# TBO VS TBR

Discussion in 'Firewall Forward / Props / Fuel system' started by Southron, Jul 13, 2012.

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1. Jul 13, 2012

### Southron

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I note that at the Centurion Aircraft Engine website they advertise that their certified 2.0s Diesel Aircraft Engine will go 1,200 hours "Time Before Replacement."

Is that their way of saying that their engines cannot be overhauled? Buying a new engine every 1,200 will get to be expensive sooner or later.

If their engines cannot be overhauled, then WHY?

2. Jul 13, 2012

### Jay Kempf

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Maybe they have a factory only refurb program so the only avenue to get zero timed is to just send the current one back and get another one. Americans really object to single source, no options sorts of programs. But in commercial aviation renting engines has become all the rage. You lease an engine and pay per hour of operation. Seems the best way for small jets.

3. Jul 13, 2012

### TFF

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It also keeps their employes employed if no one else can do the work.

4. Jul 13, 2012

### Lucrum

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Have you contacted Centurion?

5. Jul 13, 2012

### Dan Thomas

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A Centurion likely has, as with most auto conversions, a cylinder block with integral cylinders. Once they're worn out, the cylinders must be bored and honed for oversize pistons, but in many cases the cylinder walls are too thin to allow much of that, and the labour costs often outweigh the cost of a new, automatically-manufactured block. That sort of thing has happened to aircraft cylinders; often cheaper to replace than to resize, buy a new piston, redo the valves, and so on.

Dan

6. Jun 19, 2013

### DangerZone

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I heard the Centurion 2.0 is the 1.7 version with a rebored cylinder to 83mm and that the crankcase is in fact a Mercedes common rail turbo diesel engine. The TBR of the 2.0 Centurion is 1500 hours while the TBR of the 2.0S is 1200 hours. I discovered also that the engine gearbox has to be inspected every 300 hours (7800U$) and replaced every 600 hours (16000U$) which would in fact make it the most expensive certified aircraft engine in it's class due to maintenance and service costs. A Mercedes 2.0 common rail turbo diesel can easily run for more than 3000 hours and do more than 200 000 kilometers without need to change the gearbox. So what is wrong with this Centurion 2.0 engine, why does it have so much stuff that has to be changed regularly at such a high cost? Is that the reason why the company went bankrupt besides the story about the owner committing financial fraud?

The Centurion is a certified engine, so after the 1500 hours could it be bought cheap and then rebored or restored for use in a homebuilt? If a Mercedes 2.0 can, why not the Centurion, couldn't some 83mm pistons be bought from Wiseco and 83mm sleeves for a turbo engine for higher compression if there is enough space to do some more boring? In fact, wouldn't an old 1.7 Centurion engine have more space for cylinder sleeves, if the liquid cooling permits it to be bored to size? How much of an investment would it be to buy an old used Centurion engine and convert it for homebuilt use?

I'm seeing more and more of these Centurions around for sale and the 'rebuilding' seems to cost as much as a new engine. So, if a homebuilt does not need certification could it be used safely as an engine after some overhauling or rebuilding?

7. Jun 19, 2013

### Toobuilder

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At first blush, the runout engine should be viable for a homebuilt overhaul, but that gearbox may really complicate things. I understand the TV issues just tears them up inside, and unlike the basic engine, getting parts could be tough.

8. Jun 19, 2013

### DangerZone

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Apparently these engines have both a vibrational dampener and a clutch to resolve the TV issues but the price of 12K Euro for a gearbox replacement every 600 hours or so turns poeple away from considering buying the thing. I dunno about parts dough, if it is a Mercedes OM640 cylinder block the parts for the engine should be easy to find. But the gearbox/PSRU seems to be the major problem, it is quite expensive to change so often. Maybe another PSRU or a smaller diameter prop with direct drive for homebuilt use..?

9. Jun 19, 2013

### PTAirco

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If there was one engine I'd avoid even if someone gave one to me it's that one. It seems sheer folly to sell an engine to people with known problems. These kind of things should have been designed out of it at the prototype stage. Imagine selling an airplane with 1500hr wings and 300hr landing gear....It's downright ridiculous.

10. Jun 19, 2013

### DangerZone

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Absolutely true, I agree. In fact, the gearbox of the 1.7 has a TBR of 300 hours, the clutch 600 hours and the engine is 1000 hours. Yet there are people who have made more than 2000 hours with the engine before the oil consumption and compression started changing...

