pistons and compression / U dont Know!

i have answered so many posts and PM's about the mythological dangers of high compression... i figured it would be a good idea to just write a thread on the topic and be done with it... so here goes

in no particular syntax... here's the dope...

if there is truly ONE magic powerband bullet for a 4-stroke... high compression IS it...no question

4-strokes love... Love...LOVE HIGH COMPRESSION.....!!!

high compression does not cause a top end power loss in 4-strokes the way it can add a pumping load to a 2-stroke engine.... that is a very persistent hold over myth to beat down.....

high compression makes your engine perform likeit has a bigger displacement at lower RPM's....and it makes it perform like it has more camshaft at high RPM's.... more bottom...more top...and better throttle response across the board.... a beautiful thing, and very hard trick to beat.. short of forced induction

so as to not perpetuate any sort of mythological fecal fog here... it needs to be explained exactly how high compression does all of that...

high compression is NOT just a high dome that squeezes the A\F mix so tight is goes off like an atomic bomb... the tighter pressure squeeze does indeed help the power output...but it isn't all the magic

it's tough to paint an analogy in layman's terms with words alone.... . as always, i will use exaggerated illustrations for the purpose of clarity...

your piston and cylinder arrangement has now become a GIANT syringe.... the piston is the rubber plunger...and the clear tube is your cylinder.... and while we are at it..... lets give it 2 needle outlets on top too...one for intake and one for exhaust....

in our LOW compression model...we will exaggerate and say that the piston\ plunger only goes as high as half way up the tube at the top of its stroke

and the HIGH compression model goes very close to the end of the tube at the top of its stroke

that exaggeration will help with understanding all the other dynamics besides how tight the mixture gets squeezed alone....

so ...besides being used to squeeze the A\F charge before ignition.... you piston\ plunger is also important to how much vacuum is seen during the intake \ suction stroke......

let's say you could put your finger over the intake side of the LOW compression syringe ...and then feel the amount of vacuum generated as you pull the plunger\piston to the bottom of the stroke...... you will notice that the vacuum builds slowly...and doesn't become very strong until the bottom of the stroke...

doing the same test with the HIGH compression plunger \piston.... where the piston has a much smaller volume of air trapped above it to begin with.... you will see a very fast...very sharp rise in the vacumm it generates...since it has less trapped volume to dampen the vacuum in the first place....

so what does that do for a running engine?? a few things...all good!

the higher compression version provides a STRONGER and EARLIER vacuum pulse into the intake tract... which makes for better\ sharper throttle response by delivering a stronger signal to the carb's metering circuits...

and also the sharper vacuum drop makes the incoming fuel droplets break up \atomize into a better\ finer air + fuel fog.... the smaller the fuel droplets...the better the combustion...the only part that can burn is the part that comes in contact with oxygen... big droplets only have the "skin" of the drop burn away durung combstion...the reaminder of the drop not only doesn't burn...and adds unburned hydrocarbon emissions to the atmosphere....it also serves to dampen the combustion process by absorbing latent heat energy from the part that does combust...

the other thing that the stronger vacuum signal from the higher compression piston does is also wonderful....

it CREATES a HIGHER VELOCITY incoming INTAKE CHARGE....

what does that do you ask? one thing that higher velocity does is keeps atomized fuel droplets in suspensioin better than a lower velocity charge does...and we know that is a good thing....

and we sort of know that higher compression gives back a lot of the torque that a BIG duration cam loses... but most people think that the tighter squeeze of the A\F mix prior to ignition is what does this (and of course, that's part of it)...

first we need to know why a big cam actually loses bottom end power and response in the first place

a modern performance cam opens the intake some 20 to 30 degrees before the piston is all the way to the top of the EXHAUST stroke.... just prior to the beginning of the downward intake stroke....and it doesn't close the intake valve until somewhere from 50 to 70 degrees AFTER the piston has reached the bottom of the intake stroke and has started back up on the compression stroke...

