In the field of mechanical engineering, especially when discussing internal combustion engines, understanding the different ways power is measured is crucial. The question asks about the actual power generated inside an engine cylinder.

Engine Power Terminology

When an engine operates, it generates power, but this power can be measured at different points in its transmission path. The term for the power developed within the engine's cylinders before any losses due to friction is specific.

• Indicated Horsepower (IHP): This refers to the theoretical or actual power developed by the combustion of fuel inside the engine cylinders. It is calculated from the pressure-volume (P-V) diagram obtained from the cylinder. This is the power available at the piston crown before any losses due to mechanical friction in the engine components like pistons, crankshaft, bearings, etc. It represents the gross power produced within the combustion chambers.
• Brake Horsepower (BHP): This is the power available at the engine's crankshaft (or flywheel) after accounting for frictional losses within the engine itself. It is the usable power that can be delivered to an external load, such as the wheels of a vehicle or a generator. BHP is typically measured using a dynamometer (a device that measures torque and rotational speed).
• Frictional Horsepower (FHP): This is the power lost due to friction between the moving parts within the engine (e.g., pistons against cylinder walls, bearings, valve train). It is the difference between Indicated Horsepower and Brake Horsepower: \(\text{FHP} = \text{IHP} - \text{BHP}\).
• Fractional Horsepower: This is not a measure of power generation within a cylinder but rather a classification of engines. An engine is typically referred to as a "fractional horsepower" engine if its brake horsepower output is less than 1 horsepower. These are commonly found in small appliances, tools, and pumps.
• Boiler Horsepower: This is a unit of power used to measure the rate at which a boiler can convert water into steam. It is not related to the power generated by an engine cylinder. One boiler horsepower is equivalent to the evaporation of 34.5 pounds of water per hour from and at 212°F (100°C) into dry saturated steam.

Indicated Horsepower Explained

The indicated horsepower represents the total power developed by the expanding gases on the piston during the power stroke within the engine cylinder. It is a fundamental measure for evaluating the efficiency of the combustion process within the engine.

To determine the indicated horsepower, engineers often use a pressure indicator that records the cylinder pressure over the volume swept by the piston. The area of the resulting P-V diagram (indicator diagram) represents the work done per cycle per cylinder. This work, when multiplied by the number of power strokes per minute and the number of cylinders, gives the total indicated power.

Therefore, the actual power generated in an engine cylinder, before any mechanical losses, is known as indicated horsepower.