Introduction
Power is one of the most important parameters used to evaluate the performance of a marine engine. It indicates the rate at which an engine produces useful mechanical work. In marine engineering, different types of power are considered at different points of the engine and propulsion system.
Three categories of indicated horsepower (IHP), brake horsepower (BHP), and shaft horsepower (SHP) are usually under discussion. Comparing the values of these categories allows the marine engineer to evaluate engine performance, mechanical losses and the state of the propulsion system.
Indicated Horsepower is the power developed by the combustion gases inside the engine cylinders. It is calculated from the pressure acting on the piston during the power stroke and represents the power developed inside the cylinders before mechanical losses.
The basic formula is:
IHP = (P × L × A × n × k) / 4500
Where:
P = Mean effective pressure in kg/cm²
L = Stroke length in metres
A = Piston area in cm²
n = Number of power strokes per minute
k = Number of cylinders
For a four stroke engine:
n = RPM / 2
For a twostroke engine:
n = RPM
IHP is commonly determined using an indicator diagram or an electronic engine monitoring system.
Brake Horsepower is the actual power available at the engine crankshaft. It is lower than Indicated Horsepower(IHP) because some of the power developed inside the cylinders is consumed by friction and by driving engine auxiliaries such as pumps and other components.
Brake Horsepower(BHP) can be measured using a dynamometer or calculated from torque and rotational speed.
The formula is:
BHP = (2π × N × T) / 60 × 745.7
Where:
N = Engine speed in revolutions per minute
T = Torque in Nm
In practical engineering calculations:
BHP (kW) = (2π × N × T) / 60,000
BHP is therefore a useful indication of the actual mechanical output of the engine.
Shaft Horsepower is the power transmitted through the propulsion shaft from the engine or gearbox towards the propeller. In a direct drive installation, Shaft Horsepower(SHP) is measured on the propeller shaft. In geared propulsion systems, there can be additional losses between the engine crankshaft and the shaft.
Shaft Horsepower(SHP) can also be calculated using torque and shaft speed:
SHP (kW) = (2π × N × T) / 60,000
Where:
N = Shaft speed in RPM
T = Shaft torque in Nm
A torsion meter or shaft power meter is commonly used to measure shaft torque and speed.
The relationship between these powers can be understood as:
IHP > BHP ≥ SHP
The exact relationship depends on the engine and propulsion arrangement.
IHP: Power developed inside the cylinders
BHP: Power available at the engine crankshaft
SHP: Power transmitted through the propulsion shaft
The difference between IHP and BHP represents mechanical losses, while the difference between BHP and SHP can include gearbox, coupling and transmission losses.
Consider a two stroke marine diesel engine with:
Mean effective pressure = 15 kg/cm²
Stroke = 2 m
Piston diameter = 0.6 m
Speed = 100 RPM
Number of cylinders = 6
Piston area:
A = πD²/4
A = π × 0.6² / 4 ≈ 0.283 m²
After converting the required units and applying the IHP formula, the indicated power can be determined. The actual Brake Horsepower (BHP) will be lower because of mechanical losses. Similarly, Shaft Horsepower(SHP) may be slightly lower than Brake Horsepower(BHP) because of transmission losses.
Comparison of these three power values can help identify abnormalities in an engine or propulsion system.
If IHP is lower than the expected value, possible causes include:
Troubleshooting: Check cylinder pressures, fuel pump timing, injector condition, exhaust temperatures, turbocharger performance and scavenge air pressure.
If Indicated Horsepower(IHP) is normal but Brake Horsepower (BHP) is significantly lower, the engine may be developing excessive mechanical losses.
Possible causes include:
Troubleshooting: Check lubricating oil pressure and temperature, bearing condition, piston movement, crankshaft condition and engine friction related parameters.
If Brake Horsepower(BHP) is normal but Shaft Horsepower(SHP) is lower than expected, the problem may be in the transmission system.
Possible causes include:
Troubleshooting: Check gearbox condition, shaft alignment, bearing temperatures, coupling condition and shaft torque measurements.
Unequal cylinder power can indicate problems such as:
Engineers can compare cylinder pressure, exhaust gas temperature and fuel index to locate the affected cylinder.
Regular monitoring of these parameters helps marine engineers:
IHP, BHP and SHP stand for the power produced at different points in a ship’s propulsion system. IHP measures the power generated within the cylinders, BHP represents the developed power at the crankshaft and SHP stands for the delivered power through the propulsion shaft. Comparing these values can provide vital information on combustion efficiency, mechanical losses and transmission performance. As such, regular measurement and troubleshooting of power characteristics are of particular importance to ensure reliable and efficient ship operation.
IHP is the power developed inside the cylinders, while BHP is the power available at the engine crankshaft after mechanical losses.
Because some indicated power is used to overcome friction and drive engine auxiliaries.
SHP represents the power transmitted through the propulsion shaft towards the propeller.
BHP can be determined from engine torque and RPM, generally using a dynamometer or suitable torque-measuring system.
It helps engineers identify combustion problems, mechanical losses and transmission-related abnormalities.
Disclaimer :- The opinions expressed in this article belong solely to the author and may not necessarily reflect those of Merchant Navy Decoded. We cannot guarantee the accuracy of the information provided and disclaim any responsibility for it. Data and visuals used are sourced from publicly available information and may not be authenticated by any regulatory body. Reviews and comments appearing on our blogs represent the opinions of individuals and do not necessarily reflect the views of Merchant Navy Decoded. We are not responsible for any loss or damage resulting from reliance on these reviews or comments.
Reproduction, copying, sharing, or use of the article or images in any form is strictly prohibited without prior permission from both the author and Merchant Navy Decoded.
Decoded Discount Alert! up to 50% OFF
Decoded Discount Alert! up to 50% OFF
Use Coupon Code Deep50
