Any engine builders out there? Compression ratio question

[Matt Harwood]

Hi guys,

I'm having a discussion with a friend about compression ratios vs. cylinder pressures and perhaps we're thinking too much and confusing ourselves. It's been my experience that boring and stroking a motor has the net effect of lowering the compression ratio because the volume of the combustion chamber is increased by the wider bore, which offsets the added volume of the enlarged cylinder. Of course it varies from engine to engine, but on every bored motor I've ever built, I had to deck the heads to bring compression back up to desired levels.

But my friend counters by saying that it's merely the quench area that is affected by boring, not combustion chamber volume. Since the compression ratio is the ratio between the swept area and the combustion chamber size, compression should go up as swept area goes up, correct?

Then he asks, "Do dished or crowned pistons actually change the compression ratio, or just the combustion chamber "pressure?" I think the ratio actually remains constant, since the volume at bdc is increased or decreased as much as the volume at tdc."

I would argue that since a domed piston takes up a greater proportion of the combustion chamber compared to overall swept volume, that it doesn't cancel out and compression goes up. A dome on a piston may take up 40% of the combustion chamber's volume, but only 5% of the total swept volume of a cylinder, so as a ratio, they don't cancel out and compression goes up.

Can anyone elaborate? I thought I knew what I was talking about, but now I'm all confused...

Thanks.

 

[GearHead_1]

Pistons will change compression ratio! Some of your thoughts would be correct if you were compressing the mixture into a perfect cylinder. Generally however you are pushing the air into a corner so to speak (quench area) of the head. It's like killing a spider in a corner you can't do it with a steel toed boot but a pointed high heel works great. The narrower toe fills the corner more completely. Wow that's a poor analogy but I think you'll get the picture. When the piston's shape begins to mimic the shape of the combustion chamber you have the ability to compress the volume of air even further. This is true even though the dome of the piston takes up a little more space at the bottom of the stroke and there is less volume to displace. Decking your heads acompishes the same thing.

Cylinder pressures though related to compression ratio have a much more complicated combination of events to determine. Valve lift, valve timing, valve overlap and even ignition timing are just a few of the factors to be considered when talking about actual cylinder pressure. Cylinder pressure is a function of actually lighting off the compressed mixture. Compression on the other hand never has to have a spark plug fire to measure.

I know this is probably about as clear as mud.

 

[Major Ramifications]

Gearhead is correct. But one issue he didn't address is that adding to an engine's stroke (you mentioned "stroking" an engine) GREATLY increases the compression ratio, because you are pushing the piston farther into the combustion chamber. In any practical case, the stroke would be increased to the point of neccessitating different pistons with the wrist pin mounted closer to the crown. Or at least shorter rods. Also, boring an engine would yield a slight increase in compression, due the percentage of increase in swept volume vs the percentage of increase in compressed volume.

 

[GearHead_1]

Gearhead is correct. But one issue he didn't address is that adding to an engine's stroke (you mentioned "stroking" an engine) GREATLY increases the compression ratio, because you are pushing the piston farther into the combustion chamber. In any practical case, the stroke would be increased to the point of neccessitating different pistons with the wrist pin mounted closer to the crown. Or at least shorter rods. Also, boring an engine would yield a slight increase in compression, due the percentage of increase in swept volume vs the percentage of increase in compressed volume.

Major Ramifications, no offense intended here. You make some good points. Stroking an engine doesn't always mean that you lengthen the stroke. Sometimes a stroked engine is used to decrease (not sure what fun this is) the C.I.D of an engine. Racing restrictions sometimes limit the number of cubic inches for instance. Most often stroked engines require a custom set of pistons to be made. That said, wrist pin height and dome configuration (or lack of it) help to determine the compression ratio. You seldom (though it can be done depending up application) use the same piston without modification. Maybe seldom isn't the best word here but at least the term "more often not" applies. The piston, rod and crank combination are usually designed as a matched set and any (within reason) compression ratio desired can be the result. Taking this into consideration, stroking an engine doesn't necessarily mean that the compression ratio will be GREATLY increased.

I've owned/built bored and stroked engines that are both naturally asperated as well "SuperCharged" (both turbo-charged and blown). Often times the desired result is to pick up a few cubes as well as supercharge the engine at the same time. The end result of some of these engines is a compression ratio that is quite low relatively speaking with the expectaion that the blower will do it's part to "make up" for the lack of a higher natural compression ratio.

 

[Major Ramifications]

Oh yeah, Gearhead, I forgot all about reducing the stroke to meet class requirements! So, you are so correct, changing a stroke doesn't automatically raise the compression ratio, only increasing the stroke does (assuming nothing else is changed, which is highly impractical).
And don't ever worry about offending me, I am pretty much un-offendable.

 

[bdaz442]

One Quick 2cents. Valve timing , Duration, and Overlap has a dramatic effect on cylinder presure. Long duration Cams lower presure, short duration Cams Raise Presure.

 

[GearHead_1]

One Quick 2cents. Valve timing , Duration, and Overlap has a dramatic effect on cylinder presure. Long duration Cams lower presure, short duration Cams Raise Presure.

Tried to say that in my first post.

