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2 Stroke limits


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#1 SGM

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Posted 26 May 2020 - 02:58

I saw this article recently:

 

https://www.grandpri...-in-the-future/

 

It seems that two strokes won't go away, as their many benefits obviously still appeal to some people. Perhaps some of the more technically minded people might be able to fill me in on the limits of the two-stroke design in terms of power output only, we will leave emissions aside for now.

 

It looks to me that two strokes only seem to be more efficient than their 4 stroke brethren, in terms of power output, in relatively small cylinder sizes. The old 500cc Grand Prix bikes settled on 125cc per cylinder as the best compromise and after the demise of the 500's, the last of the 125cc Aprilia's were allegedly making around 55 hp. To put that into perspective, that would be around 1250hp from a 3.0 litre F1 engine! However the NSR 500 V twin which raced in the 1990's seems to have made considerably less power with its 250cc cylinders. I can only find quotes for its output at 135hp although this seems on the low side to me. I recall Alan Cathcart road testing the NSR V twin against the Roberts KR3 and a Suzuki V4 and I thought he said the outputs were something like 160hp, 170hp and 180hp respectively and he went on to say the Honda four-cylinder was making over 190hp at the time. If the V2 with its 250cc cylinders really only had 135 hp, this is only 270hp per litre, well down on the 420hp per litre of what the 125cc cylinder is capable of.  It is also less than what a four stroke engine is capable of. Although not a great example, the old 500cc motocross bikes only made a touch over 100hp per litre, so it appears the trendline for horsepower drops rapidly with cylinder capacity increase compared to a four stroke engine.

 

 My point is, if two strokes are ever to be usable as a F1 or road car engine they would need to be efficient in larger cylinder sizes. A 24 cylinder 3.0 litre two stroke F1 engine, with its attendant expansion chambers, may look and sound impressive but is not overly practical, is it?. So, if we used a 500cc cylinder as an example, what horsepower would a normally aspirated two stroke be capable of, using current, proven technology? What would be its limiting factors?

 

cheers


Edited by SGM, 26 May 2020 - 03:00.


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#2 Wuzak

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Posted 27 May 2020 - 01:52

I think efficiency is the wrong word in the context of power to capacity.

 

I suspect that the reason that the 2 strokes made ~2 times the power of the same sized 4 stroke is because they used twice the fuel - at least.

 

For road use a N/A 2 stroke would seem to be a non-starter, due to emissions.

 

A supercharged 2 stroke would probably be more likely, or one with an F1 style turbocharger/MGU.

 

Given that the current F1 V6s are 1.6L, a supercharged/turbocharged 2 stroke of the same power would be about 1L.



#3 Greg Locock

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Posted 27 May 2020 - 06:05

Didn't Orbital Engines start promoting a low(ish) emissions 2 stroke? i thought I drove a Fiesta or something with a 3 cylinder, lots of poke, not so quiet, about 20 years ago.



#4 malbear

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Posted 27 May 2020 - 08:46

I heard that the orbital 3 cylinder two stroke engines were ok but would not last any longer than 50K kilometers due to the oil starvation to meet the pollution standards  .



#5 SGM

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Posted 27 May 2020 - 10:18

I think efficiency is the wrong word in the context of power to capacity.

 

I suspect that the reason that the 2 strokes made ~2 times the power of the same sized 4 stroke is because they used twice the fuel - at least.

 

For road use a N/A 2 stroke would seem to be a non-starter, due to emissions.

 

A supercharged 2 stroke would probably be more likely, or one with an F1 style turbocharger/MGU.

 

Given that the current F1 V6s are 1.6L, a supercharged/turbocharged 2 stroke of the same power would be about 1L.

I guess the word "efficiency" can be used in many different ways. If fuel economy is your only metric perhaps the two-stroke is not as efficient as a four stroke but the way I see it if an engine of 25% less capacity which has less moving parts, weighs less and makes the same power I would argue it is more efficient as a whole.
 
 The Bombardier group have put a lot of effort in to 2 stroke development, have a look at their Evinrude ETEC. They claim their 2 stroke outboard motors create less emissions than equivalent 4 strokes and similar fuel mileage. Their snowmobile division also are biased toward 2 strokes with both a 850cc and 600cc twins which both claim 200hp/litre with low emisions. 
 
If the F1 2 stroke idea became a reality it will be interesting to see if they would be normally aspirated or blown as you suggest. I think it is likely they might be the latter.


