[Technical] Two Stroke Engine Design

PVO a écrit:

...Frits what balance factor did you run in your 500 single? I'm having trouble with engine vibration making it unpleasant to ride and my engine design professor and myself have decided to try and balance the crank to 100% forward balance to change the plane of vibration to the horizontal in hopes of the vibrations being less felt. If that doesn't work I've designed a counter balance I think I can fit into the engine however that will be a lot more machine work and I'm not sure if I can get enough weight on the counter balancer. Forgive me if it has been covered elsewhere but what percent of mass did the RSA 125 counter balancer cancel out? I've read 30-50% is enough but have no first hand knowledge.

I don't know, Paul. It's 40 years ago and I didn't have a clue about a lot of things, including balance factors. But I recognize your vibration troubles. With my bike idling, my vision was blurred as soon as I sat down on it. But once the engine started pulling, the vibration and the blurring went away.

A 100% forward balance will be more acceptible to the rider than the 100% vertical variant, but any such extreme percentage will put huge stresses on the crankshaft bearings. In the ideal case, with an infinitely long con rod, one would choose a 50% balance factor and counterbalance the remaining 50% with a counter shaft. In the real world, one would perhaps balance 60%, depending on the rigidity of the frame (kart frames, for example, have hardly any rigidity at all in the vertical plane, so one would balance a vertical kart engine with as much as 75%).

I have no data on the RSA counter balancer at hand, but it is not a question of 'being enough'; you try to eliminate any unbalance left in the crankshaft. It is important though, that crankshaft and counterbalancer shaft are as close together as possible, to keep the resulting rocking couple small. That is the reason for putting the counterbalancer bob weights wide apart, so the countershaft itself can be brought right up against the crankcase.

Regarding the question you put to Jan: I doubt that you will find a state of the art road-going cylinder. But here are some drawings to help you make up your mind.
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EDIT: the drawings I posted were fairly detailed, but apparently the forum software has drastically reduced their size. Then I zipped the drawings, but I see no option to attach a zipfile here. I will mail the zipfile to Marc and ask him if he knows a way tho make the drawings available here in their original size.

Hello Paul,

There are some kart engine manufacturers racing in superkart, many of them use Aprilia copys as their cylinders.
Maybe you could ask one of them for an unrepairable cylinder!
I think PVP in Denmark has many! These were copied from the RSA!

You can send it to me by mail, Frits, I shall give the link of the file towards of my FTP

Thank you very much Frits and Jan for your help so far and everyone else that has contributed! The drawings will most certainly be useful and I will try to get in touch with PVP. I can't tell you how nice it is to actually have found people willing to help me along and bounce ideas off of. Prior to finding this forum I had tried contacting several companies and individuals involved with two stroke engines and was given the cold shoulder by all of them.

Zip files are available HERE



Thank you Frits!



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Un énorme merci à Frits.

On aurait pu les vendre à combien en 2009 ?

Well I haven't had time to think much about the new engine design since I've gone home for the summer but I've been working feverishly trying to get my existing engine problems ironed out so I can race my bike. I experimented with a different balance factor of 75% but it did not help my vibration problem so I made a counter balancer for the engine.

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Today I finally got the bike back together after what I thought would be a quick project turned into a 3 week ordeal. I was able to balance out around 60% of the reciprocating mass which is as good as it is going to get as there is just no more room to add weight. I rode the bike today and there is still some vibration but it is definitely rideable which all I wanted!

We proceeded to dyno the bike this afternoon and start tuning it but ran into troubles. The bike isn't making anywhere near the power I believe it should and cuts out at 8300RPM. It made 52hp at ~8000RPM and I believe with the porting, pipe, and head I should be getting 70 or more. I thought it may be a pipe issue but I don't think this is the case as peak power was made around 8000RPM. I'm using the stock ignition system and believe the CDI is pulling timing in the upper rev range prior to it hitting the limiter where I need it to be working its best. If this is indeed the problem my plan is to install my Microsquirt ECU and use it to control ignition and then later on implement the fuel injection. My question is how should I go about coming up with a base ignition map?

PVO a écrit:

G.P. Blair suggested a good starting point for race engines was a bore stroke ratio of 1.2 in his book so that is another reason I've chosen it to start with.

