A pressurized four-ball friction and wear testing machine and testing method for evaluating lubricating performance of high-saturation vapor pressure fuel

Abstract

The application relates to a pressurized four-ball friction and wear testing machine and a testing method for evaluating the lubricating performance of high-saturation-vapor-pressure fuel, which mainly comprises a high-pressure fuel sample box, a magnetic coupling transmission system, a loading system and a data acquisition system. The high-pressure fuel sample box is a closed pressure-resistant container, which contains liquefied fuel, and the upper ball and the lower ball are immersed in the fuel; the fuel is pressurized to the saturation vapor pressure at the test temperature or above through a fuel pressurizing device, so that the fuel remains in liquid state during the whole test. The magnetic coupling is composed of a half-coupling driving end, a non-magnetic isolation sleeve and a half-coupling driven end. The motor drives the driving end to rotate, which drives the driven end and the chuck to drive the upper ball to rotate through magnetic coupling, and the lower ball is fixed in the lower ball fixing clamp; the non-magnetic isolation sleeve realizes contactless power transmission, and eliminates the leakage risk of traditional dynamic seals. After the test, the lubricating performance of the sample is evaluated by measuring the lower ball wear scar diameter. The application breaks through the test limitation of the existing four-ball testing machine, and provides a reliable experimental method for evaluating the lubricating performance of the fuel.

Description

Technical Field

[0001] This invention belongs to the field of tribological performance testing technology for fuels and lubricating media. Specifically, it relates to a pressurized four-ball friction and wear tester and test method for evaluating the lubrication performance of high-saturated vapor pressure fuels, lubricating oil mixtures containing high-saturated vapor pressure fuels, and corresponding additive formulations. It is applicable to the evaluation of lubrication performance of high-saturated vapor pressure fuels and the screening of lubricating oil formulations. Background Technology

[0002] With the increasing demand for low-carbon fuels in the international shipping industry, high-saturated vapor pressure fuels such as dimethyl ether (DME) are gradually becoming potential alternative fuels for marine engines due to their clean combustion characteristics, high oxygen content, and renewable production methods. However, these fuels have extremely high saturated vapor pressures and a strong tendency to vaporize, and their dynamic viscosity is much lower than that of traditional diesel fuel, resulting in extremely poor lubrication performance in fuel injection systems. This easily leads to premature wear and leakage of precision components such as fuel injectors, severely restricting the widespread application of DME in actual engines.

[0003] During engine operation, high-saturated vapor pressure fuel may also form a fuel dilution mixture with lubricating oil. The viscosity, volatility, effective additive concentration, and tribochemical behavior of such mixtures do not change. Therefore, it is necessary to evaluate their lubrication performance while maintaining the actual fuel content and liquid phase state as closely as possible.

[0004] Currently, the four-ball friction and wear testing machine is a widely used international equipment for evaluating the extreme pressure and anti-wear properties of lubricants. This equipment assesses the load-bearing capacity of lubricants under point contact pressure conditions through sliding friction, including maximum non-seizure load, sintering load, and comprehensive wear value. It can also be used for long-term anti-wear testing of lubricants. However, existing four-ball machines and their testing methods are designed specifically for incompressible liquid fuels such as diesel, and are typically conducted under open atmospheric pressure conditions, making them difficult to directly apply to high-saturation vapor pressure fuels such as dimethyl ether. The main bottleneck is that high-saturation vapor pressure fuels are gaseous or easily vaporized at normal pressure and temperature. During the test, the fuel continuously evaporates, causing the contact point between the upper and lower balls to lose its liquid immersion state, fundamentally altering the friction conditions and rendering the test results inapplicable.

[0005] Furthermore, attempting to simply modify an existing four-ball tester into a closed, pressurized form presents a technical challenge in achieving a rotary shaft dynamic seal. The rotary shaft dynamic seal under high-pressure sealing conditions is complex, carries a high risk of leakage after wear, and its long-term operational reliability is difficult to guarantee. Therefore, there is an urgent need to develop a four-ball friction and wear testing machine and method capable of achieving high-precision testing of the lubrication performance of high-saturation vapor pressure fuel-lubricating oil mixtures under pressurized and sealed conditions. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of existing four-ball machines in testing the lubrication performance of high-saturation vapor pressure fuel-lubricating oil mixtures. It provides a pressurized four-ball friction and wear tester and test method for evaluating the lubrication performance of high-saturation vapor pressure fuels. By pressurizing and liquefying the fuel and using a magnetic coupling to achieve leak-free power transmission, it enables accurate, stable and safe measurement of the lubrication performance of high-saturation vapor pressure fuels.

