Cam shaft profiles.

[makila]

I am building a twin in-line engine and having machined the cam shaft blank I studied the profiles before progressing further.

To my eye, the drawing is wrong. The shaft will rotate CCW looking rearwards from the front and the lobes themselves, positioned at 0 - 90 - 270 and 0 degrees, looks as though the cam should be rotating in the CW direction.

If I were to build the cam as shown, then the forward inlet opens at 0 followed by the exhaust at 90 degrees (180 crankshaft degrees) thus the exhaust would be open during the compression stroke. It is a similar situation for the rear cylinder lobes.

I think the lobes should be 0 - 270 - 90 - 0. Can I solicit some thoughts on the cam lobes just to be sure I am right - or wrong maybe.

Steve

 

[stevehuckss396]

Without knowing the configuration of the entire engine its hard to figure out whats what. One thing I can say is no matter what, the exhaust lobes should lead the intake lobes. Could be that the front and rear are marked wrong. Maybe the ex and in lobes are marked wrong. Like I said, hard to tell without the whole story.

 

[petertha]

Assume its this engine? If so, looking at the intake ports looks like you have the INT/EXH/INT/EXH cams identified correctly. And assuming cam drive is a belt from crankshaft to cam shaft, looks like the direction is correct. Now that leaves the cam phasing. If I roughly eyeball the front cam set (zero is vertical, positive angle clockwise), exhaust opens say 45-deg & closes 135 deg, that's 90 & 270 crankshaft equivalent. But now intake opens at say 315-deg, which is 630-deg crankshaft. Hmm... Unless I'm missing something too... that does look off?

 

[mu38&Bg#]

I'm going to guess it's a mistake. All the other engines have the exhaust toward the front of the cylinder. This is the only one with the intake toward the front. Switch the positions of the intake and exhaust. It would be worth contacting Mr. Jung to let him know.

 

[petertha]

I never noticed that before (exhaust forward, intake rear) on the opposed & V cylinder engines but you are right. Seems like an oversight surely he would have caught early on or been pointed out by others even if he inadvertently plunked the manifolds in the wrong holes on the final assembly.

Is there an idler gear between the crankshaft & cam shaft? (ie. reversing your assumed direction) I don't have these plans so only guessing it was a straight belt. Otherwise I agree - pop him an email. And let us know.

 

[makila]

Thanks for the help, this really helps. Some good advice too, swapping the intake and exhaust positions - I never thought of that.
Petertha, Haha - you tracked the engine down - great stuff.
There is no idler, the drive is via a toothed belt (usual 2 to 1 ratio) thus the crank and camshaft turn in the same direction. The prop is still in the post as I wanted to make sure that it operates in the CCW direction for confirmation, as I am sure it does.
I will email the company in Germany and advise them of the findings re the cam - assuming it was intended to drive a CCW prop, unless it was intended to be a CW engine. I will post the results.
The rest of the plan details a standard 180 degree crank configuration and uses glow plugs instead of spark ignition which simplifies things a bit.
Once I figure things out I will post in the 'work in progress' as this engine is quite simple but has an interesting quirk or two.

PS. Nowhere in the engine specs does it mention engine direction of rotation.

Thanks again for all your support.
Steve
I will post the results

 

[petertha]

It would be very rare for a typical model engine like this to not turn conventional CCW (relative to facing the front of engine). 99.99% of RC type propellers are made this way & you cant just attach them backwards The only deviation to this might be a twin engine where you need/want contra torque, but even that is quite rare for simple to reverse electric motors.

These engines are meant to operate & fly so I will place my 1-beer bet that the inlet/exhaust configuration is as designed. I say this because the carbs located on the other side of engine look like they would be close to interfering with cam assembly & pushrods. So unless I'm missing something obvious, I think the cam lobe phasing on the camshaft 3D view is not correct because it doesn't satisfy typical duration/timing as is if you say there is no idler gear between crank & cam shafts.

Glad you have the opportunity to inquire before making the parts. Everyone makes mistakes, but if it is a mistake, surprised it took this long to flush out. Keep us posted.

 

[makila]

Hi Petertha,

I did look at the suggestion that the carbs would fit over the other side, and you are right, they would interfere with the push rods and rocker assemblies.
I sent an email this morning to 'Jung' and surprisingly got a reply within a couple of hours which was very good, they are on holiday this week so he will get back to me when they return. Here is the reply -

Hello,
we will check it next week. We are this week on vacation.
Mit freundlichen Grüßen / Best regards
Dipl.-Ing. Volker Jung

I will post the reply here when I receive it.

Thanks again for your help.

Regards

Steve

 

[mu38&Bg#]

That should have read to switch the lobe positions.

 

[petertha]

Now I'm curious. I'll have to go check the cam assembly 2-cyl boxer plans I have from him. I suspect its an assembly drawing boo-boo on the in-line engine.

The other thing I was wondering about is cam shaft lubrication off on the side like that. I didn't get that far just looking at the boxer plans but just assumed maybe with cam bearing box on bottom of crank case, maybe a gravity fed weep hole? But even something like that might not work as well with side mount cam. Any insight from the instructions?

BTW - where are you sourcing some of the hardware goodies? (valve springs, timing pulley & belt). I think he prescribed Madler in Germany but I might have this mixed up with the Ohrndorf radial I'm building. Anyway, they were quite good to deal with. Some of this (mostly gears in my case) is unobtanium on my side of the pond.

 

[makila]

I enjoyed making this engine, and thinking of the boxer next myself -toying with the 4 cylinder boxer which looks really good - maybe just the twin like yours. I hope that the cam is OK now you have made yours, I thought that the rule of thumb where the exhaust leads the intake cam a great point to remember.

