Pressure vs Size

[SignalFailure]

Probably a daft question but here goes:

Given two engines of identical form made to the same tolerances and of the same materials but where one is twice the size of the other, would more steam or air pressure be required to run the larger engine? (ignoring starting the engine).

 

[AllThumbs]

More steam, less pressure to get the same torque at the wheel, at the same rpm, ignoring friction losses.

E

 

[deere_x475guy]

Signal that was a good question. I had been wondering the same thing.

 

[AllThumbs]

Horsepower is pressure x flow. If you increase either, you will get more power. For your example, since the flow will need to increase to fill the larger cylinder volume on the larger engine, the pressure will need to decrease for the same power output. If the pressure stayed the same, the HP output would be greater. This is why bigger engines produce more power but also need bigger boilers to get all that flow at reasonable pressures.

 

[steamer]

Hi

If you double the size of an engine, with the pressure constant, you will require 4 times the flow and potentially generate 4 times the power as power goes to the diameter of the piston squared ( 2 squared is 4)


Don't want to be a stick in the mud...but

Power is defined as

(P x L x A x N) Pressure x Stroke x Area X Speed (revolutions per unit time)

Pressure is the Mean Effective Pressure or the average pressure in the cylinder during the power stroke There are ways of calculating what it should be. For "Rough talk at the local pub", if you use 60% of your manifold pressure you will be close enough.

Stroke is the stroke of the engine

Area is the area of the piston ( pi D squared /4)

To find Horse Power

HP = (P x L x A x N) / 33000 with units in pound feet /minute


So based on that, the pressure remains the same but area increases by 4

Hope that clarifys a bit

Dave

 

[AllThumbs]

(P x L x A x N) Pressure x Stroke x Area X Speed (revolutions per unit time)


Yeah, that's what I said. It's pressure times flow.

Stroke, area, and speed are all what contribute to the flow required.

E