Couldn't there be a way to use these engines somehow and resolve the problems with some sort of smart engineering..?

11. Jun 20, 2013

### Dan Thomas

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As if the manufacturers haven't already spent enormous piles of money on it development?

The SMA diesel (a certified direct-drive engine) cost the developers over a billion dollars, yet when I was working on one a couple of years ago there were only about 50 flying worldwide. And the one I was working on, an early model, had numerous issues, most of them probably unforeseen.

Do the math: It's not cheap to develop a new engine, whether DD or PSRU'd, and there are always teething problems that raise the cost further. I don't imagine that Lycoming or Continental designed and built any engines that ran perfectly from the start without weaknesses showing up.

Dan

12. Jun 20, 2013

### PTAirco

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13. Jun 20, 2013

### Toobuilder

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Clearly, the TV issues are FAR from "resolved" if the gearbox chews itself to pieces in less than 1000 hours!

if nothing else, this major engineering effort by an OEM which results in nothing better than a partial solution indicates the complexity of the TV issue.

14. Jun 20, 2013

### DangerZone

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Dan, this is an excellent point, the manufacturers have indeed spent enormous piles of money on the development. And money is the key issue...

I own a BMW V8 car with around 250HP and the automatic transmission made by a company called ZF has broken two times already in the ten years that I own it even though I drive conservatively and use sportbikes if I need some speed. The reason is a design flaw that was made in 1996 when the first series came out that was never resolved on the European market but that had to be resloved in China because the commie government has put a pressure on BMW China to install Chinese parts. The trick is that the axle in question of the transmission can be bought only by some special transmission shops for a couple of hundred Euros while certified BMW shops buy the whole transmission for around a couple of thousand Euros. The Chinese axle is more robust, the holes are not rectangular (that's where these axles break, at the square hole edge corners) but round, the transmission has to be machined so only a specialised machine shop can do it. If they do it, you lose any guarantee by BMW that your car is legal/certified/good/whatever. If you install the ZF part which still hasn't resolved the issue and keeps producing the 'original' axles, your BMW is considered reliable and original in Europe. I attend the Air Expo in Friedrichshafen every year for the last five years and the ZF factory which produces these transmissions has had enormous profit so they are building new facilities for more research&development. Their profit from changing the transmission parts and transmissions is counted in billions, not millions. So quite frankly, why would they ever resolve the initial problem in Europe if they do not have to, and the customers pay for the parts and all costs? It's a matter of profit, why would anyone ever resolve a technical issue if the flaw brings billions of Euros a year?

The same goes for Thielert Centurion engines, the concept was that when the engine reaches the TBR hours it had to be sent back to the factory and engine cylinders would be exchanged, as well as the gearbox and clutch. I communicated with a guy who has done more than 2300 hours on the 1.7 Thielert engine before the compression started to go down and the engine began to consume more oil indicating the pistons and cylinders have to be changed. It is a 1000 hours TBR engine, and Thielert would charge enormous sums of money for this 'rebuilding' process, it costed around 70% to 80% of the price of a new engine. So, once you reach the 1000 hours you eaither had to buy a new engine or 'rebuild' the one you have for almost the same price, cause labor costs would make it almost the same as if you installed a completely new engine. The same goes for gearboxes and clutches that have to changed every 300 hours for the 1.7 engine, the newer 2.0 engines have water cooling gearboxes and clutches that have a TBR of 600 hours. The parts only cost a few thousand Euros, why would Thielert (or Centurion now, that's the name of the company after it went out of business and the bankruptcy managers took over) engineers ever resolve these issues if it brings them money? They'd be crazy to do that and say no to such a nice income, just do the math of a few thousand engines around the world which need parts worth a couple of thousand Euros every year, it's millions we're talking about. On the other hand I've seen some American PSRUs that have a TBO of around 2000 hours, the financing managers made sure with the engineers that the price of every gearbox and clutch would never go beyond a certan limit, because otherwise every smart engineer would come with a bright idea to use an American PSRU that lasts for a couple of thousand hours insted of a pityfull 300 hours to 600 hours. I know it's not simply a matter of doing the math, bolting it on and then forget major overhauls for 2000 hours but it is worth considering as an idea, if the prices of a good PSRU would come to be more affordable than constantly filling the pockets of a bankrupt company which does not seem to want to resolve the technical issues. And I am sure they can and that they planned in time to announce that they 'found a great solution for the gearbox and clutch assembly and now it has a TBR of 1200 hours as well as the engine'. We ain't kids, we know that money spins the world around but if we can come up with a solution to spend less and have a better product why not at least consider the idea?