#6 desmo

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Posted 27 May 2020 - 14:17

N.B. The Honda NSR 500 was a V-4, not a twin.



#7 SGM

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Posted 27 May 2020 - 22:47

N.B. The Honda NSR 500 was a V-4, not a twin.

Honda made the NSR500 and the NSR500V which was a V-twin. It was designed to be a more cost effective privateer's GP bike. 



#8 Wuzak

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Posted 28 May 2020 - 04:44

Didn't Orbital Engines start promoting a low(ish) emissions 2 stroke? i thought I drove a Fiesta or something with a 3 cylinder, lots of poke, not so quiet, about 20 years ago.

 

Yes, they did. 

 

Didn't the technology end up with Mercury Marine?



#9 Greg Locock

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Posted 28 May 2020 - 06:08

That's it, yes.



#10 mariner

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Posted 30 May 2020 - 13:41

On way of looking at a two stroke is to see it as a supercharged engine since the crankcase does the same job ( essentially) as Roots blower, i.e positive displacement related to current engine speed. 

 

Lack of valves is not a defining factor for two stroke as Uniflow two strokes can have exhaust valves in the head.

 

The GM rail diesel engines had them and a Roots blower instead of crankcase compression. 

 

I once quickly scanned the drag racing TtopFfuel rules and couldn't see that two strokes were banned (?)  It led me to thought that if you were VERY clever you could build a Top Fuel engine within the rules but running as a two strokes with the mandated two valves per head as the exhausts only. The fuel consumption might be awful but not an issue for 1000 ft. 

 

You could potentially  get lot more Nitro into it which is the key to drag engines ??



#11 gruntguru

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Posted 31 May 2020 - 22:04

I once quickly scanned the drag racing TtopFfuel rules and couldn't see that two strokes were banned (?)  It led me to thought that if you were VERY clever you could build a Top Fuel engine within the rules but running as a two strokes with the mandated two valves per head as the exhausts only. The fuel consumption might be awful but not an issue for 1000 ft. 

It would be a shame to see Top Fuel lose its reputation for excellent fuel efficiency.



#12 Greg Locock

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Posted 31 May 2020 - 23:26

Poe's Law.



#13 gruntguru

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Posted 01 June 2020 - 21:42

I'm serious. Those things can do as much as 20 gpm.


Edited by gruntguru, 01 June 2020 - 21:43.


#14 GreenMachine

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Posted 01 June 2020 - 22:01

gallons per mile!?



#15 blueprint2002

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Posted 20 June 2020 - 01:08

I saw this article recently:

 

https://www.grandpri...-in-the-future/

 

It seems that two strokes won't go away, as their many benefits obviously still appeal to some people. Perhaps some of the more technically minded people might be able to fill me in on the limits of the two-stroke design in terms of power output only, we will leave emissions aside for now.

 

It looks to me that two strokes only seem to be more efficient than their 4 stroke brethren, in terms of power output, in relatively small cylinder sizes. ……...

 

 My point is, if two strokes are ever to be usable as a F1 or road car engine they would need to be efficient in larger cylinder sizes. ……..

I seem to recall, from the days of 2-stroke racing motorcycles, that the cylinder size limitation was directly related to distortion of the cylinder bore around the very hot exhaust ports, that area expanding much more than around the much cooler transfer ports, located only a short distance away. This was first found with air-cooled cylinders (during the 60s), and further increases in power were possible only when water-cooling was adopted, this helping to equalise temperatures around the circumference, but only to some extent.

Distortion was the cause of many, if not all, of the abrupt, violent seizures that those early engines were so prone to. The Japanese makers understood this well, even their high-performance road machines never exceeded 250cc per cylinder. And you may recall Yamaha's highly successful racing TD250 and TD350 twins, which never resulted in a TD500 twin, even lower size limits being necessary at racing power levels.  They went straight to 4 cylinders when they entered the 500cc class.

To sum it up, the larger the cylinder, the lower the power output per cylinder. Hence the off-road 500 singles you have mentioned,where light weight and simplicity were more important than sheer power.



#16 gruntguru

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Posted 20 June 2020 - 01:48

Do away with ports and change to the Diesel cycle - optimum 2 stroke cylinder size seems to be about 960mm bore and 1,810,000 cc



#17 manolis

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Posted 20 June 2020 - 03:45

Hello Gruntguru.

