Small engine dyno

[Bentwings]

yes it does help. I have a set of steam tables I’ve been looking at. I think I’m beginning to under stand the various stages or conditions of steam. Pressure and temps are related. It will be interesting to see all this in action . I won’t be at the limits by any means but I think I’ll know the condition of the steam as I create and use it .
I do have an unanswered question I did a lot of searching to find origins of steam oil and it’s composition even though I’ve used steam cleaning to clean things all the way back to when I was about 12 yr old washing milk cans . Can you believe I was given insulated gloves and about a 1” I’d live steam hose to clean these things as well as the room and he tanker truck. There was about a foot of invisible steam befor there was vapor . I don’t think OSHA existed back then It definitely cleaned stuff. Even in the auto shop we had a live steam cleaner . I cleans those car and truck engines like new. Most of the paint went down the drain with the road grime .
Anyway as I got into really high performance Rc airplanes oils became important. Initially castor oil was the go to then synthetics. Of late the silicone oils have become popular in high performance Rc cars and in some of our Rc aircraft . 2 stroke gas engines like 50:1 or more oil ratios. There are high temp and very high film strength silicone oils coming along I experimented with some a while ago but it was hard to develope a baseline then add tests . It either worked or resulted in seized engines. The silicone plastics don’t seem to do well at higher temps but the gear oil really does work in differentials and miniature transmissions .
So if I use compressed air temporarily the silicone oils might be ok. But I think some testing with real steam might be in order.
I’ve got several viscosities so I thought maybe just pour some on clean silverware from the kitchen then take the boiling water from my coffee pot and see if it washes off with out soap of course . The top of my new pot is right at 200 degF and even the carfe is 200 some times a little more if I let it sit. I know this is far below what real steam might be but there would be some indication I think just wiping the piston rod where it comes through the packing gland sure made a difference in friction. None of the crankshaft bearings will see very high temps . I think maybe a little heavier oil might be better there but I’ll need to actually get this thing running. I finally got all the intake and exhaust parts put together. Those m2 screws are really tiny and there are a lot of them . Once I get the modified shaft collars and beam coupling installed I YHINK vibration will be minimum. I found Teflon tubing with the correct I’d so I can now make prongs for the packing instead of the messy tape .

you are making me happy. Thanks. I needed that today. I’ve been literally screwing around since 6:30 am with these engines. They have m2 screws holding two bolt flanges together then there is a connecting pipe between engines. Well that’s going to change. If I ordered four extra pipes i would have been fine. It’s hard enough to see as it is but those tiny screws are a nightmare. Then the connecting pipe that connects both sides ofvthe cylinders is threaded on each end. The m2 screws are just a squeak too big to just unscrew so I had to take the piston valve cylinder off one side . Now I can’t get the eccentric back on the crankshaft there is less than .0005” clearance on the shaft so the slightest misalignment means it’s not going on my son just came over and brought a nice drill press that is perfect size for me then I gave him the machining project I got the coupler I wanted so it can be the model to rework the other 6 . I got it from automation direct. Nice tech help and easy ordering on time delivery . Exactly the right part. I’ve purchased parts from them in the past so it was easy . Their prices are good too. I’m really hoping this super heater worksite way I think it will . I’ve got two more turbines ordered so I think I’ll try and team them together. I suspect they will take a lot more volume and probably not have much more power . They will be geared down considerably . I really haven’t given a lot of thought to them . I have plenty to do as it is. Too bad there isn’t an easy way to store some of this cold weather so the ac wouldn’t have to run al day in the summer.
byron

 

[Steamchick]

You need an ice store to preserve the cold. Fill a freezer with cold ice from nature, then thaw it in the summer heat?
K2

 

[ajoeiam]

You need an ice store to preserve the cold. Fill a freezer with cold ice from nature, then thaw it in the summer heat?
K2

You mean - - - build and - - - ice house?

 

[Bentwings]

You need an ice store to preserve the cold. Fill a freezer with cold ice from nature, then thaw it in the summer heat?
K2

our Wild West settlers dug deep hole lined them with prairie grass and cut ice in the wind from ponds or lakes and put in these holes then covered them with more straw . There are accounts of them having ice in the middle of our blistering summers. The local saloons or pubs often had iced tea and saspirilla. A root beer.
byron I don’t imagine it would go over well if I packed the community well opening in my back yard with ice and snow. It has a concrete cover but I can’t move it.
It s -9 deg F right now wind chill -25 f.

