Scrapman1959

Fuel Injection or Carbs on my next CT?

66 posts in this topic

I'm ready to replace my 2011 CTLS which has 1100 hours on it, all flown by yours truly.....I would like  input, good and bad, from the guys flying the Fuel Injected models. I have never had a problem with the 4 CTs I have owned, all of them carbureted. My main concern is the reliability of the FI engines, as I am a long way from anybody that could fix a FI problem. I heard the early versions had some issues and were reprogrammed etc and are now flying great? Is this the whole story? If you have had problems with your Fuel Injected CTLSi, and prefer not to air them on the forum at the risk of devaluing your AC for future sale, please call me at 608 778 0328 and I will keep your comments confidential. Thx guys.

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I love the fuel injection.  Very easy start.   Not sure you can even get carburated anymore.  Thet said it is heavier and more expensive.   And as a new engine model introduced three or four years ago I've probably had one thing every year so far that needed changing.  All covered by rotax but still a PIA.   Really looking forward though to the 915is.   

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I have the 912 ULS engine which starts first time pretty much all the time.

I went through the carb float issues a little while back and despite finding a sunk float experienced no performance issues other than a mild fuel smell after shut down. Got new floats and was reimbursed by Rotax.

I have the carbs balanced at every annual and they were overhauled at 200 hours with nothing found. Ive been using both 93 non-ethanol and lately 93E-10 for the past couple of years. Performance has been outstanding even with high DAs in the current heat-wave in SC.

I plan on 5.2 GPH usually at 5300 rpm but based on fuel used /fuel added Ive been getting 4.8gph and 4.5 GPH at 5000 rpm.

I have an RV-12 SLSA with 350 hours on it and I'm intrigued with the latest Vans announcement of a 912iS powered RV-12 in the future. I realise that Vans is keeping up with other manufacturers and that the iS engine with replace the ubiquitous ULS in the not too distant future. That said I feel my ULS still compares quite well to the iS particularly as regards the increased weight and cost plus the reports of frequent maintenance issues.

I wonder if/when I contemplate trading up to a newer technology LSA that a more reliable and more efficient model of the current iS will be available by then. In the meantime I'm happy and content with the ULS.

My two cents.

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I have flown a 2006 CTSW, a 2012 CTLS, and a 2016 CTLSi.   I'm one of those guys who gets "the itch" whenever new toys are available...  Don't worry ScrapMan - guys like us keep the economy moving forward!

My take?   Don't see too many new cars on the road with carburetors anymore... (come to think of it, not sure there have been any in 20+ years?)  Why someone would prefer carbs over fuel injection is beyond me (let alone 2 carbs).  The Fuel Injected 912 has tons of redundancy built in.  Never a good idea to be the first person with a new engine design but in this case, the 912iS Sport is no longer "new".  Enough flying in the world for engineers to make their initial sets of tweaks and to have refined the design.

It starts easy, runs smooth as silk -  I ordered one as soon as the option was available.  Knock on wood - I'm happy so far!

Speaking of carbs - I do like carbs on some things...  lol

Daytona.jpg

Cobra.jpg

cobra2.jpg

Engine.jpg

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I have about 2500 hours in big-bore Continentals, and about 100 in the CTLSi.  While I can't compare with the carbureted version of the 912, I love the 912i.  Runs like a car.  Hands-down preferred over the Continentals.

Andy

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Maybe wait for the Roush conversion. 😎

Seriously though for me the question would be do you want to do your own work on the engine. If not I see no issue (except a bit more weight) and some advantages to the Si. I think the major issues have been dealt with.

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I would not compare the fuel injected 912is engine to that in a car. I don't believe the Rotax engine has an oxygen sensor in the exhaust. I think it is using an open-loop fuel control system based on the fuel mass flow and air density. This system offers advantages over a carburetor's mechanical fuel control; including smother starts and improved combustion efficiency (lower specific fuel consumption), but it is not able to achieve the optimized fuel flow of automotive systems which have a closed-loop control based on exhaust gas oxygen level.

