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Old 11-08-2012, 10:09 PM   #11 (permalink)
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I run a 125 cc 2 stroke KTM dirt bike on the street. It is capable of over 140kph maximum speeds and sustained 120kph depending on sprocketing. You cannot just put the highest ratio on and expect good performance. Typically you have to experiment to have the maximum power arrive at the highest achievable speed. Any higher ratio lowers maximum speed and kills 1st gear performance. The 2 strokes especially are very susceptible to carb jetting. My initial highway fuel mileage was only 27-35mpg(imp) with the stock jetting. The 2 stroke would not my first choice for fuel mileage, but does prove you can still ride a small displacement bike on today's streets. Current fuel mileage is only about 55mpg(imp) maximum.




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Old 11-09-2012, 03:42 PM   #12 (permalink)
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Originally Posted by sbestca View Post
The 2 stroke would not my first choice for fuel mileage, but does prove you can still ride a small displacement bike on today's streets.
2-strokes can actually be a decent option while looking for a good fuel-efficiency while fitted with some upgrades such as direct injection. What surprises me is how they're currently neglected by mainstream motorcycle manufacturers such as Honda and even Yamaha...

workaround ideas to discuss among friends: Two-stroke engines: still a viable technology?
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Old 11-09-2012, 03:54 PM   #13 (permalink)
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Although I hate and despise 2-strokes as they are now (I'm sorry, it might be a prejudice from my behalf), I find their simplicity appealing. I'd really like to see a cleaner and more effective direct injected variant on bikes, I'm really curious what kind of FE they could achieve. And I hope that they wouldn't smell of wasted fuel like the blue-smokey oldtimers and 'modern' scooters.
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Old 11-09-2012, 04:27 PM   #14 (permalink)
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Originally Posted by alvaro84 View Post
Although I hate and despise 2-strokes as they are now (I'm sorry, it might be a prejudice from my behalf), I find their simplicity appealing. I'd really like to see a cleaner and more effective direct injected variant on bikes, I'm really curious what kind of FE they could achieve. And I hope that they wouldn't smell of wasted fuel like the blue-smokey oldtimers and 'modern' scooters.
Nowadays there are even some researches about ways to provide it a lube system similar to a 4-stroke, avoiding the smoke-belching from the oil. Anyway, vegetable-based oils such as Castor oil are good options to cut the smoke since they get more effectively blended with the gasoline and eventually with some ethanol content (E5/E10/E15 or higher). It's worth to note that many synthetic oils have the same lube properties as Castor oil and palm oil.
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Old 11-09-2012, 08:50 PM   #15 (permalink)
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Originally Posted by cRiPpLe_rOoStEr View Post
Nowadays there are even some researches about ways to provide it a lube system similar to a 4-stroke, avoiding the smoke-belching from the oil.
Nothing new about that, the Wulf engine developed by Bernard Hooper has been around for decades:

BACKGROUND TO BERNARD HOOPER ENGINEERING LTD

and there are big two-stroke diesel engines with sealed crankcases.

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Old 11-10-2012, 07:24 PM   #16 (permalink)
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Originally Posted by Michael Moore View Post
Nothing new about that, the Wulf engine developed by Bernard Hooper has been around for decades:

BACKGROUND TO BERNARD HOOPER ENGINEERING LTD
I wasn't aware of that Wulf engine, but the ones I was already aware had more conventional pistons instead of those ported ones.
http://motor2t.net/indexI.htm - the developer of this one used some chainsaw engines as the base for the project but recently was intending to convert a Honda Cub engine to 2-stroke, altough I don't know if he already did it.
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Old 11-10-2012, 10:59 PM   #17 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
2-strokes can actually be a decent option while looking for a good fuel-efficiency while fitted with some upgrades such as direct injection. What surprises me is how they're currently neglected by mainstream motorcycle manufacturers such as Honda and even Yamaha...

workaround ideas to discuss among friends: Two-stroke engines: still a viable technology?
I tried to find out just how much better the fuel efficiency is when direct injected...not that much data out there since mainstream manufacturers don't make direct injected 2 strokes.

I managed to find fuel economy numbers for I think ski doo ? snowmobiles. They had a direct injected 2 stroke and a 4 stroke. The 2 stroke was much lighter, but got worse (I think like 30% worse) fuel economy than the 4 stroke with equivalent power despite the direct injection.

I think the problem is that in a 2 stroke with fuel injection, the fuel has less than 180 crankshaft degrees to mix with the air, so the combustion efficiency is still pretty bad, just not quite as bad as in a carbureted 2 stroke.
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Old 11-11-2012, 08:08 PM   #18 (permalink)
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Originally Posted by serialk11r View Post
I managed to find fuel economy numbers for I think ski doo ? snowmobiles. They had a direct injected 2 stroke and a 4 stroke. The 2 stroke was much lighter, but got worse (I think like 30% worse) fuel economy than the 4 stroke with equivalent power despite the direct injection.
When Ford tested 2-stroke engines in the Mk3 Fiesta and the Aspire, a 2-stroke 1.2L 3-banger had better fuel-efficiency than the stock 1.3L 4-banger, and it was the same system currently used in snowmobiles and outboard motors.
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Old 11-16-2012, 05:12 PM   #19 (permalink)
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Two strokes gets very poor MPG capacity wise because of therir very low CR.

No if the MPG is related to output rather than capacity, maybe it is a little better ?
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Old 11-17-2012, 12:53 PM   #20 (permalink)
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Quote:
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Two strokes gets very poor MPG capacity wise because of therir very low CR.
No if the MPG is related to output rather than capacity, maybe it is a little better ?
I think you are right.
My KTM125EXC motorbike gets 55mpg. Not very good for a 125 motorcycle but this is a full sized dirtbike with about 35-40hp. A similar 35hp fourstroke 230 lbs dirtbike would get about 55mpg too, in the manner I drive it.

Snowmobiles cannot be given MPG figures because conditions vary so wildly. When I go snowmobiling with a crowd, the 4 strokes typically take less fuel at the pumps, but often not by much. Especially considering the performance advantage most of the 2 strokes have. Modern fuel injected 2 stroke snowmobiles are remarkably frugal on fuel compared to the snowmobiles of yesterday.

Again in outboard motors. Modern 2 stroke outboards, even carbed models, are very good on fuel, almost as good as 4 stroke models and much, much lighter.

I think there is a future for 2 stroke advancement, with lean mixtures, ecological oils and efficient designs. 2 stroke diesels make a lot of sense, oil injection targeted at the bearings, and other innovations.

I am actually doing a lot of work on a Yamaha 200cc air cooled quad engine, trying to increase power over stock. This is a 17hp motor that my son and I have increased to over 30hp without a noticeable increase in fuel use (as determined by range and running time on the small tank).

There was mention of the low compression ratio typical in modern 2 strokes.
Not really a fair comparison. A 2 stroke can use 8:1 compression to make high HP because the exhaust pipe has a supercharging effect.
A modern 4 stroke bike engine uses up to 12:1 compression because the wild high rpm cam timing bleeds off so much of the compression pressure.

So much work as gone into weight reduction in modern cars.
A 2 stroke already meets that mark. Just think if we could be driving 500cc cars with variable port timing and exhaust length to give both power and economy. That 500cc motor could weigh less than 30kg (70 lbs) and put out more than 100hp with very few moving parts.

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