Heat pump with R744 (CO₂):
The refrigerant R744 (CO₂) is increasingly used in air conditioning and heat pump systems, such as in electric vehicles. CO₂ has the property of absorbing a lot of heat during evaporation and releasing a lot of heat during condensation, making it very effective for heat transfer. However, an important limitation of CO₂ is the critical temperature of 31°C. Above this temperature, CO₂ cannot condense, making it difficult to function efficiently at summer outdoor temperatures. To address this, an additional internal heat exchanger has been added in vehicles, utilizing low-pressure refrigerant between the evaporator and compressor to condense the hot, high-pressure refrigerant in the gas cooler.
Compared to other refrigerants, such as R1234yf, CO₂ operates at much higher pressures. The standstill pressure at 20°C is 57 bar, and during operation, the pressure on the low-pressure side can rise to 90 bar, with a relief valve opening at 160 bar. This requires more robust systems, such as a compressor with thicker walls and flexible pipes with a corrugated steel jacket for protection against high pressure and thermal stress.
An important advantage of CO₂ as a refrigerant is that it can still heat effectively at extremely low outdoor temperatures, such as below -10°C. Unlike R1234yf, which boils at -29°C, CO₂ remains gaseous at temperatures below -79°C, allowing it to continue functioning in very cold environments. This makes CO₂ particularly suitable for use in vehicles as both a cooling and heating system, with better performance at low temperatures than traditional refrigerants.
Resistances:
In the following list, we see the amount of resistance the components we encounter in automotive engineering have:
- Copper wire 2 meters long and with a cross-section of 1.25 mm²: 0.028 Ω;
- Lamp (21 Watt incandescent bulb): 1.25 Ω;
- Fuel injector gasoline engine (the high-ohm version): 16 Ω;
- Relay control current section: ~ 60 Ω;
- Relay main current section: < 0.1 Ω.
The resistance of a component often depends on the temperature: for example, the resistance of the bulb is much higher while burning than during the measurement when it was cold, where the current decreases as it gets warmer.
Calculate slope resistance:
Over a distance of 100 meters, the vehicle has ascended 5 meters (see image). That means the slope is 5%. We calculate the slope angle using the tangent (tan).
Calculate tan α:
tan α = opposite / adjacent = 5 / 100
α = tan⁻¹(5/100) = 2.86°
Tip: press the shift key and then the tan button on the calculator to get tan⁻¹, and place (5/100) in parentheses. The result can be displayed in degrees or radians, depending on your calculator settings. To convert from radians to degrees, use the following formula:
Degrees = Radians * (180 / π)
Calculate gear ratio:
The transmission executed by the second system is 5.1. This is not the transmission between the motor and the wheels, but between the motor and system 1. Now we will calculate the gear ratio of system 1 using the data from system 2, as the omegas are now known:
ωZ2 = 4.1
ωD2 = 0.8
If you look at the diagram, you will see that the sun gears of systems 1 and 2 are connected. Also, the carrier of system 2 and the ring gear of system 1 are connected. The omegas of the connected parts are the same:
ωZ2 = ωZ1 = 4.1
ωD2 = ωR1 = 0.8
It is very important to pay close attention here! Always follow the lines in the diagram.
We now substitute these omegas into the calculation of system 1.