PVO, can you tell me exactly where in Blair's book you are looking, and which book? I am trying to figure out if this was from his old 1990 book that he told me was "rubbish", or in what context he was recommending to try a bore/stroke ratio of 1.2 for a racing engine in his 1996 book. The racing engine he analyses throughout the later book had a 1.0 bore/stroke (54 mm x 54 mm). If he were alive today I would be very surprised to hear him recommend 1.2 for a racing engine, for the exact reason Jan stated.

A friend of me said after racing a 620 cc Zabel enginge..it feels linke you have two bottles of beer between your fingers and not a handle bar

I dont linkt vibrations and a 500cc Honda engine have a lot of it...i like the way you did it with the balancer.

As i read this thread i have been thinking about the sense to built a 500 single cylinder engine for a raod racing bike. I thing the biggest sense is "you want it" and this is no problem. If you are going for laptimes i would also built a smaller engine..or the racetracks you ride are only a long straight.

Nice projekt!

Jannsen

Brian Callahan a écrit:

PVO a écrit:

G.P. Blair suggested a good starting point for race engines was a bore stroke ratio of 1.2 in his book so that is another reason I've chosen it to start with.

PVO, can you tell me exactly where in Blair's book you are looking, and which book? I am trying to figure out if this was from his old 1990 book that he told me was "rubbish", or in what context he was recommending to try a bore/stroke ratio of 1.2 for a racing engine in his 1996 book. The racing engine he analyses throughout the later book had a 1.0 bore/stroke (54 mm x 54 mm). If he were alive today I would be very surprised to hear him recommend 1.2 for a racing engine, for the exact reason Jan stated.

It is in Chapter 6.

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I'm back at school now after having an entire summer to work with and pick the brains of the engineers at S & S Cycle. My experience as a design engineer there was good and I learned an incredible amount in a relatively short time. Even though I primarily worked with four stroke push rod V twins it was insightful and learning about how all the engine components were manufactured was very useful.

In my spare time this summer I continued developing my road racing bike. I finally had access to a dyno, good machining tools, and knowledgeable people so I got a fair bit done though I always wish I could have done more. The first hurdle I overcame was the vibration issue which I touched on briefly in a previous post. While the balancer did not have sufficient mass (nor was there anymore room to add more mass) to cancel out all the vibration it worked well enough to make the bike rideable.

The second big step I made this summer was successfully implementing a fuel injection system on the bike. It took a long time to sort out the wiring, learn the software, and trouble shoot all the nuances of such a system. Not to mention having to make a new intake manifold and relocate some brackets on the frame. I can honestly say I like it a lot better than a carb though it does take some time to tune.

I did not get as much development done on the motor as I had wanted to though. I was hoping to try a few different cylinders and pipes but only got around to trying two different combinations. Too much of my summer was spent on getting the balancer sorted out and fuel injection.

After a bit of thought about designing a new engine I've changed my design intent to some extent. I want this project to be worthwhile, affordable, and possibly benefit more people than just myself. What I am planning on doing is designing a 500cc single utilizing as many existing components as possible in an effort to keep manufacturing costs down. I will use the transmission from a Honda CRF450 dirtbike as all the primary gear ratios should work well and it has a counter balance gear so all I would need to do is make a correctly weighted counter balance. Some gear modifications will probably have to be made but no new gears will have to be made. The piston and rod can be used from the KX500 so primarily what I'll be left with making is crankcases, cylinder, head, crank, power valve assembly, reed cage, and modifications to the Honda ignition system. What I hope to achieve is to create an engine I can actually afford to make and could potentially be used by people looking for added power in their dirt bikes, ATVs, sand rails, and maybe even shifter carts.

I understand a 500cc single may be "big" but I am curious to see how much power it will actually make. The nice thing about this design is that it can be fairly easily scaled up or down to make other displacement engines.

I will attempt to keep this thread up to date as I move through the design process.

As always thoughts and comments are always appreciated.

