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High Compression.


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I've always wondered how a bikes compression affects its performace. I know old big bore thumpers like say an xt600 have a lower coompression ratio compared to new modern 450's, but what does this mean?

All I know is that your bike sucks in fuel, and compresses it to a certain ratio, like 12:1 which means the amount it sucks in gets squashed down to a 12th of its original size. So how does that make more power over say the exact same engine but with an 11:1 comp. ratio?

I also wonder how raising a bikes compression works. If you get a high comp. piston, how does it work? All I can think of is maybe the piston sits higher in the bore, but then I think that shortens the stroke( I know it doesnt physically but the piston sits higher) and means it would suck less fuel in so whats the point?

Can someone explain this to me?

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Compression ratio is the comparison of the open cylinder and combustion chamber volume at bottom dead center (BDC) with that of the same engine at top dead center (TDC).

It can be increased or decreased by changing the combustion chamber dome or the piston crown so that there is more or less space taken up these parts at TDC.

It improves the engine's combustion efficiency and also it's efficiency as an air pump, which at the core is what it really is.

http://en.wikipedia.org/wiki/Compression_ratio

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well newer 4st bikes can have more compression and not detonate than older bikes because they have more valve over lap and leak more compression in the transition between intake and exhaust strokes, this is because they run at higher rpms

compression increased horse power in the low to mid and some up top but other perameters affect detonation such as ign timing, valve overlap combustion chamber design

to run more compression on an older bike you would have to run higher octane fuel

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Engines extract power from the fuel by igniting the fuel/air mixture, causing it to heat and expand at a rapid rate. This creates a very high pressure are in the combustion chamber, as high as 30 atmospheres (440 psi) or greater. This propels the piston down the cylinder.

A higher compression ratio allows a greater amount of heat energy to be extracted from the fuel and converted to mechanical energy, due to greater compaction of the fuel and oxygen molecules, which increases burn rate. In addition, the smaller available space for expansion means the pressure rise will be stronger and more rapid than in the same engine with a lower compression piston.

Higher compression ratios (as high as 24:1) are the primary reason diesels are more efficient than gasoline engines, as it allows them to extract a greater amount of thermal energy from the fuel.

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Compression ratio is the comparison of the open cylinder and combustion chamber volume at bottom dead center (BDC) with that of the same engine at top dead center (TDC).

It can be increased or decreased by changing the combustion chamber dome or the piston crown so that there is more or less space taken up these parts at TDC.

It improves the engine's combustion efficiency and also it's efficiency as an air pump, which at the core is what it really is.

http://en.wikipedia.org/wiki/Compression_ratio

Does increasing the compression by shaving the cylinder head base, using a high comp. piston, or reducing the squish improve the effectiveness of the engine pumping air as it would if the compression was increased by increasing the stoke?

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Does increasing the compression by shaving the cylinder head base, using a high comp. piston, or reducing the squish improve the effectiveness of the engine pumping air as it would if the compression was increased by increasing the stoke?
That would depend on the particular engine. For example, if the higher dome of a high-compression piston protrudes far enough to interfere with scavenging, it will actually hinder air flow. Cutting the head can have the same effect by effectively moving the piston dome closer to the head.
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Does increasing the compression by shaving the cylinder head base, using a high comp. piston, or reducing the squish improve the effectiveness of the engine pumping air as it would if the compression was increased by increasing the stoke?
The effect is essentially identical in a four-stroke, but two-strokes are a bit different.

A much simpler formula for calculating compression ratio can be used at least for the purpose of understanding the issue as long as the volume at TDC (V₁) and at BDC (V₂) are both known. Simply divide V₂ by V₁ and you have the picture.

If you shave the head or build up the piston crown, you reduce both V₁ and V₂ by the same amount. But the reduction in volume represents a greater relative change to V₁ than to V₂ because if the compression ratio was already, say, 12:1, V₂ was already 12 times the size of V₁.

What this means then, is that when the piston in a 12:1 engine (speaking of a 4-stroke for the moment) moves down from TDC, it will create a void 12 times the size of V₁ that has to be filled by atmospheric pressure. Increasing the compression ratio to 13:1 increases this pressure differential from 12 to 13, creating a stronger intake signal with no change in displacement. The effect on the exhaust stroke is similar.

