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Compression power increase
7

Compression power increase

Compression power increase

(OP)
Im wondering about the effects on power outbut from raising the compression ratio.

In a 4-stroke automobile engine, of course...

does the power increase raise in perfect(or near perfect) DIRECT PROPORTION with the raising of compression ration??

example-
in a 9.0:1 engine that makes 100hp, would rasing the c/r , say, 10% raise the power 10% also (9.9:1->110hp)?
(20% increase 10.8:1->120hp?)

does it keeps going to extreme amounts? (+100% c/r -- 18:1->200hp)?

i understand that in the extreme amounts as suggested, the engine that was originally designed for 9.0 c/r and 100hp is going to have a very rough time tyring to withstand those stress levels. this is all just in a theoretical mindset.

RE: Compression power increase

Compression ratio is only one aspect of a complicated picture. What you are really after is cylinder pressure - which is mainly governed by the comp ration, cam, intake efficiency and exhaust efficiency. And if the cylinder pressure gets too high preignition results. ISZ

RE: Compression power increase

Short answer

NO

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Compression power increase

the relationship is indirect, and according to most books I've read non linear. so that in the example you gave the power increase as a % is lower at the higher compression.    "theoretically"

RE: Compression power increase

The maximum compression ratio that a gasoline engine can handle is (as IceStationZebra said) goverened by many things (check his list) and also the limits imposed on controlled combustion by the onset of detonation. This depends on the fuel in use, the combustion chamber shape and the presence of any 'hot spots' in the design.

Bill

RE: Compression power increase

(OP)
Thanks for the replies, guys.

I knew it couldnt have been that mathematically simple.

RE: Compression power increase

I'm a litte surprised at some of the answers in this thread.  At a strictly theoretical level, increasing the compression rato increases the power.  At a practical level;

If heads are milled to increase the compression ratio, why is that done?

If you reduced the compression ratio from 8:1 to 6:1, what would happen?

RE: Compression power increase

8:1 down to 6:1 would result in a dramatic drop in power.     

RE: Compression power increase

the point of my earlier post was to answer op's question as to the shape of a theoretical curve. theoretically if you started at 6:1 increase to 7:1 and keep going, optimizing the whole engine along the way, power continues increasing but in diminishing amounts.

RE: Compression power increase

FoMoCoMoFo,

Now your response is crystal clear.

RE: Compression power increase

(OP)
thanks, FoMoCoMoFo, your explanation makes more sense.

that being decided,

is there such a formula to tell us how the power increase will be by raising the c/r by a given amount?

thanks for all the help guys!

RE: Compression power increase

There is a formula for the *theoretical* Otto cycle efficiency as a function of its compression ratio. Maybe someone else has it handy; I've forgotten it ... it's in a thermodynamics textbook somewhere buried in the basement in a box that I haven't opened for years.

The theoretical ideal efficiency calculation makes a lot of assumptions that are impossible to achieve in the real world, and it also makes no account of real-world effects like heat losses, pumping losses, the non-zero amount of time that it takes for combustion to take place (ideal Otto-cycle combustion is instantaneous at top-dead-centre, which would destroy a real-world engine), limitations imposed by detonation or self-ignition, etc. The real efficiency of a practical Otto-cycle engine is half or less of what the ideal-cycle efficiency is.

RE: Compression power increase

Hello BJNX2000,

One has to make assumptions regarding the background knowledge of the person who starts the thread in order to respond effectively.  So I apologize if I sound condescending - that is not my desire or intent.

Please first ask yourself - What is compression ratio?  If your answer is volume ratio in a closed system, then my following recommendation is:  Get a copy of Van Wylen and Sonntag "Classical Thermodynamics" and read chapter 9 on air standard cycles.  Then you will have sufficient info to build a simple excel model of the PdV evolution in a standard Otto Cycle.  The model will provide you with an "ideal work" answer (no engine friction consideration) which you can use to graph the ideal power response to change in CR, assuming things like constant volume addition of heat and constant ratio of specific heats.

Again, I apologize if my response seems trivial or unhelpful.

RE: Compression power increase

  HI BJNX2000, your original question and my replies dealt with theory, understanding the theory is great, but as most the other posts point out it's too big a leap to a real world rule. way too many variables. but do keep thinking about it,   Happy Motoring

RE: Compression power increase



CODE

      
REM =======================================================================
REM Compression ratios and engine efficiencies written to a file
REM to partly answer a question posed here
REM http://www.eng-tips.com/viewthread.cfm?qid=187729&page=1
REM =======================================================================
REM Formulae (see page 8 of 29) and Free Demo BASIC Interpreter from
REM http://sitemaker.umich.edu/mhross/files/fueleff_physicsautossanders.pdf
REM http://www.cix.co.uk/~rrussell/bbcwin/bbcwdemo.exe
REM =======================================================================
      
cv = 7 : REM degrees of freedom x, y, z, like propellor, like spokes & 2*pogo
cp = cv+2
gamma_minus_1 = cp/cv - 1
      
file=OPENOUT("CR_eff.txt")
phi=1                             : REM fuel-air ratio relative to stoichiometric
      