It's just a thought, I'm just doing the math 'aloud' and it seems there are great minds here with more experience that could share their points of view. I could be wrong, too, maybe it is cheaper to buy constantly a new gearbox and clutch every 300 hours. But if companies like Mercedes (the Thielert 1.7 engine was based on the Mercedes OM668 CR engine and the 2.0 on the Mercedes OM640 cylinder block) can produce gearboxes and clutches that last for more than 2000 hours, why couldn't Thielert - Centurion? The reason is simple, money. And if an engine in a homebuilt does not need certification then it could be feasible to install a better PSRU or bore the cylinders and install new Mercedes pistons and sleeves once the time comes, right?

What do you think, would it not be possible to change the TBR to a nice TBO as in old school..?

Himat likes this.
15. Jun 20, 2013

### DangerZone

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The thing is, there is a guy who has done +700 hours before replacing the gearbox and clutch and it did not chew itself to pieces even though these are 300 hours TBR parts. It's hard to be smart without knowing which parts of the gearbox or clutch give in after some time. But if the company was keeping a secret about financing they surely won't discuss these problems in public. Imagine some experts analyzing the issues and finding out it could be resolved with simple engineering as other PSRU producers have done? It's a question of trust too, if the producer (whose president is in court for some financial scam) says it should be changed every 300 hours people mostly trust these 'facts'. The TV issues exist but they seem blown out of proportion just to scare buyers they need expensive engineering and parts instead of good engineering and reliability. There are good PSRUs that cost less in the long run, but with an average of 300 hours a year or so of flying there will be many guys who will pay a couple of thousand Euros a year to avoid the hassle. And that psychological effect seems to be what the company had in mind when they designed the gearbox and clutch to last for 300/600 hours.

16. Jun 20, 2013

### stol

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3/4 of that Billion dollars when toward ENGINEERING fees........... And what did they end up with ????? Total crap...

Rule number 1..... Never trust an engineer....

17. Jun 21, 2013

### Himat

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Or the acountant's could be part of it too.
DangerZone aired some views on that. The engineers provide what the beancounters spesify.
Cheap ink jet printers with a "self destruct" counter is a known variety. Some programming in a modern car that make the engine "rough" and "misfire" a bit if you don't service it at the factory approved shop is another. And don't forget, a lot of engineers are employed by "consulting" companies. No reason to fix all problems at once then.

18. Jun 21, 2013

### Himat

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The big question is if the PSRU was designed to last 300/600 hours, if they when testing it found that this was the limit or it's a recomendation based on business strategy. One way is to do an independent test. That is costly and take a lot of time.

Another possibility is to run the engine on condition. That is, come up with an inspection sceeme, maybe togheter with instrumentation, that a monitors the PSRU and do tell when it is time for overhaul or replacement. It's like the wheel bearings on a car, in the end you hear when they are finished and then you replace them. The difficulty is to know when the PSRU is worn and no longer safe.

19. Jun 21, 2013

### DangerZone

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The Austro model of the same Mercedes OM640 engine has a solid PSRU but the weight of the engine is around 175kg while the Centurion is 135kg. This 40kg difference seems to be because the Austro has a solid PSRU and the Centurion a gearbox and clutch system to resolve TV issues. I guess the gearbox and clutch system was a compromise to reduce the weight of the PSRU since they did not have many good engineering solutions at the time they designed it. I see other Mercedes conversions like the Mercedes Smart engines use belt driven PSRUs, saving on weight but the belt changing times must also be in the 300 hour range or less. So to be safe, Thielert at first had a 300 hours TBR on the gearbox and clutch but this year it seems they will prolong this to 600 hours for both the clutch and the gearbox. Maybe in time, they will resolve all the issues and start producing a 1200 hours clutch and gearbox, who knows?

A homebuilt does not need certification and regular service as certified engines do, so if someone would resolve the problem of reduction of an used engine, be it an 'old' 1.7 or a good 'worn' 2.0 one, then it would be possible to use it after the TBR hours on an experimental. Or ditch the whole gearbox/clutch/PSRU assembly and make a direct drive with a smaller diameter multiblade propeller? I'm just thinking out loud and exploring the idea, maybe someone even already had success in such an attempt?

20. Jun 21, 2013

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