5:1 up from 10.9:1

I figured I'd aproach my problem in baby steps.

Here's the other side of this. Not only did I bump the compresion 0.6, I also upgraded the induction system. Went from LS6 intake, bone stock TB, and lid to Fast90, NW90, and Volant intake.

I have no complaints on how the car feels. SOTP dyno is off the charts (will have actuall dyno results tomorrow evening). What's getting me is that my tune, with the exception of a few idle settings, is unchanged from the previouse set up and my BEN's are unchanged. All BEN's are showing 1.00 +- 0.02. I really expected to see a difference in my logs.

[joecar]

New CR is 11.5:1 up from 10.9:1

All BEN's are showing 1.00 +- 0.02

Recalculating using your values...

Before head shave: CR = vol1/vol2 = 10.9
After head shave: CR = (vol1 - x)/(vol2 - x) = 11.5

Your total volume change = x/vol1 = 0.0052 = 0.52%

This is very small so your VE table should be almost the same, well within +/- 2%.

Does that make sense (I need to think about this over dinner with my Mrs after I get home)...?

:banana::cucumber::banana::cucumber:

These bananas/cucumbers really mess with me, mate...

[ringram]

You have increased your thermodynamic efficiency from higher compression. Volumetric is much the same but your actual engine efficiency has increased so you now get more energy from each charge.
How about that for a theory?

[VetPet]

Ahhhhh! The big bang theory. :muahaha:

:banana: :banana: :banana:

Now if they could only sing.

[TAQuickness]

Ahhhhh! The big bang theory.

:notacrook:

I'd buy that for a dollar :rockon:

Makes good sense on the CR part.

Today was a different day. Now that the brakes and clutch are working much better, I was able to drive a little more spirited. I'm starting to see in my upper PT RPM range the discrepancies I expected from the new induction system.

I still can't say enough about the raped ape feeling @ WOT

[TAQuickness]

Oh yeah - no dyno numbers today. The dyno operator had to leave unexpectedly. But the good news is, I get the dyno pull's for free at a later date :D

[joecar]

Free = good
:cheers::cheers:

[joecar]

You have increased your thermodynamic efficiency from higher compression...
That's correct; from my physics textbook, thermal efficiency for a gasoline piston engine is:

e = 1 - 1/(CR^(gamma-1))

where:
e = efficiency
CR = compression ratio
gamma = 1.4 for air (= ratio of heat capacity of air at constant pressure to that at constant volume)

As CR goes up, the term CR^(gamma-1) goes up and so 1/(CR^(gamma-1) goes down, and so 1 - 1/(CR^(gamma-1)) goes up.

Increasing CR increases efficiency.

[bink]

I still can't say enough about the raped ape feeling @ WOT


What a great expression - says it all!

"Scalded Dog" is a good one too. :D

:cheers:
joel

[TAQuickness]

I need some of your books Joe!

[ringram]

I recon the best book by which all else fall down before, is the MIT engineering text by John Heywood. http://www.amazon.co.uk/exec/obidos/ASIN/0071004998/qid=1137672997/sr=8-6/ref=sr_8_xs_ap_i6_xgl/202-0255887-2554278

Information on him and MIT course at http://web.mit.edu/mitpep/pi/courses/internal_combustion_engines.html

Its pretty chunky especilly if you like formula's. But there is also plenty of good text and the math is there to prove the text. It is by far and away the best definative tomb on engine mechanics bar none. Id say its about 20% maths, 80% text.

Here is the formula joe mentioned in one of the early chapters.

http://www.efilive.co.uk/32.jpg

[joecar]

This book seems to be available only from Amazon UK...

Edit: I found it on Amazon US when I searched later in the day...

[ringram]

.. maybe thats where all the brainy people live :)

[joecar]

:hihi: Yeah, but you eat Marmite rather than Vegemite... :banana::cucumber::cucumber:

[TAQuickness]

thank again guys. Have to wait till payday, but I'll get that one on order.

[bink]

This book seems to be available only from Amazon UK...

Edit: I found it on Amazon US when I searched later in the day...

I bought it on Amazon a couple years ago. :D
Excellent reference. FWIW.
:cheers:
joel

[joecar]

Ringram :cheers:
Joel :cheers:

Thanks for the book review; I got it on order now. :cheers:

Joe :cheers: