Compression Ratio explained by Scott The Viking

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Compression Ratio explained by Scott The Viking

Post by david58 » Wed Jan 24, 2007 7:45 pm

Compression Ratio explained by Scott The Viking
scott the viking wrote:Ya know....I really do like answering technical questions....But I feel kind of guilty for giving halfassed answers. The problem is...to give you a "wholeassed" answer would be a novel of technical jargon. In example...The compression ratio. Now...on a stock engine with a stock head (1600), I think the ratio falls somewhere around 7.8 0r 7.5 to 1. But heads are not the only thing that controls that..... Here are the things that have affect on "static compression ratio".
Bore-- The diameter of the cylinder bore
Stroke-- The distance the piston travels in the cylinder
Deck Height-- The distance between the top of the cylinder bore and the top of the piston, when the piston is at TDC (Top Dead Center, or at its highest position
Compressed thickness of the head gasket-- Usually 0.040", but varies with gasket manufacturer and application
Piston top volume-- If the piston is dished, domed, or dimpled, that has an affect on compression ratio.
Combustion chamber volume-- How much open space is in the head above the cylinder .
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So...a real quick definition of static compression ratio (the one everyone talks about) is this....Lets say your cylinder with the piston at the bottom of the stroke has 10 units of volume...This is called swept volume. And when the piston is at the top of it's stroke TDC or top dead center, the swept volume is 1...what this give you in a static compression ratio is 10 to 1. So...the piston smashed ten units of air/fuel into 1 unit.
NOW...there is another type of compression...it's called dynamic compression. Dynamic compression has the same principals as static compression BUT...dynamic compression is NOT measured from the time that your piston is at the bottom of the stroke. It rather...is measured from the time that your intake valve closes and your engine actually starts to build compression. Okay...this can get REALLY advanced, but I will keep it basic (as I can anyway). You all have an idea of how a four cycles engine works I am sure...if you had no idea...then you stopped reading this about three lines in. On the intake stroke, the camshaft opens the intake valve the piston travels down in the cylinder bore, which greats a vacuum, which pulls in the fresh air/fuel mixture....I am leaving out SOOO many details if the intake stroke, like exhaust scavenging, overlap and such...but lets just say that the piston is going down the cylinder and it is sucking air/fuel in behind it because the intake valve is open. The piston goes all the way to the bottom of the stroke with the valve open, but the valve does not shut right when the piston reaches the bottom...the intake valve stays open as the piston is starting back up the cylinder. When (and only when) the intake valve closes is where you start to measure your dynamic compression ratio. Therefore, dynamic compression ratio is always lower that static. If you have a 10 to 1 engine (static) and you install a cam with a lot of duration (again, over simplified) your dynamic ratio can drop down to 7 to 1 or much lower. If your dynamic compression ratio DCR, falls below 7.5, then it is time to rethink your cam choice or up your static compression ratio to compensate. See...when I guy spouts off that he is running a 11 to 1 compression ratio, and he just runs super..well his camshaft size may have dropped his running compression ratio down to like 9.5 to 1 or so. Now if that same guy says he has a dynamic compression ratio of 11 to 1...tha's a different animal all together. So as you can see...there is A LOT to just the compression ratio of an engine...and this did not even begin to scratch the surface of it. So...for your question of changing the compression ratio of your air-cooled motor...sure...you can...just the way I explained above. But...higher compression also builds more heat, more heat can be a cause of detonation or hot spots. So you will have to compensate for this added heat to your engine...better oil cooler, more attention to your cooling tin, run better fuel. Running an air-cooled engine through the deep sand with paddles...all day in the heat...is hard on it with a low compression ratio...you up that ratio and you will have A LOT of other issues to address...sure...it CAN be done (I do it) but at what price? The answer is a lot.
This would be good info to have in a sticky on this forum.
Hot, humid air is less dense than cooler, drier air. This can allow a golf ball to fly through the air with greater ease, as there won't be as much resistance on the ball.

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Post by sparkmaster1 » Fri Jan 26, 2007 10:31 am

Done!!
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Post by wildthings » Thu Feb 22, 2007 12:43 am

What dynamics cause the heads to run hotter with higher compression ratios? Since the thermal efficiency of the engine increases with C/R the exhaust temperatures should be less for any given power output, and this should lower heat transfer to the heads.

I can understand the head temps going up if you are getting preignition, but what would make them go up if you are not? If your engine is putting out say 100 HP at a given RPM, it seems that nearly the same amount of heat would be released directly to the heads regardless of C/R.