The problem is that the lead in our aviation fuel destroys the oxygen sensors. Once we complete the change-over to lead-free fuel I think you will see fuel injection systems become prevalent on aircraft and the benefits will be more dramatic than the 912is can achieve. However, this complete transition, worldwide, may not occur in my lifetime.

Mike Koerner

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On 7/24/2017 at 1:31 PM, Adam said:

 

My take?   Don't see too many new cars on the road with carburetors anymore... (come to think of it, not sure there have been any in 20+ years?)  Why someone would prefer carbs over fuel injection is beyond me (let alone 2 carbs).

I agree with regard to automobiles.  But in the case of a CT, there are three answers to preference for carbs:

1) Weight

2) Simplicity

3) Weight

The weight difference is ~22lb, which is almost four gallons of fuel (an hour flying) or an additional overnight bag.  Not crazy weight, but where we are against a hard limit of 1320lb, every ounce starts to count.  A CTLSi gets down to about 480lb useful in most cases, which starts to become a one person airplane for practical travel for most adult Americans.  That's a 200lb pilot, 150lb passenger, 20 gallons of fuel, and nothing else other than headsets.  

The simplicity thing comes in if you do any maintenance yourself, or if you have to do emergency repairs at a remote airport.  With a carbed airplane you can drop float bowls, look at diaphrams,  disassemble needle jets, etc with basic tools to get you flying again.  Any A&P can do it.  With fuel injection all you can do really is call a Rotax maintenance shop and pray they can come to your location.  Nobody else even has the dongle to diagnose the fuel system.  

There are significant advantages to fuel injection, and we all understand the fuel economy and efficiency arguments.  And now that the price difference has come down a LOT, the cost argument against injection is going away.  But there are still good reasons to prefer carbs on a CT, especially if you operate in areas away from easy access to Rotax certified maintenance.  

 

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Something else I would factor into that list, and in my humble opinion, of most importance is reliability.

Without question, the fuel injection is more reliable.

Just think back, over the last few years on this forum, how many problems have been fielded regarding carburation.

Countless.

So I guess, the question is . . . overall . . . what is that reliability worth?

A little less usable fuel?

 

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Fuel injection is the only way to go.  And just because it's different, doesn't mean you can't work on it yourself (provided you're qualified).

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8 hours ago, N456TS said:

Fuel injection is the only way to go.  And just because it's different, doesn't mean you can't work on it yourself (provided you're qualified).

I don't think you are going to be doing field repairs on your fuel injection system.

Don't get me wrong, I think fuel injection is great, I just think there are some advantages to carbs worth considering.  I'm not sure I agree with Bill's assessment that FI is more reliable.  My carbs have never stranded me or quit in flight, I think that is rare for either system.  Set up properly the FI unit will be more "hands off" with no tweaking, but it's also more complex and more things "have to go right" for it to keep working as intended.  I'd call reliability a wash, though that's just my subjective opinion.

Either system is great, and if they weighed the same I'd go FI.  But that weight penalty still gnaws at me.  If the CT had a 1500lb gross it would simply not matter, but at 1320lb it's a bitter pill.

 

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The way I look at that weight penalty is 5 gallons of fuel.

But with the better fuel economy, the fuel injection endurance and range numbers are commensurate with the ULS engine.

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Bill and Andy both make good points about FI vs carburetors.  One big reason why virtually all new vehicles are injected is to meet increasingly strict emissions requirements (especially in Europe).  Obviously, given advantages in fuel economy and emissions, FI will eventually replace carburetors on all but the smallest engines (e.g., chainsaws).  

My personal preference is still for carburetors.  The injected 912 was not without problems.  Maybe with another 10 years of experience, the system will be proven to be utterly reliable and will also justify the extra weight and complexity.  Until then, I'm sticking with carburetors.  I understand carburetors and I have all of the tools I need to maintain them.