This type of compression ratio increase has little or now effect on either the intake or transfer phases of the most commonly used two-strokes because none of it alters a component involved with pumping air into or out of the crankcase.

Stroke increases, though, are another matter. Say you start with a 10:1 engine. V₁ is a tenth of V₂. As we increase the stroke, we keep the same deck clearance (clearance between the the cylinder deck and piston crown at TDC) but the stroke increases V₂ to 12 times that of V₁. Surprise! You have a 12:1 engine.

In a four-stroke, that improves top end pumping ability the same way as the compression ratio did. In a two-stroke, it also changes things downstairs, because the open air volume in the crankcase at BDC just got smaller while the volume with the piston at TDC was not changed.

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Good posts on static compression and compression ratio. Another way the compression is raised is by filling the cylinder fuller. For a example a 250cc motor could be stuffed with 270cc of air, using pipe effect and good scavenging, this raises the running compression higher than what a static comp ratio might indicate. Another way the compression can rise is just by opening the throttle more. Gas engines are variable compression engines. As opposed to diesel engines which are fixed compression and run all the time with the throttle wide open, speed is controlled toggling the fuel.

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I still and unsure on one thing. If a hi comp piston has a little higher crown wouldn't that reduce the amount of air/fuel being sucked in thus cancelling out any gain in compression?

It's a very small % of total cylinder volume. Insignificant, really.

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I still and unsure on one thing. If a hi comp piston has a little higher crown wouldn't that reduce the amount of air/fuel being sucked in thus cancelling out any gain in compression?
No. The primary influence is the displacement of the engine, which has not changed. The piston in your 450 will create a 450cc hole to be filled during the intake stroke, just the same.
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I still and unsure on one thing. If a hi comp piston has a little higher crown wouldn't that reduce the amount of air/fuel being sucked in thus cancelling out any gain in compression?

the tiny amount of volume that's required for the higher compression ratio is well and truly offset by the increase in compression.

say you have an engine that's 450cc swept volume (the volume the piston displaces as it travels through it's stroke, the combustion chamber volume is in addition to this) with 10:1 compression, that means the volume between the piston crown and combustion chamber is a total of 45cc at TDC, to make this a 12:1 compression engine requires that the combustion chamber volume go's from 45cc down to 37cc so you have to somehow make the piston or head take up an extra 8cc, this 8cc is the same at TDC as it is at BDC, so in essence yes you "loose" 8cc from the engines overall capacity, but in the whole scheme of things, that's only 1.7% of the engines capacity, which is quite small, but for the loss of the 1.7% of capacity you might gain 10% more power, which makes the trade off worth it.

the big difference between between a petrol and diesel engine is that diesel burns at much lower temperatures then gasoline does, because diesel burns cooler it allows it burn at very lean air/fuel ratio's without melting the materials the engine is made of, there is no throttle restriction (in the form of throttle butterfly or slide as we have in our bikes) and throttling is controlled by controlling how much fuel is injected into the engine, this has a few benefits, firstly because there's no restriction in the intake there's much less "pumping losses" then in a gasoline engine, which is the energy the engine uses to suck the air into the cylinder, also because the fuel is injected directly into the cylinder and ignited by the hot air in the cylinder the limit for the compression ratio is set by how well the head gasket and rings can keep the combustion pressure in the cylinder, instead of by the octane on the fuel as it is in a gasoline engine.

in a gas engine, if the fuel were to be injected and ignited as it is in a diesel engine, the extremely lean fuel/air ratios would cause the gas to burn as incredibly high temperatures, which would simply melt aluminum blocks and warp cast iron/steel, so the fuel/air ratio must be very carefully controlled in a gas engine to prevent a lean condition from occurring, this is where the throttle butterfly comes in, in a gas engine instead of the throttling being controlled by how much fuel is delivered, it's controlled by restricting the air supply to the engine via either a butterfly valve, carby slide or some other similar method, once the air flow has been established by the intake then the fuel flow is controlled to suite the air flow and ensure the air/fuel ratio is right (that ideal ratio is around 13 part's oxygen (or 65 parts air, since air is generally around 20% oxygen) to 1 part fuel by weight) by the use of jet's or EFI systems, the downside is that the engine is always running a strong vacuum at low throttle opening's, which means the engine is wasting alot of energy sucking the air in, also because compression induced ignition is no good for a gas engine, the compression ratio must not be that high that is causes the fuel to ignite before the spark plug get's the chance too (which we all knew anyway)

i think i got a bit off track lol, hey someone mentioned diesel's didn't they?