 

[kartracer55]

No the compression chamber volume is not being changed, TDC is usualy level with and sometimes slightly higher than the block, its not like your actually increasing the area when the fuel mix is completly compressed, its still going to be the same asuming the head volume is the same. by boring the motor, there is more displacement in the cylinder. so whn the piston is going down on the intake stroke, there is more space to fill, so more air/fuel is comming in, but once the valve closes, there is more fuel to be compressed, adn the CR goes up

 

[GearHead_1]

No offense intended but I'm sorry to say that I have no idea what you are trying to communicate here. The whole point here is that you are pushing a greater volume into the same static space. This has nothing to do with fuel being mixed completely. In one sentence you indicate that by boring a cylinder you create more volume in the cylinder with the chamber being a static determinent and you will pick up compression. In the next you seem to be indicating that by changing the stroke you aren't picking up volume in the cylinder. That's what most of us go after when we stroke an engine is to pick up a few cubes. Are you talking about a reconfiguration of the piston and rods. More volume pushed into the same static combustion chamber equals what? I don't understand the picture you are trying to paint.

No the compression chamber volume is not being changed, TDC is usualy level with and sometimes slightly higher than the block, its not like your actually increasing the area when the fuel mix is completly compressed, its still going to be the same asuming the head volume is the same. by boring the motor, there is more displacement in the cylinder. so whn the piston is going down on the intake stroke, there is more space to fill, so more air/fuel is comming in, but once the valve closes, there is more fuel to be compressed, adn the CR goes up

 

[kartracer55]

No offense intended but I'm sorry to say that I have no idea what you are trying to communicate here. The whole point here is that you are pushing a greater volume into the same static space. This has nothing to do with fuel being mixed completely. In one sentence you indicate that by boring a cylinder you create more volume in the cylinder with the chamber being a static determinent and you will pick up compression. In the next you seem to be indicating that by changing the stroke you aren't picking up volume in the cylinder. That's what most of us go after when we stroke an engine is to pick up a few cubes. Are you talking about a reconfiguration of the piston and rods. More volume pushed into the same static combustion chamber equals what? I don't understand the picture you are trying to paint.

Ok, say your cylinder, and without the head, is 10 cubic inches. and you bore it out to 11 cubic inches, when the piston is going down on the intake stroke, 11 cubic inches of uncompressed fuel mixture is comming into the cylinder. when the valve closes at BDC the piston tavels up, 11 cubic inches of fuel mix is compressed into the same area that 10 cubic inches was originally compressed into. You therefore have a higher compression ratio.

I think this is what was originally asked, I was tired when i read it.

Jim

 

[byrdman]

GREAT THREAD!!!

I'm not the expert here, but I'd like to take a stab at this "from the top". My thoughts in bold.

Hi guys,

I'm having a discussion with a friend about compression ratios vs. cylinder pressures and perhaps we're thinking too much and confusing ourselves. It's been my experience that boring and stroking a motor has the net effect of lowering the compression ratio because the volume of the combustion chamber is increased by the wider bore, which offsets the added volume of the enlarged cylinder. Of course it varies from engine to engine, but on every bored motor I've ever built, I had to deck the heads to bring compression back up to desired levels.

hold that thought.

But my friend counters by saying that it's merely the quench area that is affected by boring, not combustion chamber volume. Since the compression ratio is the ratio between the swept area and the combustion chamber size, compression should go up as swept area goes up, correct?

If you change nothing but increasing the cylinder bore, your combustion chamber size is not increased. Going back to paragraph one above- how do you know your compression dropped when you bored it?

Then he asks, "Do dished or crowned pistons actually change the compression ratio, or just the combustion chamber "pressure?" I think the ratio actually remains constant, since the volume at bdc is increased or decreased as much as the volume at tdc."

The ratio does not remain constant. If you change nothing but the pistons (let's say you go from a flat top to a domed design), both your static compression ratio and your cylinder pressures will increase. The way I've always thought of it (again, I'm not the expert), is that the effective "combustion chamber" area is the area above the piston at TDC. This includes the area in the center of the piston. If it's a domed piston, you subtract the volume of the dome. If it's a dished piston, you add the volume of the dish. Put this total area against the swept volume of the cylinder, and you get your static compression ratio. All other things being equal, a domed piston creates a smaller combustion chamber than a flat top piston.

I would argue that since a domed piston takes up a greater proportion of the combustion chamber compared to overall swept volume, that it doesn't cancel out and compression goes up. A dome on a piston may take up 40% of the combustion chamber's volume, but only 5% of the total swept volume of a cylinder, so as a ratio, they don't cancel out and compression goes up.

That's exactly the way I see it.

Also, if the only change to a motor is stroke (up OR down), CR does not change. The difference in stroke does not change the level of the piston at TDC, so no effect on combustion chamber size, and therefore no effect on CR.

Cylinder pressure is a whole different discussion.

Can anyone elaborate? I thought I knew what I was talking about, but now I'm all confused...

Again, everyone please take what I've said with a grain of salt, as I have very little experience building motors. I have, however, been reading for a long time whilst saving money to build my garage!

Thanks.

 

[Geo]

Oh boy. A lot of stuff here. Most of it close, but no cigar.

Compression Ratio is the total volume of the cylinder, including combustion chamber volume, gasket volume (function of gasket diameter and thickness), volume from the top edge of the piston to the first ring, +/- dish/dome volume, etc., at BDC vs the total volume at TDC. Correctly calculated it's not simply the swept volume over the combustion chamber volume.

Stroke will indeed influence CR because it changes total volume at BDC (all other things being equal). Stoke, in and of itself has zero influence on how far the piston is in our out of the block at TDC. Rod lengths influence this as well.

Cylinder pressures are functions of volumetric efficiencies and in the case of a running engine, a whole host of different things including, but not limited to, ignition timing, speed at which the mixture burns, efficiency of burning the mixture, ......

Don't confuse compression ration with cylinder pressures. Compression ratio is a static measure only. Cylinder pressures are influenced by a myriad of variables.

 

[GearHead_1]

Oh boy. A lot of stuff here. Most of it close, but no cigar.

Did you read the whole thread?

 

[Geo]

Yep