Scissors

[Duke et al] Any insight into maximum compression ratios for various gasoline octanes without causing detonation?

I'm referring to N/A, of course, as the C6 is not F/I.

I noticed that the 575M is 11:1, the C6, C5 Z06, and C5 are 10.9:1, 10.5:1, and 10.1:1 respectively, and that the BMW M3 is 11.5:1. Yet they all recommend something like 93 Octane. :confused:

need-for-speed

As cylinder head design improves, they are able to increase CR, so it varies. It's a moving target. Tighter quench, improved swirl, improved atomization all help to allow higher static compression ratio w/o increasing octane requirements. Flame front travel affects it too. That is one benefit of the 4 valve per cylinder heads w/ a dome shape and the spark plug in the middle (but don't tell the DOHC guys I said that ;) . I believe a better predictor is dynamic CR which takes into account cam timing - valve overlap which can bleed off some of the cylinder pressure allowing/requiring highr static CR. But manufacturers typically don't advertise that. :chevy

SWCDuke

Achieving higher compression ratios without detonation has been a subject of intense research since the dawn of the IC engine, and it continues today. This is because the thermal efficiecy of an engine is a function of CR, and it's the greater thermal efficiency that increases power of a high compression engine, all other things being equal.

Detail design features always enter into the equation in the real world, but the higher the CR, the better the fuel efficiency. There are a lot of variables that effect detonation beyound CR such as bore size, plug location, shape of the combustion chamber, heat transfer coefficient to and from the surrounding surfaces, inlet air temperature, valve timing, and others.

Ideally, a combustion chamber would be a small sphere with the plug located at the center. This minimizes the distance from the ignition point to the edge of the chamber for a given volume and would provide the least total combustion time and the most resistance to detonation.

Nowadays, manufacturers can push CRs high because of knock sensors. High CRs maximizes fuel economy at part throttle and maximizes output if sufficient octane level is used to allow maximum spark advance for peak power.

Very high CRs can compromise combustion efficiency because of poor chamber geometry. An example is F1 engines. Because of their extremely oversquare bore/stroke ratio of 2:1 or better, the combustion volume is a thin disk with high surface area to volume ratio that transfers too much heat out of the chamber, but at 18000 revs the combustion time is only one-third the time at 6000, so they are reasonably efficient at very high revs only.

There is really no reason why all modern car engines could not be run at about 11:1, and let the customer decide how much power he wants by selecting fuel octane.

This is the case with C6. GM says that 93 PON fuel is recommended, but NOT required. On a long freeway cruise 87 octane would deliver about the same fuel economy, but the engine would not be able to develop maximum potential torque at all engine spreeds due to spark retard to keep the engine out of detonation.

Duke

LittleBoyBlu99

:cheers: :flag :flag

Mitch Alsup

Adding a few items to SWDukes posting:

"An example is F1 engines. Because of their extremely oversquare bore/stroke ratio of 2:1 or better, the combustion volume is a thin disk with high surface area to volume ratio that transfers too much heat out of the chamber, but at 18000 revs the combustion time is only one-third the time at 6000, so they are reasonably efficient at very high revs only."

F1 engines are getting closer to 2.5:1 and there are rumors of almost 3:1 bore stroke ratios. Imagine 4" pistons moving up and down 1.35" per stroke at 18,000 RPMs. The guys in the race engineering magazines think the cranks and con rods are good for at least 20,000 RPMs; its the valve train that goes south first.

The current F1 engines enter the power band around 11,000 RPMs and sign off near 18,000 RPMs or about 40% of the RPM band, and idle around 3,000 RPMs. For an engine in this state of tune (LS1 would have 1,600 HP at this level of tune) a 40% power band is "not bad". However, no one is ever going to argue that these engines were in any way designed for road car operation at legal speeds. I think your point was/is that 11,000 IS high revs, however, the engine is just getting on the cam at this point. My point, it the the point where one considere "high revs" changes based on the engine architecture; so high revs on an LS1 begins at 7,000 and on a F1 engine at 15,000. Fair?

Note, I agree with the big surface are hurts power output due to thermo issues statement. However, given the displacement limit, RPM and volumetric efficiency are the only development variables available in F1.

"Ideally, a combustion chamber would be a small sphere with the plug located at the center. This minimizes the distance from the ignition point to the edge of the chamber for a given volume and would provide the least total combustion time and the most resistance to detonation."

You mean (gasp) like all the 4 valve heads today?

Once one gets beyond the 6,000 RPM band, detonation basically doesn't have time to happen. Gasoline has a time constant from the time it has acquired enough energy to spontaneously burst into flame until it actually does burst into flame on the order of 3 milliseconds. At 6,000 RPMs the entire compression stroke takes only 5 miliseconds, and the mixture does not contain enough energy to spontaneously ignite until it gets compressed to 30 or so degrees BTDC, between this point and any actual spontaneous ignition, the remainder of the compression stroke and some of the power stroke would have transpired.

