ESE's works engine tuner

[Haufen]

I notice that two stroke competition engines usually have a 'square' bore and stroke (as opposed to chainsaws etc. which are usually very much 'oversquare') and I'm sure that there must be a very good reason for that, - what is it?

I can see a lot of win/win situations following each other in making a chainsaw engine oversquare.

I suppose one of the main reasons is vibrations and engine size. The latter is beneficial for a lower weight, too, which is very important for a hand held tool. Apart from that, the short stroke allows for low piston speeds and thus the usage of crappy oils while enabling high engine speeds. High engine speeds and a (relatively) big bore are beneficial to trapping efficiency in that high speeds provide less time for the mixture to escape the exhaust port unburned, and a big bore allows for a longer path the scavenging column has to travel inside the cylinder. This reduces fuel consumption and hydrocarbon emissions (which the latter is regulated by legislation). Running at a maximum BMEP of around 4 bar, there is no need to worry about insufficient angle area on these.

On the technical side, an even smaller displacement square engine would be interesting in terms of power to weight ratio, too, of course.

[WilDun]

Thanks for the answers on square bore and stroke and I'm sure all these questions have been addressed before somewhere in this forum but for a relative newcomer, finding these can be a bit of a mission, (but thanks to TZ for trying to make that easier).
While on the subject of chainsaws I was thinking that with the 'complete' cylinder barrelI/ head approach,this setup could possibly solve a lot of the problems associated with seperate cylinder heads. I realize it might make it difficult for experimenters etc. of course! but would it have any merit here at all?

Will.

[wobbly]

Yes the angle area issue only raises its ugly head when you are at the limits of power production for a particular engine size.
Unlike STA that takes time derived from rpm into account - angle area only describes exactly that, the area times the crank angle/displacement.
For a fixed displacement, as the bore increases the piston area increases as the square power of the diameter, but the wall area is only increasing by the bore times
a constant Pi.
Thus as we go further oversquare we increasingly loose available port area when multiplied by the correspondingly shorter stroke and its reduced piston sweep per degree
of crank rotation.
I think Frits has explained this better before somewhere.

[Frits Overmars]

For a fixed displacement, as the bore increases the piston area increases as the square power of the diameter, but the wall area is only increasing by the bore times a constant Pi. Thus as we go further oversquare we increasingly loose available port area when multiplied by the correspondingly shorter stroke and its reduced piston sweep per degree of crank rotation.
I think Frits has explained this better before somewhere.

This is what I managed to find back:

An oversquare engine is not a good idea for a two-stroke; I will try to explain this with an exaggerated example.
Let us assume an engine with a bore and stroke of 40 mm.
Now we make the bore twice as big: 80 mm. That means that all ports can be twice as wide. But if we want to keep the cubic capacity the same, the stroke must be 10 mm instead of 40 mm. And that means that all ports will be ¼ as high.
So all port areas are halved, as are the rpms of maximum torque and maximum power. If we assume that the quality of the cylinder filling remains the same, the oversquare engine develops only half as much power as the square engine...

[WilDun]

This is what I managed to find back:

Guess that all explains why the Royal Enfield two stroke racer engine of the late sixties managed to hold it's own with its competition.
As I remember, it went against the flow by having a long stroke 'under square' engine (in those days everyone was beginning to favour short stroke for more revs). I should imagine that the conrod length would also play a big part here?

[TZ350]

24mm inlet with boost bottle feeding the opening side of the inlet port just behind the RV so the returning pressure wave from the BB kick starts flow into the crankcase.

On board generator for powering the Ignitec ignition and EFI system.

New injector manifold to direct the fuel streams across the transfer ducts.

There will be three 126 g/min injectors, with the middle one firing first then the outer ones come in when the engine gets up on the pipe. The center injector is angled up so as to cool the underside of the piston crown by squirting through a slot in the piston skirt. The outer two injectors are splayed out and down so they more properly spray horizontally across the transfer streams. Before, when they were square on to the barrel the outer two squirted at the back of the cylinder sleeve.

Ball inlet valve and TPS.

After what seems like months of work the Beast is back on the dyno.



It was pretty easy to start up so hopefully that is a good sign and I will be able to dial it in easily enough, then hopefully I will be able to run it on the track at the next Bucket meet at Mt Wellington.

There were three main objectives in using the EFI system. (1) Directly cool the underside of the piston by squirting fuel on it.

(2) To be able to more accurately control the fuel curve, especially in the area of over rev where a carburetor tends to go rich,

(3) Allow for a possible return to the plenum idea as the previous problem with it was fuel dropout building up in the bottom of the plenum and affecting the fueling making the engine run rich/lean at the wrong times, and leaving a lot of oil behind that should have found its way into the engine.

If I can make the plenum idea work then the poxy 24mm carburetor restriction will mean even less than it does now, which is sweet FA.

[F5 Dave]

And yet you'll scream and grizzle if anyone suggests a mm more oversize for the 100s.

[TZ350]

And yet you'll scream and grizzle if anyone suggests a mm more oversize for the 100s.

I have won my gains by hard work, not a rule change, but for the same reasons as the 100's I would like another mm too and have no problem with the 100's wanting theirs, so long as its the same deal for all the Bucket classes.

There is another thread devoted to thrashing out this issue.

