supercharging a two stroke

Discussion in '2-Stroke Aircaft Engines' started by blane.c, Jul 25, 2018.

  1. Apr 13, 2019 #201

    Andy_RR

    Andy_RR

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    All the modern two-stroke fruit is already in use the UAV industry - that is oil injection, nikasil, liquid cooling, DI etc. The reason you don't see it for recreational aviation is that most people who fly for fun can barely afford the 50s tech at the volumes these engines are purchased. If they sold in even snowmobile volumes, things might be different
     
  2. Apr 14, 2019 #202

    Sockmonkey

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    If you add things like oil injection and stuff, doesn't it get to the point where it's simpler to just use a turbo/blower for scavenging rather than crank scavenging?
     
  3. Apr 14, 2019 #203

    pictsidhe

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    An oil injection pump is an order of magnitude easier to engineer than a turbo.
     
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  4. Apr 14, 2019 #204

    Andy_RR

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    Why is external scavenging thought to be such a good thing? It's extra components that all add weight and complexity as a substitute for engineering a crankcase pump properly. Roller bearings with oil injection are much lower friction, just as reliable. Crankcase scavenging is also usefully more efficient than external scavenging and you can lose the bottom sealing ring on the piston resulting in lighter pistons.
     
  5. Apr 14, 2019 #205

    Sockmonkey

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    Suitable turbos already exist so the engineering aspect is already done.
    If it's a single cylinder, then yes crank scavenging is better. For multiple cylinders, you may as well go for the turbo as it's one thing providing air to several cylinders. It also runs clean enough to meet environmental regs.

    For things like the self-scavenging piston I posted, the rings can be lubricated via a channel going up through the piston rod with the only caveat being that oil consumption would be a little higher than what happens in a 4-stroke.
     
  6. Apr 14, 2019 #206

    pictsidhe

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    I take it that you've never fitted a turbo to an engine that didn't previously have one...
     
  7. Apr 14, 2019 #207

    Sockmonkey

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    I'm not talking about refitting an engine that didn't have one before, but about using one meant to be turbo-scavenged. My bad I should have been clearer about that.
     
  8. Apr 14, 2019 #208

    nerobro

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    No, not very quickly it doesn't. But you've got the right idea. oil injection is low pressure, and easy. turbos are "lightweight" horsepower, and on an already lightweight engine, they make sense.

    Sealed cranks need lot of space (crank scavenged engines don't) you need an oil system, oil tank, or at least a sump, some way to maintain the oil supply, and then you start needing the positive air flow for idle and starting. At that point.. why not just go 4 stroke? Somewhere around the 100hp level, 4 strokes start to get near the weight of 2 strokes.. and it stops making sense to work on 2 strokes.
     
  9. Apr 14, 2019 #209

    Swampyankee

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    If you read the engine design literature you will see that crankcase scavenging is usually considered as less than optimal: it may not provide enough air for complete scavenging (this differs from good enough scavenging) and it's going to be less thermodynamically efficient than a purpose-built scavenging system. It also means that the mechanical design of the crankcase has to be compromised and, of course, one has the joy of a combustible fuel-air mixture going through the crankcase. It works and it's cheap; for engines where first-cost is a dominant design concern, crankcase scavenging is the first choice.

    No one is going to build a small two-stroke with anything other than crankcase scavenging because the engine will cost significantly more and probably weigh more; it's the same reason why small two-stroke engines are loop-, not uniflow-scavenged: it's cheaper. Large two-strokes are different; when your engine is expected to run several thousand hours per year, it quickly gets to the point were a fraction of a percent reduction in sfc is worth serious money up front. (as an aside, there are some quite large two-stroke engines, made by companies like MAN-B&W, Wartsila, and Fairbanks Morse (their OP engine is still in production after over 80 years)
     
    Last edited: Apr 15, 2019 at 12:00 AM
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  10. Apr 15, 2019 at 4:56 AM #210

    Andy_RR

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    I've spent a fair bit of time reading the literature and uniflow scavenging offers somewhere between very little to no advantage at all over a well designed loop scavenging system. External scavenging will always cost you more energy than a crankcase pump because it's not timed and flow losses at the ends of the event will be higher. (of course, if you get the energy from waste exhaust heat you can win back some here, but this isn't typical) High scavenging ratios are also harmful to BSFC because pumping air that isn't involved in the working cycle is expensive in energy terms.

    No doubt there are loads of uniflow scavenged giant marine engines but there is some tradition going on here as well as it being driven by the extremely undersquare designs employed in this arena. I note that the current KTM two-strokes are square-to-undersquare now which is uncommon but you can push loop scavenging a long way in this direction if you want to and with useful advantages - perhaps this is where small two strokes are heading but in the era of mass electrification, I don't think there's going to be a lot of further development in this field.
     