 

[Steamchick]

AJ. Yes. But a spare freezer can be a small ice store.
K2

 

[ajoeiam]

AJ. Yes. But a spare freezer can be a small ice store.
K2

LOL - - - - I just don't have more room for freezers.
The idea of an ice house is appealing - - - - now to make it practical!

 

[Bentwings]

LOL - - - - I just don't have more room for freezers.
The idea of an ice house is appealing - - - - now to make it practical!

It’s easy to dress to stay warm but much harder to dress to get cool you can just run under the sprinklers .
byron

 

[Bentwings]

Hi Byron, re"superheat": the prefix means "more than", or in this case "above", meaning "more temperature than the boiler", or higher temperature than the boiler. 1 degree, or 1000 degrees is "more than", but I bet you'll achieve 10 or 50 degrees "above boiler temperature" without too much bother, and this will dry the steam (not cool it!), won't destroy the lube oil inside the cylinder and significantly improve the performance, without increasing the boiler pressure above NWP for the boiler. (SO the boiler stays safe, engine performs better, and you are happier!).
Ok?
K2

I was actually trying to be humorous . My heater will not be extra hot or really super heated it will most likely heat or warm and dry the vapor from the boiler. In the process it should give an increase in effective volume at a given pressure. I was trying to avoid a complicated internal condenser but it looks like I’ll need something I’d really rather not have an inlet and outlet then external transfer tubes but I may have to compromise and use this as the means of passing steam through the unit . This little engine assembly has proven to be a real challenge due to my own shortcomings nothing I can do about that . 4 trips to the local auto parts store and getting the wrong parts has set me back some . I now have the correct hook up air line snd I made a modified connection that greatly simplifies assembly and future maintenance . The new hose is temp rated high enough to use for steam at the anticipated temps and pressures the couplings will need something better but I’ll deal with them later. The quick disconnect may have to be more permanent but I can live with that when steam comes on line. My son is making a new flywheel a little heavier that has clamp on mounts so there won’t be any wobble or tearing up the crankshafts . I’m going to try and put air in at lest one engine tonight without a flywheel . I don’t really know how this will work but I guess I’ll find out .

byron

 

[Steamchick]

Hi Byron, I understand. But other readers may not, and when it comes to correct technical accuracy, I must be clear to avoid any confusion. Although this is a "chat site", the technical information should be precise and accurate, as otherwise there could be some liability backlash form some "money grabbing lawyer". My point was simple: if your device adds heat - even in small amounts - to RAISE the temperature of the steam then it shall be called a superheater. Then we all know what you are doing.
In your reply you talk about "I was trying to avoid a complicated internal condenser but it looks like I’ll need something I’d really rather not ". Well, I simple do not understand what condenser you are talking about, unless you want to cool the steam back to water after the engine has used what heat it can? But an "internal condenser"? Internal to what? - Your workshop? Please can you clarify this one? I am doing my best to follow and help, but I sometimes find it a bit confusing: Sorry.
Sorry to be a "pain-in-the-brain" (or you may call me a "pedantic old git" - it may be more appropriate!). But correct and common technical language helps us all help each other without misunderstanding.
Cheers!
K2

 

[ajoeiam]

It’s easy to dress to stay warm but much harder to dress to get cool you can just run under the sprinklers .
byron

If you enjoy welding when you're dripping wet with only shorts on you're a far tougher man than I.
More power to you!!!!!!!!!!
(If you actually do this - - - well I would like at least a 1 minute movie of you doing something like an overhead stainless SMAW weld - - - I just gotta see that!!!)

