RV-12iS Details

[rv6ejguy]

Originally Posted by rvbuilder2002

I don't know enough about engines to explain why, but the iS engine that Rotax currently sells does indeed require additional cooling capability when compared to the ULS ( the effort required to modify the original cowl exit area, and other details wouldn't have been done if it wasn't necessary).
Representatives of Rotax have confirmed this and have said that many manufacturers have had to make changes to deal with it.

The final design of the RV-12iS allows for a climb at Vy to 10000 ft on a 100 *F day with temps staying in the normal range.

Well, physics is physics. If they wanted more cooling margin then that would explain increasing airflow. Also, it seems if would perhaps be harder to get a very efficient duct shape to feed the cylinders with the extra clutter of the EFI bits on the top of the engine.

[Crafting N112DR]

Ryan also said at the banquet that the engine is the 912iS Sport, because he said there was some problems with the iS.

[larosta]

Perhaps the difference in temperatures is,one of the measurement technique(s) and procedures themselves and not a violation of physical theories. It is common that when measurement tools, sensors, etc. have become,better and more accurate earlier assumptions and beliefs start to be questioned. This may be just an apples and oranges type of problem and not a ripple in our widely accepted laws of thermodynamics.

-larosts

[AndrewR]

Lower fuel burn for the same TAS means higher thermal efficiency and that means less heat out through the exhaust, oil and cooling systems as I said before.

physics is physics

And chemistry is chemistry

Best power mixture is cooler, because you get about 3 times the heat making CO2 than CO. However, if you are O2 limited (rich) you can make twice as much CO as CO2 from the same amount of air - so you get your pressure (power) from more gas at a cooler temperature. The rest of the energy in the fuel goes out the exhaust as CO - it is never released as heat.

I did the calculations in this thread:
http://www.vansairforce.com/communit...0&postcount=20

The bottom line is that 20% more fuel from peak EGT gives about 10% less heat with the same amount of air i.e. same RPM and manifold pressure.

[rv6ejguy]

Originally Posted by AndrewR

And chemistry is chemistry

Best power mixture is cooler, because you get about 3 times the heat making CO2 than CO. However, if you are O2 limited (rich) you can make twice as much CO as CO2 from the same amount of air - so you get your pressure (power) from more gas at a cooler temperature. The rest of the energy in the fuel goes out the exhaust as CO - it is never released as heat.

I did the calculations in this thread:
http://www.vansairforce.com/communit...0&postcount=20

The bottom line is that 20% more fuel from peak EGT gives about 10% less heat with the same amount of air i.e. same RPM and manifold pressure.

Ideally, we'd map the high rpm/ high MAP areas for around 12.5 AFR and the cruise conditions for around 17 to 1. You'll see from the graph I published that temps are roughly the same at 12.5 and 17 AFRs, although power is obviously less that far LOP. Yes, 12.5 is cooler for the same power but you'd never want to cruise here for fuel economy and the 912is clearly doesn't.

As far as the real world goes, more mpg at the same TAS means higher TE which means more fuel energy goes into turning the crank, less into the cooling system and exhaust stream. Higher TE means you can run the same TAS with a lower power setting which equals less heat. No way you need more cooling mass flow with a more efficient engine unless there are other factors at work here or you want more margin than before.

LOP operation has clearly showed 15-25% better mileage over best power mixtures while at the same time showing reduced CHTs and higher detonation margins than MBT AFRs.

More info and food for thought here: https://disciplesofflight.com/fuel-a...rcraft-engine/

[Mich48041]

Yes, physics is physics. Refrigerators work on the principle that an evaporating liquid absorbs heat. When gasoline evaporates, it absorbs heat. The ULS engine wastes fuel. That evaporating fuel absorbs heat and it is expelled out the exhaust.
Since the iS engine does not waste fuel, the heat must be transferred to the air by some other means: via the water and oil coolers.

[AndrewR]

temps are roughly the same at 12.5 and 17 AFRs, although power is obviously less that far LOP

Surely for a valid comparison you need to compare the temperature at the same HP output?

12.5 is cooler for the same power but you'd never want to cruise here for fuel economy and the 912is clearly doesn't.

Right, but the 912ULS might cruise in this range, especially since you don't have precise mixture control with the CV carbs.

If the fuel injection on the is allows more precise control, Rotax can run the engine leaner which means more heat at the same power setting. Potentially even at maximum power - Rotax presumably want to be very sure the carbed engine doesn't stray into dangerous territory at maximum power.

[AndrewR]

evaporating fuel absorbs heat and it is expelled out the exhaust

The effect of evaporating fuel is very small compared to the heat of combustion.

You have to be very rich before raw fuel is going out the exhaust. Excess fuel mainly means you are making CO instead of CO2.

The chemistry heat of reaction calculations DO factor the energy taken to go from liquid fuel to H2O, CO2 and CO gas.

You can do the calculations giving H20 in liquid or gas forms, and get different answers. I used gas of course, since water is a gas at combustion temperatures.

[rv6ejguy]

Originally Posted by Mich48041

Yes, physics is physics. Refrigerators work on the principle that an evaporating liquid absorbs heat. When gasoline evaporates, it absorbs heat. The ULS engine wastes fuel. That evaporating fuel absorbs heat and it is expelled out the exhaust.
Since the iS engine does not waste fuel, the heat must be transferred to the air by some other means: via the water and oil coolers.

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.

[rv6ejguy]

Originally Posted by AndrewR

Surely for a valid comparison you need to compare the temperature at the same HP output?



Right, but the 912ULS might cruise in this range, especially since you don't have precise mixture control with the CV carbs.

If the fuel injection on the is allows more precise control, Rotax can run the engine leaner which means more heat at the same power setting. Potentially even at maximum power - Rotax presumably want to be very sure the carbed engine doesn't stray into dangerous territory at maximum power.

Are you referring to CHTs here? With regards to cooling, the EGTs have no impact.

We know the carbed engines do cruise in this range from my testing in 2007.

We're talking about the practical considerations of why Van's and Rotax increased the cooling system inlet and exit sizes. We don't know if actual cooling mass flow was increased by these changes but that's the assumption.

Assuming no airframe or cooling system drag increases, the aircraft requires the same hp for the same TAS at a given altitude. If we're burning less fuel for the same TAS, that means less heat into the cooling system since equal energy is going into the crankshaft. In any case, cruise cooling is rarely a limiting issue, climb is.

The EFI engine may make more power than the ULS at BMT AFRs and Rotax may be able to run leaner than BMT AFR, even at high power. In this case, they could trade a bit of hp for lower fuel consumption at climb power settings. This could be an explanation for the increased heat. We don't really know what they are doing with AFRs. It would be fascinating to instrument one with a wideband.

As I stated before, the carbed 912s have unequal mixture distribution and the EFI gets rid of that so all cylinders can run at the nearly same AFRs. This on its own, is more efficient.