The oil pressure gauge is designed to monitor the oil pressure of the engine to evaluate the operating status of the engine lubrication system. It consists of two parts: the oil pressure sensor and the oil pressure indicator. The sensor is installed in the main oil circuit of the engine, while the indicator is located on the dashboard. Common types of oil pressure gauges on the market include bimetallic, electromagnetic and dynamic magnetic types, among which bimetallic oil pressure gauges are the most widely used. Next, we will take a closer look at the working principle of the oil pressure gauge.
After the power switch is turned on, the current will flow in two ways. One way flows through the positive electrode of the battery, the power switch, terminal 14, and then flows to the electric heating coil terminal 9 and contact piece 6 of the bimetal 11, while passing through the electric heating coil of the bimetal 4 in the sensor; the other way passes through the correction resistor 8. In addition, the contact spring 3 of the bimetal 4 is connected to the negative electrode of the battery to form a complete circuit loop. When the current passes through the electric heating coils on the bimetal 4 and 11, these bimetallic strips will deform due to heat. The bimetallic strip is composed of two metals with different expansion coefficients. When heated, the side with a larger expansion coefficient will bend toward the side with a smaller expansion coefficient.
When the circuit is energized, the coil wrapped around the bimetallic strip will generate heat, causing the bimetallic strip of the sensor to bend due to heat, thereby disconnecting the contacts and cutting off the circuit. At the same time, the bimetallic strip of the indicator will also bend due to heat, causing the pointer to deflect, thereby displaying the current oil pressure. When the oil pressure is extremely low, the diaphragm 2 will hardly deform, and the pressure on the contacts will be very small.
As the temperature generated by the current gradually rises, the bimetallic strip 4 will bend due to heat, causing the contacts to separate and cut off the circuit, and the heating process will stop immediately. After a period of time, the bimetallic strip gradually cools and straightens, the contacts close again, and the circuit is reconnected. Therefore, in this process, the closing time of the contacts is relatively short, while the opening time is longer. Since the average current value of the electric heating coil of the indicator is small, the bimetallic strip 11 in the indicator has a smaller curvature due to the lower temperature, resulting in a smaller deflection angle of the pointer 12, which indicates that the current oil pressure is low.
When the oil pressure rises, the diaphragm 2 will arch upward, increasing the pressure at the contact point. In this case, the bimetal 4 needs to reach a higher temperature (that is, the electric heating coil on it needs to pass a large current for a long time) to bend and separate the contacts. However, once the contacts are separated, they will close quickly after a little cooling. Therefore, at this stage, the opening time of the contacts is short and the closing time is long. As the average current value of the electric heating coil passing through the indicator increases, the deflection angle of the pointer 12 will also increase, indicating that the current oil pressure is high. In order to prevent the external temperature from affecting the oil pressure indication value, the bimetal 4 is designed in an "=" shape. The side with the electric heating coil is called the working arm, and the other side is called the compensation arm.
When the external ambient temperature changes, the additional deformation of the working arm caused by the temperature change will be offset by the corresponding deformation of the compensation arm, thereby ensuring that the indicator reading remains stable. When installing the sensor, it is necessary to ensure that the arrow on the sensor housing is facing upward and its position must not deviate by 30 degrees. This design ensures that the working arm is above the compensating arm so that when the hot air generated by the working arm rises, it will not affect the compensating arm, thus avoiding errors in the readings.
