Re: Compression Ratio explained by Scott The Viking
Posted: Sat May 29, 2010 7:38 pm
i've been reading about DCR on the bug forums, and it has been puzzling me. There sure seems like a LOT of really bright folks arguing about something which makes not much sense to me....
Compresssion causes more heat for 2 reasons. One was mentioned above - when you raise the compression ratio, you are effectively stuffing the same amount of swept volume into a smaller volume, and you get more heat and more pressure due to physics described by the Ideal Gas law. So, if you go from 8:1 up to 9:1, you will have 10 percent more heat and pressure from compressing the charge 10 percent more.
But wait. There's more.
When the sparky goes off, it will ignite the charge. Because there is more heat and pressure, the mixture WILL ignite faster. Another way to say this is the flame front will spread more quickly, and all the little fuel molecules will catch fire more quickly.
When the fuel ignites quicker, it causes even more pressure. When all done, the average temperature and pressure inside of the combustion chamber will be substantially higher. A higher temperature means more heat will get transferred to the head.
Further, because more of the fuel was ignited quicker, it is actually hotter inside the combustion chamber longer. After all, for a given RPM, the piston is moving at a fixed speed. If you light off a hotter fire and have it burn quicker, there is more time for heat to transfer to the head.
The good news is a higher, earlier pressure means more power.
So why not always run more compression? Eventually you run into detonation. Detonation is when the fuel air mixture ignites before the spark tells it to ignite.
Well, actually, this is not quite true. What is actually happening is there are many many types of carbon fuel molecules in your combustion chamber. Some of them light at low pressure/temperatures then others. Also, there are hotter spots in the combustion chamber.
If we let the fuel sit around the hot spots too long, eventually they get enough of the "lighter" carbon chains to ignite some heavier chains, and a chain reaction takes off, and the engine Knocks.
Modern engines count on a lot of turbulence to keep the mixture moving quickly around the hottest spots so the carbon chains don't start feeding off each other. If one molecule ignites, then gets blown into the next area, no big deal, as it gets "blown out" by getting cooled off in another area of the chamber. If you want to read more about my "molecules" read about free
Also, modern engines use tuned intakes and exhaust to manage the intake and exhaust flows. At some RPM the engine will be most efficient, and at this RPM your dynamic compression ratio will be the highest. The cam shaft, intake design, exhaust design, valve shrouding, and the RPM will all determine when that RPM happens.
I think the max DCR will happen at peak torque.
However, at peak torque, your RPM is high enough the light carbon molecules don't have much time to ignite, so peak torque probably is not when you have knock issues.
I can sit here and talk theory with the best of them, but when all done, the old farts who have been building race engines and street engines for many years KNOW what works, and what blew up.
I have nothing but respect for those who REALLY know what works and what doesn't. My post is nothing but theoretical guesses as to WHY some things work and some don't.
For more reading, read about free radicals detonation squish
http://www.motorcycle.com/how-to/wrench ... -3420.html
Compresssion causes more heat for 2 reasons. One was mentioned above - when you raise the compression ratio, you are effectively stuffing the same amount of swept volume into a smaller volume, and you get more heat and more pressure due to physics described by the Ideal Gas law. So, if you go from 8:1 up to 9:1, you will have 10 percent more heat and pressure from compressing the charge 10 percent more.
But wait. There's more.
When the sparky goes off, it will ignite the charge. Because there is more heat and pressure, the mixture WILL ignite faster. Another way to say this is the flame front will spread more quickly, and all the little fuel molecules will catch fire more quickly.
When the fuel ignites quicker, it causes even more pressure. When all done, the average temperature and pressure inside of the combustion chamber will be substantially higher. A higher temperature means more heat will get transferred to the head.
Further, because more of the fuel was ignited quicker, it is actually hotter inside the combustion chamber longer. After all, for a given RPM, the piston is moving at a fixed speed. If you light off a hotter fire and have it burn quicker, there is more time for heat to transfer to the head.
The good news is a higher, earlier pressure means more power.
So why not always run more compression? Eventually you run into detonation. Detonation is when the fuel air mixture ignites before the spark tells it to ignite.
Well, actually, this is not quite true. What is actually happening is there are many many types of carbon fuel molecules in your combustion chamber. Some of them light at low pressure/temperatures then others. Also, there are hotter spots in the combustion chamber.
If we let the fuel sit around the hot spots too long, eventually they get enough of the "lighter" carbon chains to ignite some heavier chains, and a chain reaction takes off, and the engine Knocks.
Modern engines count on a lot of turbulence to keep the mixture moving quickly around the hottest spots so the carbon chains don't start feeding off each other. If one molecule ignites, then gets blown into the next area, no big deal, as it gets "blown out" by getting cooled off in another area of the chamber. If you want to read more about my "molecules" read about free
Also, modern engines use tuned intakes and exhaust to manage the intake and exhaust flows. At some RPM the engine will be most efficient, and at this RPM your dynamic compression ratio will be the highest. The cam shaft, intake design, exhaust design, valve shrouding, and the RPM will all determine when that RPM happens.
I think the max DCR will happen at peak torque.
However, at peak torque, your RPM is high enough the light carbon molecules don't have much time to ignite, so peak torque probably is not when you have knock issues.
I can sit here and talk theory with the best of them, but when all done, the old farts who have been building race engines and street engines for many years KNOW what works, and what blew up.
I have nothing but respect for those who REALLY know what works and what doesn't. My post is nothing but theoretical guesses as to WHY some things work and some don't.
For more reading, read about free radicals detonation squish
http://www.motorcycle.com/how-to/wrench ... -3420.html