[0007] The present invention achieves the above objectives through the following technical solutions: This invention provides a pressurized four-ball friction and wear testing machine for evaluating the lubrication performance of high-saturation vapor pressure fuels. The machine includes a fuel tank, a shut-off valve, a fuel pressurization device, a check valve, a high-pressure fuel sample box, a fuel pressure sensor, a motor, a half-coupling driving end, a non-magnetic isolation sleeve, a half-coupling driven end, a safety valve, a clamp, an upper ball, a lower ball, a temperature sensor, a lower ball fixing fixture, and a loading mechanism. The high-pressure fuel sample box is a sealed pressure-resistant container used to contain liquefied fuel and immerse the upper and lower balls. The lower ball fixing fixture is arranged inside the high-pressure fuel sample box, and its bottom is fixed to the bottom of the high-pressure fuel sample box. The upper end of the loading mechanism is connected to the lower end of the half-coupling driven end, and the lower end of the loading mechanism is connected to the clamp. The lower ball is fixedly installed on the top of the lower ball fixing fixture, and the upper ball... Mounted at the lower end of the chuck, the loading mechanism applies a normal load to the upper ball, pressing it against the upper surface of the lower ball. The lower end of the high-pressure fuel sample box is connected to the fuel tank via a pipeline, passing through a one-way valve, a fuel pressurization device, and a shut-off valve in sequence. The motor is located outside the high-pressure fuel sample box, with its output end connected to the driving end of the half-coupling. The non-magnetic isolation sleeve is sealed on the top wall of the high-pressure fuel sample box, separating the driving end and the driven end of the half-coupling. The driven end of the half-coupling is located inside the high-pressure fuel sample box and is connected to the upper ball via the chuck. The driving end and the driven end of the half-coupling transmit torque through magnetic coupling. The fuel pressure sensor is used to collect the fuel pressure signal inside the high-pressure fuel sample box, and the temperature sensor is used to collect the fuel temperature signal. The safety valve is located at the top of the high-pressure fuel sample box for overpressure protection.

[0008] Furthermore, in this invention, the upper and lower balls have a diameter of 12.7 mm and are made of bearing steel.

[0009] The present invention also provides a method for testing the lubrication performance of high saturated vapor pressure fuels based on the above-mentioned testing machine, comprising the following steps: Step 1, Preparation Stage: Install the upper ball into the clamp, fix the lower ball into the lower ball fixing fixture, and check the status of all seals and pipeline connections; open the shut-off valve to discharge the fuel from the fuel tank, start the fuel pressurization device, and pressurize the liquid fuel to a target value not lower than the saturated vapor pressure at its test temperature, so that the liquid fuel enters the high-pressure fuel sample box through the one-way valve; monitor the fuel pressure sensor reading, and close the shut-off valve after the pressure stabilizes at the target value and the high-pressure fuel sample box is full of liquid fuel; Step 2, Loading stage: Apply a normal load to the upper ball through the loading mechanism to press the upper ball tightly against the upper surface of the lower ball; Step 3, Testing Phase: Start the motor, the motor drives the active end of the half coupling to rotate, and through magnetic coupling drives the driven end of the half coupling and the chuck to rotate, so that the upper ball rotates relative to the lower ball, and continues to test for a preset time. Step 4, Final Stage: Turn off the motor, slowly open the exhaust valve of the high-pressure fuel sample box to release pressure to atmospheric pressure, remove the lower ball, measure its wear scar characteristic dimensions, and calculate the comprehensive evaluation index of high-saturation vapor pressure fuel pressurized lubrication according to the following formula. It is used as an evaluation index for the lubrication performance of high saturated vapor pressure fuels:

[0010] in, For comprehensive evaluation of wear scar equivalent, the unit is mm; The diameter of the wear scar is parallel to the sliding direction, in mm; The diameter of the wear scar perpendicular to the sliding direction, in mm; This is the anisotropic distortion penalty coefficient for the wear scar; Thermodynamic overpressure and micro-cavitation inhibition factor are calculated based on test pressure and fuel saturated vapor pressure; This is the film thickness correction factor for low-viscosity media fluids, calculated based on the fuel dynamic viscosity and normal load under test conditions.

