Why are some engines geared to reduce prop RPM?

timwinters said:
Ummm...no.

Not according to the BSFC maps for auto engines that are posted everywhere in cyber space.
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Which ones, because I haven't seen them. I have seen the dyno data I've run, though.
 
So looking at that, the 1MZ-FE (Toyota 3.0L V6) does its best at a 0.39 BSFC, which is on par with what your 182 does in cruise.

But that's not at the same power, that's at a bit over 2000 RPM (hey, the same RPM range that aircraft engines typically operate in).

To get to a comparable power, you'd need to be at, say, a bit over 4000 RPM or higher... where it's a bit above 0.45.

So that pretty much tells me what I already said.
 
When I'm doing 77mph down the highway my Toyota is turning about 2400 rpm. My chevy truck about 2300.

So that pretty much tells me what I already said. ;)
 
Ted DuPuis said:
Your 182's engine installed I will bet is significantly lighter than your Toyota's engine installed.
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Nonetheless, Rotax 912 installations are much lighter than those of O-200 class engines, in the same airframes even.
-- Pete
 
timwinters said:
When I'm doing 77mph down the highway my Toyota is turning about 2400 rpm. My chevy truck about 2300.

So that pretty much tells me what I already said. ;)
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And what gear are you in?

And what RPM are your tires?

And what % power is that? (Probably less than 30%.)

I think most airplanes with geared engines will be flying faster than 77mph.
 
Morne said:
Tip speed is proportional to the length of the prop. Simply reduce the prop length, adding additional blades as necessary. I think that a 4-blade smaller diameter prop running at say 5500rpm would still be lighter than gearing down to a large 2-blade and running at 2750rpm.

Gearboxes have a weight (and reliability?) penalty.
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The amount of thrust is more related to the disc area created by the spinning prop. Not near as much about number of blades as you might think. So, adding a blade and making them smaller wouldnt really help in my opinion.

Not saying an extra blade or two doesnt help, but there are a lot of factors at play.
 
Morne said:
I know that some engines are geared to reduce prop RPM. My question is why? It would seem that with the correct propeller design you could utilize higher engine RPMs.
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Not really, props are more efficient at low RPM. When the tips pass 92% of the speed of sound, the drag increases horribly and efficiency falls through the floor. In order to get more horsepower out of the engine they needed more rpm while turning the prop slower so they could make it bigger to have the "bite" to transfer the power to the air. The less longer blades you have, the more efficient the prop will be. Then comes physical limitations as well such as ground clearance which restricts the length of the blade. When you can't make the blade longer and still have more power than you have traction on the air, then you add more blades.
 
dmspilot said:
And what % power is that? (Probably less than 30%.)
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A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
 
timwinters said:
A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
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??? What are you driving around in/towing and what do you consider "Highway speed"?
 
zaitcev said:
Nonetheless, Rotax 912 installations are much lighter than those of O-200 class engines, in the same airframes even.
-- Pete
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Correct. But remember the Rotax was also designed with the intent of putting it in an aircraft. Although it is water-cooled, geared, etc., they went to great pains to make it suitable for aircraft. What I don't know is how its efficiency compares with an O-200.

timwinters said:
A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
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Your arguments aren't making sense. First you state that more engines should be geared for more efficiency, and then show a reference that shows that engines tend to have lower BSFCs at higher RPM. Now your bigger concern seems to be HP/CI. But I would suspect that at your 77 mph highway speed, your Toyota is producing similar (if not lower) HP/CI than your O-470.

I don't know about you, but what I care about with respect to efficiency is how much fuel I'm burning to go my particular speed. All else equal, that comes down to BSFCs, where lower is better. HP/CI is an often used as a misguided critique of how "efficient" an engine is. I used to think that, too. Back before I was into aviation I couldn't understand how poorly designed American cars were to only get 200 HP out of a 3.8L V6 when the Mitsubishi I now own got 300 HP (easily) out of a 3.0L V6. Of course, that GM 3.8L V6 was getting 30 MPG highway while my Mitsubishi is lucky to get 25, despite being in a more aerodynamic airframe.