I am sure that this question has been answered before, but I had no luck searching for it.

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Post by Bugfuel » Thu Feb 22, 2007 10:54 pm

one suggestion:
more power = more heat, and the head cooling fin design was only designed to dissipate 30-50hp worth of heat to begin with, plus a safety margin. A high compression engine is most likely producing much more power than the cooling system can get rid of. It's not necessarily a matter of high CR either: if you produced the same power with a lower CR (bigger cylinders, longer stroke etc..) the head would still have to dissipate the same amount of heat, and it can't.


A higher CR will get more power out of the same amount of fuel, even if you do not get detonation. That is why we want as high pressure as we can get away with. Again, it is more power = more heat. You cannot avoid laws of physics :)
A car engfine is a massive waste of energy in the way it primarily turns fuel into heat, and only less than 50% goes into pushing the pistons and making the car move.

And going back to my favorite subject..... high CR means nothing. It is a value that has no meaning alone. What SHOULD be compared, is absolute cylinder pressure.

A 10:1 CR will produce LESS than stock cylinder pressure in an engine that has a big cam that bleeds off actual pressure before the valves close.

Jan

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Post by wildthings » Thu Feb 22, 2007 11:53 pm

I agree that dynamic compression ratio is what counts, but do not agree or at least not understand why cylinder head temps go up with compression. More power does not always mean more heat. For a given amount of fuel you get more power with higher compression than lower compression, this is because your engine is more efficient at turning heat into power.

Theoretically the higher the compression the more heat (energy) is used to push down on piston, and the less heat is wasted to the cylinder surfaces and to the exhaust. The energy it takes to move a 4000# bus down the road at 60 mph does not increase just because you change the C/R, so if you were getting 20mpg with a 7.5 C/R and you start getting 25 mpg with a 9.0 C/R it seems to me your entire engine should be running cooler, yet everyone says that your head temps will go up.

If I don't change the amount of air or fuel going into the engine, nor do I change my driving style, why should my head temps go up as every one seems to claim. To me it would be much easier to theoretically explain why they would go down.

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Post by Bugfuel » Fri Feb 23, 2007 7:43 am

wildthings wrote:I agree that dynamic compression ratio is what counts, but do not agree or at least not understand why cylinder head temps go up with compression. More power does not always mean more heat. For a given amount of fuel you get more power with higher compression than lower compression, this is because your engine is more efficient at turning heat into power.

Theoretically the higher the compression the more heat (energy) is used to push down on piston, and the less heat is wasted to the cylinder surfaces and to the exhaust.
I can't claim to know this as fact, but it seems to me that with higher dynamic compression, you do get more power from the same amount of fuel, and you do get more piston-pushing power, but you cannot cvreate energy out of nothing. You generate more heat energy PERIOD, some of it shows as an increase in power, and some of it shows in increase of head temperature. It has to go somewhere, and not all of it can be an improvement in power alone. Efficiency may improve, i.e. smaller percentage of heat energy is wasted on heating the head, but it is still an increase in head temps, just not linear with power output increase.

Let me try to explain with an example:
If at first your engine made 50hp and you wasted say 50% of it into heating the head, you put 25hp in heating the head.
Then you increase compression ratio, and your efficiency improves, let's say you now only waste 40%. But as a result, you make more power, let's say 70hp. 40% of 70 is 28hp. You make more absolute waste heat because you make more absolute power.



That's just one way of looking at it. I also have a gut feeling that you simply run a lot hotter because you squeeze more power out of the fuel.
I may not be any more efficient thermal distribution of energy, just more efficient use of a given volume of fuel. You get more power out of the same amount of fuel, and it naturally increases waste heat just as it increases power. Keep in mind that compressing the mixture more generates more heat even without igniting it. I don't know how big of a factor mechanical compression is in heat generation.

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Post by wildthings » Sat Feb 24, 2007 11:20 am

As I said previously I could explain easier why head temperatures would go down than go up with compression ratio. The only place where I can see that the temperatures go up is in the initial compression, and this is very minor say 30°F or so. The temperature of the burn itself is not dependent on the C/R as the specific heat of the fuel/combustion mixture does not change with compression ratio. If you put the same amount of fuel and air into the cylinder at the same air/fuel ratio you will get the same increase in mixture temperature no matter what the compression ratio, that is assuming you don't get knock. So at the completion of the burn the temperature is only the initial 30°F higher for the high ratio engine than the low ratio one, a pretty insignificant amount.