So nice project PVO
About vibrations i worked on KX 500 engine for MX the rider noticed that with a completly news cranckshaft he had less vibration and after just one fulle day of race vibration rised after doing "metrology" comparaison between a new one and saying an old one we had quite difference in alignement for solving this problem i did one point of electric weld on each side of flywhell between flywheel and cranck pin and it was better stll vibration but no more rise

good luck for your project and kind of you to share it with us

Pierre

I've been busy using our cmm machine to dimension the CRF450 cases so that I can space the gearbox appropriately. No matter what I did I could not fit the new crank the way I wanted because the clutch and crank gears were just too small and I ran into a bunch of clearance problems. I suppose that is to be expected going from a 60mm stroke to 86! What I will do is cut a new clutch gear and crank gear so that I can get the required space. This is nice because I can change the primary gear ratio too from 2.78 to 2.5.

My question is how do I size the reed cage? Jan stated that a throttle body in the range of 65mm would be about right. I think I read somewhere else here that the cage area should be another 10% of the throttle body size? So a rectangular cage with the equivalent area of 71.5mm circle?

Sketch of a mess
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My other question is in regards to the cylinder prints. I've been trying to locate the transfer ports where they start at the base of the cylinder. As you can see they are spaced way too far out right now. Were there more prints that haven't been posted here or have I just missed something?

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Some more questions for the experts as I go. The KX500 rod is 145mm, the first photo illustrates TDC and BDC using this rod. The RSA had a 125mm rod and if I scaled that number up I would need a 198mm rod which ends up being quite long. When selecting a rod for a two-stroke there are several things going on: making sure the piston can clear the crank, dealing with crankcase volume, and weight.

My thought is a longer rod would be better because it keeps the piston skirt from inhibiting the flow of the transfers. Jan and Frits both stated that crankcase compression ratio didn't seem to be of much concern so maybe the extra space doesn't hurt anything? Any guidance is appreciated.

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Dear PVO
i'm not a two stroke specialist just do like it so much but if i have the opportunity to do what you want to do i first draw a model of Rsa replica including cranckshaft rod and piston like this you could see the space between lower side of piston skirt and crankweb. For your cylinder and as Jan say you the best could be to scaling the RSA cylinder so as you have drawings of the cylinder in this topics so do a3D model and then apply a scaling factor or homothecia factor to this model
Well all of this is just my advise and maybe i'm totally wrong

Pierre

PVO a écrit:

Frits is partly right. I had a KX500 dirt bike prior to the super mono so I knew a bit about the engine and costs could be kept down in theory. The other factor was I wanted a bike that could be competitive in multiple classes. The club I race in allows singles of any displacement, two or four stroke to compete in the ultralight class which primarily has Ducati Monster 620s and 450cc dirt bikes in it so it should be easy to be competitive. The next class is lightweight (SV650s and ER6s) where I should still be competitive and the final class I want to run is middleweight with the 600cc four-strokes. Since grand prix racing seems to be dead in America for me to make a trip to the track to run one race would not be worth it.

From a development cost point of view a single cylinder is the cheapest to run and make. My dream is to see road going two strokes again one day and if I can help develop the engines to make that happen then I would be pretty happy. Once I've got a single cylinder that I'm happy with making a twin and or scaling the displacement shouldn't be too hard.

Frits what balance factor did you run in your 500 single? I'm having trouble with engine vibration making it unpleasant to ride and my engine design professor and myself have decided to try and balance the crank to 100% forward balance to change the plane of vibration to the horizontal in hopes of the vibrations being less felt. If that doesn't work I've designed a counter balance I think I can fit into the engine however that will be a lot more machine work and I'm not sure if I can get enough weight on the counter balancer. Forgive me if it has been covered elsewhere but what percent of mass did the RSA 125 counter balancer cancel out? I've read 30-50% is enough but have no first hand knowledge.

I would forget trying to engineer the vibrations away, a couple of people have successfully put CR500's into Mito frames with good success by using rubber engine and head stay mounts. A little long term project I'm doing is a KTM380 in a Mito frame, we have retained the rubber mounted footpegs and will use tungsten filled bar weights to hopefuly dampen the vibrations. You pipe looks a little small I calculated this out to be 1300mm long with a 145mm belly diameter which is as big as I could fit on the bike.Gearing the bike for a reasonable top speed is a bit of a problem. Is it possible to post your dyno graph. A KX 500 has a 'falling' power curve even as a mxer they need a special breed of madman to ride them as they have peak torque at about 3-3500rpm then they fall away, the CR 500 has peak torque and power at roughly the same RPM.
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Hey Paul, how are you getting on?
Any further updates?
Adam