The reality of BSFCs haven't changed. Yes, diesels (specifically modern diesels) do better than aircraft engines. But there are reasons why, even with all the effort Toyota put into making an aircraft engine, they still don't have one.
 
timwinters said:
A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
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BSFC at the same power and installed weight would make sense. HP/cubic inch is pretty much irrelevant - that comparison says my Rotax (1200cc, 73 cubic inches) is a WAY better aircraft engine than, say, a Continental 85 because I have the same HP but much fewer cubic inches (thanks to the gear reduction).

Traditional aircraft engines ain't bad at all. They have their advantages, they have their disadvantages. Large displacement per cylinder gives a very low surface to volume ratio - good for reduced heat transfer and improved fuel economy. Dual plugs improve the burn rate and are also good for fuel economy. High manifold pressures are good for fuel (running WOT at altitude is a good thing for economy). On the minus side, they tend to have open chambers with minimal charge motion (swirl / tumble) which makes them less lean tolerant. Leaving the mixture control in the hands of the maroon behind the stick gives mixed results. Running rich of peak is a big minus. Not adjusting the spark in response to changing mixture is a minus.
 
Ted DuPuis said:
The reality of BSFCs haven't changed. Yes, diesels (specifically modern diesels) do better than aircraft engines. But there are reasons why, even with all the effort Toyota put into making an aircraft engine, they still don't have one.
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I hung out with some of those guys in the early 90s and watched the Revlon Red Aztec with some interest. What it came down to was they could not match the reliability or fuel efficiency of the IO-540 on the other wing in several years of trying.
 
timwinters said:
A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
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At highway speeds your auto/truck engine is generating maybe 25 to 30% power. RPM is only one factor; manifold pressure is the other, and if you had an MP gauge on that auto you'd find the cruising pressure low.

A typical V8 redlines around 4800 RPM, some higher. To get 100% power out of that engine you need to be at sea level, wide open throttle, and at redline RPM. Find a long, steep hill at sea level and load up that truck and floor the throttle so that redline is reached and see what 100% power looks like. It's far beyond ordinary highway cruising speeds, and the chances of keeping the engine temps within limits is small.

The homebuilders who want to convert auto engines often make the mistake of thinking that since their Chevy runs at the same RPM as the engine in the airplanes they rent, it must be entirely suitable for an aircraft engine. Having been there and done that and finding the hard way that it really isn't worth it for the most part, and that the auto engine (Subaru, in this case) will burn far more fuel at 100% power than even Subaru would admit to, and that the engine wear is terrific at that speed and the valves burn out real easily, and that it cost as much as if we'd just bought a nice Lycoming, I can say that the typical automobile powerplant has a really easy life in the car and so its BSFCs can look pretty good at typical cruising speeds. Even an O-470 would have nice BSFCs at 30% power, but the airplane wouldn't fly too fast. Maybe not at all.

Dan
 
Dan Thomas said:
At highway speeds your auto/truck engine is generating maybe 25 to 30% power. RPM is only one factor; manifold pressure is the other, and if you had an MP gauge on that auto you'd find the cruising pressure low.

A typical V8 redlines around 4800 RPM, some higher. To get 100% power out of that engine you need to be at sea level, wide open throttle, and at redline RPM. Find a long, steep hill at sea level and load up that truck and floor the throttle so that redline is reached and see what 100% power looks like. It's far beyond ordinary highway cruising speeds, and the chances of keeping the engine temps within limits is small.

The homebuilders who want to convert auto engines often make the mistake of thinking that since their Chevy runs at the same RPM as the engine in the airplanes they rent, it must be entirely suitable for an aircraft engine. Having been there and done that and finding the hard way that it really isn't worth it for the most part, and that the auto engine (Subaru, in this case) will burn far more fuel at 100% power than even Subaru would admit to, and that the engine wear is terrific at that speed and the valves burn out real easily, and that it cost as much as if we'd just bought a nice Lycoming, I can say that the typical automobile powerplant has a really easy life in the car and so its BSFCs can look pretty good at typical cruising speeds. Even an O-470 would have nice BSFCs at 30% power, but the airplane wouldn't fly too fast. Maybe not at all.

