From what I've seen, some years back long stroke engines seemed to be more prevalent than they are today. Nowadays you only see them very exceptionally. What happened? What are the advantages of each? In general, which type is preferable in your opinion?

Regards,
Misterpanda

Longer stroke usually means bigger crank counterweight, bigger counterweight means bigger engine case. Bigger engine case - bigger gross weight.

Long strokes were designed specifically for quiet pattern use when .61s were the largest twostroke allowed in competition. After 1996 there was no need for them since standard stroke .61 are fine for sport use and much better for helicopters which were gaining popularity. All modern 61s were strong enough to grow into .80s and .90s, that is what has happened.

Panda,


The 'Long-Stroke' was never much more than a buzz-word...

The engines involved really were only .61s with a stroke of 24 mm and a bore of 23 mm.

So, yes, they were all slightly 'under-square'.
Previous .61 engines, like the OS.61FSR had a 24 mm bore and a 22 mm stroke, just like the current FX engine...

In the mid '80s, all the manufacturers fell for this craze, to provide a low-RPM .61 that would spin a relatively big prop, without the 14.5-15.5K RPM noise of earlier .61 engines.

Some do spin big props (the MVVS .61 spins a 13x8, or a 13x9 around 10,800. The OS Hanno was rumored to spin a 12x12 at 12K, but this really was a rumor...), but this in not due to the 'long-stroke', but to their timing numbers, optimized for long pipes.


The fact is all the manufacturers went back to the previous over-square dimensions. No more long-stroke.

The stroke realy is no advantage, except for engine balance. Power is design dependant, and not very much un the bore/stroke ratio. MVVS makes very potent "square" stump pullers, but they could just as well have done that in a short stroke design which allows for much better porting control and lower piston speeds (wear). Only in uniflow two stroke engines (lower cylinder porting, upper cy;inder exhaust valve) is a long stroke of advantage because the scavenging is better, and thus it will have lowest scavenge air losses and allows for supercharging two strokes.
A short stroke means lower engine weight and a much more compact engine build, smaller crankcase volume with better pumping efficiency at raised rpm. (>4000)

Hear how the long stroke engine are working with big propeller as here: http://www.youtube.com/watch?v=wmH6WUMUEJg

The long stroke engine are not hi reving engine then the bigger propeller will works better in lower rpm (more thrust).

Few, if any, engines made after maybe the early 50's could be called a true long stroke. All engines now are so close to being "square" it makes no difference. Long stroke by itself doen't make for a "torquier" engine any more than a short stroke makes for a "peakier" engine. The difference in piston speeds (more correctly piston accelerations) is of no consequence because they're all running far below the maximum acceptable anyway.

Design wise, a shorter stroke would allow a slightly longer conrod to get a better length/stroke ratio without having too tall an engine but no manufacturer seems to do that.

Engine timing the same, displacement the same , a longer stroke will place more leverage on the crank that is how it makes more torque. A bigger bore makes more rpm because the dwell at the top of the power stroke. They both have to do with the angular movement of the rod to the crank stroke. As you increase one you diminish the other and vice versa.

The true problem is we deal with small displacement engines; if you increase the stroke .007in, who cares if it makes .003 in oz more of torque.

Changing the characteristic of the engine with rod length, port timing, combustion chamber is how you broaden the power usability once you settle on your bore/stroke.

Good posts guys. It's amazing how many people think bore/stroke ratio, in and of itself, determines how torquey an engine is. People think of that big long stroke imparting more leverage on the crank but they don't think of the smaller piston required to keep the capacity the same in any valid comparison. Irrespective of the bore/stroke, it's still swept volume versus crank angle. Rod/stroke ratios make more of a difference than bore/stroke ratios.

In automotive two-strokes, undersquare rules in outright horsepower output: the slightly undersquare 54mm x 54.5mm is now pretty much the universal roadracing/MX 125cc dimension, as is the now standard 67mm x 72mm shared by almost all 250cc two-stroke singles.

Even my daily driver is undersquare, at 86mm bore x 91mm stroke. It puts out more than 100hp per litre and does over 8 grand (now there's some scary piston speed). It's certainly not a low revving torquer.

That may be the case, but the smaller piston (remember, displacement is the same) has less surface area to convert combustion pressure to downforce, evening out the equation.

It's amazing how folks misunderstand the long stroke theory of two stroke engines. Motorboy
has the right idea:
In the first place, the bore and stroke of an engine is only the starting point, or the first part
of the equation. These parameters set the design performance ideas. This is fine and dandy,
however this is only the starting point. There are other factors involved that dictate the power
band of the engine....or the RPM of the engine and just how the engine will perform in it's final
configuration.

Factually, oversquare....or short stroke engines are designed for high RPM's. Short stroke=higher
RPM....no argument there. Long stroke engines....undersquare engines are primarily designed for
torque and lower RPM's....yes there are exceptions.

The primary design feature of a two stroke engine, as it pertains to the way it produces power is
the port timing, not the bore and stroke. And thirdly....the pipe on the engine ultimately
determines the RPM range in which the engine will run....all these things in harmony with the design.