 

[Bentwings]

Hi Byron, I understand. But other readers may not, and when it comes to correct technical accuracy, I must be clear to avoid any confusion. Although this is a "chat site", the technical information should be precise and accurate, as otherwise there could be some liability backlash form some "money grabbing lawyer". My point was simple: if your device adds heat - even in small amounts - to RAISE the temperature of the steam then it shall be called a superheater. Then we all know what you are doing.
In your reply you talk about "I was trying to avoid a complicated internal condenser but it looks like I’ll need something I’d really rather not ". Well, I simple do not understand what condenser you are talking about, unless you want to cool the steam back to water after the engine has used what heat it can? But an "internal condenser"? Internal to what? - Your workshop? Please can you clarify this one? I am doing my best to follow and help, but I sometimes find it a bit confusing: Sorry.
Sorry to be a "pain-in-the-brain" (or you may call me a "pedantic old git" - it may be more appropriate!). But correct and common technical language helps us all help each other without misunderstanding.
Cheers!
K2

Ok I’m corrected I’ll try and be more precise. We had another snow storm yesterday and I had to walk up to another auto parts store to get the remaining fittings I needed. So I YHINK I was just trying to calm down a little it was my 4th trip up there in a week. I won’t interject the political issues. Anyway I believe what I’m realy trying to do is create something on the order of the economizer that seems to be part of commercial steam systems I’ve been reading about the way I’m understanding these is that they are returning condensate to the boiler that still has heat value so in effect reduces the input required from the sctusl boiler as well as capture vapor that would be vented. .
I really didn’t want to get into super heated steam but the way I see it , if I want dry steam or dry volume of “air I must dehumidify the steam vapor to remove the moisture. This means pressurizing and higher temps within the steam charts. If I want to limit my discharge dry volume I must condense moisture out then bleed off this condensed moisture leaving me with hot gas or dry steam. If I now cool is by reducing heat input , pressure will go down to some value. If there is still moisture in this gas it will condense somewhere that I can drain it off now I have some value of moisture in the remaining gas. Picking some numbers say I choose 150 psi available for use in the engines. I can regulate flow to the speed or torque needed to do the work required, then exit this gas with the heat used to provide energy needed. In the engines it will cool further then go into the collection cyclone device. Then exit to atmosphere. It’s clean essentially air at a much lower temp. I’ve used as much energy as it contained. . If you were to exit it to a tank it might get to 75 psi choosing some number. These numbers I will have to test and quantify . That’s why I’m planning on a number of gages at different locations. My supercharged street rod had 12 permanent gages and several temporary gages available it really was an easy and fun car to drive I put 35k near trouble free miles on it . Even going on decal extended vacation tours . Mostvgages were for tuning . Once tuned the standard 6 automotive gages we’re just like most ordinary cars

so this internal condenser is a dehumidifier for more accurate terms as well as a device to allow input of additional heat energy . Excuse me I have a docs appt. I’ll be back. Got clean bill of health no changes required. Revisit in 6 months . Doc said I should ease up on hand exercises he said I have the strongest hand shake he has ever seen. I do try to stay in shape.
Anyway I hope I clarified what I’m trying to do.
I had to leave as my ride showed up. I hope I’m way out in the woods here.
mice looked at many operating model steamers but I don’t see many with real super heat except the railroad engines I see slimy exhaust condensate dribbling out with foamy oil it’s really just hot vapor ladened with oil some pipe it to a vortex type of tank others just let it dribble out.
my intent is to make this more realistic. I don’t plan on excessive pressure or volume as I want to limit rpm to usable sustainable reasonable speed. This little engine does not have great displacement so rpm will be the governing factor I think . I’ll come up with a vortex separator and the cooled exhaust will be into a container of some sort. We go to a big farm show in the spring. There are lots of hit and miss engines ans steamers there . I’ll find out more shortly and plan on some pictures .
I’m waiting now for my modified parts . My so had one part blow up on the lathe when he snagged the set screw with the boring bar. I have 6 more couplers so one down is not the end of the world. I may have to change coupler style to get a little more room for modifications . The new flywheel should have them built in.

Byron

 

[Steamchick]

