engine compression
panzershreck
04-21-2004, 12:30 AM
just trying to take everything in...
ok so my understanding is that higher compression ratio = more likely to combust fuel and therefore higher octane is required vs. lower compression ratio = less likely to combust and therefore lower octane is required... so in basic terms, a brand new engine would hypothetically (as an example) require higher octane gas, but after 15 years or so, due to wear of parts & etc., loses its compression ratio, and that as time goes on, less and less octane requirement is nessecary to get performance and longevity out of an engine... correct?
just as a future reference guide: which is higher compression: 8:1 or 10:1? (example)
also, is it true or not that an engine with lower or (as an example) 91 octane requirements overtime would result in accelerated carbon buildup if 91 octane for both lower and originally 91 required engines thus causing lower engine performance? is it also true in any way that after a long period of time using one type of fuel that the engine runs best on that same type of fuel? and would carbon buildup affect the ability of the engine to knock?
thanks
ok so my understanding is that higher compression ratio = more likely to combust fuel and therefore higher octane is required vs. lower compression ratio = less likely to combust and therefore lower octane is required... so in basic terms, a brand new engine would hypothetically (as an example) require higher octane gas, but after 15 years or so, due to wear of parts & etc., loses its compression ratio, and that as time goes on, less and less octane requirement is nessecary to get performance and longevity out of an engine... correct?
just as a future reference guide: which is higher compression: 8:1 or 10:1? (example)
also, is it true or not that an engine with lower or (as an example) 91 octane requirements overtime would result in accelerated carbon buildup if 91 octane for both lower and originally 91 required engines thus causing lower engine performance? is it also true in any way that after a long period of time using one type of fuel that the engine runs best on that same type of fuel? and would carbon buildup affect the ability of the engine to knock?
thanks
Sluttypatton
04-21-2004, 03:23 AM
Almost...First off, be sure you make the distinction that the fuel isn't simply more or less likely to combust in a high or low compression motor, it's more or less likely to combust spontaniously due to heat of compression. It's important that you realize that the higher the compression ratio the motor has, the more the air/fuel is being compressed, so the hotter it gets. This increase in heat of compression is what leads to the fuel spontaneously combusting before spark (detonation/knock). Higher octane fuel is just less likely to ignite before spark.
10:1 would be higher than 8:1...think of it this way; 10:1 means the air/fuel is compressed 10 times at TDC, 8:1 means the air/fuel is compressed 8 times. Clearly 10 times is more highly compressed.
The other mistake you made is about engine wear. As engines wear they don't lose compression ratio, they lose compression. Compression ratio and compression are related but different. Compression ratio is how compressed the a/f mixture gets from BDC to TDC, and is expressed as a ratio. Compression is a measure of how tightly the combustion chamber seals in the air/fuel when it is compressed, and is typically measured in psi. In other words; the compression ratio will always be the same, because the amount the air/fuel is compressed never changes, but compression does decrease with wear because the piston rings and valves wear, allowing air/fuel to squish out of the combustion chamber as the piston compresses it (this is known as blowby). This does not effect the octane rating of the fuel you use to fill the car with, you will always need the same octane rating.
Carbon buildup can effect the motors tendancy to knock, as the carbon can build up on the tops of the pistons over time, artificially increasing the compression ratio. As we know, higher compression ratio's mean higher heat of compression and a higher tendancy to knock.
Other than that you seem to be on the right track. Feel free to ask any more questions you may have.
10:1 would be higher than 8:1...think of it this way; 10:1 means the air/fuel is compressed 10 times at TDC, 8:1 means the air/fuel is compressed 8 times. Clearly 10 times is more highly compressed.
The other mistake you made is about engine wear. As engines wear they don't lose compression ratio, they lose compression. Compression ratio and compression are related but different. Compression ratio is how compressed the a/f mixture gets from BDC to TDC, and is expressed as a ratio. Compression is a measure of how tightly the combustion chamber seals in the air/fuel when it is compressed, and is typically measured in psi. In other words; the compression ratio will always be the same, because the amount the air/fuel is compressed never changes, but compression does decrease with wear because the piston rings and valves wear, allowing air/fuel to squish out of the combustion chamber as the piston compresses it (this is known as blowby). This does not effect the octane rating of the fuel you use to fill the car with, you will always need the same octane rating.
Carbon buildup can effect the motors tendancy to knock, as the carbon can build up on the tops of the pistons over time, artificially increasing the compression ratio. As we know, higher compression ratio's mean higher heat of compression and a higher tendancy to knock.
Other than that you seem to be on the right track. Feel free to ask any more questions you may have.
camaroincal
04-21-2004, 10:22 AM
How high does a compression ratio need to be in order to see benefits of running 91 octane? Like 9.5 or higher?
MagicRat
04-21-2004, 09:07 PM
How high does a compression ratio need to be in order to see benefits of running 91 octane? Like 9.5 or higher?
Two things:
Just to add to what Sluttypatton wrote.....brand new car engines do not always produce the highest comression pressures. As the new engine is run for a few thousand miles, the piston rings will improve their sealing ability and compression will often increase, at least a little.
Second thing:
Some engines require a higher octane fuel than others, even though their compression ratios are the same. This is due to the shape of the combustion chamber. A 'wedge' shaped chamber (usually found in pushrod engines) will require a higher octane fuel than a hemispherical chamber (found in virtually any overhead cam engine and Chrysler Hemi's), given an equal compression ratio.
However, there are many other factors that make the octane requirement vary from one car to another. So to answer your question......if your car pings or detonates on 89 octane gas, then there will be a benefit in going to 91. If it runs fine on 89, then there is no benefit to moving up, regardless of the compression ratio.
Two things:
Just to add to what Sluttypatton wrote.....brand new car engines do not always produce the highest comression pressures. As the new engine is run for a few thousand miles, the piston rings will improve their sealing ability and compression will often increase, at least a little.
Second thing:
Some engines require a higher octane fuel than others, even though their compression ratios are the same. This is due to the shape of the combustion chamber. A 'wedge' shaped chamber (usually found in pushrod engines) will require a higher octane fuel than a hemispherical chamber (found in virtually any overhead cam engine and Chrysler Hemi's), given an equal compression ratio.
However, there are many other factors that make the octane requirement vary from one car to another. So to answer your question......if your car pings or detonates on 89 octane gas, then there will be a benefit in going to 91. If it runs fine on 89, then there is no benefit to moving up, regardless of the compression ratio.
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