Dan
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Mine does, but it has 2 of them.:D I did 127 on 12.5 combined.
 
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Ted DuPuis said:
Your arguments aren't making sense. First you state that more engines should be geared for more efficiency, and then show a reference that shows that engines tend to have lower BSFCs at higher RPM. Now your bigger concern seems to be HP/CI. But I would suspect that at your 77 mph highway speed, your Toyota is producing similar (if not lower) HP/CI than your O-470.
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You're right. That's because you, the BSFC maps, and the fact that cars are geared to "cruise" at anywhere from 2000-2500 rpm ( which happens to be where the BSFC maps would appear to indicate that most engines are the most efficient) have all convinced me that higher RPMs probably wouldn't mean greater efficiency for aircraft engines. The prop turning at lower speed would need to be far more efficient to adequately compensate.

But, you've yet to convince me that aircraft engines have BSFCs "significantly lower than what you'd get out of the engine in Tim's Toyota" (but keep trying). From what I've seen, at "highway speeds (ie 75 mph +-), the BFSC maps indicate that car engines are more effiecient than my o-470 is at cruise power. Some significantly so.

OTOH, I haven't seen the BSFC map for my o-470 and am relying on what you said re efficiencies. The only data I find on an aircraft engine is a turbo charged engine listed on Wiki with a BSFC of .40 & 243 & 33.7%. And I'd think that my engine would be less efficient since I don't have a turbo and assume lower compression.
 
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Dan,

The point is strictly efficiency and not the airworthiness of auto engines. I am in no way advocating the use of car engines in aircraft or arguing for same.

Dan Thomas said:
At highway speeds your auto/truck engine is generating maybe 25 to 30% power.
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Truth be known, my O-470 is probably running at 30% power also when in cruise. 30% of the power that it could produce if it's RPMs weren't limited by the propeller RPM. At redline (2500 RPM) my engine is barely breaking a sweat.

Really, how hard would we laugh if Detroit produced a 470 CI engine that was rated at 230 HP? I've always thought that it's simply a function of the prop limiting engine speed and engine speed limiting engine power...but Ted my prove me wrong on this also. ;)
 
timwinters said:
Dan,

The point is strictly efficiency and not the airworthiness of auto engines. I am in no way advocating the use of car engines in aircraft or arguing for same.



Truth be known, my O-470 is probably running at 30% power also when in cruise. 30% of the power that it could produce if it's RPMs weren't limited by the propeller RPM. At redline (2500 RPM) my engine is barely breaking a sweat.

Really, how hard would we laugh if Detroit produced a 470 CI engine that was rated at 230 HP? I've always thought that it's simply a function of the prop limiting engine speed and engine speed limiting engine power...but Ted my prove me wrong on this also. ;)
Click to expand...

It's prop RPM that limits it, alright. That O-470 could produce a lot more power if its RPM was higher, but the aircooling would now be inadequate.

Dan
 
timwinters said:
You're right. That's because you, the BSFC maps, and the fact that cars are geared to "cruise" at anywhere from 2000-2500 rpm ( which happens to be where the BSFC maps would appear to indicate that most engines are the most efficient) have all convinced me that higher RPMs probably wouldn't mean greater efficiency for aircraft engines. The prop turning at lower speed would need to be far more efficient to adequately compensate.

But, you've yet to convince me that aircraft engines have BSFCs "significantly lower than what you'd get out of the engine in Tim's Toyota" (but keep trying). From what I've seen, at "highway speeds (ie 75 mph +-), the BFSC maps indicate that car engines are more effiecient than my o-470 is at cruise power. Some significantly so.

