tech talk about compression

[chad502ex]

I've read plenty of threads lately regarding some misconception on static compression; so, I'd thought I'd start posting tech talk....


OK, before I start, these tech talk threads are open for discussion or questions from anyone as long as they relate to the topic at hand... I encourage peeps to ask questions, cause if you don't, you'll never advance knowledge of your THUMPER.


Often times questions are asked,

"How much will I gain if I go with more compression"?

Although its a complex answer, due to other variables such as dynamic compression (discussed later), here's a chart that provides some generalization....

[400exrider707]

Ok here's a quick question. Does this table relate to a certain engine. For instance a single cylinder motor? Would the results be different on a multi-cylinder motor? Going from 10:1 to 12:1 on a single cylinder should be less of an improvement than going from 10:1 to 12:1 on a four cylinder application correct?

[chad502ex]

I like this illustration and have previously posted,.... it points out what when you increase compression your able to siffen harder through the ports to signal the carb..

A higher compression piston creates not only more squeeze on the compression stroke, but also pulls in an exponential decay fashion on the intake stroke when siffening in the fuel into the cylinder...

Notice how the curves of each piston illustrated do not follow the same slope on the graph?

[chad502ex]

Originally posted by 400exrider707
Ok here's a quick question. Does this table relate to a certain engine. For instance a single cylinder motor? Would the results be different on a multi-cylinder motor? Going from 10:1 to 12:1 on a single cylinder should be less of an improvement than going from 10:1 to 12:1 on a four cylinder application correct?

The chart is based off an increase in cylinder pressure per cylinder.

the chart is calculated from the formula below

Where E = the Otto cycle (thermal) efficiency, R the compression ratio and k the coefficient of adiabatic expansion for air, which is nominally 1.4.

[Chino886]

Good stuff Chad as always!

Sorry to hear that you left the Tech

[chad502ex]

Originally posted by Chino886
Good stuff Chad as always!

Sorry to hear that you left the Tech

Thanks chino for the kudos.

(off topic) not trying to drum up heartache, but I left tech because of the double standard that Corey was setting allowing other builders, not being a site sponcor, to have an avatar with phone numbers and business names and talking about their products and services, when I was harrassed even after all the tech I posted for its members. Its not about a stupid avatar, but more about the double standard being set. It took me about three hours to deleted just tech talk postings when I decided to hit the road...

Too much of a double standard to live by with that sites bottom feeding left after the big fish feastings.... It's not like that site was suppose to be about obtaining business, but 'suppose' to be about tech... forgotten roots... besides its ATVRIDERS where drama is minimal and all the members care about is the quads they ride.... getting away from the back room sponcor led decision-making is a good thing...

[400exrider707]

My brain is going hungry for more tech info!!!! I need numbers and formulas and graphs and detailed pictures!!! On with it!:blah:

[chad502ex]

not to mention the increase fuel economy with increasing compression.....


But hey... back to compression/octane requirements...

This chart is about dynamic compression not static.....

dynamic compression is cylinder compression when the engine is running. Static compression is when the engine is off. Pistons are identified with static compression. Intake valve closing timing determines dynamic compression ratio.

DYNAMIC COMPRESSION RATIO IS ALWAYS LOWER THAN STATIC COMPRESSION!

Generally, the bigger the camshaft, used for top end hp, the longer the duration and the more overlap timing it will have. Longer duration intake timing generally pushes the closing later on the compression stroke. The later the intake valve closing the less the cylinder compression that can develop on the compression stroke thereby netting less running compression. Dynamic compression can be calculated or determined using cam timing and other engine specifications,...

Looking at the chart again notice the x-axis lable being "CR at Valve Closure". Also, note that the Y-axis specifies RON + MON/2.
------------------

more technical explanation of DCR:

RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = Stroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs

RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)

[chad502ex]

the top row is normal combustion process...

the second row illustrates pre-ignition where hot spotting from sharp edges glow self igniting the air-fuel mixture before the primary ignition occurs

the third row is where detonation exists where inreased pressure/excessive heat/and fast burning low octane fuel causes a secondary ignition wavefront to spread toward the primary wavefront created by the spark plug. when these two wavefronts collide, the created an audible ping as the sound wave energy reverberates off the cylinder walls. this is known as pinging..

[400exrider707]

Excellent info:macho

[Chino886]

Post away Chad, as you can see people are reading!

Nice pictures to represent deto.......