FOR cr = 7 TO 22                  : REM compression ratio
  PRINT# file, STR$(cr)+" "+STR$(FN_FuelAirEfficiency(FN_OttoEfficiency(cr),phi))
  BPUT# file, CHR$(13)
NEXT cr
CLOSE# file
END
      
DEF FN_OttoEfficiency(cr)
= 1 - (1/(cr^gamma_minus_1))
      
DEF FN_FuelAirEfficiency(otto_eff,phi)
= (1 - 0.25*phi)*otto_eff

CODE

7 0.319865101
8 0.335966433
9 0.349667779
10 0.361539399
11 0.371975023
12 0.381257017
13 0.389594231
14 0.397145135
15 0.404032591
16 0.410353626
17 0.416186091
18 0.421593325
19 0.426627495
20 0.431332035
21 0.435743462
22 0.439892746

RE: Compression power increase

So, I wrote my little program and ran it with phi=1 (stoichiometric air fuel ratio).

Now I just pasted some of those numbers as the following line of text
0.371975023/0.361539399=
into C:\windows\calc.exe and it gave me
1.02886441707007429085204625236432

Now I can finally answer the original question.

Changing a compression ratio from 10:1 to 11:1 on an engine running at stoichiometric would give an expected power increase of about 3%, eg 100 HP to 103 HP.

  • My earlier post has a picture generated on the fly by a computer that is usually offline, so if it doesn't show, don't worry. It is just the formulae for engine efficiency from the reference.

RE: Compression power increase

I have used an 'online' program to interpret some of my dyno runs on the Mini.  It's a 1380cc Austin A series engine. Don't get too excited if the numbers don't match theory, in my experience, they never do. All things remained the same (except for pistons) for all but the last run.  That engine was not mine, but the same type and configuration.  Fuel was Sunoco 108 for test 1 thru 3, VP 110 for test 4, unknown for test 5 and 6 (most likely Sunoco 116). All tests on same dyno and corrected to std. norms.

10.5:1 Compression ratio---94 hp.
11.5:1 ---97hp
12.5:1---99hp
13.87:1---110hp---107hp (two different runs)
14.5:1---117hp
16.0:1---122(approximate as I did not do this run)

I settled on 13.87:1 (it just worked out that way) at 110.48hp @ 7200rpm.

I have yet to find a computer program that works for my cars, or at least for me (pre computer geek).

Obviously, I guess, I went off track with that "all things remained the same", except...

Rod

RE: Compression power increase

Thanks.

10.5:1 Compression ratio---94 hp.
11.5:1 ---97hp

We are in the same ball park, 3 HP in 100 HP for adding 1 to the compression ratio at around 10:1, 10.5 in your case.

RE: Compression power increase

to give you an example of ranges toyota 1zzge, 1.8 liter 10:1 compresion gives 130 HP a 2 2gzze, 1.8 liter 11.5 : 1 is 180 HP.  At 12.5 : 1 compression ratio   you are approaching the limit for 92 octane gasoline at 100 F.

the problem in converting compression ratio to HP is all kinds of things, the Toyota engines above have ibtirely different storkes, Max RPM, heads/cams/vvt, and ready for this, similar torques...?

On a carburated or fixed injection, or fixed head/cam, the HP cam match equations list above.

RE: Compression power increase

Back to the OP

I stick by my original answer, however I have 10 min to kill, so in addition to an answer I will explain why, even though that was not asked.

Increasing the compression ratio effects a number of factors.

1) Fuel octane requirement. Higher octane fuel will be a different chemical composition and this will change other properties as well as the octane rating.

2) Increasing compression to the point of detonation increases power, however once exceeding this point, the engine detonates and looses power as well as beating itself to death.

3) To a point, you can retard the spark to avoid detonation, but the retarded spark costs more power than the compression gave.

4) Combustion chamber and or piston top change of shape. This affects flame travel, onset of detonation and combustion speed.

5)  Change of combustion chamber surface area which influences energy lost to cooling system, which also effects onset of detonation.

6) Changes to expansion ratio. The greater the expansion of the gas during the power stroke before the exhaust opens, the greater the amount of energy extracted.

7) Effect on manifold pressures. If the chamber volume is smaller at TDC, the engine does not need as many degrees of rotation to double the volume over the piston. This means the pressure drops quicker in the exhaust port, promoting more gas flow. Smaller volume also means a greater amount of the spent gas is sent out the exhaust.

