How about a 180 (30cc) glow motor?

[Ian Inverarity]

Unfortunately the pics that I posted ages ago in this thread of the potential design of a two cylinder 180 have gone, but the idea of that layout was two 15cc cylinders in a common crankcase side by side which have step up gearing to a common output shaft. Using the simple assumption that two 90's will have twice the power of 1, by gearing up to double the cylinder speed the output shaft torque would be the same as a 90, but at twice the revs!!

This means that the engine would actually have to run props smaller than 90's currently use.

This engine layout would be lighter than two seperate 90's and could fit in a 6 inch tub. The carbs would be at the front and two pipes out the back as normal.

If the torque levels are kept reasonable by the step up gear ratio then such an engine may be able to be used in conventional monos. Riggers may present a problem for fuel capacity in modern narrow tub designs.

I still think this approach is worth persuing.

Ian.

 

[TimD]

Agree totally with Ian.

An electric boat ran 120mph with a small diameter prop. How? 48,000 rpm at the prop. Thats a big part of it. I honestly believe a single prop overdriven from the engine is the way to go to take advantage of the 30cc limit. Overdriving the prop requires a huge amount of torque. This torque is reduced at the prop thru the gearing. Imagine swinging a H50 at 35,000rpm or more? There is no way a direct driven 90 engine can do that (or 2 of them in a twin as we see them now days).

Does anyone have the ability / resourses to make a strong enough gearbox to link 2 x 90's together and see if it works or not? I'm betting that when the correct ratio and props are found - look out speed records.

This is good posting guys - please keep it up!

Tim.

 

[Mark]

I don't think a gearbox would be that difficult to build to run that sort of rpm as an 1/8th scale road car runs 35,000 to 40,000rpm with a plastic spur and a steel pinion with no problems and that's taking into account that the clutch is also on the pinion gear and they run fairly course teeth on the gears.

I remember Ian's CAD picture of his idea/setup, I'm guessing this would be the cheapest and easiest way of getting the 2 cylinder setup working. Ian, do you think that handling would suffer because of the width of the side by side layout as opposed to a inline setup? Would the extra speed be a concern when cornering when comparing the 2 different engine layouts?

 

[Mark]

With Ian's idea you also get the option of running the cylinders either 180 degrees apart or firing at the same time or you could experiment if you wanted to.

 

[Mark]

Actually now that I've thought about it a bit more the plastic spur in the middle could possibly strip the teeth if one engine quit but they are very cheap. You would have to test it to be sure I guess.

 

[Ian Inverarity]

Okay, I have found my CAD layouts for this design, the motor is only 90mm wide with no engine lugs, and about the same length and height as a normal 90.

The gearbox would be integral to the design of the case. As I have stated earlier in this thread the gears would need to be heat treated precision made high tensile steel to have a chance of survival. And yes, depending on how the engine is assemebled the firing can be 180 degrees apart something else!

The problem with an inline layout is the arrangement of exhausts and carby's becomes more arkward and the engine is much longer.

If a 180 was made that worked well then it may need new hull designs to cope with the power and speed. But who here DOESN'T want to go faster?

Ian.

 

[Mark]

Ian,

Was your intention to have the gears run in a shallow oil bath. I'm gathering you would use a counter rotating prop because of the direction change from the gearing.

You could mount 2 OPS style engines side by side by milling off the mounting lugs so as they sit as closely as possible then bolting the engines to a gearbox from the rear of the crankcase, of course the gearbox would have to support the main crank bearings but that way you wouldn't have to change the crank and all is required is to secure the gears onto the crank end. Still a little costly but much cheaper than manufacturing an entire engine which I've got the feeling won't happen any time soon. I find it hard to believe that with the rules as they are a proper racing 2 cylinder isn't already available.

 

[Ian Inverarity]

Mark,

It was my intent to use very small passages from the case volumes to use the fuel/oil mix to lube the gears rather than a seperate oil reservoir and having to have seals etc.

By simply having the timing of the inlet system the cylinders can rotate in the opposite direction to normal but the output shaft and prop runs in the normal direction.

I would prefer to do a proper two cylinder engine than adapt existing engines.

Ian.