at high speeds you need to have the intake valve open those long periods of time to simply have enough time @ high rpm to get any sort of decent cylinder fill...and at high piston speeds @ high rpm you will get a stronger vacuum pull into the intake port.... and the velocity generated in the port can sort of "ram charge" the incoming mix into the cylinder even though the intake valve is still open as the piston is traveling upwards for as much as 70 degrees of rotation

BUT at lower speeds.... you not only don't get as much piston speed generated vacuum signal ...with a BIG cam you are still leaving the intake open long enough after bottom ... that the piston is able to push charge that has already entered the cylinder back up through the open intake valve... i've said many times that you can't compress a charge in a cylinder that isn't sealed...

SO...

as we have already discussed....the high compression piston imparts more vaccum...and more signal...and more velocity into the intake tract...in a BIG cammed engine...that added intake velocity helps to give enough inertia to the incoming charge that it helps to counter act tha low speed reversion of the intake flow....

high comprression one-two punch to help with low end loss on big cams.... tighter squeeze is always bigger boom...PLUS higher velocity \ earlier acceleration of the intake charge making for more cylinder fill AND less reversion loss of that charger by virtue of that greater velocity...

so could high compression possibly do anything else ...beyond the wonderful stuff outlined already??

you bet it does!

on the exhaust stroke it is more effective at getting more of the burned charge out of the cylinder....think of the 2 different piston\ plunger\ syringe's again.... the one that leaves the least space at the top of the cylinder is the one that pushed the most spent charge out the exhaust.....

and it did it with higher velocity too..... and since higher exhaust velocity has more inertia heading in the OUT direction...it creates a stronger vacuum in its wake....

which brings us to another good thing....

at top dead center \ piston at its highest point...at he end of the exhaust stroke...and beginning of the intake stroke...it is during the period known as "cam overlap".... for a brief segment of time ...just before and just after the top...the intake AND exhaust valves are open just a little bit...and for very good reason....

the exiting high velocity exhaust...and subsequent vacuum tail it leaves in its wake....will pull the last bit of spent charge out of the cylinder... AND use its energy to begin pulling the intyake charge into the cylinder...even BEFORE the piston begins its downward intake stroke... it couldn't vacuum the rest of the combustion chamber out completely...OR begin the movenent of the fresh charge inward from the intake tract unless both intake and exhaust valves were open simultaneously @ TDC...which is exactly why there is overlap timing in high performance cams in the first place.....

NOW....

which would take better advantage of a strong exhaust vacuum signal....and both clean out the combustion chamber AND transfer some of that vacuum energy effectively to the intake port??? the large combustion chamber volume of low compression OR the small\ efficient combustion chamber volume of the high compression piston??

once again..... ADVANTAGE HIGH COMPRESSION.....

i hope i was effective at illustrating the MANY unseen...and largely unknown...advantages of how a high compression setup works...well beyond the simple "tighter squeeze of the charge" ( which is wonderful in and of itself BTW)

now...to debunk the RELIABILITY VS HIGH COMPRESSION myth...hopefully for the last time....

horsepower and torque are a direct reflection of the combustion pressures seen inside an engine......

ANYTHING that makes your engine have a higher output is a result of it creating more combustion pressure within your engine...... whether the power came from a jet kit...pipe...cam...special fuel...etc...etc...

as far as the stress on your engine components....they have not the slightest idea wher the pressure comes from...and they wouldn't really care either...more pressure = more power = more stress on everything...

a 50 hp pump gas setup ..... is putting out more stress on the engine components ....than a high compression engine delivering 47hp.... the compression isn't what is the stress...the actual pressure from combustion is.... and combustion pressure is MANY times greater than cranking compression in any event....

increased power = stress and accelerated wear.... that is the bottom line....it doesn't have anything to do with what compression you have..aside from the actual power it adds to the engine..

and BTW....on the piston reliability thing...compression notwithstanding... there are design and material components that will make one piston\ ring setup better in the reliability and longevity arena's

nobody want to talk tech??

I think you scared everyone off!

