Bob Roach - "The Radial Man" Chief Engineer
At the age of 13, he designed his first engine a 4 cylinder gasoline motor. At the age of 15, a 6 cylinder..........................(more details later).
History
The beginning of his work at age 15....................
..................The last project: The Bushmaster Armoured Vehicle
A lesson from Mr. engineer - Bob Roach "The Radial Man"
Let me explain the Bore to Stroke relationship in the ¼ scale radial engines:
The combustion chamber needs to be hemi- spherical, and to suit valves at 35 degrees [P&W standard] needs to be deeper than say 25 or 30 degree. Deeper means bigger and more volume. But reducing valve angle means the towers stand up more and the engine gets a bigger dia.
I think we can say we will stay with 35 degrees as per P&W as we wish to have look-a-like cylinders.
To calculate the compression ratio, you add the capacity of the combustion chamber to the cylinder capacity, and divide this total by the combustion chamber capacity.
The cylinder capacity is constant at about 50cc above, and say the combustion chamber is 10cc, then we have 10 + 50 = 60 divide by 10 and we get 6 to 1 ratio.
If we make the compression chamber 7 + 50 = 57 divide by 7 = 8.14 to 1.
Once the compression chamber radius is fixed by the valve angle, I think you can see that as the bore gets larger, the height of the compression chamber will be taller, therefore the compression chamber is bigger, and in the above example we need to keep the chamber at 7cc, and the only way to do this is to add metal to the top of the piston. To keep the mass of the piston down, we need to scoop out metal from the inside of the piston. Making a piston from Solid Bar, limits the ability of the cutting tool to do this compared with casting the piston insides.
I think you can see that to help the situation, we need to keep the bore small in diameter, BUT smaller diameter piston means longer stroke. Longer stroke means the cylinder barrel is longer and the diameter of the crankcase is larger, and this means the overall engine is larger in diameter.
When Wright were designing the R3350 for the B-29, some one fixed the diameter of the cowling [ guess Boeing to save on drag], and the only way by this point in time, was to shorten the piston, which shortened the cylinder, which reduced the engine dia. The pistons became so short, that in service they turned inside the barrel, and no more engine, well if one piston did this and there are 17 more driving, the engine just keeps going with no doubt large noises.
So there is a lot of juggling between lots of criteria, and this takes some time to do, so you will have to leave it to me to juggle these things, but I will do this later, not now.
It is nice to have the 50 engines in ¼ scale at 13” diameter.
The R2800 had a compression ratio of 6.65 and this is because it was super charged and this boosts the compression ratio [ie raises the internal pressure].
The bore x stroke of the R2800, is 5.8 dia x 6 inches or 147 dia x 152mm .
In 1/5 scale this is 29.4 dia x 30.4. In ¼ scale this is 36.7dia x 38. This is 40.2cc a pot, so to get 50cc a pot into 13” dia is a bit squeeze.
The 29 dia x 30 stroke is what I am using in the V12.
So ends the lesson.
You will be tested on this next class.
Bob Roach
MS Mechanical Engineering
The combustion chamber needs to be hemi- spherical, and to suit valves at 35 degrees [P&W standard] needs to be deeper than say 25 or 30 degree. Deeper means bigger and more volume. But reducing valve angle means the towers stand up more and the engine gets a bigger dia.
I think we can say we will stay with 35 degrees as per P&W as we wish to have look-a-like cylinders.
To calculate the compression ratio, you add the capacity of the combustion chamber to the cylinder capacity, and divide this total by the combustion chamber capacity.
The cylinder capacity is constant at about 50cc above, and say the combustion chamber is 10cc, then we have 10 + 50 = 60 divide by 10 and we get 6 to 1 ratio.
If we make the compression chamber 7 + 50 = 57 divide by 7 = 8.14 to 1.
Once the compression chamber radius is fixed by the valve angle, I think you can see that as the bore gets larger, the height of the compression chamber will be taller, therefore the compression chamber is bigger, and in the above example we need to keep the chamber at 7cc, and the only way to do this is to add metal to the top of the piston. To keep the mass of the piston down, we need to scoop out metal from the inside of the piston. Making a piston from Solid Bar, limits the ability of the cutting tool to do this compared with casting the piston insides.
I think you can see that to help the situation, we need to keep the bore small in diameter, BUT smaller diameter piston means longer stroke. Longer stroke means the cylinder barrel is longer and the diameter of the crankcase is larger, and this means the overall engine is larger in diameter.
When Wright were designing the R3350 for the B-29, some one fixed the diameter of the cowling [ guess Boeing to save on drag], and the only way by this point in time, was to shorten the piston, which shortened the cylinder, which reduced the engine dia. The pistons became so short, that in service they turned inside the barrel, and no more engine, well if one piston did this and there are 17 more driving, the engine just keeps going with no doubt large noises.
So there is a lot of juggling between lots of criteria, and this takes some time to do, so you will have to leave it to me to juggle these things, but I will do this later, not now.
It is nice to have the 50 engines in ¼ scale at 13” diameter.
The R2800 had a compression ratio of 6.65 and this is because it was super charged and this boosts the compression ratio [ie raises the internal pressure].
The bore x stroke of the R2800, is 5.8 dia x 6 inches or 147 dia x 152mm .
In 1/5 scale this is 29.4 dia x 30.4. In ¼ scale this is 36.7dia x 38. This is 40.2cc a pot, so to get 50cc a pot into 13” dia is a bit squeeze.
The 29 dia x 30 stroke is what I am using in the V12.
So ends the lesson.
You will be tested on this next class.
Bob Roach
MS Mechanical Engineering