OTOH, I haven't seen the BSFC map for my o-470 and am relying on what you said re efficiencies. The only data I find on an aircraft engine is a turbo charged engine listed on Wiki with a BSFC of .40 & 243 & 33.7%. And I'd think that my engine would be less efficient since I don't have a turbo and assume lower compression.
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How are you calculating that? Where are you getting the load factor/HP produced number? Have you been running "Tim's Toyota" on a dyno? Why could the engineers from Toyota not reach the same conclusion you do? Do you think Lycoming paid them off to not come out with the aircraft engine they had spent millions of dollars and several years developing? Why has it been kept secret in the 20 years since?
 
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Read the entire thread (and cites) Henning and most, if not all, of your legit questions will be answered.

And WTF did I ever say about Toyota's aircraft engines in this thread? Exactly nothing, so I have no clue why you'd even tie that to any of my statements...except that you're smokin' sumthin'...or just playing troll.
 
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Tim - Sure your Toyota may be efficient at 70 mph - but that means nothing.

How much HP is your Toyota putting out at 70 mph? How fast do you think your 182 would be going driven by that much HP? I think you'd be quite disappointed.

Now ask your Toyota to output enough horsepower to get your 182 up to the speeds you want to fly at and look at how efficient it no longer is. Also look at how long it'll last. Now look at how much it weighs. Now try to add the gearbox. Doesn't work.

Aircraft companies build engines that work the best for aircraft. It's not that they're not smart. It's that the requirements make a car engine just not work well. How many people have tried to do conversions. A lot. How many of those in the end were better? Pretty much none.

I guess I'm extremely confused as to what the argument here is. Clearly your truck is a horrible engine for aircraft and it's not going to beat an aircraft engine in any airframe I can think of.
 
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timwinters said:
A comparison of % rated HP is irrelevant. A comparison of HP/CI is more appropriate. At highways speeds my vehicles are producing as many or more HP/CI than my o470 is at cruise power.
Click to expand...

Well I'm having trouble understanding how your engine rpm driving down the highway is "relevant" to propeller efficiency. That was my point.
 
Jesse,

A simple statement was made (paraphrasing) "1940s aircraft engines are far more efficient than today's car engines". And that simply isn't true from the data I've seen. My argument has nothing to do w/ using auto engines in planes (as I've said numerous times now but it seems to keep coming back to that). It's simply a comparison of which has the best BSFC number at their relative "sweet spots" or relative "cruise speed". And again, from the data I've seen (and provided links to earlier in the thread), the car engine wins.

Ask my Toyota engine to produce enough HP to fly my 182? What would that have to do w/ anything? My 182 has 470 cubes in it...almost 3 times the 164 cubes my Toyota has in it.

How's the training going?
 
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timwinters said:
Read the entire thread (and cites) Henning and most, if not all, of your legit questions will be answered.

And WTF did I ever say about Toyota's aircraft engines in this thread? Exactly nothing, so I have no clue why you'd even tie that to any of my statements...except that you're smokin' sumthin'...or just playing troll.
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Because Toyota's aircraft engine was the same automotive V-8 you buy in Toyota/Lexus land vehicles. You compared a car engine to an aircraft engine making claims of superiority in efficiency. I was bringing forth an example (the only one I'm aware of) where a full on multi million dollar project was run my an automotive engine manufacturer cited for their efficiency was unable in several years of trying to match your theory.

IOW, Toyota FAILED at proving your claim and the data you cite no matter how hard they tried and how much money they threw at it. They proved it false by not being able to achieve the results that you bring forth. They work good in theory, but they could make it work in reality.
 
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I've been involved in over half a dozen auto conversions. First off they all need some bolster for the end of the crank to eliminate the gyroscopic forces from the crank and rear main cap. They are not designed to take it. I've seen 3 VW and 1 Corvair crank broken in this fashion. The PSRU is typically the way this is accomplished. I have not seen one automotive installation that was near the efficiency or reliability of our Lyconentalsauruses, and that includes a small block Chevy.

What are engines need to improve them is modern electronic engine management controls like we have on the cars, not the engines themselves. If we are going to create a new aircraft engine, it needs to be diesel.
 
Henning said:
What are engines need to improve them is modern electronic engine management controls like we have on the cars, not the engines themselves. If we are going to create a new aircraft engine, it needs to be diesel.
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Amen.
 