You pretty much sum'd everything up right there. I do have to say if i had a dollar for every period, i could buy a new bike.

While this is a good article -- kudos to you mixxer -- shouldn't this be posted in the tech forum just once instead of posted several times in different forums? I'm just sayin...

I think you scared everyone off!

no way charlie...?! not you though.. i was counting on you!!

You pretty much sum'd everything up right there. I do have to say if i had a dollar for every period, i could buy a new bike.

lol...! im a two finger typer :smirk: ...and i made up my own writing/typing style

While this is a good article -- kudos to you mixxer -- shouldn't this be posted in the tech forum just once instead of posted several times in different forums? I'm just sayin...

probably.... but what fun is that :smirk:

Awesome write up this is what I was looking for.You helped me make up my mind about my next engine mod.Thanks!

no way charlie...?! not you though.. i was counting on you!!

If we got going, we'd get fined for using up too much bandwidth! It was an excellent write up! The 2 finger method had to take a good while!

For Nerds:

The first law of thermodynamics says that any change in energy is equal to the heat added to the system minus the work done by the system. This equates to: ΔE = Qin − Wout

Analysis of the four-stroke engine can be simplified by ignoring the intake and exhaust cycles and simply considering what happens when both valves are shut. During this phase, the four-stroke is most like the Otto Cycle. No, not AUTO like automobile and not cycle like bicycle or motorcycle, but a cycle of heat and energy exchange, first considered by some dude named Otto.

If you hate me already, look it up in Wikipedia; http://en.wikipedia.org/wiki/Otto_cycle.

Anyway, after you analyze the otto cycle using the first law, breaking it into its four constituative parts, what you can derive is the thermal efficiency of the cycle, or engine in our case....

From Fundamentals of Engineering Thermodynamics, 5th Edition, Moran & Shapiro, the thermal efficiency of an Otto cycle is equal to:

n = 1 - 1/(r^(k-1)) [eq. 9.8]

where

n is the thermal efficiency of the cycle

r is the compression ratio of the cycle

and k is the specific heat ratio of the fuel/air mixture (assume this is constant)

Simply put, if the compression ratio (r) goes up, you subtract less from '1' and the closer the thermal efficiency gets to 1, it's optimal value.

End game: higher compression ratio = higher thermal efficiency = higher power. If you put the same amount of fuel into an engine with low thermal efficiency as and engine with high thermal efficiency, you get more power out of the engine with higher thermal efficiency. If you keep power at a constant value, as is the drive of all of these fuel-efficient cars nowadays, you use less fuel with the higher efficiency engine.

However, from a structural and fatigue point of view, a higher compression ratio causes higher stresses and temperatures within the engine, so expect components to wear out faster.

From a fun point of view, a bike's compression ratio is directly proportional to the size of the grin on your face.

That pretty much did me in. Waaaaaaaay too much information on this thread. Back to reality, I'm leaving!

Some points are excellent - but there is some poor info too regarding power output and compression.

Around 10:1 compression ratio the gains from higher compression ratio on a percentage basis begin to diminish very quickly. Talking purely power output influences on intake charge notwithstanding (expansion ratio)

The change in thermal efficiency from 12:1 to 16:1 is very small - only 4 percent improvement according to various scientific studies and published papers.

The change in power output from 12:1 to 16:1 is almost nill - you almost go backwards due to mechanical losses (pumping losses and heat production).

Now - in a high performance four stroke calculated/static compression ratio and dynamic compression ratio are very different. Some use different terms here, but what we are saying for compression ratio is what i call static. There is then the "dynamic" which is what the engine actually experiences - and there is calculated dynamic which is our "best" estimation of what the engine experiences based on measured volumetric efficiency.

So if an engine has a VE of 120 percent - it is consuming 1.2 times the air it displaces. In doing so the dynamic compression ratio will be MUCH higher than the static compression ratio.

IE instead of 250cc+20 CC/20cc = 13.5:1 CR

You would have 250cc*1.2 = 300cc+20cc/20cc = 16:1 CR

BUT even further - since the intake valve is open upwards of 30 degrees ABDC - and true compression function can not occur until it closes - the true dynamic CR is less than 16:1....