[0011] Furthermore, in this invention, thermodynamic overpressure and micro-cavitation inhibition factor The calculation formula is:

[0012] in, The actual test pressure inside the high-pressure fuel sample box (5) is expressed in MPa. The pressure is the saturated vapor pressure of the fuel under test at the test temperature T, in MPa. This represents the overpressure lubrication response coefficient.

[0013] Furthermore, in this invention, the low-viscosity medium fluid film thickness correction factor The calculation formula is:

[0014] in, For the fuel to be tested at the test temperature T and pressure P The dynamic viscosity at the specified unit ; The reference dynamic viscosity constant is expressed in units of... ; The normal load applied to the loading mechanism (17), in N; The standard reference load for the four-ball method is 392 N; m and n This is the hydrodynamic correction index.

[0015] Furthermore, in this invention, the test conditions are adapted to the standard test conditions of the four-ball method, and the test temperature is set according to the actual working conditions of the fuel to be tested or with reference to the standard lubricant test method; the fuel pressure corresponding to the test temperature is adjusted by the fuel pressurization device to ensure that the fuel remains liquid throughout the test.

[0016] In the implementation of this invention, the shut-off valve is opened to discharge the fuel from the fuel tank. After being pressurized by the fuel pressurizing device, the fuel is injected into the high-pressure fuel sample box through a one-way valve until the high-pressure fuel sample box is filled with liquid fuel and completely submerges the upper and lower balls. The fuel pressurizing device pressurizes the high-saturated vapor pressure fuel to a pressure not lower than its saturated vapor pressure at the test temperature to ensure that the fuel remains liquid throughout the test. A normal load is applied to the upper ball through the loading mechanism, pressing the upper ball tightly against the upper surface of the lower ball. The motor is started, and the motor drives the driving end of the half-coupling to rotate. Through magnetic coupling, the driven end of the half-coupling and the chuck rotate, causing the upper ball to rotate relative to the lower ball 14. After the test, the lower ball is removed, and its wear scar diameter is measured under an optical microscope. The high-saturated vapor pressure fuel pressurized lubrication comprehensive evaluation index ( () is used as an evaluation index for the lubrication performance of fuels with high saturated vapor pressure.

[0017] Compared with the prior art, the present invention has the following beneficial effects: First, the present invention uses a high-pressure fuel sample box to pressurize and liquefy high-saturation vapor pressure fuel, so that the fuel remains in a liquid state throughout the test, ensuring that the contact point between the upper and lower balls is always immersed in the fuel, thus truly reflecting the lubrication performance of the fuel under actual high-pressure injection system conditions.

[0018] Secondly, the present invention uses a magnetic coupling to achieve contactless power transmission. The driving end and driven end of the half coupling are driven by magnetic coupling. The non-magnetic isolation sleeve completely isolates the inside of the high-pressure fuel sample box from the outside, completely eliminating the leakage risk of traditional rotary shaft dynamic seals under high pressure, and significantly improving the sealing reliability.

[0019] Third, this invention is an improvement on the existing four-ball tester, retaining the mature testing standards and evaluation system of the four-ball method. The test results are highly comparable to those of traditional lubricant tests and can be quickly adapted to the testing needs of different high-saturation vapor pressure fuels and additive formulations.

[0020] In summary, this invention effectively solves the limitation of existing four-ball testing machines in being unable to test the lubrication performance of high-saturation vapor pressure fuels and their mixtures with lubricating oils, providing a reliable experimental means for evaluating the lubrication performance of such fuels. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a pressure-applied four-ball friction and wear test device; Among them, 1. fuel tank, 2. shut-off valve, 3. fuel pressurization device, 4. check valve, 5. high-pressure fuel sample box, 6. fuel pressure sensor, 7. motor, 8. half-coupling driving end, 9. non-magnetic isolation sleeve, 10. half-coupling driven end, 11. safety valve, 12. chuck, 13. upper ball, 14. lower ball, 15. temperature sensor, 16. lower ball fixing clamp, 17. loading mechanism. Detailed Implementation

