Long stroke / Short Stroke

A questions about engine..

Can you tell me the different characteristics of a long / short stroke engine.

My first thoughts are that a short stroke will turn higher RPM and less torque.. The inverse for long stroke.

is there a coloration with pipe length?

From my experience the short stroke produces more RPM but less torque when compared to along stroke. i run a slightly longer pipe on the short stroke.

Unlike in four stroke engines, our two stroke engine's port area is proportional to the cylinder wall area, not the cylinder head area. The wall area is proportional to the bore times the stroke. The displacement is proportional to the bore squared times the stroke. That means that increasing the bore will increase the displacement a lot faster than the port area. If breathing is the only limitation, a long stroke engine will be able to rev higher than a short stroke engine. This is the opposite of a four stroke.

In our small two stroke engines, scale effects give us a large port area for the displacement. That means breathing isn't the only limiting factor. A shorter stroke with its lower inertial effects will rev better. Even so, the most powerful small two strokes usually have a bore equal to or slightly larger than the stroke.

Lohring Miller

Dead square will give the best of both worlds.

Ran a .060 Pontiac 455HO It was dead square 4.21B x 4.21S = 472 CID surprised many big B/S,R/S engines at the stop lights on Friday night.

Timed area is what it is all about with a two stroke.

Stroke and rod length all contribute to piston speed. Not just the speed over all but the speed at different points in the travel across the port.

How much time is most of the port open. not just total but the accretion on the way down, sitting at the bottom(dwell) and the acieration on the way back up. when will the pipe draw the cyl down will also come into play. The intake timing also comes in to play as this will change the case pressure.

There is no strait forward answer as it all needs to work together.

Two stroke engs are a living breathing animal that need lots of testing time to understand and tame.

Lohing

You have much more of a grasp on this than most can you elaborate on this some for us?

Check out the new Propwash. Lohing will make head spin.

Lohring, is there an "optimal" rod ratio that has the speed & dwell times - while still being easy on hard parts, esp. piston side loading? 4 strokes are so much simpler, imho.

Anthony, supper Great thread starter !!!!! This is the kinda of stuff that intrigues me about engine theory. I have to think that bigger bore and less stroke would make a more powerful 2 stroke like Lohring Says, but the fact that the most modern higher horse power delivering engines made by manufacturers today have longer stroke and less bore. it dose deify Physics. so what Dave Wilfong says has to have a big play too. look, I've played with this stuff since I was 13yrs old and am no pro to this day, but why are the longer stroke less bore engines kicking tail. I bought engines based on large bore and small stroke. one was a K&B which had no ponies out of the box. and caused me to get my but smoked for many seasons, and the other was a OS Max RZM with a MAC .21 nitro pipe that still Kicks ass with today's motors. this stuff really throws me for a loop. love to learn more. and Ray where do I find the new prop wash? Andy You had the MAC made to your specs, whats your Take on this? This comes at a good time because I am trying to buy a new 20 motor these days, and the woman is getting sick of me compering specs instead of listening to her. hahaha. anyway this is a great thread, Chime in all you great elders Please.

Jim

Oh ya, Steve as far as using a longer pipe on shorter storke confuses me too because the sonic wave would need to be faster Right? well I guess exhaust Temp would Change things but help me with this. not being a smarty I am trying to learn something here.

Jim

I believe this is a tricky question. My opinion. Short stroke engine has larger piston top area, can get more down force from expanding flame, but shorter moment arm. While long stroke, things reversed. I believe this not related to torque but the efficiency of scavenging process.

In about late 198X's. .21 engines branched to on road and off road lines. They differ mainly by height of exhaust port. On road engine has higher exhaust port, this make MES possible, while there is little charge and burnt gas mix. But MES in off road engine is almost no advantage, NR P5XLT use single exhaust. Because low exhaust angle make exhaust port very close to the transfer port, that gas max is more likely, means lost of fresh charge.

To reduce gas mix in off road engine and make MES possible. Extra long stroke engines are made, NR bore 15.88mm, stroke 17.6mm. Cylinder diameter is smaller, height is greater, so the distance between exhaust port and transfer port is greater. Even with MES, gas mix in off road engine is less. It seems longer stroke has more advantage, interms of gas mix. No. A small bore big height cylinder make scavenging process less efficient. Gas need to travel a higher distance to refresh. Less gas mix and efficient refresh, cannot gain both. New NR KEEP-OFF.21-7XL and O.S. R2101 are extra long stroke. I believe they are approaching the limit.

Best regards,


Leo

The standard "ideal" rod length seems to be around twice the stroke. There's an argument about that going on now about the effects on engine timing, but I'd stick with something like twice the stroke. In Zenoahs, the 50 mm rod is definitely too short for the 30 mm stroke engines if you expect the rod big end to live very long at racing rpm. There start to be issues with the 28 mm stroke at over 20,000 rpm.

