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07-26-2017, 07:01 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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Not enough information on this "heat"
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Originally Posted by rv6ejguy
Higher thermal efficiency means more energy into the crank, less into the cooling system. How else could it burn 20% less fuel? The iS must be running leaner cruise mixtures and more optimized timing to get the same TAS on less fuel.
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Not necessarily - If you think about it, there a huge number of variables that affect the jacket water heat rejection. So, for the same bore/stroke, ring/bore package, valve events, intake/exhaust systems, cooling jackets, cooling jets, coolant/flow/temperature, and combustion chamber swirl/tumble (and more) that would be true. What other changes are in this engine vs the one it replaced?
As example, higher swirl and tumble will allow higher cr, faster heat release, resulting in better BSFC but will also increase heat rejection to the head and cylinder. 20% claim would have to be carefully defined, though.
One can drive better power to the crank with better expansion ratios, and might yield higher JW heat rejection at the same time, but with less exhaust energy content/losses through less blowdown energy.
Also, what are the comparisons of the earlier models' installation temperatures relative to ambient.
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
Last edited by BillL : 07-26-2017 at 07:09 AM.
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07-26-2017, 07:14 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by BillL
Not necessarily - If you think about it, there a huge number of variables that affect the jacket water heat rejection. So, for the same bore/stroke, ring/bore package, valve events, intake/exhaust systems, cooling jackets, cooling jets, coolant/flow/temperature, and combustion chamber swirl/tumble (and more) that would be true. What other changes are in this engine vs the one it replaced?
As example, higher swirl and tumble will allow higher cr, faster heat release, resulting in better BSFC but will also increase heat rejection to the head and cylinder. 20% claim would have to be carefully defined, though.
One can drive better power to the crank with better expansion ratios, and might yield higher JW heat rejection at the same time, but with less exhaust energy content/losses through less blowdown energy.
Also, what are the comparisons of the earlier models' installation temperatures relative to ambient.
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I agree, there could be multiple internal changes to the injected engine with regards to the chamber design, CR etc. All interesting questions.
Your last point follows my thinking- they may have done this to increase cooling margin on hot days.
Last edited by rv6ejguy : 07-27-2017 at 08:31 AM.
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07-26-2017, 03:59 PM
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Join Date: Jan 2005
Location: Irvine, CA
Posts: 873
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Quote:
Originally Posted by RV8JD
Thanks for the info. What is the difference between the iS and the iS Sport?
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This explains it:
http://www.flyingmag.com/aircraft/ls...ngine-unveiled
__________________
Mark
RV-12iS Fuselage
RV-9A Project: Sold
VAF donation made for 2020
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07-27-2017, 05:03 AM
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Join Date: Jun 2017
Location: Ballarat, VIC
Posts: 50
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Quote:
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Are you referring to CHTs here? With regards to cooling, the EGTs have no impact.
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I was referring to the chart you referenced. Without seeing the source of the chart, I guess the thermal plug is the important temperature. If you want to compare temperatures at different mixtures, you need to keep power constant not airflow i.e. the IMEP line should be flat.
Quote:
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If we're burning less fuel for the same TAS, that means less heat into the cooling system
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This is not correct. Yes the power into the crankshaft is the same, however the temperature is very different.
The increase in pressure in the combustion chamber comes from 2 sources when fuel is burnt - increase in temperature and increase in number of gas molecules.
Lean of stoich the temperature and power will be reasonably linear because you are converting all fuel to CO2.
Rich of stoich you start making CO instead. Combustion to CO gives you much less heat, but you get twice as many CO molecules per O2 as CO2.
So you are getting more of your power from the increase in gas molecules, and less from increase in heat.
From 14.7 -> 12.5 AFR you get much less heat released from the fuel, even though power increases. Less heat released means less cooling needed.
So if Rotax can control the AFR more precisely and as a result don't need to run as rich to give a margin for error, it is not surprising that more cooling is needed. Leaner will give you more heat at any equivalent power setting.
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07-27-2017, 07:23 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by AndrewR
I was referring to the chart you referenced. Without seeing the source of the chart, I guess the thermal plug is the important temperature. If you want to compare temperatures at different mixtures, you need to keep power constant not airflow i.e. the IMEP line should be flat.
This is not correct. Yes the power into the crankshaft is the same, however the temperature is very different.