BTW, my airplane is E-LSA and I do all of the maintenance, including the fuel system.  

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5 hours ago, FredG said:

Bill and Andy both make good points about FI vs carburetors.  One big reason why virtually all new vehicles are injected is to meet increasingly strict emissions requirements (especially in Europe).  Obviously, given advantages in fuel economy and emissions, FI will eventually replace carburetors on all but the smallest engines (e.g., chainsaws).  

 

I totally agree with this...within the next few years, it will probably be illegal to sell non-FI engines for most purposes, including aviation.  I don't see them outlawing the ones that are in service, but it's possible that if enough carbed airplanes remain in service long enough, that a requirement that they be converted to FI might come about.  In that case a sub-industry of FI conversions for everything from the C-65 to the 912ULS will probably spring up.

I do agree that carbs are terribly inefficient; it's kind of a miracle that the 912ULS gets the miserly fuel burns it does with two of them installed. 

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On 8/4/2017 at 6:09 PM, Mike Koerner said:

I would not compare the fuel injected 912is engine to that in a car. I don't believe the Rotax engine has an oxygen sensor in the exhaust. I think it is using an open-loop fuel control system based on the fuel mass flow and air density. This system offers advantages over a carburetor's mechanical fuel control; including smother starts and improved combustion efficiency (lower specific fuel consumption), but it is not able to achieve the optimized fuel flow of automotive systems which have a closed-loop control based on exhaust gas oxygen level.

The problem is that the lead in our aviation fuel destroys the oxygen sensors. Once we complete the change-over to lead-free fuel I think you will see fuel injection systems become prevalent on aircraft and the benefits will be more dramatic than the 912is can achieve. However, this complete transition, worldwide, may not occur in my lifetime.

Mike Koerner

New designs are almost all exclusively injected, and fuel injection was something that was rather common even in the 80's for what were higher end aircraft of the era. My 1981 M20J was fuel injected.

Rotax's fuel injection is closer to cars than aircraft though because it is not constant, it uses pulsed width modulation to control timing and amount of fuel injected. Most aircraft are just continuous injection. Basically, they are just spray nozzles and the pressure in the fuel distribution manifold dictates the amount sprayed, but it sprays even when the intake is closed. The advantage to the continuous system, is fewer points of failure, which rotax makes up for by using two injectors per cylinder.

O2 sensors won't make a tremendous difference in fuel efficiency in common vehicle applications, their primary purpose is emissions control. An open loop system, such as using a mass air flow sensor, already gets REALLY CLOSE to the target, and are needed to set initial parameters, and to fault-check the o2 sensors. What the MAF can't do, is account for cylinder conditions, which is what the O2 sensors correct for.

Rotax uses a Manifold Air Pressure/Intake Air Temperature/RPM combination to roughly guess the mass of air going into the engine to set the fuel injection rate. I'm kind of surprised they didn't just go with a MAF. But perhaps there is a reason.

 

6 hours ago, FlyingMonkey said:

I do agree that carbs are terribly inefficient; it's kind of a miracle that the 912ULS gets the miserly fuel burns it does with two of them installed. 

It's the way the engine is built. Short stroke is generally more efficient. Bore size is a big deal in this regard too.

Cars with turned carbs have pretty good mpg ratings too. Not as high as fuel injection for sure, but I don't know if I would start saying carbs are *terribly* inefficient as there's a lot of other factors involved.

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One thing to consider is that while the 912is weighs 22 pounds more, the 912is does fly more efficiently, and I've heard startling numbers like 3.5-4 gph burn where a carbureted 912uls would take 4.5 - 5 gallons, negating the 22 pound difference in 4 hours of flight.

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10 hours ago, rtk said:

One thing to consider is that while the 912is weighs 22 pounds more, the 912is does fly more efficiently, and I've heard startling numbers like 3.5-4 gph burn where a carbureted 912uls would take 4.5 - 5 gallons, negating the 22 pound difference in 4 hours of flight.