 

[Mark]

A totally new engine is best I know but the expense is staggering. How do you end up with the prop shaft rotating the right direction? Are you using more than 3 gears? Doesn't this rob power when things are spinning so fast. I don't know the calcs but doesn't frictional load increase as speed increases.

 

[TimD]

A totally new engine is best I know but the expense is staggering. How do you end up with the prop shaft rotating the right direction? Are you using more than 3 gears? Doesn't this rob power when things are spinning so fast. I don't know the calcs but doesn't frictional load increase as speed increases.

Yes, and if gearing up the torque is reduced proportionally. Keep in mind though that there is ample torque available and sacrificing some of it in order to increase prop rotational speed. gear ratio's would be optimised to suit commercially available props...

As for direction - use of 3 gears would only require the inlet port timing to be cut in such a way to allow the motors to run backwards - just like an outboard motor.

Tim.

 

[Mark]

TimD,

Yeh, I realise this as I raced superkarts or shifter karts as they're called in the USA and did all my own engine mods including reversing the barrel (not the rotation of the engine). It's not difficult but it takes time and money to make things from scratch especially precision stuff. If someone has a mate or another model boater from here who has access to CNC machines they should talk with Ian (nitrocrazed) to try and work something out. I'm pretty sure he knows his stuff and has the technical side covered. The problem I've found with people is they want to see it work before they support it. Once it's up and going I'm sure everyone will want one.

 

[Ian Inverarity]

Well designed, well made and properly lubricacted gears should not have too excessive power loss, but it would be there.

My design utilised a lot of parts from the A-90HP engines which I use. Some of the other parts I can make or have made where I work, like cyl heads and water jackets, the case and gear cover case, drum housings, flywheel, carb mounts etc. Other parts would have to be custom made like the crankshafts, gears, and induction drums.

Ian.

 

[Mark]

Sounds like your work only deals with aluminium. Why can't you make the gears and crank, etc? Gears are often made on milling machines with an indexing head. I only thought this because all the parts you mentioned (custom made) are steel parts, aren't all the parts custom made crankcase, housings, etc?

 

[Mark]

BTW, I'm still a big fan of the inline engine though, in a "V" similar to the 180V but with a barrel at the front and centre of the engine which would be the middle of the crankshaft for the carbs, it just seems to me to be a lot simpler in design and less weight.

 

[Ian Inverarity]

Mark,

We work with steel also, but do not have the fancy grinding and honing capabilities required for cranks, drums etc. Gears is a whole different story!!

I prefer the side by side layout for compactness. The problame for any in-line system is coupling the cranks. This is done ith the 180V, but not well, as that is where it broke!!

Ian

 

[Mark]

Yeh, I read about that. I was thinking in pressing the cranks together and putting a tiny weld on the end of the pins to stop them from slipping like we had done on the 1000cc 4 cylinder sidecar I used to race with that had roller big end bearings. It was the usual practise other wise the cranks would twist sometimes on the starts.

One thing though is you would need to run an "S" shape flex outer because the engine would stick up to much at the front.

 

[Mark]

Ian,

Have you thought about using the oil from the exhaust for lubrication of the gears.

 

[Mark]

I just re-read one of your posts, have you got some parts already made? If so can you post or email me some pictures of them.

[email protected]

 

[Ian Inverarity]

Mark,

The problem with welding cranks is taking them apart again for changing rods etc... Also the heat of welding can mess up the hard surface of the crank pins! Certainly the Evo type CMB cranks I have seen that were welded didn't look so good after a while.

For the geared layout the output shaft does not need to be that high if the gears are small enough.

My thoughts were to use fuel lubrication because of the proximity to the case volumes and any contamination from oil drawn back into the cases should not be an issue as it is just fule. But if the gears start to waer and there are metal fragments in there it si a different story!

I dont have any parts made, as I said many parts can be used from A-90's, ie P&L's, rods, and carbs.

Ian.

 

[Mark]

Actually Ian, I was thinking that a gearbox version would enable the final drive to be much lower than a conventional engine shaft. As you could have the gears close together forcing the centre gear to be lower in order to fit, if you get my drift. It's just a matter of finding the right diameter gears to get the ratio you would be after.

As for a non-lube geardrive I saw a plastic and steel geardrive for a CMB 45 on a U.K. website. The gears obviously have to be wide but it's a retail item so it must stand up to the stress I would hope.