[Frits Overmars]

Matching the teeth orientation on the primary and clutch gears might give you another couple of horses

The center injector is angled up so as to cool the underside of the piston crown by squirting through a slot in the piston skirt.

The piston will appreciate it, but don't forget to cool the big end .

If I can make the plenum idea work then the poxy 24mm carburetor restriction will mean even less than it does now.

The bellmouth throttle allows you to make the inlet tract really short and then you don't need the boost bottle.
You can have a 38 mm (or thereabouts) entry to the inlet disk and a bellmouth with a 24 mm restriction at the plenum entry where the air flow can be almost continuous.

[TZ350]

Matching the teeth orientation on the primary and clutch gears might give you another couple of horses

Good spotting, it turns out that a Chinese Honda clone has straight cut primary gears with the same center distance between them as my 125.

You can have a 38 mm (or thereabouts) entry to the inlet disk and a bellmouth with a 24 mm restriction at the plenum entry where the air flow can be almost continuous.

Yes, that was the original plan some 600 pages ago and I think the plenum is going to be worth another visit when I get the EFI thing working properly.

http://youtu.be/YxiEo8cgopg

http://youtu.be/p4ef-WUO1Qs

A couple of early clips of the magic vacuum cleaners 1st ever start up with the plenum.

[TZ350]

The plenum re visited ...

You can have a 38 mm (or thereabouts) entry to the inlet disk and a bellmouth with a 24 mm restriction at the plenum entry where the air flow can be almost continuous.

Pretty much what we tried. The problem was, with the carb on the outside we got excessive fuel dropout in the plenum, EFI should cure that.

The bellmouth throttle allows you to make the inlet tract really short

With a real short inlet tract there were problems with resonance in the plenum at low RPM that I called the Blarrrs and that the motor could barely pull through and it needed a longer inlet tract to cure that.



Then I tried an automated way of halving the area of the inlet tract at low RPM. This worked a treat and it was surprising how high the power curve would go before falling over (Blue line).

[TZ350]

Plenum Inlet Revisited ...

The bellmouth throttle allows you to make the inlet tract really short. You can have a 38 mm (or thereabouts) entry to the inlet disk and a bellmouth with a 24 mm restriction at the plenum entry where the air flow can be almost continuous.

I think I can do this and make it work without increasing the inlet tract length by fitting a variation of the variable inlet tract idea behind the Bellmouth inlet. And having a stop that only allows the Bellmouth Inlet to open 50% until the revs are approaching maximum torque at which point the throttle will be able to be opened all the way.



One of the things Thomas and I found when we were exploring ways to improve the flow through a 24mm carb, was that a big carb with a 24mm insert flowed much more air than a smaller one did, but any carb with an insert flowed better than a plain insert with a bellmouth entry did.



We figured that a bigger carb with a 24mm insert acted like a flow straightener aligning the air column before it negotiated the 24mm restriction. But in practice the 38mm carb was to big and stalled the fuel flow when opened to quickly at lower rpm. A 28 or 30mm carb with a 24mm venture insert worked best on the Dyno and Track.



If it does not have to meter fuel, the big carb with a 24mm insert could be worth a revisit for the inlet side to the plenum as it flows a lot of air and as it would not have to meter fuel it could be left fully open all the time.

Anyway for my plenum with a 38mm RV inlet to work I have to get the fuel injection idea going properly first.

Tomorrow ....... hopefully.

[jasonu]

New injector manifold to direct the fuel streams across the transfer ducts.

There will be three 126 g/min injectors, with the middle one firing first then the outer ones come in when the engine gets up on the pipe. The center injector is angled up so as to cool the underside of the piston crown by squirting through a slot in the piston skirt. The outer two injectors are splayed out and down so they more properly spray horizontally across the transfer streams. Before, when they were square on to the barrel the outer two squirted at the back of the cylinder sleeve.

Ball inlet valve and TPS.

After what seems like months of work the Beast is back on the dyno.



It was pretty easy to start up so hopefully that is a good sign and I will be able to dial it in easily enough, then hopefully I will be able to run it on the track at the next Bucket meet at Mt Wellington.

There were three main objectives in using the EFI system. (1) Directly cool the underside of the piston by squirting fuel on it.

(2) To be able to more accurately control the fuel curve, especially in the area of over rev where a carburetor tends to go rich,

(3) Allow for a possible return to the plenum idea as the previous problem with it was fuel dropout building up in the bottom of the plenum and affecting the fueling making the engine run rich/lean at the wrong times, and leaving a lot of oil behind that should have found its way into the engine.

If I can make the plenum idea work then the poxy 24mm carburetor restriction will mean even less than it does now, which is sweet FA.

It looks to me that the 24mm restrictor on the RV inlet will mean sweet FA because you have other injectors that feed into the transfer ducts that are not restricted in any way. Do all of these injectors combined flow meet the 'equivalent to a 24mm carb' rule and will you be able to prove it?

[Drew]

It looks to me that the 24mm restrictor on the RV inlet will mean sweet FA because you have other injectors that feed into the transfer ducts that are not restricted in any way. Do all of these injectors combined flow meet the 'equivalent to a 24mm carb' rule and will you be able to prove it?

Pretty confident no one is going to protest if he's running it too rich.

[Yow Ling]

Pretty confident no one is going to protest if he's running it too rich.

Why would it run rich ? it has plenty of air going through the 38mm inlet