Short Answer:

An indicated diagram is a graph that shows the pressure changes inside the engine cylinder during one complete cycle. It is obtained by using an instrument called an indicator, which records the relationship between pressure and piston movement. This diagram helps in understanding the performance of an internal combustion engine by showing how pressure varies with volume during the engine operation.

The indicated diagram is very useful for engineers to study the engine’s working process and to calculate important parameters like indicated power, mean effective pressure, and efficiency. It provides valuable information about combustion quality, timing, and engine health.

Detailed Explanation :

Indicated Diagram

An indicated diagram is one of the most important tools in internal combustion engine analysis. It is a graphical representation of the pressure inside the engine cylinder plotted against the piston displacement or cylinder volume during a complete operating cycle. This diagram is taken by using a mechanical or electronic instrument known as an engine indicator. The purpose of this diagram is to study how pressure changes in the combustion chamber as the piston moves from the top dead center (TDC) to the bottom dead center (BDC) and back.

The indicated diagram shows the actual working of the gases inside the cylinder, including compression, combustion, expansion, and exhaust. It helps in understanding the thermodynamic processes that occur during engine operation. By studying the shape and area of this diagram, engineers can determine whether the engine is operating efficiently or if there are faults such as improper valve timing, knocking, or leakage.

Parts and Shape of Indicated Diagram

The indicated diagram typically consists of a closed loop curve drawn on a coordinate system where the horizontal axis represents piston displacement (or volume) and the vertical axis represents pressure.

• The compression curve shows how pressure increases when the air-fuel mixture is compressed.
• The expansion curve represents how pressure drops after combustion as the gases expand and do work on the piston.
• The exhaust and suction lines are usually near atmospheric pressure.

The area enclosed by this diagram is proportional to the work done by the gases on the piston during one cycle. Hence, this area is used to calculate indicated power of the engine.

Types of Indicated Diagrams

There are mainly two types of indicated diagrams:

1. Theoretical Indicated Diagram – This shows the ideal pressure-volume relationship assuming perfect combustion, no heat loss, and no friction. It is used for comparison and analysis.
2. Actual Indicated Diagram – This is obtained from a real engine using an indicator instrument. It considers all real conditions such as heat loss, valve delay, and combustion irregularities. The actual diagram is always smaller and irregular compared to the theoretical one because of real-world losses.

Uses of Indicated Diagram

The indicated diagram provides several important pieces of information about an engine:

• It helps to determine Indicated Power (IP), which is the total power developed inside the cylinder.
• It gives the Indicated Mean Effective Pressure (IMEP), which helps to compare engine performance under different conditions.
• It helps in detecting faults like early or late ignition, improper fuel-air ratio, leakage through valves or piston rings, and knocking.
• It is used to analyze combustion characteristics and efficiency.
• It helps in tuning the engine for better performance by studying the shape and area of the diagram.

Instrument Used for Indicated Diagram

The indicator is the device used to draw the indicated diagram. In older engines, a mechanical indicator consisting of a small piston, spring, and recording drum was used. The piston senses the pressure inside the cylinder, and the drum moves in proportion to the piston’s stroke. A pen attached to the system draws the pressure-volume graph on a paper. In modern engines, electronic indicators or pressure transducers connected to computers are used to record and analyze the diagram digitally with high accuracy.

Importance of Indicated Diagram

The indicated diagram plays a vital role in engine testing and development. It allows engineers to study engine performance without dismantling the engine. Any deviation in the normal shape of the diagram immediately signals an issue in engine operation, such as valve malfunction, wrong ignition timing, or poor compression. It also helps in comparing different engines and optimizing their performance.

Conclusion

In conclusion, an indicated diagram is a pressure-volume graph that gives a clear picture of the internal working of an engine cylinder during one complete cycle. It is obtained using an indicator and is very useful for calculating indicated power, mean effective pressure, and efficiency. By studying this diagram, engineers can easily detect engine faults and improve performance. Thus, the indicated diagram is an essential tool for analyzing and improving the performance of internal combustion engines.