Another fallacy about long stroke engines is the RPM range where they will run. Have you ever seen,
for instance, the RPM range of a Hanno Special with a small prop ? It will knock your socks off !!! [X(]

Factually, you can take two OS Max engines....a .61 FSR, and a .61 RF (long stroke) engine and see
the differences in the way they run....stock. The FSR runs typically at 14K, and the RF engine runs at
11,400 per design. The only difference is a small change in the bore and stroke. You can however,
open up the pipe on the FSR and run a bigger prop slower....or shorten it up....close up the stinger,
and get 16K out of it....no problem. The same thing with the long stroke RF engine or the Hanno type.

Shorten up the pipe, put a smaller prop....it will scream bloody murder. The size of the piston, one or
2mm smaller would be the least likely item in the performance equasion....not to mention the nitro content
or the compression ratio.

FBD.

Quote:

ORIGINAL: TFF

Engine timing the same, displacement the same , a longer stroke will place more leverage on the crank that is how it makes more torque. A bigger bore makes more RPM because the dwell at the top of the power stroke. They both have to do with the angular movement of the rod to the crank stroke. As you increase one you diminish the other and vice versa.

TFF,


Please reread your statement...

Where does the greater piston top area (larger bore - on which the combustion force is applied) come in!? It doesn't...

A small child on a long lever does not necessarily apply greater torque than does Hulk on a short lever...

Dwell??? Work is done only as the piston is moved and when 'dwelling' around TDC, movement is virtually nil...
...And this is a very small effect of the con-rod length to stroke ratio - not dependent on whether the engine is over-square, or under-square...

It looks like you are attempting to confuse all of us...


Please get some better comprehension of this issue and rephrase your statement.

Good response Dar,

For those who still believe in the long stroke story:
I have been MX racing 50cc engines when young and fly-weight, and long tried a long stroke 38/43 Sachs. It never could get the torque that a slightly overbore Kreidler produced 40/39.5 IIRC. Our shed was full of tuned pipes that worked only so-so. Lots of header length juggling and carb intake duct tuning to get these baby engines on the pipe like the gear ratio wanted it to.
Except for MVVS, all modern gas engines are very much short strokes, and they pull as hard on large props as the square stroke engines.
We all know that a prop needs torque to turn, don't we?
The long stroke is not a torquer by definition. It is however larger and heavier. Size can only be reduced by a shorter conrod, which increases piston side loads. Port area in a longstroke engine cannot be large enough to suit engine tuning demands, so port angles have to be increased. Just the opposite of the requirments for low rpm torque.
Fairy tales are nice to believe in. Reality is when you wake up, and realise you have been dreaming

Displacement being equal. Take two engines same displacement, one with a .5 in stroke and one with a 1 in stroke. As the crank pin travels around, the 1 in goes a longer distance than the .5 in. Because of the longer distance, the 1 in rod makes a faster angular change at the pin than does the .5 in, it dwells less around TDC. At around 90 deg the piston slows down spending more time at the best leverage point. On a short stroke engine, the distance the crank pin moves at around TDC is small; it is almost a straight line. The piston dwells longer absorbing the energy. As it reaches 90 deg it speeds up spending less time there so it does not dwell at the best leverage point.
The con rod length as the arm doing the pushing comes in because of stress. A short rod forces the piston face into the cylinder wall, creating friction or if weak punching a whole in the cylinder wall. A long rod does not allow the angle between the piston and the rod to become great putting more force downward than sideways in the bore. You cant have too long a rod if weight and size are not a factor.

conrod length normally is a function of crank throw, with a value of 1.8 times the stroke as a good starting point. With these ratios being equal, and angular velocity (rpm) the same, all other values are the same as well, including what you call "dwell" and conrod angles. Because of this, and because the long stroke engine has a smaller surface piston, the side loads of the piston are smaller, but this again is about canceled out by the smaller projected surface of the piston side, so specific loads (lbs per square inch) will be about the same.
If piston dwell is defined as the angle 15°BTDC to 15° ATDC. ie the angle when the piston moves very little, dwell is rpm depending, and not stroke depending.
The longer stroke just is a scaled-up version of the short stroke as far as timing is concerned. To alter timing, you need desaxing the cylinder (cylinder offset to the crank center line). Again, this has nothing to do with the bore/stroke ratio.

TFF,


You are senselessly repeating yourself.

The side forces applied by the piston on the sleeve are a function of the 'con-rod length to stroke ratio'.

They are completely non-dependent on the stroke to bore ratio.


Two engines of equal displacement, working at the same brake-mean-effective-pressure, will make the same torque, even if one is an extreme long-stroke design and the other an extreme short-stroke design.

Even the side-forces resulting from the 'con-rod length to stroke ratio', which you so heavily emphasize, are minuscule when compared to the explosive force provided by combustion.

...And this item in both engine would be equal, if they are running at the same BMEP (a function of the engine's volumetric efficiency - VE).

In reality, the big torquers are over-square engines, mainly because they allow larger valves (in four-strokes), or larger ports (in two-strokes).