The arguments about bore/stroke ratios have been going on a long time. The dimensional issues outlined above mean that cylinder wall area increases a lot slower than displacement as engines get bigger. The big marine two strokes are breathing limited due to this and are low rpm engines. Small engines still have a lot of available port area for their displacement so breathing isn't the only limitation. Please don't confuse overhead valve four strokes with this. Formula 1 engines are very short stroke for their displacement because the valves are in the cylinder head. There the valve area is related to the cylinder head area, not the cylinder wall area.

My favorite two stroke example comes from the old days with the rivalry between the Dooling and McCoy 60s. The Dooling was significantly more over-square than any of its earlier rivals, having bore and stroke measurements of 1.015 in (25.78 mm) and 0.750" (19.05 mm) respectively for a displacement of 0.607 cu. in. (9.94 cc). The resulting bore/stroke ratio of 1.35:1 was unprecedented at the time. The McCoy 60 had a bore of .940" (23.88 mm) and a stroke of .875" (22.22 mm) for a displacement of .607 cu. in. as well. It's bore/stroke ratio was a more modern 1.074:1. The McCoy lacked the Dooling's elegant machining and casting, but ultimately held all the control line speed records for cross flow, open exhaust engines.

Two stroke motorcycle and kart engines evolved to close to equal bore and strokes. That seems to be the best compromise in this size range. We may be able to run a little bigger bores because we have a little more port area for our displacement. I'll work on a graph of this relationship.

Lohring Miller

PS Thanks for the complements. I just try to pass along things that will help understand the very sophisticated operation of our seemingly simple engines.

lohring said:

The standard "ideal" rod length seems to be around twice the stroke. There's an argument about that going on now about the effects on engine timing, but I'd stick with something like twice the stroke. In Zenoahs, the 50 mm rod is definitely too short for the 30 mm stroke engines if you expect the rod big end to live very long at racing rpm. There start to be issues with the 28 mm stroke at over 20,000 rpm.

Click to expand...

Is there a way to bandaid this issue? We went through this with one of our gas motors. By far the fastest we had ever had running on the boat, but every half a year or so we would lose the rod bearing or snap a rod until eventually destroying the engine. Obviously you cant put a longer rod in them, but is it possible to find a way to avoid it?

So If I understand what has bin put out there so far. The further the gases have to travel the less efficient the scavenging of the cyl is.

So in a smaller size eng a bigger boar will make it easer to scavenge the cyl as there is more diameter making a shorter vertical lift for the gasses to travel.

So there is a sweet spot between port area controlled by stroke and distance the gases have to travel?

Lohring

I will love to see this graft.

This kind of discussion is very good as it helps for those wanting to learn the ability to ask questions.

Not all of us can under stand with out some feed back as we all have different levels of comprehension.

This is what makes IW what it is.

dwilfong said:

So If I understand what has bin put out there so far. The further the gases have to travel the less efficient the scavenging of the cyl is.

So in a smaller size eng a bigger boar will make it easer to scavenge the cyl as there is more diameter making a shorter vertical lift for the gasses to travel.

So there is a sweet spot between port area controlled by stroke and distance the gases have to travel?

Lohring

I will love to see this graft.

This kind of discussion is very good as it helps for those wanting to learn the ability to ask questions.

Not all of us can under stand with out some feed back as we all have different levels of comprehension.

This is what makes IW what it is.

Click to expand...

X2....Im tired of the Negative post's. Let's keep this one alive.

I have tried different transfer port widths on my CMB 1.01 eng and found that there is a limit to it. the closer the transfer port is to the ex the better the eng ran then I went to far and it fell on its face.

That is what lead me to the VAC .91 with the 1.01 P/L in it to gain some timed area. This also helped increase the blow down time in the VAC .91 as the transfers are lower with the 1.01 P/L.

I will be setting the timings here soon and this discussion will be very helpful in my timing choices.

I don't remember exactly. As what I know. From 199X to 2010, O.S. .21 engine use 16.6mm/16mm bore stroke ratio. While NR REX was square engine. Then MES was invented. NR use 16.26mm/16.8mm bore stroke ratio. The O.S. way make MES is almost no advantage in buggy engine. The O.S. VZ(16.6mm/16mm bore stroke) did not use MES, but a rib in the centre of the exhaust port. Until recent years, 201X. O.S. launch the XZ, 16.27mm/16.8mm bore stroke ratio, the first smaller than 1 design. XZ has MES. Longer stroke make MES in buggy engine possible/efficient. The NR bore stroke ratio continue to decrease. The latest NR .21 engine use 15.88mm/17.6mm bore stroke ratio. We can see the bore stroke ratio is decreasing. I believe the designer want to maximise exhaust and scavenging efficiency. They are Contradictions, cannot gain both.