Hi Byron, GOOD TO HEAR YOU ARE HEALTHY. I wonder if the Doctor is saying your strong right hand is a symptom of what you are doing with it, rather than a problem in itself?
I follow some of what you are trying to do. but you seem to have a mis-understanding - IMHO - that you need to cool the steam to separate the moisture before the steam is superheated. - I.E. " If I want to limit my discharge dry volume I must condense moisture out then bleed off this condensed moisture leaving me with hot gas or dry steam." The way it works for the rest of the world, is not to "condense the water out" as you put it, but to "boil the water droplets into steam" - BY ADDING HEAT. Which is what the superheater does. The added heat dries the steam.
I do think for your engines (rated for 50psi - or whatever - by the manufacturer) should have the NWP of the boiler set at 50psi. (NOT 150psi.) THEN a superheater to dry the "wet" steam (by boiling any water droplets) and adding some extra heat so the steam gets to the engine at a higher temperature than it leaves the boiler. That way, the steam in the boiler that condenses during expansion will be blown through to the exhaust in normal running, without having to cope with additional water from condensed steam from the feed pipe, etc. (But only when the engine is HOT! - You must still take care to turn the cold engine by hand with a little steam to heat it until you reach running temperature, to avoid the hydraulic lock often realised by wrong start-up technique. Forcing a cold engine that has an hydraulic lock is likely to damage rods and bearings.).
I understand a cyclone separator for separating the oil-water emulsion droplets from the exhaust - then pass the residual steam into the condenser to drain into a tank for recirculation into the boiler. This is what I do, and many others, in boats that cannot carry enough water to do a sensible run. (My boats did 10mins max from a full boiler to min water level, but with re-cycling condensate using an engine driven pump I have run for 20 mins or more, and the boiler came back 1/2 full! (Gas was nearly empty!).
Have fun!
K2

 

[Steamchick]

Hi Byron, Just to explain, for anyone who doesn't understand yet, why you should NOT have the boiler at 150psi and the engine rated (strength) for 50 psi.

1. If you make a mistake, and feed steam at too high a pressure to your little engine, then it is likely to damage something, or at least to dramatically reduce the lifetime of some part. I often see someone start an engine and apply "full steam" to an unloaded engine when it "revs its head off" - sometimes quite literally strips the head bolts, or damages components not intended for such speed or pressure.
2. The thermodynamics of water and steam do not like to be messed around, the way you can with air. An air receiver at 150psi can have a regulator set to supply 50psi air to the engine. - I do it regularly! The air doesn't change, it is still just air. But - to put it simply - if you expand STEAM at 150psi through an orifice/valve/restriction so the engine is taking the supply at 50psi (continuously) then all you will have is a lot of water (condensed steam) and some steam at the temperature of wet steam at 50psi. That's what steam does. Expand it to a lower pressure (below the "wet-point") and it condenses, so you are pumping a mix of steam and water into the engine. Not good. Not what you intend.

So please do not make steam in the boiler at a higher pressure than the engine is rated. Then add a superheater to dry the steam and make sure it gets to the engine without converting back to water... Even the cheapest tin models have boilers with a pipe through the flames as a superheater = they ADD HEAT after the steam has been taken from the boiler, at the pressure that the engine has been designed to take, durably and safely.
I hope this helps?
K2

 

[Bentwings]

Hi Byron, GOOD TO HEAR YOU ARE HEALTHY. I wonder if the Doctor is saying your strong right hand is a symptom of what you are doing with it, rather than a problem in itself?
I follow some of what you are trying to do. but you seem to have a mis-understanding - IMHO - that you need to cool the steam to separate the moisture before the steam is superheated. - I.E. " If I want to limit my discharge dry volume I must condense moisture out then bleed off this condensed moisture leaving me with hot gas or dry steam." The way it works for the rest of the world, is not to "condense the water out" as you put it, but to "boil the water droplets into steam" - BY ADDING HEAT. Which is what the superheater does. The added heat dries the steam.
I do think for your engines (rated for 50psi - or whatever - by the manufacturer) should have the NWP of the boiler set at 50psi. (NOT 150psi.) THEN a superheater to dry the "wet" steam (by boiling any water droplets) and adding some extra heat so the steam gets to the engine at a higher temperature than it leaves the boiler. That way, the steam in the boiler that condenses during expansion will be blown through to the exhaust in normal running, without having to cope with additional water from condensed steam from the feed pipe, etc. (But only when the engine is HOT! - You must still take care to turn the cold engine by hand with a little steam to heat it until you reach running temperature, to avoid the hydraulic lock often realised by wrong start-up technique. Forcing a cold engine that has an hydraulic lock is likely to damage rods and bearings.).
I understand a cyclone separator for separating the oil-water emulsion droplets from the exhaust - then pass the residual steam into the condenser to drain into a tank for recirculation into the boiler. This is what I do, and many others, in boats that cannot carry enough water to do a sensible run. (My boats did 10mins max from a full boiler to min water level, but with re-cycling condensate using an engine driven pump I have run for 20 mins or more, and the boiler came back 1/2 full! (Gas was nearly empty!).
Have fun!
K2