Edited by Ttoks
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I think there are some major misconceptions about lean fuel/air mixtures causing high combustion temperatures. Peak combustion temperatures are the highest with a stoichiometric fuel/air ratio. They are also higher with higher compression ratios. But... engines run cooler with high compression ratios and stoicheometric fuel/air ratios? How can this be? It has to do with expansion ratio. Stoichiometric ratios and high compression ratios encourage fast burns. The peak temperature is high but all of the fuel is consumed in the very small clearance volume of the combustion chamber. As soon as the piston starts moving downwards, the volume increases rapidly and the temperature drops rapidly. In a lean burning engine, combustion is slow and the fuel is still burning as the piston travels downwards. Hot combustion products forming late in the down stroke don't see the same expansion ratio and don't see the same temperature drop. Therefore, they are hotter when more of the cylinder is exposed to the heat, they are hotter as the valve opens, and they are hotter as they travel into the exhaust. This is what causes overheated piston crowns, burned valves, and high exhaust gas temperatures.

Disclaimer, this is just one of my hypotheses but I have yet to hear a better explanation of why lean engines run hot.

Edited by 1987CR250R
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There are a number of flaws in your hypothesis:

AFR's (air fuel ratios) leaner than stoichiometric (~14.7:1) do in fact burn at a higher temperature than stoichiometric. On top of that, they burn faster. The reason they are harder on the engine (assuming the engine is set up to run stoichiometric ratios) is that the piston crown is heated more quickly and by a hotter source. The higher burn speed encourages detonation, and we know where that goes.

Normal combustion at WOT with a stoichiometric AFR continues through the entire power stroke and into the exhaust port as the exhaust valve opens near 120-140 degrees ATDC (or the port in a two stroke). The bulk of the useful pressure is spent within the first 90 degrees (depending on RPM, mostly), but the burn is a long way from finished. Richer AFR's lead to lower combustion temps, but can cause higher exhaust temps due to a higher volume of fuel burning in the exhaust (if there is air present), normally at lighter loads.

The term stoichiometric refers only to a balance between oxygen and fuel on a chemical level, so that there is precisely enough of each to have none of either left over after the burn. It is not an indication of the mixture that will produce either the most heat nor the most energy of any AFR.

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I think there are some major misconceptions about lean fuel/air mixtures causing high combustion temperatures. Peak combustion temperatures are the highest with a stoicheometric fuel/air ratio. They are also higher with higher compression ratios. But... engines run cooler with high compression ratios and stoicheometric fuel/air ratios? How can this be? It has to do with expansion ratio. Stoicheometric ratios and high compression ratios encourage fast burns. The peak temperature is high but all of the fuel is consumed in the very small clearance volume of the combustion chamber. As soon as the piston starts moving downwards, the volume increases rapidly and the temperature drops rapidly. In a lean burning engine, combustion is slow and the fuel is still burning as the piston travels downwards. Hot combustion products forming late in the down stroke don't see the same expansion ratio and don't see the same temperature drop. Therefore, they are hotter when more of the cylinder is exposed to the heat, they are hotter as the valve opens, and they are hotter as they travel into the exhaust. This is what causes overheated piston crowns, burned valves, and high exhaust gas temperatures.

Disclaimer, this is just one of my hypotheses but I have yet to hear a better explanation of why lean engines run hot.

Kudos for thinking about this. Most people give this much thought to their engine: the JiffyLube sticker says I have 100 miles till I need to change the oil (and wiper blades).

To the 14 year old, if it's a 4-stroke motor, buy a high-compression piston. Ask the parts counter guy what other changes he recommends. If it's a 2-stroke, the easy and cheap route, but not always the best, is to bring your head to a machine shop and have them remove a small amount of material from the head. The really cheap route is to use a thin head gasket, if you're patient, you can make one from a soda can for experimental purposes, but without knowing your application, I can't say if I'd venture too far from the tool box with this set up.

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