The question asks for the instrument used to measure the revolution carried by an IC engine per unit time. This measurement tells us how fast the engine's crankshaft is rotating. This speed is commonly expressed in Revolutions Per Minute (RPM).

Understanding Engine Speed Measurement

An Internal Combustion (IC) engine generates power by the rotation of its crankshaft. The rate at which this crankshaft rotates is a key indicator of the engine's performance and operating condition. Measuring this rotational speed per unit time is essential for various purposes, including monitoring engine health, optimizing fuel efficiency, and ensuring safe operation.

Instrument for Measuring Engine Revolutions

Let's look at the given options and see which instrument is designed to measure the rotational speed of an engine shaft:

• Tachometer: A tachometer is specifically designed to measure the rotation speed of a shaft or disk. It displays the rotational speed, typically in revolutions per minute (RPM). This is exactly what is needed to measure the revolutions carried by an IC engine per unit time.
• Velocimeter: A velocimeter measures linear velocity, which is the rate of change of position with respect to time in a specific direction. It is not used for measuring rotational speed.
• Accelerometer: An accelerometer measures acceleration, which is the rate of change of velocity. It measures how quickly speed changes, not the speed itself or rotational speed.
• Speedometer: A speedometer measures the instantaneous speed of a vehicle. While vehicle speed is related to engine speed (especially in vehicles with fixed gear ratios), a speedometer measures the speed of the wheels or the vehicle's movement over the ground, not the engine's rotational speed directly.

Based on the functions of these instruments, the tachometer is the correct instrument for measuring the revolution carried by an IC engine per unit time (RPM).

Why Tachometer is the Right Choice

A tachometer uses various methods, such as counting pulses from an ignition system or using magnetic sensors near a rotating part, to determine how many revolutions the engine completes in a minute. This direct measurement of rotational speed makes it the appropriate instrument for monitoring the engine's revolutions per unit time.