Compression Ratio and Thermal Efficiency

[MolaKule]

What compression ratio gives the highest thermal efficiency in an Internal Combustion Engine?

Comment with respect to both SI and CI engines.

[ October 30, 2003, 12:47 AM: Message edited by: rugerman1 ]

 

[Jim]

The higher the better. But in practice the shape of the combustion chamber becomes the limiting factor.

 

[novadude]

From what I remember about Otto cycle curves, I'll agree with Jim.

 

[MolaKule]

Okay, consider a very simple rectangular combustion chamber.

 

[dickwells]

As an uninformed but inquisive collector of information I must ask: wouldn't it depend on the pre-ignition characteristics of the fuel?

 

[userfriendly]

I'm gonna' have some fun with this thread later on.

 

[MolaKule]

We're concentrating on, "What compression ratio gives the highest thermal efficiency in an Internal Combustion Engine?"

The greater the Thermal Efficiency of any engine, the greater the output power per unit of fuel.

I.E, given a graph of Thermal Efficiency (in %) on the vertical scale [Work Output/ Heat energy input] vs. Compression Ratio (1:1 to 20:1) [horizontal scale]:

1. what Compression Ratio gives the optimum Thermal Efficiency of an engine?
2. then we can discuss fuels vs. SI and DI engines.

[ October 29, 2003, 05:06 PM: Message edited by: MolaKule ]

 

[userfriendly]

Then throw in volumetric efficiency, boost pressures, mechanical efficiency and peak combustion pressure at X rod angle ... and the whole discussion becomes quite interesting or meaningless.
Take your pick.

 

[sbc350gearhead]

I thought that thermal efficiency varied, depending on CR, combustion chamber material and design, ignition timing, plug location and intake air temperature?

[ October 29, 2003, 05:37 PM: Message edited by: sbc350gearhead ]

 

[userfriendly]

SBC350;
That and about 20 other variables.
If there are not going to be any relivant or applicable examples we can only offer educated guesses and informed opinions instead of scientific reasoning to a real condition.
Without that we might as well talk about the viscosity of golf balls.
I wonder what the date on this SAE paper is going to turn out to be?
I'll guess 1940ish.

 

[MolaKule]

The optimum compression ratio is 16.25 for a maximum thermal efficiency of approx. 40%.

Ratios of 12 to 18 are typical for direct injection diesel engines. For Indirect-Iinjection Diesel's the compression ratios are higher than optimum (>16) to assist in cold starting.

Gasoline engines cannot run at the optimum ratio because of knock, and usually run between 8.5 and 11. Their thermal efficiencies are approx. 34%.

Data obtained from Ferguson and Kirkpatrick, "Internal Combustion Engines: Applied Thermosciences," 2001, John Wiley and Sons.

 

[sbc350gearhead]

I am not an educated man, but how can someone give an optimum "static" CR for thermal efficiency? Wouldn't the calculation have to give a "dynamic" CR?

Or is the given spec, a dynamic CR?

[ October 30, 2003, 10:27 PM: Message edited by: sbc350gearhead ]

 

[MolaKule]

SBC,

Don't sell yourself short since your question is an educated one.

The thermal efficiences vs CR is a nominal value.

The authors' say, "...increasing the compression ratio decreases the both the indicated and the brake specific fuel consumption... The compression ratios depicted and their underlying causes, however, are typical to all engines, compression or spark ignited, two or four stroke...The indicated specific fuel consumption improves faster with increasing compression ratio, because the both friction and and heat losses are increasing with compression ratio. In fact there is an optimum compression ratio due to these effects [as shown in the graph]..."

 

[oilyriser]

On a previous car, I had to slightly retard
timing to get it to pass an emission test, and
I felt the engine had less torque. Since it was
probably still burning the same amount of fuel,
I supposed the thermal eficiency had been reduced.

If my current car, which uses a knock sensor,
pings slightly on 87 octane gas, should it get
better gas mileage on gasoline of a higher octane
rating, assuming the energy content of that gas is
the same? I'm hoping that the computer will
advance the timing, which might give a higher
thermal efficiency.

 

[sbc350gearhead]

As compression ratio and thermal efficiency increases, so do Nitrogen Oxide emissions.

 

[userfriendly]

Thetansedan;
The link you supplied....
At least the authors could have givin credit to the article they stole from Eng-Tips forum.
Posted about a year ago under KJB2002.

 

[TheTanSedan]

I've read or remembered from somewhere that 10.25:1 for a V8 was optimal, given usual combustion chamber shapes, carburetion, etc (albeit, on 1969 fuel, etc).

Beyond this point the benefits decreased dramatically.

Dynamic compression ratio is ****ed interesting

http://cochise.uia.net/pkelley2/DynamicCR.html

The question of 87-89 octane is still open, on this site at least. My friends who also run iron-head B/RB Chryslers V8's of 383 to 440 cid, 29" tires on 2.76 to 3.23 gears in cars weighing between 4200 and 5800-lbs have not yet come up with a solution for vac-sec carbs and electronic ignition. (While there are those willing to run 93-octane, it is not available in all places; nor is it likely to remain. Most engine rebuilding -- at least for us -- concentrates on 89-90 octane as the max that will be available).