Institute of TwoStrokes a écrit:

I would forget trying to engineer the vibrations away, a couple of people have successfully put CR500's into Mito frames with good success by using rubber engine and head stay mounts. A little long term project I'm doing is a KTM380 in a Mito frame, we have retained the rubber mounted footpegs and will use tungsten filled bar weights to hopefuly dampen the vibrations. You pipe looks a little small I calculated this out to be 1300mm long with a 145mm belly diameter which is as big as I could fit on the bike.Gearing the bike for a reasonable top speed is a bit of a problem. Is it possible to post your dyno graph. A KX 500 has a 'falling' power curve even as a mxer they need a special breed of madman to ride them as they have peak torque at about 3-3500rpm then they fall away, the CR 500 has peak torque and power at roughly the same RPM.

I have to disagree, I feel the solutions you mentioned are only band-aids to the problem and not a very good solution. The vibrations are hard on everything, not just the rider, so to me it is worthwhile to stop them at their source. Though it will cost me some power to run the balancer the additional weight of the solution is in a better place and frame stiffness will not be affected as it would be with rubber dampers.

Here is the KX500 power curve:
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The pipe I made was a replica of a Pro Circuit Platinum II series pipe. It is thin and probably in many ways not optimized.
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I think a bit of an update is due

This project and my final year of university engulfed my life so it ended up being hard updating this as I went along. Fortunately I've submitted my final paper on engine design and apart from some exams I don't have any more coursework that requires attention so I can get on here more often.
To start with the design intent of the engine was broadened. I decided that I would try and design an engine platform which is somewhat universal and can be used in all sorts of applications (on and off-road motorcycles, atvs, sand rails, shifter karts, etc.). My design criteria became:

1. Multiple applications
2. Use as many existing parts as possible
3. Eliminate vibration issues by incorporating a counterbalance
4. Ensure the engine fits in the CRF250 and 450 series dirt bikes with minimal modifications

After scaling up some of the RSA dimensions it quickly became apparent that a 500cc engine would be very challenging to design because it would end up a lot bigger than the current 500cc mx engines unless a lot of compromises were made. Sourcing the correct size reed valve, carb, and other various parts was also more difficult. For these reason I decided that a 400cc engine was a better idea and more economical.

I started by scaling up the RSA cylinder as best I could. I've not designed the power valve assembly yet and intend to run the first engine without a valve.

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I’ve chosen to use the Honda CRF450 gear box and used a CMM machine to obtain the critical dimensions of the gearbox and other parts. Due to the increase in stroke it was not possible to reuse the clutch and crank gears. The primary gear ratio of the 450 was also incorrect for the RPM at which the 400 would be running. I designed new gears for the crank and clutch which also allowed the freedom of choosing the primary ratio depending on intended use of the engine.

The gears driving the counterbalance were also unusable as the center to center distance was too short to clear the crankcase. A new balance assembly utilizing removable weights which could be lightened to change the percentage of overall engine balance was designed.

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Laying out the components within the crankcases was just a matter of trying different positions until I felt the design was optimized. There were some limitations imposed by the requirements of the Honda gearbox that had to be considered as well. Once I had a layout I thought would work I printed my drawings to scale and test fit them as best I could to our CRF450.

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Once I had the initial geometry set I began designing the crankcases in 3D. I started with the gearbox and crankcase as separate parts within the same model and eventually tied them together. Keeping them separate as long as possible allowed repositioning of things to be easier. Eventually, I ended up with the finished design.

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My timing with finishing the initial designs worked out well and my previous employer helped me out in a big way by making rapid prototypes, making them affordable to me, and having them ready for when I returned home over Easter. I purchased a CRF250 frame and started fitting the cases. From the photo it probably doesn’t look like the engine fits that well since I’ve had to chop the engine cradle out of the frame but this is considered standard practice when doing the 500cc conversions. I designed and fabricated a new cradle to suit and apart from a few small design changes that will be required the fitment is pretty good.