Obviously I’m not really putting what I want to do in the right context. So thanks for the help. I’ve had to take a breake due to outside things another snow storm then warm thawing weather . Waiting for Amazon parts and a few others . My son is bringing my flywheels back either to night or tomorrow morning . I’ve ordered parts three times on Amazon and still have the wrong stuff. I made a sketch of the eccentric modification I want but I think I’m goingvto have to get out to dons shop so I can make it. The connecting rod is a very close fit and if it is off by much it will either seize or wear out quickly it’s essentially a skinny bore with tight size on a wide shaft area so if it gets cocked even slightly it binds also the it’s pin, grudgeon pin is very small it doesn’t carry much load but if out of line far the load is not centered. Chilertern must have some extremely good machine to make these parts that fit so perfectly . I could make a new wider rod that would be more stable but it’s a lot of fussy work. It should operate as designed. They have made a bunch of these engines so I don’t feel that I should interfere in the design work. . It turns over easily it’s just the timing is more “ about” than exact. I’m used to being able to degree a cam or eccentric in right to the numbers so I suppose I’m just making things harder than necessary. Once I get the minor mods done timing can be set exactly on all cylinders in both engines as well as timed to each other. It’s really an easy thing but just takes a bit of work to make the parts. They have to have very fine surface finish so it takes a little finessing to get there . The boiler calculations you gave have helped me a lot. I think I understand but just not putting it out in the right words. I have three pressure gages coming and another temp gage so I’ll be able to monitor things better. Once I get these little parts completed I’ll try and post pictures I’m helping my younger son build a couple Rc super scale warbirds so I’m actually having fun helping him. He is a new generation of “ builders” there are not many of them we put a lo of miles under us while he was growing flying and building. He has much of my building and flying notes. He said it’s hard to believe I kept

Byronrecords like that. So he is now following . Nice to think you made a difference . When kids are growing up .

 

[Bentwings]

Well another day broken up
I was supposed to have an in home doc visit yesterday . They called about an hour after the time which put it right in the middle of the day . Had errands to call about so I got that done. Sat down to do hobby stuff and clinic called wants the in home doc visit today instead. I said ok let’s get it over with already spent two days on these visits. It is nice for in home as I don’t have to go anywhere but a little more notice would be good. So middle of the day today the doc is coming. About an hour visit. It’s pouring rain which is better than white out blizzard I guess. But I’m not getting anything done fast.
I’ve come up with a way to set the engine piston valve timing and engine to engine timing so it’s precise as needed. We tried machining a part but a slight miscalculation and the boring bar snagged the forgotten set screw and the fancy aluminum coupling blew up . It’s outside diameter is just too small for what is needed. I ordered new one larger but it’s a week or more out. I came up with a coumpound bore coupling 12 mm by 6mm that will work I think it should arrive tomorrow. My son brought all the pieces back Sunday so I’m sorting through them . I need a 1/4” 40 TPI ME PLUG well for practical purposes they don’t exist excep order from UK I YHINK I can drill # 10 32 but out and tap it then screw a piece of brass pipe in and fill the hole ith solder. That’s today’s project. I have new more flat mount board so I’ll remount the engines my “ new” drill press will come in handy for this . I’m looking for a small machinist vice for it. I have a monster 4” vice that weighs almost as much as the drill press. I did get some nifty 1/4” copper elbows. They are so small they look like lego parts. I thought I should get another bag of them . Yikes the price has almost doubled in two weeks.
I’ll refrain from getting on my political soap box. Once this visit is done I’ll get back to work.
I’m still thinking about the proposed “ super heater” . I have a 10k watt portable generator that probably needs a carb o’hsul but that’s something I can do. It’s at a friends shop so he would come and get me then we cal fill up with gas and have lunch then I can finish the thing I also want to test myself again TIG welding. There is a process called TIG brazing for copper. I have some silicone bronze brazing rod that might wok on my mini copper elbows. I suspect that I might not need this as I YHINK the hot steam may cool before it gets to the exhaust system . . So more testing to be done . I’m still thinking of the condenser for the final cooling . I’ve seen a couple on the internet tat look interesting .