The 10 min is up.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Compression power increase

Pat,

A very good post.  A question though on point 2 that you made:

If I remember correctly, we were determining the detonation limit of an engine a few years back using pressure transducers.  I believe we found that maximum power was derived just after the onset of detonation.  This maximum power point was very impractical, as the engine "ran away" from us a couple of times and did some internal damage.  Has anybody else experienced such a phenomena of maximum power just after the detonation limit?

Again, this is more theory, since you would never design an engine to run in this region of your detonation curve.

-Reidh



RE: Compression power increase

"Detonation" aka knock is a continuum, from very small energy release near the end of combustion, to simultaneous combustion of most of the charge, shortly after ignition.
For any given engine and fuel, MBT timing may be with or without some detonation.

RE: Compression power increase

Has anybody else experienced such a phenomena of maximum power just after the detonation limit?

Yes. I had a car with a four speed manual gearbox and regularly did a journey with a very steep hill and a dual carriageway that everybody treated as a motorway. Top speeds were about 40 MPH in 1st, a bit over 70 MPH (motorway speed limit) in second, around 105 in third, and I don't know in fourth because a blue light started flashing behind me when I reached 116 MPH (according to the man with the blue light).

Normally top speed is in top gear, but some vehicles with fuel economy gears can have their top speeds in lower gears. This hill produced that effect: changing up to fourth meant lowering the RPM so much that the work done lifting the car up the hill at that speed required more power than the engine would produce in fourth, and I would charge up the hill with the needle in the red in third, somewhere short of hitting the rev limiter.

Heavily loaded engines are more inclined to knock than unloaded engines, and so although the car would (happily?) hit the rev limiter at 40 MPH in 1st, it would start pinging or knocking blasting up this steep hill at around 100 MPH in third.

This must have been at or close to maximum power. Maximum power certainly wasn't "normal" engine operation, as I had accelerated past that. I never felt comfortable at that point, and would ease off a little, not liking the knocking, and not wanting to hit the rev limiter (which was quite brutal in its operation).

=

I agree with what pat is saying about engines being normally engineered close to a limit. It is therefore difficult to raise the compression ratio without having to make other changes.

I don't think that affects the maths of 'how much more power is due to a compression ratio change?'

Suppose we take an engine that is engineered to the limit. We can always ask the question how much would the power drop if the compression ratio was dropped? That's really the same question viewed differently, but avoids his points about engines normally being already "at the limit" in some sense.

But I think it is valid to look at the contribution compression ratio makes to power.

Let's just take his first comment - there is no point in me boringly going through them all one by one.

1) Fuel octane requirement. Higher octane fuel will be a different chemical composition and this will change other properties as well as the octane rating.

Suppose an engine is built so it will run on regular and premium - low and high octane fuels. Now suppose the owner wants the head skimming a little and accepts that the low octane fuel will no longer be suitable. How much more power will he get from skimming the heads? I think that is a valid question.

Suppose the guy decides to switch fuels to a special high octance fuel. How much power will his new engine produce?
Well, he could switch fuels before skimming his head. The power change for that can be calculated from the new air fuel ratio, and the energy content of the new fuel. Or he could switch fuels, and measure the power with the new fuel.
Then, when skimming the heads, the power changes as a result of the different compression ratio and we are back to the original question, but without the complications of the new fuel's energy content and changed air fuel ratio.

=

There is a tendency on the forum to think of racing engines, engineered right to the limit on a number of fronts, and then when a question is asked, to say "it's too complicated because everything affects everything else".
I don't think that is a healthy approach to understanding the basics. Surely it is valid to say that there are production engines engineered with sufficient margin to avoid day to day problems in the normal world, and to ask and answer what are the effects of minor tweaks to individual parameters of the engine.

Having an idea of the expected performance increase from doing some work, could surely sometimes prevent performing an expensive engine modification, only to find that the performance gain wasn't quite what you had hoped for.

RE: Compression power increase

Great thread.

It's somewhat unusual to relate an increase in compression
alone to an increase in power. When power output is the
objective, the increase in compression is combined with and
increase in volumetric efficiency to realize full benefit.

It's more common to relate increased compression alone to
engine efficiency. Here's a few graphs you may find
interesting:



RE: Compression power increase

I tried to keep it simple, each point could be opened up to a major topic in it's own right, and many have been in previous threads here.

Point 2

The compression ratio for onset of detonation with a specific fuel varies with conditions during operation and with a:f ratio. also at lower rpm detonation quickly destroys an engine, but at F1 engine speeds, some detonation is the way to et the fuel to burn fast enough.

Point 3

I think modern engines with knock sensors can maintain optimum advance by constantly advancing to onset of knock then retarding just enough for no damage to occur.

Also increasing compression ratio increases charge temperature before ignition and that effects evaporation rate and combustion speed.