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At this point I need to incorporate the changes into the CAD models, make drawings, and figure out who is going to manufacture the engine. So that’s the short story of where I'm at with this. There's certainly been a lot of late nights, head scratching, and problem solving going on in the past six months as I figured out what I think is the right way to design an engine and the various components. I don’t anticipate that I’ll be able to afford to make the engine until at least the end of the year so things will be dormant for a while.

I’d like to thank Jan, Frits, Howard, and everyone else on the forum that has contributed to the success of this project. Without your help I would not be nearly as far along as I am and there is a very good chance that my engine would be much less likely to succeed.

As always advice, criticism, questions, and comments are welcome as that is how we learn and improve.

Best regards,

Paul

The only successful CR500 into twin spar frame needed a large rubber mount on the head stay without it the bike was unrideable.

The counterbalancer is the ideal solution.Is the crank going to be bob weight or full circle?

Institute of TwoStrokes a écrit:

The only successful CR500 into twin spar frame needed a large rubber mount on the head stay without it the bike was unrideable.

The counterbalancer is the ideal solution.Is the crank going to be bob weight or full circle?

I'm going to try a bob weight design first. I can go to a full circle design later if excessive crankcase volume is an issue.

Great great job!

Thank you for sharing!

Cogratulations, your project has evolved enormously. I think it is a good idea to start this with the possibility to use as much standard gear as possible. But may i ask why you chose the honda 450 ? Honda is notoriously expensive when it comes to buying parts. (on the other hand, honda parts are available everywhere...) Also, honda has no standard 6 speed gearbox. That could be a serious limitation (road racing, enduro... ) where a 6 gearbox is more common.
I know of only one aftermarket firm who makes a 6 speed for the honda, and they are not cheap. [Vous devez être inscrit et connecté pour voir ce lien]
Have you considered using KTM parts ? Much more affordable, and they have different gearbox ratio's for their 300, which could be applicable to your 400 cc project.
Another question : do you intend to work with billet - CNC made crankcases ? So that you could alter your drawings when new idea's come up ? (a casting mould is expensive, so changing things afterwards is difficult)

Good luck with your final exams, this project shows that you will be a fine engineer !

peter1962 a écrit:

Cogratulations, your project has evolved enormously. I think it is a good idea to start this with the possibility to use as much standard gear as possible. But may i ask why you chose the honda 450 ? Honda is notoriously expensive when it comes to buying parts. (on the other hand, honda parts are available everywhere...) Also, honda has no standard 6 speed gearbox. That could be a serious limitation (road racing, enduro... ) where a 6 gearbox is more common.
I know of only one aftermarket firm who makes a 6 speed for the honda, and they are not cheap. [Vous devez être inscrit et connecté pour voir ce lien]
Have you considered using KTM parts ? Much more affordable, and they have different gearbox ratio's for their 300, which could be applicable to your 400 cc project.
Another question : do you intend to work with billet - CNC made crankcases ? So that you could alter your drawings when new idea's come up ? (a casting mould is expensive, so changing things afterwards is difficult)

Good luck with your final exams, this project shows that you will be a fine engineer !

Thanks for the kind words. Used parts availability was the biggest consideration along with how long a model stayed the same. I believe the Honda 450 parts that could be used in my engine are from 2002-2007 (possibly longer) so that is a long time for used parts to accumulate. I believe most of the Honda CRF250 parts are interchangeable with the 450 which also increases the number of available parts. I don't anticipate many folks going out and buying brand new Honda parts so the availability of used parts is more important than OEM prices.

KTMs are less popular in the United States (which would be a big market) and used parts are much more difficult to get so to me it was not a good idea to use the KTM parts in the first iteration of the engine. This doesn't mean a KTM gearbox couldn't be used though. Almost all the dirt bike engines follow the same layout and I'm guessing it would be possible to reposition things within the cases to accommodate almost any gearbox. As far as manufacturing goes being able to use other gearboxes is a consideration so a manufacturing method which allows this will be used. I have not determined exactly which way the cases will be manufactured if they were to go into production. I think my prototypes will be machined from billet or rapid prototyped via DLMS. For low volume production a generic casting could probably be done which still allows for machining of different gearbox configurations. I have a long ways to go before that step so there is plenty of time to figure it all out. At this point I'd just be happy if I could get the funds together to make some prototypes!