Henning said:
IOW, Toyota FAILED at proving your claim and the data you cite no matter how hard they tried and how much money they threw at it. They proved it false by not being able to achieve the results that you bring forth. They work good in theory, but they could make it work in reality.
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No, because I never made any claims as to how my Toyota engine would perform in an aircraft and I have repeated that many times but apparently some people can't read.

I'm done, another potential chance to learn from a thread (thanks for teaching, Ted) ruined by arrogant games.
 
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Russian colleagues, who were intimately linked to Raikhlin, report that the varying cetane number is a problem, despite Henning handwaving it away. BTW, here's the site:
http://www.raikhlin.com/
http://finam.aero/news/144/ -- report about a completion of a stage of flight testing

They also say that generally 3 aviation diesels exist today, aside from Raikhlin:
- Centurion nee Thielert -- well known for installations at Diamond
- SMA SR305-230 -- hello $44k firewall-forward STC for 172
- Continental TD-300 -- clone of SMA engine which TCM is unable to get running right, funny how that works
- АЕ-200 -- in-house Diamond engine that they bought after falling off with Thielert

So yeah. It's not like we need to "create" a diesel. There's a bunch of them. Neither works too well or costs anywhere near gas engines.
 
Capt. Geoffrey Thorpe said:
On the minus side, they tend to have open chambers with minimal charge motion (swirl / tumble) which makes them less lean tolerant.
Click to expand...


I believe this is the biggest area where current aircraft engines could be improved.

Leaving the mixture control in the hands of the maroon behind the stick gives mixed results. Running rich of peak is a big minus. Not adjusting the spark in response to changing mixture is a minus.
Click to expand...

I do agree that adjusting the spark in response to mixture has benefits on a lot of automotive engines, but in the aircraft engines I've seen... not so much.
 
The varying cetane is the primary running issue when it comes to aircraft diesels running on Jet A (because Jet A is not diesel, and has different components controlled).

The big reason why diesels would be beneficial to GA is that it solves the whole issue of 100LL. They will also show better BSFCs, which is (of course) very nice.
 
Ted DuPuis said:
I believe this is the biggest area where current aircraft engines could be improved.



I do agree that adjusting the spark in response to mixture has benefits on a lot of automotive engines, but in the aircraft engines I've seen... not so much.
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I agree with Ted.... On an engine that operates in the 1000-6000 rpm range, a variable ignition and cam timing option will benefit the motor and its output..... On a motor that spends it life between 2200 and 2700 rpms the cam timing option does not make any sense,,,, The ignition can make some additional gains when coupled with intake pressure readings and rpm of the motor..... Perhaps a knock sensor would add a bit of safety too but having a sensor that could tell the difference between a true knock and piston slap in a 5 1/4" bore motor would be hard to configure. IMHO

On the Diesel end....... Yup, there are a few manufactures in the market, most have bellied up or failed to deliver a good product... Last I heard the SMA in a 172 was closer to 75,000 + for the FFW......

Too bad the Duetz guys cannot get their foot in the door with a lightweight aero diesel, they have the stationary market figured out.


Ben.
www.haaspowerair.com
 
timwinters said:
A simple statement was made (paraphrasing) "1940s aircraft engines are far more efficient than today's car engines". And that simply isn't true from the data I've seen. My argument has nothing to do w/ using auto engines in planes (as I've said numerous times now but it seems to keep coming back to that). It's simply a comparison of which has the best BSFC number at their relative "sweet spots" or relative "cruise speed". And again, from the data I've seen (and provided links to earlier in the thread), the car engine wins.
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That's not a relevant comparison, though. The relevant comparison is taking engines of equivalent powers working at equivalent % powers, i.e. make engine A work in application B. That was my point (and I think everyone else's). And in that mindset, the data you showed states it as accurate.

Ask my Toyota engine to produce enough HP to fly my 182? What would that have to do w/ anything? My 182 has 470 cubes in it...almost 3 times the 164 cubes my Toyota has in it.
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Which is why looking at the sweet spot by itself is a worthless piece of information. Engines are designed for particular applications. even you probably don't run your Toyota at its sweet spot on the highway, so it'd be interesting to see what your BSFC on the highway actually is. You may not run there, and may not really have the capacity to.

