Diesel vs miller cycle

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stanislavz

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Again: one of the reasons that pretty much ALL newly certified piston engines for some time are diesels, not gassers.
Is int the real reason of just end of lifetime of av-gas itself?
 

PMD

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The Rotax 912iS and 915iS are SI engines where the EFI/ variable EI tech replaced carbs and fixed timing EI, resulting in a 15-20% reduction in fuel flow for the same TAS. These are the best selling aero engines in the world where CI engines have barely scratched the surface of market share in the past decade.
In defense of CI vs SI: EVERY diesel now certified has a dismal power/weight ratio. Once someone comes up with Rotax power density (that is IMHO the benchmark) things will change.
 

stanislavz

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Once someone comes up with Rotax power density (that is IMHO the benchmark) things will change.
You have to compare of total weight for your mission - engine + fuel. I did this for Rotax 912 UL / ULS / 914 / DV6

And its all depends on you mission at the end.
 
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rv6ejguy

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In defense of CI vs SI: EVERY diesel now certified has a dismal power/weight ratio. Once someone comes up with Rotax power density (that is IMHO the benchmark) things will change.
The CD155 with aluminum block isn't bad at 295 pounds. The others are pretty heavy.
 

stanislavz

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Numbers on 912 ul/uls / 914 /dv6

Uls/Ul - 88kg firewallforward,
914 - 104kg
Dv6 - 107kg.

SI/CI shows 25% of better bsf. For Cl. So we want to save 19kg of fuel per you mission to be on pair with SI.

And it looks like if our mission needs 76kg of fuel - CI will be on par with it.

And, yes it will hurt us on shorter trips..
 

Vigilant1

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That's some very good performance from the fuel injected Rotax. From the article:
+++++++++++++
"The 912 iS ECU is programmed to run in eco mode at 77 percent power or less, at which point the engine will operate at what European engineers know as Lambda 1, or a stoichiometric air-to-fuel ratio of 14 to 1. Without providing much detail, Rotax said the 912 iS is also capable Lambda 1.3 or lean-of-peak operation. In power mode, it runs at about Lambda 0.8 or 0.9."
++++++++++++++++

From the operator manual, the ECU depends on the following sensors:
"Throttle valve position, engine speed, intake air temp, ambient pressure, manifold pressure, exhaust temperature."

So, in keeping with their discussion of lean-of-peak operations and Lambda, how does the system determine the AFR without using an O2 sensor? Is it operating in a closed loop mode, but using EGT as the feedback sensor (just as a pilot might adjust mixture manually by reference to EGT)?

The Rotax redundant ECU setup seems very sophisticated (aka a thicket of complexity to troubleshoot). The CANBUS system can help, I guess, or it can introduce another batch of complexity.
 
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stanislavz

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So, in keeping with their discussion of lean-of-peak operations and Lambda, how does the system determine the AFR without using an O2 sensor? Is it operating in a closed loop mode, but using EGT as the feedback sensor (just as a pilot might adjust mixture manually by reference to EGT)?
Open loop. Alpha-N alhorithm. Less precise than lambda one, but more robust.
 

dwalker

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Open loop. Alpha-N alhorithm. Less precise than lambda one, but more robust.
I would not use Alpha-N, and I would be surprised if many do.

Almost all efficient motors use a speed/density or a MAF system with 02 feedback. In the case of using Leaded fuel- which I have no idea why anyone would want to do that in a modern engine- the wideband 02 would be used for initial and perhaps periodic calibration- say every hundred hours or so- and then removed from the system and plugged.