The increase in pressure in the combustion chamber comes from 2 sources when fuel is burnt - increase in temperature and increase in number of gas molecules.
Lean of stoich the temperature and power will be reasonably linear because you are converting all fuel to CO2.
Rich of stoich you start making CO instead. Combustion to CO gives you much less heat, but you get twice as many CO molecules per O2 as CO2.
So you are getting more of your power from the increase in gas molecules, and less from increase in heat.
From 14.7 -> 12.5 AFR you get much less heat released from the fuel, even though power increases. Less heat released means less cooling needed.
So if Rotax can control the AFR more precisely and as a result don't need to run as rich to give a margin for error, it is not surprising that more cooling is needed. Leaner will give you more heat at any equivalent power setting.
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"The CHT drops more quickly on the lean-of-peak side of 15 GPH, than it does on the rich-of-peak side."
Superimpose this chart with my previous AFR vs. IMEP chart.
There is plenty of real world data from APS and others showing that your assertion is incorrect. Despite what chemical theory says, CHT vs. TAS tells us the actual story. If TAS is the same, power must be the same. If CHT is lower under these conditions, then there is less heat to dissipate from the cooling system. See the links below.
APS recommends adding back a bit of MAP in turbo engines to restore speed lost from leaning. The CHTs are still lower, FFs are way down and TAS is the same.
Many of our clients climb LOP as well to keep CHTs down and in cruise at altitude, LOP can sometimes result in the CHTs getting down near the minimum allowed on Lycoming engines.
I have trouble buying theory when it's contradicted by actual real world data.
http://www.gami.com/paulferraris_leanofpeaksaga.pdf
http://www.gami.com/articles/bttfpart1.php
Last edited by rv6ejguy : 07-27-2017 at 08:16 PM.
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07-27-2017, 08:18 AM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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While you are at Oshkosh, has anybody asked Van whether he plans to upgrade his personal RV-12?
__________________
Alan Bishop
Mechanical Engineer
PPL since '66
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07-27-2017, 12:18 PM
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Join Date: Aug 2008
Location: Texas, Fort Worth
Posts: 1,237
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You cannot upgrade an old RV 12 to the new Fuel Injected engine through Vans!  If you mean is he going to build the complete
new Version of the RV12 then Yes he is.
__________________
John
RV12 N1212K
Flying Since June 2010
1020 Hours as of 9/30/2017
Johnrv12@icloud.com
RV14 Wing, arrived and building at Rdog's new Hanger at 16X
S/N 140014
Last edited by Jetguy : 07-27-2017 at 12:52 PM.
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07-27-2017, 01:22 PM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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I understand what you are saying, John. There apparently are people with fully completed forward fuselages who are selling them and presumably using the tail, rear fuselage, and wings as the basis the what they are building, so I would call that an upgrade. But it certainly makes more sense to me for Van to start from scratch and sell his existing RV-12. I would probably want to do the same thing.
I really like the improvements they have made. I do think being able to lock up the canopy is important. They probably had a reason for leaving out that feature, however. Someone who became locked in could quickly die in the heat.
__________________
Alan Bishop
Mechanical Engineer
PPL since '66
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07-27-2017, 01:38 PM
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Join Date: Dec 2008
Location: Lacey, WA
Posts: 1,365
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Quote:
Originally Posted by AlanTN
**************snip***********
I really like the improvements they have made. I do think being able to lock up the canopy is important. They probably had a reason for leaving out that feature, however. Someone who became locked in could quickly die in the heat.
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Alan,
There is/has been an excellent after-market lock available by Aircraft Specialties:
I don't think it's possible to lock yourself in with this lock.
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Tony
E-LSA RV-12 ULS
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07-27-2017, 08:57 PM
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Join Date: Oct 2013
Location: Powell, OH
Posts: 250
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That's me, I'm selling my old style fuselage. (feels funny saying that) Will use the wings (will need to install a landing light in the Left wing) and the empennage.
__________________
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David Rohrlick
PPSEL, A&P
RV-12iS Completed
Airworthiness signed off by the FAA 1/23/2020
First Flight on 3/14/2020
Phase I Finished on 5/7/2020
Need:
Paint?
Serial#12-0864
N112DR
https://youtube.com/c/CraftingN112DR
VAF donation made for 2020
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