http://www.rcuniverse.com/magazine/article_display.cfm?article_id=770
Model Name: O.S. 21VZ-B V-Spec
Displacement: 0.211ci (3.46cc)
Bore: 0.654" (16.6mm)
Stroke: 0.630" (16.0mm)
Practical RPM: 3000-40,000rpm
Output: 2.48hp @ 33,000rpm
Weight: 12.7 oz. (360g)
Carburetor: 21C Slide-Valve
Glow Plug: O.S. P3 Turbo

http://www.osengines.com/engines-boat/osmg1621/index.html
O.S. 21XZ-M Inboard Marine Engine
Stock Number: OSMG1621
Displacement: 0.21 cu in (3.5 cc)
Bore: 0.640 in (16.27 mm)
Stroke: 0.661 in (16.8 mm)
Practical rpm: 4,000-45,000
Output: 2.71 hp @ 33,000 rpm

http://www.osengines.com/engines-car/osmg2043/index.html
O.S. Speed R2101 Engine
Stock Number: OSMG2043
Displacement: 0.213 cu in (3.49 cc)
Bore: 0.633 in (16.08 mm)
Stroke: 0.677 in (17.2 mm)
Practical RPM range: 4,000-45,000
Output: 2.7 hp @ 33,000 rpm

http://www.novarossi.it/2012/index.php/en/products/off-road-engine-nova/3-5-cc/keep-off-21-4.html
NR KEEP-OFF.21-4
Displacement: 3,49 cc
R.P.M. (max power): 36.000 RPM
Practical range: 6.000-38.000 R.P.M.
Bore x stroke: 16,26x16,80 mm
Sleeve: 4 ports
Rear ball bearings: ceramic
Front ball bearings: steel
Crankshaft: 14 mm - turbo - tuned
Carburetor: plastic - 9 mm - reverse
Glowplug: Turbo (C6TGC)
Exhaust position: rear
Starting: -
Weight: 360 g
Fuel type: 25% nitro

http://www.novarossi.it/2012/index.php/en/products/off-road-engine-nova/3-5-cc/keep-off-21-7xl.html
NR KEEP-OFF.21-7XL
Displacement: 3,49 cc
R.P.M. (max power): 36.800 RPM
Practical range: 6.000-40.000 R.P.M.
Bore x stroke: 15,88x17,60 mm
Sleeve: 7 ports
Rear ball bearings: ceramic
Front ball bearings: steel
Crankshaft: 14,5 mm - turbo
Carburetor: plastic - 9 mm - slide
Glowplug: Turbo (C6TGC)
Exhaust position: rear
Starting: -
Weight: 370 g
Fuel type: 25% nitro

Best regards,

Leo

I calculated the relationship between displacement and a port/valve area for different displacements and bore/stroke ratios. I always felt that the relationship shown on the graph was true, but never tried to get actual numbers. I picked the flat cylinder head area as an index of possible four stroke valve area and 0.3 times the total cylinder wall area as an index of the possible two stroke valve area. I know four strokes don't have this much flow area from their valves, even with pent roof combustion chambers. Four stroke piston speed is also limited by the flow velocity through the valves. The FOS and triple port exhaust two stroke engines may have more possible port area as well. However, the trends still hold.

The graph shows the scale effect of bores from 15 mm to 1000 mm. Small engines have a huge advantage and aren't as limited by breathing as large engines. The four stroke's head area to displacement ratio increases with increasing bore to stroke ratio; the opposite of a two stroke's wall area. I know a lot more enters into engine design than these factors, but it was instructive for me to actually see what happens. The plot shows the area to displacement ratio plotted against displacement. This area ratio is an indication of the flow velocity needed to fill a cylinder in a given time.

Lohring Miller

Lohring

So what the graft shows is the 1/1 B/S (boar the same as stroke) "dead square" has the closes relation ship?

Leo

What dose MES refer to?

So we all have a understanding on what is what.

Sorry. Check out the edited graph. The graph shows the ratio of potential port or valve area to displacement plotted against displacement. At a 10 cc displacement an engine with a bore twice as big as the stroke would have a head area in square mm about 68 times the displacement in cc. A 10 cc engine with a bore 1/2 the stroke would have port area factor (0.3 times the total cylinder wall area) in square mm about 64 times the displacement in cc. That is to say a 10 cc four stroke with a bore twice the stroke has potentially about the same valve area as a 10 cc two stroke with a bore that's half the stroke.

This opposite trend between two stroke and four stroke engines leads to a lot of confusion. On top of this, the flow velocity through a four stroke's intake valve needs to be kept below the speed of sound. That limits piston speed, another reason high power four strokes have short strokes for their displacement. Two strokes aren't as limited, but scavenging and inertial bearing loads will limit the rpm of small two strokes.

A 100 cc two stroke only has a port area 30/64 or a little less than 1/2 that of a 10 cc engine. A 1000 cc two stroke has about 14/64 or around 1/5 the potential port area of a 10 cc engine. That means breathing isn't the problem for our engines that it is for larger engines. It's also the reason high speed engines aren't huge. Smaller cylinders can run higher rpm than larger cylinders where breathing limits the rpm.

Lohring Miller