Likewise, your O-470 (which does a great job in your 182), would be terrible in your car.

Now, what I took issue with in the first place:

timwinters said:
Higher RPM allow for more HP/CI. Thus the weight penalty of the gearbox would be considerably offset by the weight reduction of the engine (or increased HP) and the engine would be more fuel efficient running at the higher RPMs.
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Which would be incorrect, as was shown by the data you shared.

If I misunderstood you somehow, then please let me know. I'm not trying to be arrogant, just trying to be informative.

Best,

Professor Ted, Ph.Dyno.
 
By the way - said it before, will say it again. If I were to build an experimental, I'd put in an automotive-based engine. But I'd do things a bit differently than you find them in cars. And I wouldn't expect it to be more efficient than a purpose-built aircraft engine. I'd do it for different reasons.
 
zaitcev said:
They also say that generally 3 aviation diesels exist today, aside from Raikhlin:
- Centurion nee Thielert -- well known for installations at Diamond
- SMA SR305-230 -- hello $44k firewall-forward STC for 172
- Continental TD-300 -- clone of SMA engine which TCM is unable to get running right, funny how that works
- АЕ-200 -- in-house Diamond engine that they bought after falling off with Thielert

So yeah. It's not like we need to "create" a diesel. There's a bunch of them. Neither works too well or costs anywhere near gas engines.
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The SMA305-230 is an engine for the 182Q, not the 172. We have one here and it's been a pain. Cracked brackets, leaking case seams and head gaskets (the head is partially oil-cooled), vibration issues, on and on. Expensive tools to calibrate various things. Injection lines that can only be disconnected and reconnected once before replacement, at big $$$. And it's a lot more than $44K for the conversion, too. Still, it runs nicely once it's set up properly and has a unique snarl to it. Redlines at 2200. SMA is working the bugs out and Continental will, too. New aircraft engines cost a fortune to get certified and they're not going to be cheap, ever.

SMA's engine was a from-the-ground-up aircraft engine.Thielert's diesel was a converted Mercedes. Thielert has already had too many ADs against it and there was another just the other day. Some airplanes have crashed for very stupid reasons, like the computers that control it falling offline when the system voltage fell a little too far for a millisecond or two. Laugh all you like about magnetos or mechanical injection systems, but those things don't care much about electrical system problems; they just keep going. Since electrical system issues tend to make up a huge portion of aircraft glitches (up to 90%), adding more electrical supply-dependent systems is adding more failure points.

The Thielert's gearbox has a 30-hour TBR (Time Before Replacement). Isn't that a nice technological advance? The engine itself has a TBR, not a TBO, so guess how much it might cost at "overhaul" time?

The Austro? I don't know how it's doing. It was designed and built and certified really quickly, so I would expect some unforseen stuff to show up. I think it, too, is another converted Mercedes. More gearbox stuff.

Dan
 
Dan has summarized the issues with the current diesel offerings well. I haven't deal with the SMA design. I like it on paper, but figured there had to be some issues with its practicality since they aren't being gobbled up left and right. The Thielert engine is an interesting concept, but the reality of it has spoken for itself. There are a number of things that I'd do differently if I were building it.

Developing any engine is very expensive for a lot of reasons. Diesels, especially so. I remember talking to Cummins engineers who explained to me that the lines for the common-rail direct injection system in one particular engine (I forget which - I want to say the 6BT in the Dodge Ram) required some incredible tweaking, to the point of a little bend here and there just to make the pressure pulses in the system (operating at some ludicrous PSI - I want to say 40,000, but forget) function properly. Just having fuel at that high of a pressure is a lot to contain by itself. Then you add in trying to get it to operate on a fuel that's not design to run that kind of engine, but does kinda sorta work...

Then you have to certify it. Which is a lot harder when you can't certify it "by similarity," as most of the engines in our aircraft have been.
 
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