KACR & Cold Starting
#81
01-17-2013, 10:31 PM
OK, I got a response back from the U of IL physics department. The Q (question) and A (answer) are at the bottom of this post or in the next post by me.
We will use the calculation that they made using the 1:12 compression, as that is more of a hard number than the 135psi compression test info. Remember that these numbers are theoretical and actual numbers will vary, but not by more than a factor of .5 I would imagine.
Based on this the compression temperature if full compression would be about 1400degF.
I revised the numbers to reflect starting at 32deg F (273Kevin).... 273*2.7=737C=1359F
Using a reduced compression pressure of 80psi due to the KACR & assuming no loss in air (this is not actually true due to the exhaust valve lifting and letting air out of the cylinder) the temperature would be about 840degF.
273*1.64=448C=837F 1.64=80/14 'TO THE POWER OF' .285
THEREFORE IN CONCLUSION: It appears that the KACR is in fact lowering the chamber temperature quite a bit on the initial rotation. These temperatures would be less due to some of the heat being transmitted to the cylinder, head, piston, & final vaporization of fuel.
THOUGHTS: I think the KACR is limiting cold starting a little, but once started the engine is very cold blooded, which again points to the fact that there is not enough fuel in the air mixture at these temperatures. If you want to start at low temps, you will need to change the starting jet considerably or go to a pumper carb where you can add fuel by twisting the throttle.
Thanks & good night.
Q: I am trying to calculate the increase in temperature in a combustion engine due to compression. 250cc volume, 12 to 1 compression, initial pressure 14#/sqin, compression pressure 135#/sqin. Beginning temperature 30 degrees. Using ideal gas laws I come up with 4300 degrees, that cannot be right. Air is not ideal gas, but it should not be that far off.
- darrel kruger (age 62)
cottonwood, AZ, USA
A: The numbers you give aren't quite consistent. This is close to an adiabatic compression, meaning that it happens too rapidly for there to be much heat flow out of the gas. For an ideal gas with a temperature-independent heat capacity, we have that pV 'TO THE POWER OF' (γ) stays constant, where p is pressure and V is volume. Those conditions are pretty well obeyed by air, with γ =1.4. So for a factor of 12 compression you expect p to go up a factor of 12 'TO THE POWER OF' (1.4)=32.4. Yet you seem to say that p only goes up by 135/14= 9.6.
At any rate, for an ideal gas the absolute temperature T is proportional to pV, or in our ideal case V 'TO THE POWER OF' (1- γ) or p 'TO THE POWER OF' (γ-1)/γ. If we use your x12 compression in V, we'd get a factor of 2.7 for the increase in T. If we use the x9.6 in p, we'd get a factor of 1.9 increase in T.
When you say"Beginning temperature 30 degrees" I assume that's in °C, or T=303K. So you end up with T of 2.7 *303 K or 1.9*303K, depending on whether your V or p numbers for compression were right. You can do the calculations more carefully, but that's in the range of about 300°C to 540°C.
Mike W.
We will use the calculation that they made using the 1:12 compression, as that is more of a hard number than the 135psi compression test info. Remember that these numbers are theoretical and actual numbers will vary, but not by more than a factor of .5 I would imagine.
Based on this the compression temperature if full compression would be about 1400degF.
I revised the numbers to reflect starting at 32deg F (273Kevin).... 273*2.7=737C=1359F
Using a reduced compression pressure of 80psi due to the KACR & assuming no loss in air (this is not actually true due to the exhaust valve lifting and letting air out of the cylinder) the temperature would be about 840degF.
273*1.64=448C=837F 1.64=80/14 'TO THE POWER OF' .285
THEREFORE IN CONCLUSION: It appears that the KACR is in fact lowering the chamber temperature quite a bit on the initial rotation. These temperatures would be less due to some of the heat being transmitted to the cylinder, head, piston, & final vaporization of fuel.
THOUGHTS: I think the KACR is limiting cold starting a little, but once started the engine is very cold blooded, which again points to the fact that there is not enough fuel in the air mixture at these temperatures. If you want to start at low temps, you will need to change the starting jet considerably or go to a pumper carb where you can add fuel by twisting the throttle.
Thanks & good night.
Q: I am trying to calculate the increase in temperature in a combustion engine due to compression. 250cc volume, 12 to 1 compression, initial pressure 14#/sqin, compression pressure 135#/sqin. Beginning temperature 30 degrees. Using ideal gas laws I come up with 4300 degrees, that cannot be right. Air is not ideal gas, but it should not be that far off.
- darrel kruger (age 62)
cottonwood, AZ, USA
A: The numbers you give aren't quite consistent. This is close to an adiabatic compression, meaning that it happens too rapidly for there to be much heat flow out of the gas. For an ideal gas with a temperature-independent heat capacity, we have that pV 'TO THE POWER OF' (γ) stays constant, where p is pressure and V is volume. Those conditions are pretty well obeyed by air, with γ =1.4. So for a factor of 12 compression you expect p to go up a factor of 12 'TO THE POWER OF' (1.4)=32.4. Yet you seem to say that p only goes up by 135/14= 9.6.
At any rate, for an ideal gas the absolute temperature T is proportional to pV, or in our ideal case V 'TO THE POWER OF' (1- γ) or p 'TO THE POWER OF' (γ-1)/γ. If we use your x12 compression in V, we'd get a factor of 2.7 for the increase in T. If we use the x9.6 in p, we'd get a factor of 1.9 increase in T.
