Calculating Power Gains/Loses with compression ratios.
Calculating Power Gains/Loses with compression ratios.
Does anyone know/know where to find the calculations for power differences with different compression ratios?
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Q45tech
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http://naca.larc.nasa.gov/repo...-205/
"The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.
With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke."
"The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.
With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke."
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maxnix
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1995 Infiniti Q45t
2000 Infiniti Q45
Brake Mean Effective Pressure
Perhaps this will.toki wrote:that tells me nothing.
http://www.factorypipe.com/Tec....html
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Q45tech
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1995 G20t 5 speed 334,000 miles 16" 2002 wheels - 205/50/16 Sr20ve vvl
PUBLISHED Static compression ratio is a marketing term to warn you of a high compression engine.
In reality the running compression is closer to 8.2:1 except at what ever the torque peak rpm is where the intake tuning allows more air to stay in the cylinder [doesn't leak back out into the intake when the intake valve is still open up to 48 degrees after the piston starts its upward compression motion.
You multiply the VE at each rpm and load by the static CR to get the real effective running CR............the Q 10.2 becomes 9.2-9.3 at 4,000 rpm WOT.......pretty much the limit for 91-93 octane.As the VE declines due to filling time [restriction of valves and intake to incoming atmosperic pressure] the CR begins to decline again.........to something like 7.2-7.4 at redline..........only 70% of cylinder volume get full of air compared to 90% at 4,000 rpm or 15-20% at idle or cruise.
Guess you could say the CR of what ever amount of air that flows in stays the same..........but the BMEP sure changes a lot!
In reality the running compression is closer to 8.2:1 except at what ever the torque peak rpm is where the intake tuning allows more air to stay in the cylinder [doesn't leak back out into the intake when the intake valve is still open up to 48 degrees after the piston starts its upward compression motion.
You multiply the VE at each rpm and load by the static CR to get the real effective running CR............the Q 10.2 becomes 9.2-9.3 at 4,000 rpm WOT.......pretty much the limit for 91-93 octane.As the VE declines due to filling time [restriction of valves and intake to incoming atmosperic pressure] the CR begins to decline again.........to something like 7.2-7.4 at redline..........only 70% of cylinder volume get full of air compared to 90% at 4,000 rpm or 15-20% at idle or cruise.
Guess you could say the CR of what ever amount of air that flows in stays the same..........but the BMEP sure changes a lot!
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Q45tech
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1995 G20t 5 speed 334,000 miles 16" 2002 wheels - 205/50/16 Sr20ve vvl
For efficiency with best pump gasoline and good knock sensor control aluminum engines are limited to 10.5 CR at most [though some are claiming 11:1], the Q 10.2 CR static was set so that it might grow to 10.5-10.7 with deposits.
http://www.tpub.com/mechanics.htmhttp:/ ... po...-232/
"The compression ratio was increased from 9.3:1 to 12.0:1 to take advantage of the high octane number of Ed85 (85% denatured ethanol/15% gasoline), increasing efficiency about 8% compared with the baseline engine."
It appears that for some engines you could extrpolate each point increase in CR with a 2% increase in thermodynamic effeciency.
But you are fuel limited and will need to change fuel types.
Lot easier to change cams than to change CR and the results are the same at certain rpms.
http://www.tpub.com/mechanics.htmhttp:/ ... po...-232/
"The compression ratio was increased from 9.3:1 to 12.0:1 to take advantage of the high octane number of Ed85 (85% denatured ethanol/15% gasoline), increasing efficiency about 8% compared with the baseline engine."
It appears that for some engines you could extrpolate each point increase in CR with a 2% increase in thermodynamic effeciency.
But you are fuel limited and will need to change fuel types.
Lot easier to change cams than to change CR and the results are the same at certain rpms.