Questions for engine engineers
 

Got a few questions for some of the gas engine engineers out there.

What gives the most power, top end or toque

Longer stroke bigger bore or Shorter stroke bigger bore, I know that port timing and a lot of other things make a big difference but lets just say all is right with either one.

I know back in the old day’s of outlaw drag racing when you could show up with anything and race it that shorter stroke engines got you off the line faster but not have good high end and longer stroke had better high end but did not get off the line as fast, Yes I know we are talking about 4 stroke verses two stroke but just using this as a example.

Example

YDA bore 44mm stroke 37mm
DLE bore 45mm stroke 35mm
Redlion bore 47.5mm stroke 33mm.

In what way should these engines perform different

RE: Questions for engine engineers
 

In truth, mathematically there is basically little difference between two engines of the same size but different bore stroke ratios within reason. However, many aviation engines tend to be of a short stroke big bore design because that type of engine can be built lighter than a long stroke engine, and in a smaller overall package. In two stroke engines, a larger bore allows more room for your port layout, which of course can help power. Long stroke engines will reach their critical piston speed at a lower rpm than a short stroke engine in high rpm max effort engines. That is probably where the old "long stroke = low rpm" idea came from.

Many people think the dual ignition on full size airplanes is only for safety. Safety is actually a side benefit. The very large bore aviation engines rather need dual plugs for efficient fuel charge ignition and to avoid detonation. These engines have a relatively large bore and short stroke, yet produce max power at around a lowly 2700 rpms. So much for short strokes being only for high rpms....

AV8TOR

RE: Questions for engine engineers
 

The thing that got me thinking about this is if Redlion had made there engines a 35mm stroke it would be a 62cc engine.

Just wondering why they made it a 33mm stroke.

A G62 bore 47.5 and stroke 35mm. ??

RE: Questions for engine engineers
 

Maybe Redlion kept the stroke shorter for less viabration.Just a wild guess. Capt,n

RE: Questions for engine engineers
 

Could be.

One thing for sure, nobody can call it a DLE clone. LOL

RE: Questions for engine engineers
 

Must be a 3W Clone then LOL

Ok, with crankcase transfer 2 strokes (aka - what we use in our "toys") the bore/Stroke ratio is actually very flexible, as has been mentioned, the larger bore / shorter stroke makes a more compact unit with a power band that works well "down low", a smaller bore / long stroke wil give you a higher "top end" torque (think of the Nelson Racing engines).

However, just the bore / stroke ratio os only a small part of the overall equation.

Crankcase compression ratio is also a very large part. this is not just a simple ratio of crankcase volume and overall piston displacement, Rather it is the crankcase volume whn the ports are fully covered versus the remaining piston displacement after that time. This ratio also affects the induction cycle.

True Cylinder compression ratio - same thing. It is not about overall Piston displacement and Combustion chamber volume, (ok - for the purists, swept and unswept volume versus unswept volume) - it is about volume once the ports are fully covered and combustion chamber volume (In fact, most of our engines heve very low "true" compression ratios)

Then of course port timing, desax etc all come into it.

I am lucky to have some VERY good engine modelling software in my system here. it is actually quite interesting to change various factors around and see the final results on expected torque curves. (and yes, Real world data matches the model within less than 8% MOE in most cases when all factors are considered properly). Most of our enignes have a bore/stroke ratio of 1.15 - 1.3, that is where the power curves are best suited for our requirements.

Larger bore also means more reciprocating weight. Shorter stroke means greater acceleration figures on the reciprocating parts, combine those two itmes and you have a need for a MUCH beefier connecting rod, gudgeon pin, wrist bearing and big end bearing at any given speed. (thus adding to reciprocating weight).

For power - keeping all other ratios equal, the oversquare (large bore :small stroke) WILL get you off the line quicker and give you a better low end power. The "Undersquare" (longer stroke : smaller bore) will give you better "legs" in the top end. Close to "Square" will give you the best of both worlds.

Recently I have had cause to model a off the shelf 48mm cylinder with an off the shelf 35mm crank with the intent of using an "off the shelf" cylinder for an engine made from "off the shelf" parts for very respected engine guy who I consider to be a good friend of mine. Even though the cylinder porting was designed for a different stroke, the basic unit still worked very well for the model. It is just a matter of finding the right piston to suit the deck height involved (still working on that side of it).

The best part of this hobby? - ALL products suit someone in it, some of us like to wonder, others like to tinker, others like to get the simple instant gratifaction - we ALL have fun either way, so no matter what the Chinese throw at us (lets face it - right or Wrong, Deserved or not, They are our major suppliers) someone will enjoy the new products.

RE: Questions for engine engineers
 

Thanks for the reply Steve.

Then from what you say the old theory of shorter stroke is quicker off the line it true for two stroke engines also.

