In this column, I will discuss the measurement of engine rotation speed in automobiles, motorcycles, and other vehicles.
What is engine rotation speed?
This refers to the number of rotations an engine makes per minute (or every minute), and its unit is r/min.
*This is sometimes written as rpm.
This is an abbreviation for "Revolutions Per Minute," and it represents the number of rotations per minute of a rotating body, including an engine. For example, if it rotates 3000 times per minute, it is written as "3000 r/min."
Why measure engine speed?
Engine rotational speed is one of the important pieces of information for understanding the engine's performance and condition.
Changes in rotational speed, whether increasing or decreasing, are related to fuel efficiency. Furthermore, by considering this in conjunction with other measurement data such as torque and power output, it is possible to evaluate the overall performance and condition of the engine.
Regarding the engine speed detection method
Our detection methods are broadly classified into two types: pulse measurement methods and FFT calculation methods.
Pulse measurement method (pulse count, period calculation)
There are three types of this method:
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A method that detects pulse signals from the ignition cable.
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A method that detects pulse signals from engine vibrations.
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A method that detects pulse signals from the pulsation of the fuel injection pipe.
Here's a brief introduction to each detection method.
A method that detects pulse signals from the ignition cable.
The device detects current by clamping onto the low-voltage primary wire of the ignition coil, the high-voltage secondary wire, and the current wire of the electronic distributor. Ignition sparks are generated in sync with the changes in current flowing through each wire. These current changes generate changes in the magnetic field. This change in the magnetic field is detected by a detection coil (core winding) and extracted as a voltage signal.
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Figure 1: Example of installation of a rotational speed sensor using ignition pulse detection.
A method that detects pulse signals from engine vibrations.
By installing detectors on the engine's cylinder head and other parts, the system detects vibrations caused by combustion explosions and imbalances in various components.
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Figure 2. Example of installation of a rotational speed sensor using an engine vibration detection method.
A method that detects pulse signals from the pulsation of the fuel injection pipe.
The fuel injection pipe is clamped, a detector is attached, and the pulsation of the injection pipe is detected.
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Figure 3: Example of installation of a rotational speed sensor using a fuel injection pipe pulsation detection method.
FFT calculation method (detection of rotational frequency)
This method does not measure pulses caused by the rotational speed of a rotating object, but rather employs the FFT (Fast Fourier Transform) function in the calculation unit of the rotation indicator to extract frequency components corresponding to rotational speed even from complex waveform signals from microphones, sound level meters, and vibration sensors, and calculates and displays the rotational speed. This method is effective when it is not possible to detect relatively clean pulses.
Measurement algorithms include the maximum peak frequency method and the frequency interval method.
Maximum Peak Frequency Method
This method uses the frequency component of the maximum peak in the power spectrum for calculation.
This is the usual method.
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Figure 4: Image of the maximum peak frequency method
Frequency interval method
This method involves sequentially determining the frequency intervals of each order component of rotation, and identifying the most frequently occurring frequency interval (Figure 5, blue arrow) as the first-order component of the rotational speed, thereby determining the rotational speed.
This is effective when the primary peak is unstable.
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Figure 5: Image of the frequency interval method
Detection methods by engine type
The detection method differs depending on the type of engine, as follows:
Gasoline engine
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Engine rotational speed is detected from the ignition pulse (pulse measurement method, FFT calculation method).
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Engine rotational speed is detected from engine vibration and sound (pulse measurement method, FFT calculation method).
Diesel engine
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Engine rotational speed is detected from engine vibration and sound (pulse measurement method, FFT calculation method).
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Engine speed is detected from the rotation of the alternator (FFT calculation method).
*Alternator: A generator that produces alternating current.
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Engine rotational speed is detected from the pulsation of the fuel injection pipe (pulse measurement method).
Motor rotation speed detection for EVs/HEVs, etc.
Although it deviates slightly from the main topic of this column, I will also briefly introduce the detection of rotational speed of motors installed in HEVs and EVs.
This method detects the rotational speed by detecting the leakage magnetic flux from the motor, and is also called the electromagnetic induction method. An electromagnetic induction detector is mounted perpendicular to the motor's rotation axis to detect the magnetic flux leaking out to the outside of the motor (called leakage magnetic flux), and the motor's rotational speed is calculated.
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Figure 6: Mounting image of the rotational speed sensor for EV/HEV motors.
This time, we briefly explained how to measure engine rotational speed as part of rotational speed measurement. We would appreciate it if you would refer to the following URL in conjunction with this column.
[For your reference]
Engine Tachometer and Related Measuring Instruments Overview
(Excerpt from the email newsletter issued on March 15, 2023)