When you say"Beginning temperature 30 degrees" I assume that's in °C, or T=303K. So you end up with T of 2.7 *303 K or 1.9*303K, depending on whether your V or p numbers for compression were right. You can do the calculations more carefully, but that's in the range of about 300°C to 540°C.
Mike W.
What I do know is quite often fuel quality degrades over a short time and can make a single a hard starter in some cases. Mine was one. Drain gas, instant ignition.
Besides, all the KACR does is cut compression briefly to enable the engine to spin up some momentum. Without the release, the engine may struggle to get spinning at first.
#82
01-17-2013, 10:50 PM
So does this mean the KACR is not cutting compression after the starter gets the motor spinning so that full compression is achieved?
#83
01-17-2013, 11:08 PM
My observation when I cranked & ran the motor with the valve cover off is that the KACR is engaged at cranking speeds but not at idle & above.
Last edited by go cytocis; 01-17-2013 at 11:11 PM.
#87
01-19-2013, 02:06 PM
As for spinning, Kawasaki engineering figured there needs to be a bit of relief to allow the engine to spin over TDC to gain momentum when temps are low enough to affect the drag of the oil on the engine components.
Same thing with using a manual release to ease the piston over TDC on an old kick start single, but it is needed virtually all the time. Ease the piston past TDC then the crank has nearly a full 720 degrees of spin to gain momentum before hitting compression. If you ever tried to start a 500 without easing the piston past TDC you will know what I mean and you may have broken bones. If you know what you're doing you could start the bike fairly readily (well at least if it doesn't have Lucas electrics).
Using an auto release will allow the piston to crest TDC a bit easier at least the first time over to gain momentum. All it takes is enough spark to light the charge and the combustion process starts. Even an engine with 20% leak down (aka an engine with bad rings or slightly tweaked valves that don't quite seat) can start if ignition is right and the engine can spin fast enough with a good fuel charge. That was my problem with gas that is old, it won't fire. My recent problem was a plug that wouldn't spark adequately if at all. One has been recurrent if I did not drain fuel, the other was remedied with a plug. I was fortunate - easy and cheap.
#88
01-19-2013, 02:36 PM
The spring that acts against the centrifugal weights is VERY light so it doesn't take a lot of revs to disengage the KACR.*The motor can't be spinning at much more than ~2-300rpm when cranking whereas it idles at ~1100rpm so there's a big jump in centrifugal forces between cranking & idling states, which easily overcome the resistance from the little spring.
I also remember having a manual compression release on an old XS650 and I recall that mechanism would allow partial engagement as you describe, but I am quite confident that on the KLX a properly functioning KACR will be TOTALLY disengaged once the engine's idling.
#89
01-20-2013, 07:37 PM
My observation from running the motor with the valve cover off was that the KACR does in fact operate in a pretty immediate & binary way.*
The spring that acts against the centrifugal weights is VERY light so it doesn't take a lot of revs to disengage the KACR.*The motor can't be spinning at much more than ~2-300rpm when cranking whereas it idles at ~1100rpm so there's a big jump in centrifugal forces between cranking & idling states, which easily overcome the resistance from the little spring.
I also remember having a manual compression release on an old XS650 and I recall that mechanism would allow partial engagement as you describe, but I am quite confident that on the KLX a properly functioning KACR will be TOTALLY disengaged once the engine's idling.
The spring that acts against the centrifugal weights is VERY light so it doesn't take a lot of revs to disengage the KACR.*The motor can't be spinning at much more than ~2-300rpm when cranking whereas it idles at ~1100rpm so there's a big jump in centrifugal forces between cranking & idling states, which easily overcome the resistance from the little spring.
I also remember having a manual compression release on an old XS650 and I recall that mechanism would allow partial engagement as you describe, but I am quite confident that on the KLX a properly functioning KACR will be TOTALLY disengaged once the engine's idling.
As for starting the big inch single, the compression release is only used to get the piston just over TDC compression before kicking and then kicked like you mean to do it. Otherwise you might find it painful when the engine kicks back if it hits TDC compression and pops back. That is the only time the release is used on the SR, not while actually kicking. I have no idea about the 650 twin, if it worked one or both cylinders and if it was held in all the time so as to more or less make the starting like kicking over a 325 single when it came to compression. How did it work?
#90
01-20-2013, 11:53 PM
klx678, It's tough to describe the KACR operation without illustrations, but the 'lobe' of the KACR that actuates the exhaust valve at cranking speeds slips into a little recessed pocket when the fly weights open. The profiles of the backside of the lobe, and the pocket it recesses into, don't permit partial activation. *I agree that the spring will have a range of rpms across which it stretches, but the lobe itself recesses either all or nothing.
On the old XS, the manual compression release was essentially like a second clutch lever that operated a cable which entered the left side valve cover and then activated a 'rocker' which depressed the left exhaust valve. *That little 650 could give you a sprained ankle if you didn't plan the kick right. *Same thing on my BMW 75/5 for that matter!
On the old XS, the manual compression release was essentially like a second clutch lever that operated a cable which entered the left side valve cover and then activated a 'rocker' which depressed the left exhaust valve. *That little 650 could give you a sprained ankle if you didn't plan the kick right. *Same thing on my BMW 75/5 for that matter!
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