How about it, userfriendly?

 

[userfriendly]

You know, the common assumption with BB Mopars is that you have to run a tight lobe seperation camshaft.
If you look at the valve timing comparison between a tight lobe sep cam and a wide lobe sep cam of equal duration, you will see that:
1. The overlap is much greater with the tight lobe sep cam. (the overlap "area" is what counts)
Valve overlap causes rough idle and poor low rpm performance.
Vacuume secondary carbs and power brakes do not like overlap.
And vacuume operated windshield wipers I guess.
The other event that changes is the intake valve closing point and "dynamic" compresion ratio as described in the above link.

I think the reason why BB Mopar guys go for the tight sep cams is to prop-up a small intake port with an early intake valve opening point.
Opening the intake valve early slows the intake port velocity at low and medium RPMs, making the signal to the carb weak.
Match that to a late exhaust valve closing point and you end up with overlap, poor low rpm intake port velocity, rough idle and poor carburation.
Now the intake closing point, give or take a little valve lash, and resulting dynamic compression ratio, I think is overplayed by magizine writers as the end all and be-all to the entire camshaft specs.
I suppose if that is all they know, that is all they can write about.
Achieving high intake port velocity with valve timing goes something like this...
Delay the intake valve opening point until the piston has moved down the bore a little causing a low pressure area in the cylinder.
The higher the compression is, the lower the cylinder pressure will be before the intake valve is opened.
Now slap open the intake (large) valve with an agressive lobe and a high ratio rocker.
Nascar FT lobes are good lifter and valve slappers especially with 2:1 rocker ratios attached to the formula.
The "late start" intake will cause a very high port velocity that will continue to feed the cylinder after BDC "ramming" the cylinder beyond 100% VE.
That is called the "late start better finnish" ram theory.
The above is the Reader's Digest 1.1 version.

[ December 05, 2003, 01:07 AM: Message edited by: userfriendly ]

 

[TheTanSedan]

I appreciate your having addressed it.

I've learned what I can over the years -- and while there seems little secret (so-to-speak) about what will run well in lower-geared, far lighter muscle car -- few seem willing to take time to consider what would be optimum given todays fuel (cat-**** ) in a high-geared, very heavy car where the 46-degrees overlap of the old TNT 440 cam (approx 205/221 at .050; .450/.458 lift; I/E) seems to be the limit for the stock convertor/effective rear gearing/weight combination, reasonably-speaking; in cams able to operate efficently over a wide band with low torque and horsepower peaks . . all the while trying to avoid detonation AND trying to run a (again, relatively) fast ignition advance curve.

Dynamic compression ratio appears to be an effective tool, especially as throttle response and fuel useage (engine life) is mighty important in "replicating" the fun of old-fashioned big American cars. Having to drop true compression, delay ignition timing, put up with weak carb signals and all, well, the search for an "optimum" CR and cam profile continues.

The only other advice I have seen that makes "sense" was in having a cam grinder esentially replicate the TNT cam with a few more degrees of lobe separation. I'll look into the later I-opening event, and thank you again for the shot at the problem.

Folks used to think that the old barges were gas-guzzling, unroadworthy, etc. Too bad they never then got to drive a well-tuned one, or, today, get to ride in one with all the "modern" advances:
gas-pressurized shock absorbers, radial tires, electronic ignition, anti-roll bars, etc.

A friend with a beaut of a 1967 Imperial coupe (5,250# with driver and fuel on 29" tires with 2.94 Sure-Grip), 42k oem 440-4V and dual exhaust regularly gets 16-18 mpg at 75 mph. Also regularly p.o.'s the diesel pickem-up trucks who try "bad" passing manuevers on I-45 out of Houston: they haven't got the cojones or the breathing. (At least, he hasn't run into one yet that has been modded enough to make a difference; but, hey, a strokin' highway-only cammed RB would take care of that.)

It's a helluva lot better ride than any SUV will ever come up with. Not to mention handling, acceleration and braking.

CR is the key to making the rest of the combination work well. Nice as that Imperial is, what it will do with some VP Racing Fuel is even more impressive; just tweaking the fuel/timing curves for weekend fun shows a little of "what once was" (jet fuel!!!).

 

[userfriendly]

Call me wierd, but I would look for a cam around 220/228 @115 sep for 361s, 383s, and 400s,
-3/43 51/-3, and 228/234 @ 117 0/48 57/-3 for 413s, 440s, and strokers.
A 1.6 rocker to get the lift up to .500" would not hurt a thing.
Those examples should handle 10:1 cr without backing down the timing.
I'd try a ignition curve with about 6* initial and another 28* all in no sooner than 4000 rpm.
Vacuume advance hooked up to the intake manifold, another 14* starting at 10" and max in at 17".
IMO