Also simply shaving a cylinder head changes 3 things. It increases CR, it changes surface area and mostly increases squish or quench area, each of which effect power and maximum sustainable CR.

!0 min is up again

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Compression power increase





automotivebreath
The equation used in your second graph is the same as the equation I used, so there is agreement there. The difference is that I took k=CP/CV to be (7+2)/7=1.3 instead of (5+2)/5=1.4. They use this warm air value of k=9/7=1.3 in your first graph, but for some reason have chosen a room temperature value k=7/5=1.4 for the second.

It is easy to differentiate the equation.
-1/rk-1 is -1*r1-k and its derivative is -1*(1-k)*r(1-k)-1= (k-1)*r-k

So the efficiency increase with changes in compression ratio r, divided by the thermal efficiency is
(k-1)*r-k/(1-1/rk-1).

Trying this with compression ratio r=10 and k=1.3
0.3*10-1.3 / (1-1/100.3) gives 0.03.

That is, at 10:1 power is going up 3% per unit of compression ratio. eg 100 HP at 10:1 -> ~103 HP at 11:1.

This is almost the same calculation as I did before.
Previously I calculated by interpolating between points at CR=10:1 and CR=11:1.
Today I have differentiated and used a gradient of your graph at exactly 10:1, (but using my k=1.3 instead of 1.4).
The results are similar as your curve is almost linear for small changes in compression ratio.

RE: Compression power increase

crysta1c1ear, very clever!

Raising static compression is something I often use in practice. With street driven vehicles the objective is increased efficiency for reductions in fuel consumption. It becomes a careful balance; mentioned earlier when the cylinder pressure gets too high the auto-ignition point of the fuel is reached before the charge is consumed by normal combustion.

At the drag strip the objective is increased power. Combining increased static compression with an increase in VE the operating RPM range is raised, additional power can be generated at the higher RPM. Although the static compression is raised, the dynamic compression can be maintained to avoid the need for high octane fuel. The delayed intake valve closing point of the long duration camshaft effectively maintains cylinder pressure to a level below the auto-ignition point of the fuel.

“Has anybody else experienced such a phenomena of maximum
power just after the detonation limit?”


This is something I use at the drag strip. The objective is to create a situation where a small portion of the end gas auto-ignites. This is primarily done to assure the entire charge is consumed when the crankshaft is in a position to create power. A secondary benefit is reduced intake reversion from exhaust gas expansion during camshaft overlap.

RE: Compression power increase

Homogeneous Charge Compression Ignition:
http://www.me.berkeley.edu/cal/HCCI/

The faster the combustion, the closer to the ideal Otto-cycle and the higher the efficiency.

RE: Compression power increase

Thanks kitabel for that; the information provided by Jeffrey Diamond is consistent with the math of crysta1c1ear.

RE: Compression power increase

Many years ago, the rule of thumb was that CR affected the peak torque much more than the peak HP--not that peak torque means much anyway.

"When the eagles are silent, the parrots begin to jabber."
    Winston Churchill

RE: Compression power increase

Hi there:

Here are two plots for an ideal the Otto Cycle -- efficiency and power output:





Thanks,

Gordan Feric, PE
Engineering Software
http://members.aol.com/engware

RE: Compression power increase

An example for the record: the 6/07 Automotive Engineering International has a writeup on the 2008 Jeep Grand Cherokees 4.7L OHC V8 upgrades.  "New head castings improve the flow and accomodate dual spark plugs that permit a higher CR- 9.8:1 vs. the old engine's 9.0:1.  The result is 305 hp, a 30% increase, and 334 lb.ft, a 10% increase. . . fuel economy is improved by 5%, an engineer said.  Of that, 2% is attributable to the boosted CR and dual spark plugs, and the rest (to transmission changes)"

RE: Compression power increase

In reference to Evelrod’s Mini example, there seem to be a lot of variables that can affect the HP outcome.  Beyond just raising compression, piston shapes such as dished, flat or domed and combustion chamber design can have a major affect.

A case in point, the 35 year old 1973 Porsche 2.0 liter VW T4 based 4 cylinder engine had it’s own unique heads and was available in two compression ratios. The world market GB engine developed 100 HP with 8:1 CR using flat top pistons and the US market GA engine developed 95 HP with 7.6:1 CR using dished pistons.  The two engines were identical all other ways.  With a 5.26% increase in compression the GB engine benefited in a 5.26% increase in horse power.  

In Evelrod’s Mini example, when the CR was increased from 10.5 to 11.5 (a 9.5% increase) the HP only increased from 94 to 97 HP(a 3.2% increase).  The 914 engine was still only making 50 HP/liter whereas the 1300cc Mini engine was making 75 HP/liter. If the Mini uses domed postons, there may be other flame front considerations, and there may be also some diminishing return factors too with CR increases in general.

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