I just got back from a 1377nm tip in mt CTSW with the 912ULS.  I burned 52.9 gallons for 12.5 hours total flight time, which gives a flow rate of 4.23gph.  Now, my airplane lacks a fuel flow gauge and so this is a calculated value based on fuel added at each stop.  But given some inaccuracy, I'd think the actual burn was 4.6gph or less.  That was cruising at 5300-5400rpm and 5500-7500 feet most of the way, with some lower altitude operation to get under weather at various points along the route.

I definitely think the 912iS will have better fuel economy, but I have never seen the 5gph+ numbers published in some places for the 912ULS.  A question for the FI engine owners:  what fuel burns do you see in cruise at say 5500-7500ft and 5400rpm?   Just curious what the actual difference is in the real world.

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13 hours ago, rtk said:

One thing to consider is that while the 912is weighs 22 pounds more, the 912is does fly more efficiently, and I've heard startling numbers like 3.5-4 gph burn where a carbureted 912uls would take 4.5 - 5 gallons, negating the 22 pound difference in 4 hours of flight.

If you consider that the iS engine also costs about $5k to 5.5k more to buy I figure using E1093 auto gas prices in my area it'd take around 520 hours to break even before it becomes more economical on fuel cost versus the ULS engine. That's about 7 years of flying if the average sport pilot flies 75 hours per year.

If the iS engine were magically installed in my airplane today I'd get about 55 minutes extra flight time on the same tank.

Many of the LSAs with the iS installed already have heavier empty weights. The rule of thumb with most LSAs is that solo pilots can usually fly with full tanks and usually some baggage . Add a passenger and typically the fuel capacity has to be reduced in order to maintain gross weight limit. Quite often when you crunch the numbers this means that the actual range with two passengers in a higher empty weight iS engine equipped LSA is the same and often less than an LSA with the ULS engine.

Add other options such as the chute and amount of fuel that can be carried may be further reduced.

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44 minutes ago, FlyingMonkey said:

I just got back from a 1377nm tip in mt CTSW with the 912ULS.  I burned 52.9 gallons for 12.5 hours total flight time, which gives a flow rate of 4.23gph.  Now, my airplane lacks a fuel flow gauge and so this is a calculated value based on fuel added at each stop.  But given some inaccuracy, I'd think the actual burn was 4.6gph or less.  That was cruising at 5300-5400rpm and 5500-7500 feet most of the way, with some lower altitude operation to get under weather at various points along the route.

I definitely think the 912iS will have better fuel economy, but I have never seen the 5gph+ numbers published in some places for the 912ULS.  A question for the FI engine owners:  what fuel burns do you see in cruise at say 5500-7500ft and 5400rpm?   Just curious what the actual difference is in the real world.

I've been getting similar numbers in my RV-12 SLSA. I've been paying particular attention to fuel burns lately as I've been tweaking the  'k factor' on my fuel flow gauge.

I plan on 5.2gph at 5300 rpm 912ULS. However, based on fuel added and time flown I typically burn 4.8 gph at 5300 and 4.5 at 5000 rpm.

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I have been flight planning 120kts and 5gal/hr for 11 years now and have 't been far off yet.

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I think it's finally time for me to reduce my fuel burn estimates.  I have been conservatively using 5.5gph at 5400rpm for long-distance travel for several years, but I have never actually even hit 5gph when I do the actual calculations.  I think I will start using 5gph.   

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We just bought a 2014 CTLSi last month (after having a 2008 for seven years.)  The increase in fuel economy is fantastic and it starts like a car.  We only have 25 hours on it but no complaints at all.

We also love the Dynon 10" screens.  The biggest headache was getting the screens unlocked so we could upgrade the software from version 6.X to 15.2.  That took some "doing" but we're happy now.

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