Saying otherwise would be 'barking up the wrong tree'...

Dar:

I've always heard/read that the general rule concerning engine stroke length is:

short stroke: smaller prop for higher revs.

long stroke: bigger prop for lower revs. (and more efficiency)

Supposing the above assertions were correct, what would the mechanical explanation be?

Or maybe the answer could be: "not necessarily".


Misterpanda

Panda,


It is a one-way relationship...

An over-square engine uses a larger, but slower moving piston, with lower piston acceleration figures.

In an under-square engine, the higher piston speeds and acceleration levels are limiting factors, which join the smaller ports in defining an engine as a low-RPM power unit.

As engine manufacturers did in the mid '80s, the lower RPM range used by these engines can be boosted by using long tuned pipes and more modest porting numbers, to get the best from these engine, without blowing off the RPM lid...


An over-square engine can also utilize long tuned pipes and more modest porting numbers, to get similar low-rpm performance, but it can also use more radical porting numbers and shorter pipes, since it is not RPM limited limited by high piston speeds and accelerations... It is also lighter in weight.

It seems the long-stroke engine practically has no advantages, nor did it ever have any in the '80s...

I remember the standard stroke Supertigre X60 pulling oversized props as well as long strokes could when a pipe was used. Carburetors became terribly oversized at lower rpms and required a pump or extreme tank pressure (like YS) to work in an airplane.

I have an old 4 bolt HP .61 that has the cylinder offset to the left of center, (looking from the front of the engine) not buy much but you can clearly see it.
I believe it was a Frenchman by the name of Desake..? (I think) that first came up with it.

"Apparently" the geometry offers significant benefits in that an increased mechanical advantage is imparted at certain parts of the cycle, and improved thrust angularity. The offset was supposed to add a significant increase in power and torque, and improve economy. Offsetting the crank can be used to advance or retard the timing.

The frenchman was called Desaxe, and it made him famous.
I wrote about that in post #14.
It offers an opportunity to lower the piston side thrust, but also to have the exhaust port close earlier than it's opening for a better trapped compression and larger blowdown. Both of these are required for good low rpm torque.
see also http://members.lycos.nl/autovraagbaa.../zuigerweg.htm

Rightttttttttt....sounds good in theory (which is all it is) but how about some real world numbers? Seeing you mentioned it earlier we'll use the LS Hanno 61 as a basis for some acceptable piston speeds and accelerations and compare them to the "short stroke" 61FSR. I don't have an actual figure for rod length on the FSR so I'll assume it uses the same rod as the Hanno.

OK, on test (a real test) a Hanno with open exhaust develops 1.85HP @ 19,000 RPM. It didn't blow up/destroy itself so we can assume the loads were within safe limits. At these revs and with a rod length of 42mm the maximum piston speed is 4893 feet/minute (FSR is 4456) while maximum piston acceleration is 6234 G (5607 for the FSR). If these loads are safe for the Hanno then the FSR could run to 23,500 before reaching the same loads.

So our "long stroke" Hanno is limited to "only" 19,000 revs.

Now back to the real world because no one is going to run either a Hanno or an FSR at those revs. Let's pick 13,000 as a figure. The Hanno now has a max piston speed of 3348 (FSR 3048) and piston acceleration has dropped way down to 3348 G (FSR 3049).

So in the real world long or short stroke makes no difference as far as useable rev range goes.

Here is a chart from a Clarence Lee evaluation of the MOKI .61 vs the MOKI .61 LS, I have a MOKI .61 LS and an MVVS .61. They have the same 23mm bore and 24mm stroke. My Fox Eagle .60s at .905 bore and .937 stroke are very close to these demensions. The MVVS converted to Diesel turns an APC 14x7 at 9,500, its no slouch.


23mm borex24mm stroke-------------24mm borex22.5mm stroke
Moki 61 Long Stroke ------------------ Moki 61 Standard Stroke
11 x 7== 12,800---------------------------11 x 7== 12,900
11 x 8== 12,100 --------------------------11 x 8== 11,450
11 x 10= 10,300-------------------------11 x 10=== 9,900
12 x 6== 11,600---------------------------12 x 6== 11,300
12 x 8== 10,300---------------------------12 x 8== 9,950
13 x 6== 11,100 -------------------------13 x 6== 10,500
13 x 8== 8,200----------------------------13 x 8== 7,750
14 x 6== 9,000----------------------------14 x 6=== 8,400

Oops, Desaxe popped up again while I was doing that last post
One of the things I check when I'm degreeing the porting on an engine is to see if the exhaust/transfer timings are symmetrical (same figures with the piston coming down as with going back up). If they vary by 2-3 degrees then it's a Desaxe arrangement. All Mercos from the .29 to the .61 are Desaxe (I don't know about the later 40 though) and the Fox 35 is as well. From memory the Fox was offset in the opposite direction to the Mercos. The SC 1.08 is Desaxe but the 1.20 isn't. These are just engines I've checked myself but I know there are a few others.

So according to these figures for the Moki, they would tend to lend credence to the 'theory' expressed in my post #17, I suppose at least in stock form.