Most people can tell you what happens, but like they say, pictures are worth a 1,000 words! :cool:

[IMSROLL450R]

Used a lot of those types of equations in fluid mechanics..........great info man!!!!!!!! that first chart is very helpful I had never saw that one before even in my auto classes. THANKS!!!!!!

[MX MaNiAc 06]

Very good info!

[chad502ex]

WOW, I'm rather surprised,... I thought i was talking to myself a bit,... LOL...

Great thanks,... let's continue some more..

[chad502ex]

Speaking of ‘pinging’, this chart relates the frequency or tone of the knock according to cylinder bore size.

Each line on the graph is another tone of the original knock tone. These multiple tones are called harmonics. Each harmonic ping is produced at the same instance when the knock occurs. The lowest audible tone that can be heard by your ear has the most amount of energy associated with it. Each harmonic of the original knock frequency contains a lower amount of destructive energy. This means that if you had a microphone bolted to the engine case, or specifically knock sensor, and used an amplifier to amplify the ping, you could determine if your engine had any destructive engine knock using a lower octane fuel.

Why would you want to do that? Well, all cars nowadays have ‘knock’ detector circuits that change ignition timing when it senses internal cylinder pinging and will automatically in real time advance or retard ignition timing to helps reduce the damaging effects.

Too many ppl guess at what octane is required in our little 450cc engines… the plan will be to perform a knock index test to determine the required engine octane for compression as high as 16:1

[d3ktrix]

Very good stuff, I'm learning a lot, keep it coming =)

[chad502ex]

Some of the images I’m posting many may have seen me post elsewhere, but I think they are extremely informative and could drum up further discussions…..

So here are two illustrations on wave-front propagation. Really nothing unusual about the first overhead wave-front propagation except, notice that the spread of the wave-front energy is isotropic. Isotropic is sphere-like, almost equal distance from the center radiating outward equally until its obstructed by an object

[chad502ex]

Now, the second illustration is typically what would be expected when the piston dome comes in close towards the ignition wave-front … the ball-like wave-front becomes distorted and flattens on the piston dome surface outward toward the cylinder walls.

What’s also worth noting in the illustration is the squish band. This zone is extremely important as it provides turbulence to cool the hot mixture and prevent pre-ignition from occurring. When I designed my piston I set this thickness up as small as possible without compromising reliability. The squish zone can enhance performance too by increasing the compression ratio- more bang for the buck!

On the compression stroke the piston moves upward toward TDC. As the squish zone gets smaller, the squeezed mixture begins to move faster and faster as the squeeze gets smaller. This fast moving air cools this area of the piston which reduced hot-spotting… The minimum spacing of squish zones have been tested to provide maximum gains at about 0.035”.

[chad502ex]

Domed piston high speed photograph of cylinder filling and the effect of piston dome shape on cylinder filling…

[chad502ex]

How many realize that when the spark plug fires, there is a delay before peak power develops?

This illustration is great for showing that after the spark, it takes time for fuel burn to occur in a 94mm bore….

Higher compression pistons lift this curve upward with higher cylinder pressures…

[chad502ex]

Moving the position of the ignition point, done by re-programming the Dynatek ignition module, can advance or retard the ignition to maximize the propagation timing and cylinder pressures.

this is an excellent thread!

[TheNewn]

Defiantly some useful info if you understand it.
Which leads me to ask...am i the only that has about 75% of this going over their head? i'll have to read it a few times.

Chad, as for the first table you posted about compression ratios. can you give an example of an actual quad and expected HP increase?

Like if you went to 11:1 comp in a stock 400ex (8.5 i think) it would net you roughly 7 hp? am i understanding this correctly? assuming you changed nothing else?

I'm pretty sure i'm thinking of it wrong, because that's a lot for one mod.

[chad502ex]

Originally posted by TheNewn
Chad, as for the first table you posted about compression ratios. can you give an example of an actual quad and expected HP increase?


For example, going from 10:1 to 11:1 would 'theoretically' yield 2.5% gain in power.

So, if your machines is putting down 30hp to the wheels,... multiple 2.5% to 30hp your new hp is 30.75hp,.... but if your machine is making 60hp then that same increase in compression would provide a larger gain.... either way a gain is a gain...

[TheNewn]

Alright, makes perfect sense now, thanks a lot.

[DnB_racing]

boy there is some good info here.... got to love the search button oldie but goodie!!!!!!