[0022] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are based on the technical solutions of the present invention and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0023] Example like Figure 1 As shown, a pressurized four-ball friction and wear testing machine for evaluating the lubrication performance of high saturated vapor pressure fuel includes: a fuel tank 1, a shut-off valve 2, a fuel pressurization device 3, a one-way valve 4, a high-pressure fuel sample box 5, a fuel pressure sensor 6, a motor 7, a half-coupling driving end 8, a non-magnetic isolation sleeve 9, a half-coupling driven end 10, a safety valve 11, a chuck 12, an upper ball 13, a lower ball 14, a temperature sensor 15, a lower ball fixing clamp 16, and a loading mechanism 17.

[0024] The high-pressure fuel sample box 5 is a sealed pressure-resistant container with a design pressure of not less than 5 MPa, used to contain fuel and immerse the upper ball 13 and the lower ball 14. The lower ball 14 is fixedly installed in the lower ball fixing clamp 16, and the upper ball 13 is installed at the lower end of the clamp 12. The clamp 12 is connected to the loading mechanism 17, which applies a normal load to the upper ball 13 to press it tightly against the upper surface of the lower ball 14. The upper ball 13 and the lower ball 14 have a diameter of 12.7 mm and are made of bearing steel, meeting the requirements of the four-ball method standard. The loading mechanism 17 can adopt a weight lever loading method, hydraulic loading, or pneumatic loading method; in this embodiment, a weight lever loading mechanism is used.

[0025] The high-pressure fuel sample box 5 is connected to the fuel tank 1 via a pipeline that sequentially passes through a one-way valve 4, a fuel pressurizing device 3, and a shut-off valve 2. During testing, the shut-off valve 2 is opened to discharge the fuel from the fuel tank 1. The discharged liquid fuel is pressurized by the fuel pressurizing device 3 to a target pressure value not lower than the saturated vapor pressure at its test temperature (for example, for dimethyl ether, pressurization to above 0.6 MPa when tested at 25 ℃; pressurization to above 1.5 MPa when tested at 50 ℃), and then enters the high-pressure fuel sample box 5 through the one-way valve 4. When the fuel pressure sensor 6 displays that the pressure has reached the preset value and the high-pressure fuel sample box 5 is full of liquid fuel, the shut-off valve 2 is closed.

[0026] The motor 7 is located outside the high-pressure fuel sample box 5, and its output end is connected to the driving end 8 of the half-coupling via a key. A non-magnetic isolation sleeve 9 is sealed to the wall of the high-pressure fuel sample box 5, separating the driving end 8 of the half-coupling from the driven end 10. The driven end 10 of the half-coupling is located inside the high-pressure fuel sample box 5 and is connected to the upper ball 13 via a clamp 12. Both the driving end 8 and the driven end 10 of the half-coupling are embedded with permanent magnets, transmitting torque through magnetic coupling. After the motor 7 starts, the driving end 8 of the half-coupling rotates, and the magnetic force passes through the non-magnetic isolation sleeve 9, driving the driven end 10 of the half-coupling to rotate synchronously, thereby causing the upper ball 13 to rotate.

[0027] Safety valve 11 is located at the top of high-pressure fuel sample box 5, with a set opening pressure of 4 MPa. When the pressure inside the box exceeds the set value, the safety valve automatically releases pressure to ensure system safety.

[0028] The testing method in this embodiment is as follows: Preparation Phase: Install the upper ball 13 into the clamp 12, and fix the lower ball 14 into the lower ball fixing clamp 16. Check the status of all seals and pipeline connections. Open the shut-off valve 2 to discharge the fuel from the fuel tank 1. Start the fuel pressurization device 3 to pressurize the liquid fuel to a target value not lower than the saturated vapor pressure at its test temperature, allowing the liquid fuel to enter the high-pressure fuel sample box 5 through the one-way valve 4. Monitor the reading of the fuel pressure sensor 6. After the pressure stabilizes at the target value and the high-pressure fuel sample box 5 is full of liquid fuel, close the shut-off valve 2.

[0029] Loading phase: A normal load is applied to the upper ball 13 via the loading mechanism 17. In this embodiment, a normal load of 392 N is applied, referring to the GB / T 12583 standard.