In an ideal world, compression ratio COMPLETELY out the window - if we could design the ultimate COMBUSTION chamber - it would be very small with spark initiation in the center of the fresh charge. Imagine a soccer ball (or any other ball) with the spark plug dead center of the ball...for quickest combustion process possible.

We may also want to use latent heat from the exhaust valves to speed ignition time....so we might concentrate the charge near the exhaust valves...

Sadly - we can not always go exactly what we want - as the smallest combustion chamber shape may bring compression too high for the fuel - or the mechanical losses will go through the roof to get that compression etc.. etc..

Engine builders and designers are faced with HUGE challenges in juggling design.

For instance - what if we WANTED 20:1 compression ratio in a performance four stroke? Well - in order to accomplish it we would have to considerably alter cam profiles and rid ourselves of valve overlap - and kill hp to do it because the valve pockets in the piston would have to be removed.

Valve sizes may have to be made smaller - or the head shape different which would kill flow...and thus hurt hp again.

It's all a juggle!

If we got going, we'd get fined for using up too much bandwidth! It was an excellent write up! The 2 finger method had to take a good while!

who doesn't like burning up bandwidth ?! :smirk:

Mr. Harris and Mr. Kong

very nice adds to the thread...!!

Mr. Harris.... what a can of worms you open when trying to discuss dynamic compression with most people....

i have seen so many times in the atv world... where the atv is pretty close to stock...or has components that aren't working very well.... so overall volumetric efficiency is not good...

but then they tell people they have no problem running high compression pistons.... i try to explain that if the cylinder isnt getting effective fill.... you still arent compressing enough charge to have a high dynamic compression.... so of course you don't have a detonation problem until your VE goes up....

balancing and juggling are the name of the game in high performance for sure.....

everyone has performance goals...and unfortunately...a budget that moderates them

i was being heckled for not covering detonation and compression in another thread...

that is a big big topic on its own.... brief answer from the other thread;

detonation is similar to picking the right burning rate for doing your own bullet loading....

a fast burn rate compared to the mass it has to accelerate will result in higher pressure and that in turn creates a faster burn rate.... pressure spikes come from all of it literally igniting at once....

same deal with octane as detonation supressant..... higher octane is like slower burning powders... can handle more compression/ heat/ leaner mixtures and still burn in a controlled manner...instead of auto igniting the whole mixture....

i will write way more than 500 words to fully cover the topic of detonation and fuels...

latent heat refers to the thermal mass and the caloric value a compond can absorb ... you don't want big droplets of fuel for many reasons...unburned fuel ...besides being unburned / not adding to power / and dampening the burn of the fuel that will ignite...also washes the lubricating film from your cylinder wall...not a good way to slow and control a combustion event

Mixxer, I throughly enjoyed this thread and when I have to read it a few times I know I'm learning something. Could you elaborate on how 10% ethanol will effect our engines. I'm learning, ethanol decreases mileage up to 25% because of lack of BTU's and it does nothing to improve octane. Will higher octane have to be added to fuels that contained ethanol? The laws require all fuel to contain at least 10% and are threatening to increase it to 15%....very bad for all.

I enjoyed. I love articles that break down the big words into little ones. I can do a top end on my CRF250 and actually diagnose and repair common engine problems. However how it works, I have not one clue. Appreciate you posting that and also might as well thank Charlie C while I'm at it. I always learn a thing or two from your posts.

thanks for the kind words gentlemen.... i enjoy breaking down tech and then talking about it...

alcohol fuels have oxygen in them naturally....you can burn more of it because it carries its own oxy...that is part of how you make more power with lower btu

high octane...oxygen carrying....cool burning.... all good points of alcohol

but..... you need to take advantage of it all via high compression...proper jetting...and advanced timing curves....

BTW.... on all of my dyno driven testing...all other things being equal....

jumping from 12:1 to 14:1 range on 450 engines yields +2HP

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