Discussion on Thermal Balance of Loader Power System

Water-cooled diesel engines are widely used as power sources in loaders. Diesel engines are economically efficient, with thermal efficiency generally reaching 30-40%. However, about one-third of the heat is still transferred to various components of the diesel engine through various heat transfer methods. If not cooled, various faults will occur, especially for components such as the cylinder head, piston, cylinder liner and valves that are in direct contact with the combustion gas. Therefore, a diesel engine cannot operate without cooling.

If the cooling is insufficient, the following will occur: ① The temperature inside the cylinder is too high. The working medium drawn in expands due to heat, reducing the inflation volume and resulting in a decrease in power. ② When the temperature is too high, each part expands to varying degrees, causing changes in the gaps between them. ③ The viscosity of the engine oil will also decrease and may undergo a qualitative change, deteriorating the lubrication between the friction surfaces and leading to accelerated wear of parts.

However, excessive cooling will result in: ① Low temperature inside the cylinder, poor atomization of fuel, and poor combustion; ② The heat carried away by the cooling system increases, leading to higher heat dissipation losses and greater power consumption by the cooling system. ③ High viscosity of engine oil increases the resistance of mechanical operation. All of the above will lead to a decrease in power and an increase in fuel consumption.

Practical experience has shown that if diesel engines are frequently used under conditions of overheating or overcooling, it will affect their service life, power performance and economy. When a loader is in operation, how to enable the cooling system to dissipate an appropriate amount of heat into the atmosphere and keep the diesel engine operating between 80 and 98 degrees Celsius is the main issue to be addressed in the thermal balance of the loader's power system.

Considering economic factors, the cooling system of a loader is generally not designed to be overly strong. How to enhance the cooling capacity is the main content discussed in this article.

I. Thermal Balance of a Single diesel Engine

1. The heat distribution emitted by the combustion of diesel engine fuel

In the formula:

Q - Total heat generated by complete fuel combustion (KJ)

G - Mass of Fuel burned (kg)

Hu - The calorific value of the fuel. For diesel, the lower value of 42.7 ×10 (KJ/kg) is usually taken.

Ii. Simulation Test of Thermal Balance Bench for Loader Cooling System

Based on the overall configuration of the machine, CFD technology is utilized for simulation calculation and analysis. According to the analysis results, an optimization plan is proposed and improved. Then, the effectiveness of the optimization plan is verified through simulation bench tests and vehicle tests.

1. Simulation calculation analysis

Through CFD analysis and calculation, the main factors affecting the thermal balance of the entire vehicle are wind side factors, which are as follows:

Intake air resistance, radiator assembly resistance, exhaust air resistance and return air inside the hood.

The influence of the return air inside the hood: It refers to the "hot air" that has already passed through the radiator. This return air flow has no effect on heat dissipation and will cause the temperature inside the hood to be higher.

CFD analysis and calculation show that the influence of the wind side is significant.

2 Simulation test bench test

Simulate the wind tunnel frame, with a controlled wind speed of 15km/h and an ambient temperature of 30-50℃. Simulate the overall vehicle structure layout on the test bench according to the loader structure.

The simulation test bench tests show that the optimization measures on the wind side are the main direction to solve the problem. There are several solutions as follows:

The use of hard brazed tube belt water tanks can increase the capacity by 6-8 ℃, but the cost of hard brazed tube belt water tanks is about 40% higher than that of tube sheet water tanks. Therefore, they are generally not recommended for use on medium-sized general-purpose loaders.

The sealing of the gaps on both sides of the water tank can enhance the system's capacity by about 2℃.

The optimization of the hood structure enhances the system's capacity by 4 to 5 degrees Celsius.

Increasing the distance between the fan and the diesel engine can enhance the system's capacity by 3 to 5 degrees Celsius.

The optimization measures on the water side have limited effect on enhancing the system's capacity:

The increase in fan speed ratio is linearly related to the improvement of system capacity. Considering the overall performance of diesel engines, the use of speed-increasing fan mechanisms is generally not recommended. The adoption of speed-increasing fans will lead to a decline in power performance and fuel economy, an increase in noise, a reduction in the service life of the fan bearing housing, and a decrease in the reliability of the diesel engine.

The optimization of oil dispersion for the water pump, thermostat and torque converter respectively has a relatively small impact on the improvement of system capacity.

Iii. Improvement of Thermal Balance in the Cooling System of Loaders

Through CFD analysis and calculation as well as simulation test bench tests, taking into account factors such as economy and reliability, the focus of the thermal balance improvement of the loader's cooling system is placed on the following aspects:

Reduce wind resistance to improve the resistance of the water tank and oil dispersion

Reduce the return air volume

1. Hood structure

2 Fill the gap between the water tank and the hood with fillers

3 A wind deflector is added to the water tank's air outlet

• Increase the air intake volume by 1 Increase the distance between the fan and the diesel engine

2 Optimize the structure of the air deflector

Based on our years of research on the matching of the cooling system of loaders and thermal balance tests, as well as the experience of the whole machine thermal balance tests conducted at the user's location, and in combination with the actual situation of the XCMG LW521F loader, in order to make the cooling system of the LW521F loader adapt to more severe working conditions, we have made improvements to the cooling system of the LW521F loader in the following aspects:

1. The hood has been optimized by using a metal hood and adjusting the window position to improve the air intake conditions.

2 The return air inside the hood is optimized to eliminate the large gap between the hood and the side of the water tank, reduce the return air volume and improve the heat dissipation efficiency.

3 Increase the distance between the fan and the diesel engine block to enhance the air intake capacity. At the same time, increase the distance from the radiator core to improve the uniformity of the air intake to the radiator.

The improved LW521F passed the road subgrade removal work test. When the ambient temperature was 32 degrees, the water temperature was maintained at 90 degrees for two hours during the test run, and the torque converter oil temperature was 100 degrees. The improvement was successful.