Now in aircooled engines, I think the challenge in an EFI, turbocharged, flight engine would be the predictable high CHT at full throttle and boost. Aerovee limits full power use to I think 3 minutes? There are a couple of things that can be done to help. Right out of the gate a water/alcohol injection system would allow for higher boost levels with greatly reduced detonation risk with the side benefit of cleaning the combustion chambers. Yeah, I know, its one more thing to check, like fuel and oil, but with modern ECUs the boost level simply cannot be reached if there is no water to inject. Aerovee is not intercooling the compressed charge, and I think that is a huge mistake on an aircooled engine. Regardless of what the intake air temp is the act of compressing it heats that air up and that is all bad when you are trying to limit risk of detonation. A very simple air/air intercooler would go a long way. Lastly, in an aircooled engine, E85 fuel instead of 100LL will likely offer a lot of benefits over 100LL. With the E85 the engine can be run very lean without overheating the cyl heads. The risk of detonation is almost non-existent in high boost modes, and of course, it keeps the engine very clean and removes carbon buildup which in itself helps lower the risk of detonation. Yes, I am very well aware of the propensity to need to burn more of it- roughly 1/3 more in some situations- however with careful tuning in high compression engines the fuel usage was very comparable to "pump gas".

I like the idea of small and powerfull diesel engines, and the Mazda skyactiv diesel is really cool.
 

Vigilant1

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Maybe Rotax is getting these BSFC numbers using relatively good open loop logic (MAP, baro, RPM and temp) and they've just built their fuel tables to appropriately lean the injection when the engine isn't being asked to make much power (e.g low MAP relative to RPM). I dunno. But they aren't warning against leaded fuel (except as it affects valves), and they apparently aren't using an O2 sensor in normal ops.
 

stanislavz

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like the idea of small and powerfull diesel engines, and the Mazda skyactiv diesel is really cool.
Exactly. Sad that it is not available in 1.4-1.6 liter size. Coolnes due to its lower compression ratio...

On maf or map - agree. But is still a game of how much we need sensor to run it in high efficiency, and how much for limp home mode.
 

dwalker

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Maybe Rotax is getting these BSFC numbers using relatively good open loop logic (MAP, baro, RPM and temp) and they've just built their fuel tables to appropriately lean the injection when the engine isn't being asked to make much power (e.g low MAP relative to RPM). I dunno. But they aren't warning against leaded fuel (except as it affects valves), and they apparently aren't using an O2 sensor in normal ops.
That is exactly how it works.

In the engines that ran on leaded fuel (few and far between these days) if the wideband dies it simply defaults to open loop and runs off of "hopefully" well done calibrations. In general the engine will not run as lean as it might in closed loop, but with good base calibration and trim tables it should be close.
 

dwalker

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Exactly. Sad that it is not available in 1.4-1.6 liter size. Coolnes due to its lower compression ratio...

On maf or map - agree. But is still a game of how much we need sensor to run it in high efficiency, and how much for limp home mode.
In most modern EFI, the only sensors 100% necessary for the engine to run are the crank and cam sensors. The other sensors have default values that, if they fail the ECU uses that value and trim tables in thier place.

Of all the things I think about when considering EFI system failure sensor issues are pretty far down the list.
 

Vigilant1

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But we all want drip carburator as our fadec unit. With magnetos..
In a plane that can accommodate gravity feed from the fuel tank, I'd think a floatless carburetor (e.g. Lake, POSA, RevFlow, AeroInjector, etc) could make an easy, light, reliable backup to an EFI system. It's little more than a calibrated fuel leak (tapered needle that admits more fuel as the slide throttle is opened) that varies mostly by throttle position, a little bit due to airflow rate past the needle. Since the EFI needs a throttle body anyway, these carbs can serve that purpose. No venturi (so little chance of icing), no fuel pump (so no electricity needed). If the EFI crumps out, just open the mixture knob to the carb and let that fuel start flowing.
With a well designed and installed EFI, it would probably never be needed.
 
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rv6ejguy

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Exactly. Sad that it is not available in 1.4-1.6 liter size. Coolnes due to its lower compression ratio...

On maf or map - agree. But is still a game of how much we need sensor to run it in high efficiency, and how much for limp home mode.
Dual ECUs with dual sensor suites as Rotax does on their iS engines and we do on Lyconentals. No limp mode in aircraft ECUs since you need relatively high power to maintain flight compared to automotive applications where you only need 20-30 hp to limp home. Most sensors outside of crank sensors are not needed to keep the engine running to get you home safely if you mitigate with defaults.

ecu2000.jpg
 
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