Guess what you need to get significantly beyond the 6,000 RPM range? (Gasp) 4 valve heads!

"There is really no reason why all modern car engines could not be run at about 11:1"

Small motorcycle engines are in the 12.5:1 on 91 octane today, larger motorcycle engines are in the high 11.x:1 range also on 91 octane. Smaller engines can achieve higher compression ratios because the flamefront has less distance to travel and therefore there is less chance of detonation between the time of ignition and the time the flame front reaches the sides of the cylinder walls.

Ferraris, Honda, Porsches all run about 11:1 CRs in their most advanced engines, and have since the mid 1990s. It seems to me, just by watching the car magazine data over the last 15 years, that the smaller displacement engines (like 3 litre V8s or 3 litre flat 6s) can run 0.5 to 1.0 more compression than the larger displacement engines (5.7 to 7.0 litre engines). Really tiny cylinders (e.g. motorcycles) can run even higher compression.

The Ferrari, honda, and Porsche engines use ceramic coatings on the combustion chamber and piston top to minimize the unproductive thermal heat transfer through these surfaces. I believe that these coatings have the ability to allow anothe 0.5 compression (all by themselves).

Scissors

:skep: There are no production engines with spherical chambers and a spark in the exact center.

DngrZne

Isn't a "Hemi" pretty much this design?

WhiteDiamond

I believe the Hemi is very close to a perfect sphere, but Dodge ran into some problems with oxides of nitrogen in getting the new Hemi certified. Maybe Duke can expand on this, but it has something to do with slow flame front and cooler combustion, I think. On a side note, I also believe the slower burn is good for torque generation.

Todd

SWCDuke

I dont' buy the "40 percent power band" on F1 engines. No way!!! If that were the case they wouldn't have seven speed gearboxes with 10 percent gear spacing. Gear spacing is established to keep the engine between the torque peak and rev limit which is the usual power curve "sweet spot". On vintage SHP small blocks gear spacing was 25-30 percent, but the modern smallblock can handle 40 percent.

It's tough to get reliable data on F1 engines because nobody wants to talk. Magazine writers may be well intentioned, but most lack the background to make good judgments about the detail design features of IC engines.

Combustion start lag time from the time the spark is initiated and total combustion time is not the limiting factor. It's the time it takes to fill the cylinder with fresh charge, which is a function of the speed of sound and inlet runner length. Even with the shortest runners possible, F1 engines are running out of breath at over 5500 FPM mean piston speed. It's the time it takes for rarifaction waves to travel from the cylinder to the inlet runner opening, so the only alternative is to shorten the stroke and hope there is not so much heat loss that the engine will make more power at 20,000 revs.

Unrestricted NASCAR engines are highly developed pushrod engines that develop over 750 HP at around 8500-9000 revs. With a 3.5" stroke, the mean piston speed is 4958-5250 FPM, so they are operating hear the same mean piston speed range as F1 engines - not bad for pushrod technology!!!

Pure hemispherical combustion chamber designs are not very detonation resistant. Most "hemis" are really semi-hemipherical with some quench
area(s) to reduce flame propagation distance and induce more turbulence. All the variables I mentioned in a previous post, and more, come into play, so it comes down to juggling them all to come up with an optimum design.

The 4V pentroof chamber is thermally efficient, detonation resistant, and allows generous valve area with a good port layout. It's a no-brainer, but GM keeps tweaking the pushrod wedge design and gets almost as much out of it as a good 4V design. The "wedge chamber" is now the "compact wedge" with the plug pushed closer to the center, and GM has been able to figure out how to make the ports and valves flow almost as well as a 4V even though they still have to neck down for the pushrod passage, and it's all stuffed into a lighter, smaller, and lower cost package than a 4V design.

I like 4V engines. That's why I bought a Cosworth Vega 25 years ago and still own it. The trouble is, no one has really improved on the basic narrow angle 4V design that Keith Duckworth laid out nearly fortyyears ago. Meanwhile GM keeps reinventing the pushrod engine (three times now by my count), and I have no doubt that the 3V design of the upcoming LS7 will put the specific output on a par with similar displacement, naturally aspirated, 32V V-8s while retaining the pushrod engines low mass and package volume, low cost, and excellent fuel efficiency.

Way to go, guys! :flag

Regarding burn rate, most homogeneous mixtures of hydrocarbon fuels with air have about the same flame propagation speed in an initially quiescent mixture, however, in an IC engine, flame propagation speed is enhanced by turbulence. This is why quench is good because it creates tremendous turbulence near TDC, which speeds combution improving thermal efficiency and resisting detonation. A pure hemi design has no quench, so average flame propagation speed is low. This enhances detonation since the last of the unburned charge is at high temperature and pressure for a longer period of time. "Slow" combustion also enhances heat transfer to the cooling system, which means lower thermal efficiency.