[0030] Testing phase: Start motor 7 to drive upper ball 13 to rotate relative to fixed lower ball 14 at 1200 r / min, and the test lasts for 60 min. Temperature sensor 15 monitors fuel temperature in real time to ensure that the test temperature is stable at the set value (25 ℃ in this embodiment).

[0031] Final Stage: After 60 minutes, turn off motor 7. Slowly open the exhaust valve of high-pressure fuel sample box 5 to release pressure to atmospheric pressure. Remove the lower ball 14 and measure its wear scar diameter under an optical microscope. Calculate the comprehensive evaluation index of high-saturation vapor pressure fuel pressurized lubrication according to the following formula. It is used as an indicator to evaluate the lubrication performance of fuel.

[0032]

[0033] in, The diameter of the wear scar is parallel to the sliding direction, in mm; The diameter of the wear scar perpendicular to the sliding direction, in mm; This is the penalty coefficient for anisotropic distortion of the wear scar.

[0034] Among them, thermodynamic overpressure and micro-cavitation inhibition factor The calculation formula is:

[0035] in, The actual test pressure inside the high-pressure fuel sample box 5 is expressed in MPa. The pressure is the saturated vapor pressure of the fuel under test at the test temperature T, in MPa. This represents the overpressure lubrication response coefficient.

[0036] Low viscosity medium fluid film thickness correction factor The calculation formula is:

[0037] in, For the fuel to be tested at the test temperature T and pressure P The dynamic viscosity at the specified unit ; The reference dynamic viscosity constant is expressed in units of... ; Normal load applied to loading mechanism 17, in N; The standard reference load for the four-ball method is 392 N; m and n This is the hydrodynamic correction index.

[0038] The test parameters in this embodiment (load 392 N, rotation speed 1200 r / min, steel ball diameter 12.7 mm, duration 60 min) are consistent with the four-ball method standards such as GB / T 12583 and ASTM D2783, ensuring the comparability of test results with traditional lubricant test results. Considering the characteristics of high saturated vapor pressure fuels, the fuel pressurization device 3 pressurizes it to a pressure not lower than the saturated vapor pressure at the test temperature, ensuring that the fuel remains liquid throughout the test. This ensures that the contact point between the upper ball 13 and the lower ball 14 is always immersed in liquid fuel, thus truly reflecting the lubricating performance of the fuel under high-pressure injection system conditions.

[0039] The specific operation of the present invention has been described above. It should be understood that the present invention is not limited to the specific operation described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the substantive content of the present invention.

Claims

1. A pressurized four-ball friction and wear testing machine for evaluating the lubrication performance of high-saturated vapor pressure fuels, comprising a fuel tank (1), a shut-off valve (2), a fuel pressurization device (3), and a one-way valve (4), characterized in that, It also includes a high-pressure fuel sample box (5), a fuel pressure sensor (6), a motor (7), a half-coupling driving end (8), a non-magnetic isolation sleeve (9), a half-coupling driven end (10), a safety valve (11), a chuck (12), an upper ball (13), a lower ball (14), a temperature sensor (15), a lower ball fixing clamp (16), and a loading mechanism (17). The high-pressure fuel sample box (5) is a sealed pressure-resistant container, which is used to contain liquefied fuel and immerse the upper ball (13) and the lower ball (14); the lower ball fixing clamp (16) is arranged inside the high-pressure fuel sample box (5), and the bottom of the lower ball fixing clamp (16) is fixed to the bottom of the high-pressure fuel sample box (5); the upper end of the loading mechanism (17) is connected to the lower end of the driven end (10) of the half coupling, and the lower end of the loading mechanism (17) is connected to the chuck (12); the lower ball (14) is fixedly installed on the top of the lower ball fixing clamp (16), the upper ball (13) is installed on the lower end of the chuck (12), and the loading mechanism (17) applies a normal load to the upper ball (13) to press it against the upper surface of the lower ball (14); the lower end of the high-pressure fuel sample box (5) is connected to the fuel tank (1) through a pipeline via a one-way valve (4), a fuel pressurization device (3) and a shut-off valve (2); The motor (7) is located outside the high-pressure fuel sample box (5), and its output end is connected to the active end (8) of the half-coupling; the non-magnetic isolation sleeve (9) is sealed on the top wall of the high-pressure fuel sample box (5) to separate the active end (8) of the half-coupling from the driven end (10) of the half-coupling; the driven end (10) of the half-coupling is located inside the high-pressure fuel sample box (5) and is connected to the upper ball (13) through the clamp (12); the active end (8) of the half-coupling and the driven end (10) of the half-coupling transmit torque through magnetic coupling; The fuel pressure sensor (6) is used to collect the fuel pressure signal in the high-pressure fuel sample box (5), and the temperature sensor (15) is used to collect the fuel temperature signal; the safety valve (11) is located on the top of the high-pressure fuel sample box (5) for overpressure protection.