Did something today just for kicks

Got a little bored today so thought I would have some fun.

I have a lot of JC engines crank cases that I can not get cranks, piston and cylinders for but have a lot of parts for DLE.

So I took a crank piston and cylinder and installed it in one of the JC crank cases, All I had to do was shorten the crank bearing spacer about 1/2mm, move the sensor and drill out the cylinders for 5mm bolts and all went together good, Even the squash came out good and has good compression.

Kind of looks funny because the crank case is a little wider than the base of the cylinder.

Got to late for me to run it today but will give it a try in the next couple day’s and see how it does.

RE: Questions for engine engineers
 

Double post

RE: Questions for engine engineers
 

Now that is something I would really like to try. There are so many small engines made for RC & for small power products. Seems like many different combos could be experimented with. I wish we some one would start a forum witha thread just on parts interchange people have found. Thanks Very Much. Capt,nhttp://www.rcuniverse.com/forum/js/f...ngue_smile.gif

RE: Questions for engine engineers
 

Going to have to see how it runs before I get to excited about it. LOL

There are a lot of the china parts that will interchange with other engines.

One main part are the reed blocks, most of the are off the shelve parts.

Found out today working on a 3w 100 that the China made reeds will work with it.

RE: Questions for engine engineers
 

Look at the new OS 55cc, it has a very long stroke compared to other engines except Evolution. This was necessary in order to get a large enough gas passage through the crank. They (OS and Evolution) also made a greater effort to keep the rod and piston as light as possible. Its just a lot easier and cheaper to keep weight and vibration low by using a shorter stroke with performance differences being minimal. Plus the whole concept of "low end torque" only amounts to slightly faster spool up since little power can be transfered at low rpm.

RE: Questions for engine engineers
 

As Kweasel has pointed out - 2 of the engines that currently have the highest "top end power" (Actual useful torque to turn large props and make large amounts of thrust) are both "under square".

Captinjohn, it sure would be an interesting thread. So many engines have such similar dimensions, there is a plethora of useful parts around. some may require very minor mods to make them work well.

Badazzmax - I have recently used the exact same crank as you to replace an "early JC" one. Didn't try the cylinder change out but it sure makes a good idea for obtaining good quality parts for the future.
(Now I wonder what I could use to replace a 28cc crank - shoud I ever need to - might investigate that in the next week or so - not that I need to of course ;)).

I wonder if it would also work with the old DL50 bits. I have a couple of very low run time of the "Version one" of those (the one with the weak crankcase lugs) I might have a bit of a play also - like you, I have a few spare crankcases laying around that are not useful for anything else.

RE: Questions for engine engineers
 

The DLE style crank rods are 2mm longer than the JC rod but the length of the DLE style cylinder is 2mm longer from the base to the squash point of the cylinder so it all worked out good.

From outside to outside case bearings, The JC is 1mm shorter so I had to take 1mm off the bearing spacer that goes on the crank between the case bearings, I think I posted 1/2mm but it was 1mm

Got the squash down to 1mm, Would have liked for it to have been about a 1/2mm but will see what kind of power I get out of it.

Just trying to find something to do with these JC engine crank cases I can not get parts for. LOL

RE: Questions for engine engineers
 

steve

very interesting - i am by no means any form of engine engineer, but love to know what makes what do what and why

why i find it very interesting is this - i have raced pylon for some years and through that learnt a bit more about engines.
Nelson racing engines are also oversquare in design / short stroke motors. I have always understood that the longer the stroke, generally, the bigger prop it will turn, through the fact it has better torque at lower rpm. Nelsons will develop really impressive HP ratings for their size, but only at 30000 rpm which is of no use to the average modeller.

also always thought the shorter stroke helped with the fact that the piston travel speed / inertia/ stresses imposed was less ata given high rpm.

Maybe i am confusing things here, but i have always thought longer stroke was better for low end torque. the downside being long stroke = more vibration. so there is always a trade off to be had.

you also say
"Larger bore also means more reciprocating weight. Shorter stroke means greater acceleration figures on the reciprocating parts, combine those two itmes and you have a need for a MUCH beefier connecting rod, gudgeon pin, wrist bearing and big end bearing at any given speed. (thus adding to reciprocating weight)."
is this not becuase oversquare designs are required to run at higher rpm to achieve usable torque?
I agree undersquare design wont be any good for us, there is a trade-off somewhere between, and many other factors at play like port timig etc.

this is where i disagree with you (again i am no expert, maybe you can explain it to me)
"For power - keeping all other ratios equal, the oversquare (large bore :small stroke) WILL get you off the line quicker and give you a better low end power. The "Undersquare" (longer stroke : smaller bore) will give you better "legs" in the top end. Close to "Square" will give you the best of both worlds."
Racing engines are always oversquare in design to allow them to rev higher which is where they develop their Horsepower. they have no usable power at low rpm.

for me this is an ever changing, ever learning,fascinating hobby...........just dont talk to me about electrics

RE: Questions for engine engineers
 

Hey Clive - yes you are right in many aspects.

