Time-Area Question

[ids987]

Since there have been so many good topics on engine timing etc recently.....

After reading some of Gordon Jennings' material, I took some time-area measurements on a couple of engines I had lying around at home. I made the assumption that the transfer port time-area should be the sum of the time-area of all the transfer and boost ports (using mean area in the way Jennings suggests), but in both cases, the transfer time-area came out very high (at least in relation to the Naitoh-Nomura numbers quoted by Jennings). The exhaust time-area, however, seemed to be more as expected.

Just wondering whether anyone here may know if I made the wrong assumption, or whether the numbers may just be different for the engine sizes and RPMs for our applications ???

Happy New Year One and All:

Ian

 

[DaveMarles]

Which engine did you measure ? On high performance nitro motors I get comparable figures. I measure exact TA not the mean TA and for example my modified 91RS using the same units as Jennings has ex TA as .000166 and transfer TA as .000088 which is not too far from his figures. Don't forget that when measuring TA you have to use the peak hp rpm or at least an estimate of it which is not the same as the rpm on the water. If you are measuring a modded Zenoah or similar gas engine then the figures could easily be way out depending on who modded it.

Dave

 

[ids987]

Which engine did you measure ? On high performance nitro motors I get comparable figures. I measure exact TA not the mean TA and for example my modified 91RS using the same units as Jennings has ex TA as .000166 and transfer TA as .000088 which is not too far from his figures. Don't forget that when measuring TA you have to use the peak hp rpm or at least an estimate of it which is not the same as the rpm on the water. If you are measuring a modded Zenoah or similar gas engine then the figures could easily be way out depending on who modded it.
Dave

Dave, Thanks for the reply. I started off with a OPS .40 I had lying around, and the sum of the measured mean intake and boost time-areas (at least as I measured them), came out to around .00010 at about 30,000 RPM - I started by working out the time-area figures at 25,000 RPM, and the intake was way out of range, so I then worked out the RPM which would bring it back into the quoted range (0.00008 - 0.00010). I then looked at a CMB .45 EVO2. This has considerably more intake/boost port area, and longer duration, so it pushes the figures further out. In both cases I used crank angle equal to half open duration to estimate mean area by mean "height".

I did wonder if the (much) richer mixture in glow engines (over gas engines) might need more intake time-area due to more density/viscosity, but your figures show that is not the case.

I must be doing, or maybe just measuring something wrong.......

Ian

 

[DaveMarles]

Which engine did you measure ? On high performance nitro motors I get comparable figures. I measure exact TA not the mean TA and for example my modified 91RS using the same units as Jennings has ex TA as .000166 and transfer TA as .000088 which is not too far from his figures. Don't forget that when measuring TA you have to use the peak hp rpm or at least an estimate of it which is not the same as the rpm on the water. If you are measuring a modded Zenoah or similar gas engine then the figures could easily be way out depending on who modded it.
Dave

Dave, Thanks for the reply. I started off with a OPS .40 I had lying around, and the sum of the measured mean intake and boost time-areas (at least as I measured them), came out to around .00010 at about 30,000 RPM - I started by working out the time-area figures at 25,000 RPM, and the intake was way out of range, so I then worked out the RPM which would bring it back into the quoted range (0.00008 - 0.00010). I then looked at a CMB .45 EVO2. This has considerably more intake/boost port area, and longer duration, so it pushes the figures further out. In both cases I used crank angle equal to half open duration to estimate mean area by mean "height".

I did wonder if the (much) richer mixture in glow engines (over gas engines) might need more intake time-area due to more density/viscosity, but your figures show that is not the case.

I must be doing, or maybe just measuring something wrong.......

Ian

By intakes I hope you mean transfers! What TA figures did you have for 25,000 rpm?

 

[ids987]

By intakes I hope you mean transfers! What TA figures did you have for 25,000 rpm?

Sorry, transfers.

For 25,000 RPM I got around 0.00013 - comprised of 0.0000494 for each transfer, and 0.0000299 for the boost port. For exhaust I got 0.00015. Timing I made 167 (exhaust), 127 (transfer), 122 (boost). Areas of .89 sq cm (exhaust), .38 sq cm (each transfer), .24 sq cm (boost).

I started by converting piston position (height) to angles, then halved the angle and converted back to height - to obtain the mean height of the port, so I guess errors could have been compounded. These are all for the OPS .40.

The transfer/boost port area / volume for the CMB .45 looks quite similar. I haven't gone much further with the CMB .45 numbers, as I started comparing differences between different sleeves. Looks like on the whole the transfer port area is smaller, but the boost port area is much greater. Divided by the greater volume seems to give a similar number.

Ian