  11. Apr 15, 2019 at 1:11 PM #211

    Swampyankee

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    I have no doubt what you say is true, but I think it’s also relevant that the vast majority of small two-stoke engines are for applications which are much more sensitive to first cost than fuel consumption. Where fuel consumption is highly important, makers of small engines have tended to go to four-strokes
     
  12. Apr 15, 2019 at 1:33 PM #212

    Andy_RR

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    Care to remind me of an application for small engines where fuel consumption is important?

    BTW, I'm pretty confident that a 300cc/cyl two-stroke could be running around the 250g/kWh BSFC mark without too much effort. In a project I was working on some time ago, we were burning kerosene fuels at less than 300g/kWh.
     
  13. Apr 15, 2019 at 2:06 PM #213

    BJC

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    Aren't most small generators (portable, household size) concerned about fuel consumption?


    BJC
     
  14. Apr 15, 2019 at 2:29 PM #214

    blane.c

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    I want a fuel efficient small engine. I can't print how I feel about those that don't.
     
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  15. Apr 15, 2019 at 2:40 PM #215

    Andy_RR

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    Not really - for (random) example, a Honda EU30is runs a GX200 and has a massively oversquare engine at 68x54 and an 8.5:1 compression ratio. The real design requirements are to be able to burn just about any goat's piss that falls to hand.
     
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  16. Apr 15, 2019 at 4:34 PM #216

    nerobro

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    Not even close. "It must start" and "It must keep running with 3 year old 90% varnish mixed with some 2 stroke gas that I found" are more important.

    BIG engines, are very concerned with fuel consumption. Trucks, boats, power stations. You'll see CRAAAAZY methods for sucking all the energy out of fuel on those things. Very low speed, crosshead supported pistons, with big, damned, turbochargers. And then, secondary heat recovery from the exhaust to drive a steam turbine. Running the heaviest, cheapest, densest fuel.

    Peaking power plants, also do the secondary heat recovery, and they exist to run turbines at their most efficient (that is full throttle).
     
  17. Apr 16, 2019 at 4:47 AM #217

    Andy_RR

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    Agreed, and immature too!

    Let's do the math, Blane, rather than staging an inappropriate hissy-fit, hey?

    A 60hp engine flown for 100hrs/year is probably a well used ultralight. At 75% power, that's 3357kWh of energy. If you have a bad engine it might do 400g/kWh. A very good engine at that power level would be, say 250g/kWh, hence a 150g/kWh delta. That represents 680L or about 180USG. Assuming you're burning MOGAS at $2.50 a gallon, that's only $450 per year you have to spend on technology to improve the fuel consumption by a huge amount - just to break even!

    For an hour's flight, that's less than two gallons extra you need to carry with you.

    On an engine that might do 500-1000hrs before overhaul that's a lifetime spend of up to $4500 just to break even, even at pretty elevated flight hours. For most, the delta is much less, as is the SFC improvement.

    The real-world savings will be more like $1500 over the engine's lifetime.

    So, how much more are you prepared to pay for your 60hp engine over a simple carb-fed, magneto-ignited design just to have better efficiency?
     
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  18. Apr 16, 2019 at 1:27 PM #218

    blane.c

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    My hissy-fit is about a green footprint. We all have to do more to reduce carbon emissions and small engines are some of the worst offenders. Making it about how much gas you can financially afford to partially burn through an engine is petty and selfish in my opinion. Reducing the SFC of engines is a important matter for industry's and businesses that burn millions of gallons of fuel a year because it directly effects there bottom line. The knowledge and technology is directly transferable to small engines but is not generally done for them for a myriad of reasons like burning any goat piss that happens along. This is however no excuse for aviation, we don't burn goat piss.
     
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  19. Apr 16, 2019 at 2:44 PM #219

    pictsidhe

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    Most people only look at the up front cost and don't consider running costs. I like the newer EPA mowers, I can cut all my grass without a refill half way round. My chainsaws cut more wood before they need a refill. I'm planning to try and achieve good economy on my aircraft. Partly because I want my descendants to have a habitable planet, partly because I hate letting the moths out of my wallet...
     
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  20. Apr 17, 2019 at 12:14 AM #220

    Andy_RR

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    I share your ambition to improve CO2 emissions but are you aware there is pretty much 100% correlation between GDP and CO2 emissions and energy consumption. That is, every dollar you spend represents an equivalent amount of CO2 and energy consumed. If you spend more money to do the same job your CO2 will be higher. Until non-carbon based energy production becomes widespread (which is not the case yet!) the dollar/CO2 correlation will continue to hold
     

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