The cam case has two small 3mm holes, one on the fore mounting point and the other on the aft, so that should get some oil from the crankcase during running.

The hardware goodies are all available in the UK from the Accu Group who sell all kinds of components, have a look at the site and get some part numbers and descriptions to find an outlet where you are. The timing wheels and belt are HTD type and all standard sizes are available and quite cheap. I found that the belt tension was a bit loose when offering the parts together but this was fixed with a piece of thin gasket material between the cam case mounting points which makes a perfect cam belt tension.

Another small point was the attachment of the small cam HTD drive wheel attached to the rear of the crankshaft - as this turns clockwise when looking at it from the rear - I drilled and tapped a left hand thread to prevent it loosening.

The timing wheels are standard makes, called HTD, and should be available on your side of the pond. HTD wheels and belts are based on their particular size and you should find them retailed quite easily. Try robotics etc - they are in quite common use in that sphere. I make my own springs on the lathe - I can do a article on the site some time but look on You Tube, there are loads of spring making techniques by various posters made from piano wire. All gauges of piano wire are obtainable in small quantities on Ebay.

I really must get this engine build onto the forum, apart from the heads and overly technical crankshaft - it is quite an easy build. I just did'nt take many pictures of the machining techniques I used so it might have to go in the completed build section.

Steve

 

[petertha]

Thanks for vendor link. I hadn't realized you have built already. Yes! Get your pics up! (I'm guilty of same).
Just to clarify, I'm building 5-cylinder radial, not Jung but Ohrndorf. I purchased some Jung plans a while back but they are on the 'future prospects' list.

What did you choose for carburetors?

 

[makila]

The carbs are manufactured by ASP and they are the series 3 type - chosen because they are quite cheap. No idea how they perform yet. There are some details on www.justengines.co.uk

 

[petertha]

Re ASP carb, good to know. That's what I ordered for my radial too. They look like lower cost Chinese clones of OS design for the most part. ASP about 20$, OS about 90$. On the ASP 4S engines they make a downdraft version (also OS clone) which looks to be a different body & mixture control design. But it looked to be a bit more fussy mounting as they have lateral through bolts which attach to crankcase & specific manifold tubes with O-ring seals. I opted for a simpler 2S carb with slightly reduced venturi orfice to match what see on similar 4S displacement engines. It will point axially into rear intake manifold, but that's exactly what the running design uses.

 

[mu38&Bg#]

If you handle an OS vs ASP carb you'll see why there is a cost difference. The main needle alone tells most of the story. Still, ASP carb will probably be good enough to run as most of their engines do run OK.

 

[petertha]

Thanks dieselpilot. I figured as much. Like many things 'from over there', you have to assume its a 'kit' that requires additional re-working & TLC. I'm also wondering out loud about their low end mixture control (or lack thereof). I guess I'll try it as a low cost sizing carb & go from there. Sorry to venture of this 'cam shaft' post but do you have much experience with the downdraft carbs on 4S methanol RC engines? The bodies & internals look different in a few respects but I cant quite determine if that's a function of them being for 4S or for downdraft mode. Some of the earlier 4S engines looked more like 2S carbs, just slightly smaller venturi size for displacement.

 

[mu38&Bg#]

"Downdraft" types are used because it's convenient for four stroke model aircraft engines (brings the carb closer to the fuel tank centerline in the model). Operation and design is not different between 2T or 4T. There are two basic types of glow carbs. You have air bleed types which are quite simple and tolerant of fuel type. And, there are metering (two needle) types with several methods employed to meter fuel at part throttle. Be aware that metering carbs may be tuned for certain fuels and a large change in nitromethane or running gasoline may not yield a suitable mid range mixture. IE., avoid heli, boat, or car carbs as many are designed for 30% nitromethane. In some cases glow carbs work just fine with gasoline(when people convert glow to spark ignition with gas), but I have not looked at any specifically. Often gas conversions require modification of the metering device. 2T and 4T carbs can differ in what the engine needs for mid range mixture. Two needle carbs allow for perfect WOT and idle mixture, but mid range is a function of the metering component and is fixed by it's geometry. Venturi size has an impact on engine power and fuel draw, with the latter also changing part throttle metering needs. I've run a wide range of fuels in glow engines and their carbs.


Greg

 

[napoleonb]

It could be an error on their behalve with just one drawing or its just because they use another projection method.
Truth be told if the drawings are professionally made nowadays there should be a drawing of 2 cones in the title block which show the projection method used.
German drawings are mirrored to what most of the world is used to:
http://www.vista-industrial.com/blog/u-s-vs-european-manufacturing-drawings/

Its a quite common mistake where I live to mix up projection methods in engineering which can be quite costly.
So whenever you buy drawings or make parts to European standards be sure to check for the projection method used before beginning.
As most European country's use the same projection method as the U.S. but there are some exceptions like the Germans.

 

[petertha]

Good thought. For the same reason I got tricked on some German drawings until I figured that out. But in this case he is working off 3D assembly rendering of the entire camshaft, not 2D projections extending from it. I don think there is any other way to view them differently in 3D is there? Plus the camshaft clearly has a front & rear, this corresponds to a different 3D view so the direction appears to be correctly defined relative to prop direction & belt > pulleys > camshaft. So that just leaves the lobes relative orientation to one another. Guess will hear something from designer sooner or later.

 

[makila]

Hi,
The prop has arrived and I can confirm it operates in the CCW direction as viewed from the front.

The reference taken from the 2D drawings does not indicate the orientation of the two centre lobes, this information can only be obtained from the 3D view which was where the issue was discovered, as you mention above.

Still awaiting a response from 'Jung'. I may have to send a reminder email.

Steve