I’ll know a little more later today

Byron

 

[kf2qd]

There are 2 uses of a dyno. Most here just seem to be thinking about getting a torque/rpm plot to calculate horsepower. The other use of a dyno is to provide a controllable load so testing can be done over a longer period of time under controlled conditions. Break-in monitoring, tuning induction, fuel delivery, ignition timing, cooling...

 

[lohring]

We used our inertial dyno to setup engines for the seasons racing. With a few short runs the proper operation could easily be confirmed under racing conditions. Boat race engines are being pulled down and then accelerate. Inertial dynos simulate that. The same is true for kart engines. Engines reach normal operating temperature very quickly at full throttle. Simple brake dynos would be useful for endurance testing, but modern ones have the loads computer controlled to simulate the expected conditions. An inertial dyno is a simple way to do some of this.

Lohring Miller

 

[dazz]

We used our inertial dyno to setup engines for the seasons racing. With a few short runs the proper operation could easily be confirmed under racing conditions. Boat race engines are being pulled down and then accelerate. Inertial dynos simulate that. The same is true for kart engines. Engines reach normal operating temperature very quickly at full throttle. Simple brake dynos would be useful for endurance testing, but modern ones have the loads computer controlled to simulate the expected conditions. An inertial dyno is a simple way to do some of this.

Lohring Miller

Hi
I race karts and although I don't have a dyno, I have looked into building one. An inertial dyno is definitely the way to go. Ideally it needs to be fully instrumented to record not just rpm & torque curves, but also all the other parameters including:
barometric pressure,
ambient air temp,
humidity (not really important),
throttle position,
EGT / CHT,
Lamda,
Air mass flow,
Fuel flow rate,

but even a basic dyno is better than no dyno.

 

[Ken I]

Here's a small inertial dyno for testing slotcar motors :-

A flywheel (which has holes for photocell pickup - not visible in photo) is accelerated by the motor and the various parameters calculated and displayed vis :-

FYI

Regards, Ken

 

[Steamchick]

Well another day broken up
I was supposed to have an in home doc visit yesterday . They called about an hour after the time which put it right in the middle of the day . Had errands to call about so I got that done. Sat down to do hobby stuff and clinic called wants the in home doc visit today instead. I said ok let’s get it over with already spent two days on these visits. It is nice for in home as I don’t have to go anywhere but a little more notice would be good. So middle of the day today the doc is coming. About an hour visit. It’s pouring rain which is better than white out blizzard I guess. But I’m not getting anything done fast.
I’ve come up with a way to set the engine piston valve timing and engine to engine timing so it’s precise as needed. We tried machining a part but a slight miscalculation and the boring bar snagged the forgotten set screw and the fancy aluminum coupling blew up . It’s outside diameter is just too small for what is needed. I ordered new one larger but it’s a week or more out. I came up with a coumpound bore coupling 12 mm by 6mm that will work I think it should arrive tomorrow. My son brought all the pieces back Sunday so I’m sorting through them . I need a 1/4” 40 TPI ME PLUG well for practical purposes they don’t exist excep order from UK I YHINK I can drill # 10 32 but out and tap it then screw a piece of brass pipe in and fill the hole ith solder. That’s today’s project. I have new more flat mount board so I’ll remount the engines my “ new” drill press will come in handy for this . I’m looking for a small machinist vice for it. I have a monster 4” vice that weighs almost as much as the drill press. I did get some nifty 1/4” copper elbows. They are so small they look like lego parts. I thought I should get another bag of them . Yikes the price has almost doubled in two weeks.
I’ll refrain from getting on my political soap box. Once this visit is done I’ll get back to work.
I’m still thinking about the proposed “ super heater” . I have a 10k watt portable generator that probably needs a carb o’hsul but that’s something I can do. It’s at a friends shop so he would come and get me then we cal fill up with gas and have lunch then I can finish the thing I also want to test myself again TIG welding. There is a process called TIG brazing for copper. I have some silicone bronze brazing rod that might wok on my mini copper elbows. I suspect that I might not need this as I YHINK the hot steam may cool before it gets to the exhaust system . . So more testing to be done . I’m still thinking of the condenser for the final cooling . I’ve seen a couple on the internet tat look interesting .

I’ll know a little more later today

Byron

Hi Byron,
Do not TIG Braze the copper. Just Silver solder. Just as effective on your models, ands a lower temperature melt of the silver solder. Worth paying for the right stuff.
K2