Based on the following input:

Engine: CAT 3512 – 3158 displacement

Min Cranking RPM: 120

Number of Teeth on Flywheel: 183

Number of Teeth on Starter Pinion: 11

Ambient Temp: 100°F (560°Rankin)

Accumulator Precharge: 1650psi w/N2

Accumulator Gross Volume: 60 gal ((4) 15 gal units)

Starter Motor Displacement: 5.1 in³/rev

Accumulator Pressure – Hydraulic (fully charged): 2800psi

Cranking Time Required: 30 seconds ((3) 10 second cranks)

Customer prefers (3) accumulators, (4) max

Cranking Oil Volume Required:

Cranking speed times the ratio of teeth in the drive gears

120 rev/min x 183/11 = 1996 rpm

Flow Rate of Oil Required:

Displacement of the cranking motor divided by 231 to convert to gallons times motor rpm

5.1 in³/rev x 1996 rpm / 231 in³ per gallon = 44 gpm

Gallons of Oil Required:

Flow rate times cranking time required

44 gal/min x 30 seconds / 60 seconds per min = 22 gallons

Accumulator Volume Available:

Using the equation: (P1V1) / T1 = (P2V2) / T2

Where P = Pressure is in psi

V = Volume is in gallons

T = Temperature is in degrees Rankin (F+460 = Degrees Rankin) to derive the nitrogen volume.

Nitrogen volume from fully charged to fully discharged is:

2800V1 / 560 = 1650 x 60 / 528

V1 = 37.5

Oil volume from fully charged to fully discharged is:

60 total volume – nitrogen volume = 60 – 37.5 gallons = 22.5 gallons


With 22.5 gallons available and 22 gallons required, the use of (4) 15 gallon accumulators is adequate for the required cranking speed and time.

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