Single cylinder engine

[John Rudd]

I'm embarking on a single cylider engine designed to run off petrol...
So far my specs are:
Bore               1 inch
Stroke            1 inch
Piston length   1 inch

Compression ratio Yet to be determined, around 5:1 or as advised by the group.

Cylinder length ermm I think it needs to be around 2 inch Could someone verify this for me please?

Length of conrod...ermm again tbd..I'm guessing its a function of stroke..Could someone offer guidance on this too please?

So far, I have my piston blank machined to diameter, going with 2 ci rings, cast iron liner with an aluminium barrel/cylinder and aluminium cyl head.

Crank maybe overhung or I may go for a fully supported unit and a bolted big end cap.

Need to figure out lubrication too..  :-\

Any comments or offerings appreciated  ;)

[David Jupp]

Con rod will have to be long enough that piston skirt doesn't foul the crank or balance weights at BDC.  The detail of your crank design will influence the minimum length quite a bit.  You can also cut an arc out of the piston skirt to clear crank parts.  Height of gudgeon pin in piston will have some influence.

Sounds like a job for paper/scissors, or parametric CAD.

Others can probably give more precise guidance - my comments are only based on having stripped a few car and lawnmower engines in my time.

[John Rudd]

Sounds like a job for paper/scissors, or parametric CAD.

Dave cheers for that..
Yeah I guess so...Dont have CAD or the time to learn it  ( nor the inclination  )

[Spurry]

John

I'm pleased that you are not idling your time away.

I just did a quick sketch to see what was involved. This is a metric drawing, so bore is 25, stroke is 25. The blue piston is TDC, red in the middle at 3 o'clock and 9 o'clock and the black BDC. So that the crank clears the bottom of the liner a con rod length of about 43 should work.

I have never designed an engine, so would be inclined to crib the important data from a working version.

Pete

[John Rudd]

Hi Pete,

Well the weather hasnt been too great here for any flying...

So I thought I'd make use of the time to do something constructive and nowt to do with aeroplanes

Although I've a couple of steam engines on the go, I thought I'd try something different to stop boredom setting in..

The sketch you provided is great and many thanks for it....I'll probably go for a con rod length of 45mm or whatever that is in Imperial just to keep things neat..

[Spurry]

Good luck with it John

I'm the pictures will be appreciated. 

Pete

[John Rudd]

Pictures will be forthcoming, as soon as 'Er indoors re-appears with my camera (grrrrr...)

Meanwhile keep on turning 

[PekkaNF]

This is a topic I have very vague ideas, but I remember reading something like that:
1) You may want to consider offsetting piston or crank shaft, to reduce wear on power stroke, not sure if it used on model engines, but is is used to some extent on live size engines.
2) Longer than minimum con rod offers less wear (same mechanics than #1) also I heard one engineer claimining that it gives better torque, but I can't think off-hand any other factor than less friction. Dunno.

Woow, I just realized that I was probably 15 to 16 years old when I was reading books about engines and couple of decades later I still can remember some fragmets that I have never utilized.

PekkaNF

[cfellows]

Hi John,

I've built a several IC engines, a single cylinder hit n miss, a single cylinder throttled engine, and a two cylinder throttled engine.  The hit n miss engine was my first and I was pretty lucky with it.  The only problem I had was that my battery voltage wasn't high enough.  I was trying to use a six volt battery with a motorcycle coil and regular points.  It wouldn't run so I finally bought a 9.6 volt RC battery and it worked great.

The next two engines presented a number of issues.  I had a lot of trouble trying to get the valves to seat properly.  They have to be a dead nutz seal or the engine won't run.  Good compression is vital and in both cases, I used an o-ring on the piston.  Once I got the valves to seal the o-rings gave me great compression.

The next problem I had on both engines was carburetion.  I tried a vapor carburetor and had reasonably good success, but I couldn't get the mixture rich enough... maybe too many air leaks in the intake system.  I've tried various carburetors, again with mixed results.  It seems that these two throttled engines are easier to make run good at low speeds if you have a load on them.  For that reason, I'm adding a generator and a 60 watt light bulb to my 2 cylinder engine.

I've never tried a hall effect ignition.  I have used transister ignition but used standard points to make it fire.  Ignition is something you'll have to deal with.  There are some good alternatives for under $50 if you want to use electronic ignition.  S&S engineering has a unit which includes the coil for about $50.  I've used that one on my horizontal single cylinder engine.  The unit I have fires on both the make and break of the points, but that doesn't seem to be an issue.

Chuck

[John Rudd]

Chuck,
Thanks for the pointers on some of the pitfalls you encountered. I'll bear them in mind if I have problems starting the engine( when its completed...my big lathe is busted at the moment to the engine is on hold.... )
Ignition systems shouldnt present any problems for me, unlike mechanical stuff...I'm pretty handy with a soldering iron and a few electronic components. 

[John Rudd]

In the mean time here's a couple of photos...

A shot of the piston blank which needs finishing and the liner it will go into...


and here's a shot of the cylinder/crank flywheel blanks.piston and liner