In this article, we will focus on the rig engine power consumption and overall engine efficiency and there are few examples for you to get more understand on how to use the formulas as well.

**Input Power of Engine**

The formula is listed below;

Where;

Pi = input power in horse power (hp)

H = fuel heating value in BTU/lb

Q_{f} = fuel consumption rate in lbm/hr

**Fuel Consumption**

The formula is listed below;

Where;

Q_{f} = fuel consumption rate in lb/hr

N = rotary speed of engine in revolution per minute, rpm

T = output torque, ft-lb

n = engine efficiency

H = fuel heating value in BTU/lb

**Output Power of Engine**

The formula is listed below;

** **

Where;

N = rotary speed of engine in revolution per minute, rpm

T = output torque, ft-lb

**Engine Efficiency**

The formula is listed below;

**Example#1: **Rig engine runs at 1,600 rpm and the average output torque is 1,900 ft-lb. The engine consumes fuel about 200 lb mass per hour. Heating value of the fuel is 20,000 BTU/lb.

From the given information, determine the following values;

- Input Power (Pi)
- Output Power (Po)
- Engine Efficiency

**Input Power (Pi)**

Pi = 1572 hp

**Output Power (Po)**

Po = 579 hp

**Engine Efficiency **

Efficiency (%) =36.8 %

**Example#2: **Diesel engine on the rig runs at 1,700 rpm and its output torque is 2,500 ft-lb. The average engine efficiency is about 35%.

Diesel fuel data: Heating value = 20,000 BTU/lb. Price per gallon = 1.1 $/gal. Weight = 7.15 lb/gal.

Determine daily fuel cost from the rig diesel engine based on the given information.

**Output Power (Po)**

Po = 809 hp

**Input Power (Pi) based on the engine efficiency **

Pi = 2,311 hp

Based on the input power formula, we can determine fuel rate consumption.

Re-arrange the equation

Q_{f} = 292 lbm/hr

Convert Q_{f} from lbm/hr to gal/day

Q_{f} = 980 gal/day

**Total fuel cost = Q _{f} x Fuel price**

Total fuel cost = 980 x 1.1 =1078 $/day

**Ref books: **

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

In fuel consumption :

Qf = 48.46 ( N x T)/n x H

How does 48.46 has been derived

This is the conversion factor. We will share it later.