In 2 strokes especially, there are many apsects of the design that affect the final torque curve. Combined changes make it even more interesting. For the Nelsons, I am sure you wil agree that the Longer stroke was a more successful engine in racing and I never claimed them to be undersquare - (or if it appeared that way, it was certainly not the intent - the example was to do with the Bore:Stroke ratio).

Yes - ALL other things being equal, (which is near impossible to achieve if you alter the bore:stroke ratio by much) the longer stroke does give a higher torque. However, it also creates timing issues (port opening time, transfer time, power transfer time). Short strokes create different issues - BOTH will work.

HP by itself is very much a meaningless figure as it is a derivative of Work performed and time - HP = (Torque in ft lbs x RPM)/5250. The old "40,000 hp honda" is a good example of that. Torque is the real measurement of power - it is just a matter of deciding where one wants the torque to be peaking at when deciding what design criteria to use.

However as Pe rightly pointed out in the other thread - there are other advantages that come into play - the "Port Time Ratio" is just one aspect. Port timing affecting true Cylinder compression ratio and Crankcase compression ratios are other aspects.

My example on Stroke length was to discuss acceleration forces, not ultimate speeds. It is the acceleration forces that break things such as connecting rods, pins and bearings - not the Ultimate speed (slowing down is also an acceleration force)

Ever wondered why so many of the more successful RC engines are close to 1.2:1 Undersquare? - it is simply because it is what works best for us. That is not to say other ratios don't work because there are no doubt many examples of them working.

It is a hobby for goodness sake - everything is likely to work just fine for someone out there (I think it is why there are so many Chinese engines on the market - someone must buy them and be happy with what they got). In any engine design there are tradeoffs. The perfect internal combustion engine has yet to be developed and I doubt it ever will be.

Here is a link to some simple explanations of the fundamentals of what works and why. Gordon Jennings is a pretty experienced guy and even though the publication may be a couple of years old, it is still very accurate.

[link]http://edj.net/2stroke/jennings/2stroketunershandbook.pdf[/link]

I use a significantly good set of Engine modelling software when I "play" with designs But Gordons publication (in that link) is the closest I have seen to the models and real life (probably because it is based on real life experiences). I only use these tools as a hobby these days. IF you have a design you want modelled, Send me the details and I wil model it for you and send you the Likely torque curves.

RE: Questions for engine engineers
 

steve,

ok thx, im with you. just thought for a minute i may have missed something. thanks for the link to Gordons book. will have to do some serious reading. trade-offs are always present and it is getting to know what is best for each application that counts.
Exactly why one guy will love Engine XXX an another hates it. the most popular engines are I suppose those that get the best trade-offs and work for most people. they become the most 'user friendly' and therefore popular.

cheers
clive

RE: Questions for engine engineers
 

Cool stuff.

Post 10,000 !!!!!

RE: Questions for engine engineers
 

HI How can I HOP UP a OS 160 a little bit and it still be reliable.
thanks

RE: Questions for engine engineers
 

A OS 160 is normally a glow plug type engine. Does yours have ignition with a CM-6 10mm spark plug and Walbro carburettor?

RE: Questions for engine engineers
 

no it is the nitro version

RE: Questions for engine engineers
 

This is the Gas engine forum

Most of these guys know a lot about glow fuel engines but there is another forum for glow engines . . .

http://www.rcuniverse.com/forum/forumid_114/tt.htm

The OS 160 is probably one of the best engines OShas made in recent years. I'm sure you enjoy yours.

RE: Questions for engine engineers
 

I love my OS 160, wouldnt trade it for gas just yet. Anything above this size, a few gassers really start being a reality and work well.
I have a Taurus 52cc, Bill seems to have this one right, not sure how the stroke to bore ratio is, but its proven itself to be a great choice for all model applications. Yeah, its a bit spendy, but it aint gonna give you any grief for years

RE: Questions for engine engineers
 

Very fun discussion, reminds me of years gone by building auto engines and trying to beat the next guy at all costs. I never learned it all that is for sure:) and learned a lot that wasn't true:( Nothing like being young and dumb

RE: Questions for engine engineers
 

大家好<div> 我是æ�¥è‡ªä¸*国的å�‘动机设计者，懂ä¸*æ– ‡çš„è¿›æ�¥æˆ‘们一起探讨。



Hello everybody,

I'm from China, the engine designers,we come to understand Chinese to explore


Translation provided by moderator</div>

RE: Questions for engine engineers
 

good. QQ 326401420