1/4 Scale Benz Engine (Inch) Model

[Ken Brunskill]

Jason,

Bingo! You are absolutely correct I'd mis-read the volume numbers initially the smaller version is 88% smaller. Original is .33 cu in, smaller is .04 cu in.

About the welding, did a small welding experiment today, finding plain silver soldering is perfect see photo. Left side is Safety Silv, Middle is plain silver solder I think provides the best fillet, and is what I'll go with & Right side is Tack welded with TIG which leaves too large a fillet (hidden by pin) with my welder (low end controls are not so good). If I want a larger fillet that will be done with JB weld.
I have a small furnace and believe that will be my best bet for doing the silver soldering, as I can flux the parts, place a small piece of silver solder in the crevasse and heat to a known temperature, these were done with a torch.

 

[Jasonb]

Another reason the TS engine may be low on compression is that their website says 44mm stroke but the original engine was 150mm stroke so at 1/3rd scale that would give an extra 6mm of piston travel which would make a fair bit of difference. Bore is OK at 30mm as that is 1/3rd of the 90mm original.

 

[Ken Brunskill]

Interrupted a cabinet project to noodle out the compression ratio on the inch 1/4 scale benz at top dead center the volume of the combustion chamber with the reduced cavity is .57 cu inch and the volume of the chamber at bottom dead center is 2.945 cu in, that comes out to be north of 5:1, if I am doing this right (my first time). Thought is to have a 1" bore and a 2 3/8" stroke. Maybe bring the top of the piston up some by making a chamfer cut to allow the spark to ignite the gas, any comments?

 

[Cddeshon]

I don't have a lot of experience in compression ratios and designing engines. I did design and build a Witte hit and miss a while back and I set the compression ration at around four to one by design. The engine fired every other available available power cycle and I couldn't seem to 'fix' this without changing the compression ratio. Seeing I couldn't really get the governor to control the engine either this was probably a good thing as hitting every time it would have just run away.

 

[Jasonb]

Some of these early engines did have quite low compression ratios, my I F Allman for example is about 2:1 (1.75cu.in at TDC, 3.5cu.in at BDC) and it even runs better bleeding out some of the compression. So if you allowed for that extra bit of stroke I expect the TS kit would come out similar rather than the 1.5:1 mentioned earlier. We need to remember that these things ran at 2-300 rpm so need to think along the lines of hit and miss engines not higher reving tether or RC engines with their built for speed ethos.

2 3/8" stroke would be very long for the 1/4 scale engine scaling upto 240mm rather than the original 150mm so unless the engine frame is stretched you may get into problems with the angle of the conrod being too large and hitting the sides. 1" bore is 1/3.5 scale so 43mm stroke would be the same scale so maybe 1 3/4" to 1 7/8" would keep proportions and still up compression slightly. A "packer" could always be added to the underside of the head or the top of the piston both with suitable clearances to bring the CR up if it was found to be needed.

 

[Badhippie]

Hello
Don’t forget it is possible to change C/R by the placement of the top compression ring. Also you can change the top of the piston to change the C/R
Thanks
Tom

 

[Ken Brunskill]

Great comments! Thanks

 

[Ken Brunskill]

Some of these early engines did have quite low compression ratios, my I F Allman for example is about 2:1 (1.75cu.in at TDC, 3.5cu.in at BDC) and it even runs better bleeding out some of the compression. So if you allowed for that extra bit of stroke I expect the TS kit would come out similar rather than the 1.5:1 mentioned earlier. We need to remember that these things ran at 2-300 rpm so need to think along the lines of hit and miss engines not higher reving tether or RC engines with their built for speed ethos.

2 3/8" stroke would be very long for the 1/4 scale engine scaling upto 240mm rather than the original 150mm so unless the engine frame is stretched you may get into problems with the angle of the conrod being too large and hitting the sides. 1" bore is 1/3.5 scale so 43mm stroke would be the same scale so maybe 1 3/4" to 1 7/8" would keep proportions and still up compression slightly. A "packer" could always be added to the underside of the head or the top of the piston both with suitable clearances to bring the CR up if it was found to be needed.

This design has been accomplished more from visual observation of photo's and bit's & pieces of the metric plans posted way back . . . my thoughts are shortening the Cylinder Base, would not look right, of course I could make the piston taller (move the wrist pin down) to accomplish that, however think the wrist pin needs to be as near the top of the piston as feasible.

 

[Jasonb]

A longer conrod would have the same effect as it will move the piston closer to the head and have the added bonus of reducing any risk of the rod hitting the sides with any increase in stroke. I would aim to have the top face of the piston come right to the edge of the spark plug hole at TDC, from what I can see the TS design has it stop quite a bit short of that. I also think the TS version has the spark plug a bit too far down the cylinder when compared to full size photos (mostly replicas), so a bit more compression ratio can be gained if that is nearer the head by 1/8" on the 1/3rd model so maybe 3/32" on a 1/4 scale. Combined with a true scaling of the 150mm stroke and reducing the head void should put it at about 4:1 CR.

I did find a German hobbiests site yesterday who had made a 1/3rd scale one, this was before TS brought their kit out. He mentions that the original engine had a compression ratio of 2.5:1 but he did increase it on his model