Compression power increase 7

[WDWalker]

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.

 

[reidh]

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

 

[Lou Scannon]

"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.

 

[crysta1c1ear]

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.

 

[automotivebreath]

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:

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[crysta1c1ear]

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=C_P/C_V to be (7+2)/[highlight]7[/highlight]=1.3 instead of (5+2)/[highlight]5[/highlight]=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/r^k-1 is -1*r^1-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/r^k-1).

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

 

[automotivebreath]

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.

 

[globi5]

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.

 

[kitabel]

More thoughts:

http://victorylibrary.com/brit/compression.htm

 

[automotivebreath]

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

 

[Metalguy]

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

 

[Feric]

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

 

[drwebb]

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)"

 

[ahb77]

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.