2. The testing machine according to claim 1, characterized in that, The upper ball (13) and the lower ball (14) have a diameter of 12.7 mm and are made of bearing steel.

3. A method for testing the lubrication performance of high-saturated vapor pressure fuels using a testing machine as described in any one of claims 1 or 2, characterized in that, Includes the following steps: Step 1, Preparation stage: Install the upper ball (13) into the clamp (12), fix the lower ball (14) into the lower ball fixing clamp (16), and check the status of all seals and pipeline connections; open the shut-off valve (2) to discharge the fuel in the fuel tank (1), start the fuel pressurization device (3) to pressurize the liquid fuel to a target value not lower than the saturated vapor pressure at its test temperature, so that the liquid fuel enters the high-pressure fuel sample box (5) through the one-way valve (4); monitor the reading of the fuel pressure sensor (6), and close the shut-off valve (2) after the pressure stabilizes at the target value and the high-pressure fuel sample box (5) is filled with liquid fuel. Step 2, loading stage: Apply a normal load to the upper ball (13) through the loading mechanism (17) so that the upper ball (13) is pressed against the upper surface of the lower ball (14); Step 3, Test phase: Start the motor (7), the motor (7) drives the active end (8) of the half coupling to rotate, and through magnetic coupling drives the driven end (10) and the chuck (12) of the half coupling to rotate, so that the upper ball (13) rotates relative to the lower ball (14), and continues to test for a preset time. Step 4, End Stage: Turn off the motor (7), slowly open the exhaust valve of the high-pressure fuel sample box (5) to release pressure to atmospheric pressure, remove the lower ball (14), measure its wear scar characteristic size, and calculate the comprehensive evaluation index of high saturated vapor pressure fuel pressurized lubrication according to the following formula. It is used as an evaluation index for the lubrication performance of high saturated vapor pressure fuels: ； in, For comprehensive evaluation of wear scar equivalent, the unit is mm; The diameter of the wear scar is parallel to the sliding direction, in mm; The diameter of the wear scar perpendicular to the sliding direction, in mm; This is the anisotropic distortion penalty coefficient for the wear scar; Thermodynamic overpressure and micro-cavitation inhibition factor are calculated based on test pressure and fuel saturated vapor pressure; This is the film thickness correction factor for low-viscosity media fluids, calculated based on the fuel dynamic viscosity and normal load under test conditions.

4. The test method according to claim 3, characterized in that, Thermodynamic overpressure and micro-cavitation inhibition factor The calculation formula is: ； in, The actual test pressure inside the high-pressure fuel sample box (5) is expressed in MPa. The pressure is the saturated vapor pressure of the fuel under test at the test temperature T, in MPa. This represents the overpressure lubrication response coefficient.

5. The test method according to claim 3, characterized in that, The low viscosity medium fluid film thickness correction factor The calculation formula is: ； in, For the fuel to be tested at the test temperature T and pressure P The dynamic viscosity at the specified value, in units ; The reference dynamic viscosity constant is expressed in units of... ; The normal load applied to the loading mechanism (17), in N; The standard reference load for the four-ball method is 392 N; m and n This is the hydrodynamic correction index.

6. The test method according to claim 3, characterized in that, The test conditions are adapted to the standard test conditions of the four-ball method. The test temperature is set according to the actual working conditions of the fuel to be tested or with reference to the standard lubricant test method. The fuel pressure corresponding to the test temperature is adjusted by the fuel pressurization device (3) to ensure that the fuel remains liquid throughout the test.

Images