But quench tends to increase hydrocarbon generation in the native exhaust, so modern designs tend to have less quench area. Oxides of nitrogen are created in the flame front's high temperature - about 4500F, but the disassociation rate is much slower, so as the burned gases expand and cool the NOx level tends to be frozen at their flame front temperature equilibrium concentration. The only effective way to control them is to reduce peak flame front temperature, and this is what EGR does. Nowadays external EGR valves are being replaced with cam phasing. By retarding the exhaust cam at cruise speed, the exhaust gas residual in the cylinder is increased. (and the expanding combustion gas can work on the piston for a few more crank degrees). NOx is only an issue at cruise speed - not with rich mixtures at WOT or at idle.

This is one reason I still think there's a reasonable chance that the LS7 will have two inblock cams. Phasing the exhaust controls NOx and phasing the inlet broadens the torque curve. Independent cam phasing is certainly a good argument for the DOHC layout, but GM has one-upped that architecture with the "twin-cam" small block. There's no doubt that it exists. It's just a matter of when and on what engine it will enter production.

Duke
'

Scissors

No, the Hemi is hemispherical, as the name suggests. You won't get a sphere unless both the part of the chamber in the head and the piston are both hemispherical. I've never seen a hemispherical piston. Not saying they don't exists, but I know of no production car that has one.

WhiteDiamond

Duke,

Is there a particular book that you believe explains the combustion process as it relates to modern gasoline engines in very good detail? I would enjoy reading such a book to try and learn some of this stuff you so generously provide. Thank you.

Todd

Mitch Alsup

"I dont' buy the "40 percent power band" on F1 engines. No way!!! "

Then go watch the tape of the 2002 F1 race at Indianapolis. There is a cut of Ruben's car for several laps in a row where he takes T6. Rubens enters the turn at 11,000 RPMs and runs all the way up to 18,000 without shifting. If the powerband was not this wide he would be forced to shift mid turn, and more than likely, uspet the chassis in the critical mid turn area

"With a 3.5" stroke, the mean piston speed is 4958-5250 FPM, so they are operating hear the same mean piston speed range as F1 engines - not bad for pushrod technology!!!"

Except that the stroke and piston speed has nothing to do with pushrods.

"GM keeps tweaking the pushrod wedge design and gets almost as much out of it as a good 4V design."

Once again you are confused, unless you are taking the position that 405 HP (LS6) is "almost as much" as 514 HP, or that 380 lb-ft is "almost as good" as 435 lb-ft.

"it's all stuffed into a lighter, smaller"

The LS1 is neither smaller not lighter than the F355 or F360 engines.

Scissors

Incorrect. The engines in the F355 and F360 weigh ~370 lbs. whereas the LS1/6 weigh ~345 lbs.

Wadoka

What about the effects of special items like thermal barrier coatings on pistons? I remember a "Corvette Fever" article on a carburated engine build they did, where they had the piston crowns coated to improve their combustion for emissions testing purposes.

SWCDuke

About the only thing I could recommend are textbooks. The "bible" in my opinion is the late Charles Fayette Taylor's "Internal Combustion Engines, Theory and Practice". It's two volumes. Vol. I is heavy on math and thermodyanmics and may not be for the average reader, but has a good chapter on simple mathematical combustion models.

Vol. II has a lot of practical information on detail design.

Another textbook - a bit newer than the most recent revisions of Taylor is Internal Combustion Fundamentals by John Heywood (MIT).

These volumes are available from SAE and can also be ordered through any college bookstore.

Coatings can reduce heat transfer out of the combustion chamber, but I think they have a tough time passing OE durability tests.

Duke


[Modified by SWCDuke, 5:47 PM 1/28/2004]

TTRotary

This is incorrect, as Scissors points out. Furthermore, you pick a megabuck engine that makes liberal use of exotic metals including TI con rods, ultra-thin castings, dry-sump oiling etc. In short, it's a race engine ina $150K car.

As a more reasonable (but still expensive) example, lets use the BMW 4.4L V-8 that makes 395HP in M-Power trim. 455lb!!!

Furthermore, the LS-6 packages in 2/3 the volume of the DOHC engines...even the diminutive-displacement 3.6L Ferrari V8. In fact, I have seen the fully-dressed LS-6 and 1.3L Mazda twin-turbo rotary engine on stands next to each other in a race shop. The LS-6 was smaller. And it weighs only 5lb more than the mighty-mouse rotary.

Truly, the Gen III engines are engineering marvels which prove that pushrods deliver a superior solution. They are lighter, smaller, and more powerful than most exotic engines, and they do all that while delivering 30mpg.

Now THAT's real hi-tech.

And it pretty much proves how irrelevant and antiquated the old "hp per liter" bragging rights really are.


[Modified by TTRotary, 9:51 PM 1/28/2004]

Scissors

Excellent points, TT.

If people and magazines were logical they'd ignore peak HP and hp/liter numbers and instead wonder about:
Torque area:engine weight
Torque area:MPG
Torque area:engine total volume

need-for-speed

and don't forget the all important: unit of area under the curve for torque and HP per $$ :yesnod:

Scissors

Oops, forgot that one. :cheers:

need-for-speed

:cheers: