BMW R7 Motorcycle Engine

[gbravo]

Sorry guys,
In this moment, the compression problem has been solved.

Continuing with my history, after the troubles with new liners, I recheck the compression with compress air, with that I can see that the cylinders continue leaking to the crankcase, not from valves.

As I have reading many posts about the difficult in to make good rings, I assumed the problem was in the rings, and make new rings, after assembly the engine as the problem continue, then I make more rings.

As the problem continued and the previous rings was made with Chaddok/Walshaw method, I decided change to the Trimble method and make more rings, but the compression not Up, even turning the engine with drill for obtain more adjustment between rings and cylinders, continues leaking.

At this point, the only part that I not have made new was the pistons, well, I make new pistons. With those new pistons, oh surprise, the compression raises magically with minimum leakage. But, what was the problem?

Investigating in the web I concluded that the problem was in the groves rings. When I made the groves I turn the grove with a tool more narrow respect to the grove and then I adjust the grove width with the same tool. With this method and so thin tool, the grove walls are not straight, they are some tapered and the seal between piston and ring is not right. In the new pistons the groves was turning with tool with the same grove width and with only one tool pass.

After those changes, I could hear by first time some explosions in the exhaust pipes.
Now, the engine should be start, but I discovered an ignition problem. Because I connected the CDI to one coil and then to both sparkplugs in parallel, with this configuration, I could start the engine with only one cylinder running and the other with the sparkplug removed. At this moment, at least the engine start with one cylinder but start …

Investigating this new problem, some persons tell me that this configuration with both sparkplugs in parallel and with waste spark, not work because the difference of conductivity in both compress chambers, weak the spark in the compressed chamber.

Then the solution is to buy another coil and connect each sparkplug with corresponding coil.

In the following picture you can see the old cylinder liners a lot ot rings for different liners and with different methods.and pistons

 

[Rayanth]

I am glad to hear you solved your compression problem! It's interesting how it often turns out to be something we never even thought of, and we were so SURE we knew what the problem was...

I am not sure if you would need a separate coil for each cylinder, that doesn't sound right for some reason to me, but i'm not the expert on ignitions... I am certain someon will be along to help you soon on that

- Ryan

 

[gbravo]

Ryan,
In this topic you can see the discussion about two coils,

http://www.homemodelenginemachinist.com/index.php?topic=14434.msg148516#msg148516

 

[Rayanth]

just knew I'd be proven wrong, thanks

 

[Admiral_dk]

You're in trouble with two plug in parallel .... Just as you discovered :

When you do it that way, you have a fast rising voltage when the system fires. At some point in time this voltage will be high enough for one (or the other) plug to create a ionizing path resulting in a spark. At this point, the plug goes from being a very high impedance (AC resistance) to a very low impedance. This reduces the voltage from the coil from several thousands to a few hundred volts and this effectively prevents the other plug from firing.
Since the voltage where the ionizing occurs need a higher voltage, the higher the pressure in cylinder is - you're effectively only firing the "waste" plug and never the one that needs to be fired - the one with a compressed gas :-\

That's why I suggested that you connected one end of the coil to one plug and the other end to the other plug - this is the way ALL four cylinder Japanese motorcycles worked before electronic ignition ...!
It might give you a lot of EMI problems - but if this is a stationary display model, it might not give you any problems - If it weren't for the fact that you use a Hall sensor as the trigger.

Looking forward to see a video of it running - You've done a work of art so far :bow:

Best wishes

Per

 

[Lakc]

In order to use the single coil, you need a somewhat special kind without a grounded secondary. Many automobiles use these, often found in banks of two or three coils for waste spark. Each end of the secondary has its own coil terminal to run to a spark plug.

 

[gbritnell]

When I built my V-twin engine I was lucky enough to have a coil with 2 high tension leads coming out of it. I had purchased it from Jerry Howell many years ago and wondered if I'd ever have a use for it. They discontinued making them a couple of years back. Shame! I don't know what I'll do if it burns out. I watching to see what kind of answers you get.
gbritnell

 

[RonC9876]

Guys: The single spark ignition system as sold by Roy Scholl can be wired to produce two sparks at once as used on a waste spark twin. I am using it to fire both plugs at once for my Hercules engine. The coil on this system usually has one end grounded to the engine block. You simply remove this ground wire and connect it to the second plug. This coil is double ended and isolated from ground in this configuration. Both plugs are now in series and they both HAVE to fire together. This set up really works well. Just remember to do away with the ground wire all together. Ron Colonna P.S. Ask Roy about it if you think I'm wrong. He had been selling a dual spark system with two coils but I think it was Doug Kelly that told him about this way to wire his single spark coil to produce dual sparks at the same time.

 

[gbravo]

Ron,
When you used this system, your sensor was hall or mechanical contacts?