From this point on the high C/R ratio engine should cool more rapidly than the low ratio one since the mixture is expanding more rapidly and goes further into turning the heat and pressure into usable power. By the time the exhaust valve opens this should be in the order of several hundred degrees cooler, meaning the average cylinder temperatures and the exhaust temperatures are both sifnificantly lower for a high C/R engine.

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Post by david58 » Sat Feb 24, 2007 2:09 pm

I typed in "compression ratio versus heat" in a google search here is what it came up with.
http://kb-silvolite.com/article.php?action=read&A_id=64
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KB has always liked the idea of cooling the underside of the piston. Anything to keep the top of the piston below 600 degrees reduces the chance of detonation. Oil jet cooling reduces the temperature of the whole piston and was originally developed for lubricating wrist pins. The tool sold by BLP is something every engine builder should have in their tool box. BLP can be reached for more information and applications. Phone 1-800-624-1358. The image above shows the jet installed in the crankcase and the lower image is the basic kit.

Image
This looks like a good idea to me. Has anyone every used this method on a flat four?

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Post by wildthings » Sat Feb 24, 2007 2:28 pm

This looks like a good idea to me. Has anyone every used this method on a flat four?
I think that Ray Greenwood has done something like this. You might search his post.

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Post by scott the viking » Sat Feb 24, 2007 8:39 pm

It was brought to my attention that there are some questions...or at least a question...It can be answered pretty simply.
Compression builds heat because the compressing of air (or gas) creates heat. Even it you take the fuel explosion out of the picture, the spark plug out of the picture...hell just take the engine out of the picture and lets talk about a turbo charger.
A turbo charger compresses air. the compression of the air builds heat, the more it compresses the air, the more heat it builds. Thus the reason for an inter-cooler.
We can also look at a diesel engine for a good example. The diesel engine does not need a spark plug to ignite it's fuel. It ignites it's fuel by compressing the air to a point where it gets so hot that it will burn the fuel that is sprayed into it. Some of them get to 1600 degrees fahrenheit. This is also based on something called the "combined gas law"...I won't bore you to death with it...but you can go and look it up if you like reading physics formulas.
So in short...the engine is an air pump...compressing air makes air hot, the more you compress it, the hotter it gets. More compressing means hotter air, hotter air means more temperature in an air pump.

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Post by MNAirHead » Sat Feb 24, 2007 8:48 pm

Dumb question.. with this much pressure why doesn't it reconstitue to a liquid

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Post by scott the viking » Sat Feb 24, 2007 9:17 pm

MNAirHead wrote:Dumb question.. with this much pressure why doesn't it reconstitue to a liquid
You mean liquid oxygen?
I am not totally up on what all is in to making liquid oxygen...but I think it has to be cooled to like 300 (and some) degrees below zero...then compressed to make it into liquid.

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Post by MNAirHead » Sat Feb 24, 2007 9:19 pm

Sorry..

Meaning .. why dont the small droplets recompress into larger droplets?

Sorry to be an idiot.. I'm qualified more to chart the Gross Domestic Product vs engines.

Sincere thanks.

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Post by wildthings » Sat Feb 24, 2007 11:56 pm

Compression builds heat because the compressing of air (or gas) creates heat
Yes compression builds heat, as I said this accounts for about a 30F temperature rise between a low and high compression engine, we aren't talking about a diesel with a 21:1 C/R we are only talking about a small change from 7.5 to 9 or so, and the effective increase is likely even less.

Remember that expansion also occurs, this is what is happening on the power stroke, and expansion reduces heat, but since you are starting at a much higher pressure and temperature the cooling from the expansion is way more than the little bit of extra heat that gets added by the compression, probably by a factor of 10 or so. This is why high compression make your engine run more efficiently, you get way more energy out of the mixture as it expands and cools than you put into it by compressing it.

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Post by scott the viking » Sun Feb 25, 2007 12:19 am

I am absolutely willing to listen to this theory of high compression running cooler. But I would like to see some numbers, real numbers showing this. They don't have to be your own testing...just good test on the same engine, ran under similar circumstances on high compression and then on lower compression...and I am not be in the least bit sarcastic...I would find that fascinating...I mean there is no doubt that higher compression is much more efficient. I would like to see a conclusive test to show it will build less heat due to that efficiency.

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