 

You write:

"Do away with ports and change to the Diesel cycle - optimum 2 stroke cylinder size seems to be about 960mm bore and 1,810,000 cc"

 

 

The state-of-the-art giant 2-stroke marine Diesels:

 

WartilaRTA84RT_crankshaft.jpg

 

keep on using ports for the transfer: 

 

Wartsila_Cylinder_Ports.jpg

 

and a poppet valve per cylinder for the exhaust

 

Wartsila_X92_Cylinder_Head.gif

 

 

 

 

Hello Blueprint2002

 

You write:

"I seem to recall, from the days of 2-stroke racing motorcycles, that the cylinder size limitation was directly related to distortion of the cylinder bore around the very hot exhaust ports, that area expanding much more than around the much cooler transfer ports, located only a short distance away. This was first found with air-cooled cylinders (during the 60s), and further increases in power were possible only when water-cooling was adopted, this helping to equalise temperatures around the circumference, but only to some extent."

 

 

Here is the PatATE wherein the same ports are utilized for both, the exhaust and the transfer:

 

PatATE_Rotary_Port_Opening.gif

 

Note: the rotating cylinder (the red part) is away from high pressure, away from thrust loads, away from the piston and from the piston rings. 

 

  

Besides the significant lowering of the temperature differences on the cylinder liner (and on the piston), the PatATE also provides a substantially increase of the port area (or port-time area) and a strong assymetry on the timing (the exhaust opens first and closes first):

 

PatATE_Rotary_vs_Conventional_Timing.png

 

More at http://pattakon.com/pattakonPatATE.htm and at https://forums.autos...stroke-engines/

 

Thanks

Manolis Pattakos 



#18 blueprint2002

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Posted 20 June 2020 - 06:20

Do away with ports and change to the Diesel cycle - optimum 2 stroke cylinder size seems to be about 960mm bore and 1,810,000cc

 

 

 

 

 

 

 

 

Hello Blueprint2002

 

You write:

"I seem to recall, from the days of 2-stroke racing motorcycles, that the cylinder size limitation was directly related to distortion of the cylinder bore around the very hot exhaust ports, that area expanding much more than around the much cooler transfer ports, located only a short distance away. This was first found with air-cooled cylinders (during the 60s), and further increases in power were possible only when water-cooling was adopted, this helping to equalise temperatures around the circumference, but only to some extent."

 

 

Here is the PatATE wherein the same ports are utilized for both, the exhaust and the transfer:

 

 

 

Note: the rotating cylinder (the red part) is away from high pressure, away from thrust loads, away from the piston and from the piston rings. 

 

  

Besides the significant lowering of the temperature differences on the cylinder liner (and on the piston), the PatATE also provides a substantially increase of the port area (or port-time area) and a strong assymetry on the timing (the exhaust opens first and closes first):

 

More at http://pattakon.com/pattakonPatATE.htm and at https://forums.autos...stroke-engines/

 

Thanks

Manolis Pattakos 

 

Thanks for these responses.

Those large marine diesels operate at very low speeds; the biggest at 60-80 RPM, the smallest (350-400 bore) at below 200 RPM.

And they are the most efficient engines in the world, as expressed in terms of specific fuel consumption, or thermal efficiency.

Manolis I don't know if that engine has been built and tested, but lubrication of that valve might cause higher oil consumption, as compared to the current state of the art?



#19 manolis

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Posted 20 June 2020 - 09:32

Hello Blueprint2002

 

You write:

"Manolis I don't know if that engine has been built and tested, but lubrication of that valve might cause higher oil consumption, as compared to the current state of the art?"

 
 
 
The contrary is the case.
 
In the conventional 2-stroke the scuffing starts at the side of the piston that slides over the "red-hot" exhaust port. There is where plenty of oil (for lubrication and cooling) is required. But in order to have plenty of lubricant around the exhaust port, the rest engine is over-lubricated and the specific lube consumption increases.. 
 
Without the hot exhaust port of the conventional 2-stroke, the specific lube consumption reduces.
The red part, which is substantially rid of loads, is lubricated as the rest parts into the crankcase.
 
So, the PatATE lowers the specific lube consumption.
 
 
With the transfer closing substantially after the exhaust:
 
PatATE_Rotary_Timing.gif
 
the supercharging is an interesting option.
 
Thanks
Manolis Pattakos