I asked Roy about this connection and his response was:

"I have recently had two modelers successfully use one of our single spark CDI systems on twin cylinder waste spark engines. What I did was leave the ground wire off from the secondary back to the ground on the board. They used 2 spark plug wires, 1 on each of the spark coil terminals ( this does not use a ground wire ) to each of the spark plugs. The ground path is through the engine from one spark plug to the other. In both cases this worked very well on their engines."

My only doubt about this connection is what happend with the hall sensor, while the sensor has a separate ground connection, it is glued to the crankcase, where have a high voltage and the hall signal wires are very thin not designed to work with high voltages, then could have an electric shock that burn this sensor, due to those wires are near to this high voltage surface.
Am I right or my doubt is exaggerated?

Regards,
German

 

[nfk]

Hi,
Why not use an original BMW coil?

Or maybe use any other wasted spark coil, my old kawasaki z650 has 2 double output coils in a wasted spark configuration and those where kept in use after upgrading to CDI ignition.
The link below ir for a couple of old Honda CB coils, if those fit an old CB600 then one of those should work for your engine/CDI.
http://articulo.mercadolibre.cl/MLC-34976742-bobinas-honda-cb-_JM

Another option could be to wire the sparks in "series".
Wire the live end of the coil to one of the plug and wire the ground terminal to the other coil.
Beware that way there is no ground reference on the engine and if your CDI is using any part of the engine for ground reference it wont work. (and you could burn the CDI)

Cheers,
Norberto

 

[gbravo]

Because my plan is locate the coils below the upper engine cover as you can see in the picture showed in reply 89.
The coils from CNC-engines go in this place, even two coils.

 

[Lakc]

My only doubt about this connection is what happend with the hall sensor, while the sensor has a separate ground connection, it is glued to the crankcase, where have a high voltage and the hall signal wires are very thin not designed to work with high voltages, then could have an electric shock that burn this sensor, due to those wires are near to this high voltage surface.
Am I right or my doubt is exaggerated?

German,
High voltage electricity can behave in funny ways, but if you understand whats happening you can take the proper precautions. Hall sensors are notorious for failing on these type of ignitions, and you are right to be careful, but understanding electricity in this case will show you how to be safe.

Your engine is probably ground potential, 0 volts, to the battery and ignition system. The ignition transformer turns the battery voltage to 300-500 volts and flows across the ignition coil primary windings. This sets up a magnetic field with the current flow that collapses when the ignition controller wants a spark. That magnetic field moving across the secondary coil creates a new voltage, say 20,000 volts, which is enough to jump the gap in the spark plug gap. This 20,000 volts is only between the two ends of the coil secondary, if you could measure the voltage to the coil primary or ground, it would be 0. Now when you take one end of the coil wire and go to the cyl 1 plug, and the other end to cyl 2 plug, your spark jumps from plug 1 ----shell>-->block--->shell of plug 2--->plug 2 electrode---> back to the coil secondary.
Although the block is part of the path of the voltage, as long as the hall sensor is not near either plug electrode, it should be at 0 volts to the secondary 20,000 volts.
There are many variables that determine if you can blow a hall sensor, usually from the ignition board itself, or allowing the hall wiring too near the spark plug wires or coil wires, but you shouldnt blow the hall from a waste spark system like I just described.

 

[RonC9876]

Guys: My Hercules uses a Hall sensor to trigger the system. It is well isolated from any high voltages and I haven't had any problem with any burnouts. In fact I have never had a Hall sensor fail on any of my myriad of engines that use them. I take that back. One did fail due to vibration when one of the leads to it broke off. No problems due to high voltage spikes though. Best of luck. By the way the two modelers Roy spoke of were me and Doug Kelly. Ron Colonna

 

[gbravo]

Finally my engine starts,

[ame]http://www.youtube.com/watch?v=K7UzI7dA8wc[/ame]

In this video, the engine is running with his two cilinders and two coils located below the engine.
German

 

[willburrrr2003]

WOW :bow: :bow: you did a hell of a job, It looks fantastic and sounds great too!!!

Regards,

Will R.

 

[steamer]

Outstanding job German! Well Done!

Dave

:bow: :bow: :bow:

 

[ozzie46]

Way to go! I knew you would get it. :bow: :bow:
Love the sound.

Ron

 

[RManley]

Very very nice and sounds great too! Those rockers are too nice to cover up ;D

Are you going to try the waste spark method? interested because thats probably what im going to do.

There cant be much left to do to finish it now (except a motorcycle to put it in )

I would love to see some models from this forum in person, shame I cant afford a round the world ticket!

Rob.

 

[coopertje]

Congratulations with this beautiful engine, its a true state of (modelling) art :bow: :bow: :bow:

I hope to see this engine soon on top if this site as project of the month, it does not deserve less!

Regards Jeroen

 

[nfk]

Very nice engine!
Congratulations! :bow: