How does A Bus Air Conditioning System work? (Wikipedia)

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A bus air conditioning system, also known as an HVAC (heating, ventilation, and air conditioning) system, is designed to provide comfort and maintain a pleasant temperature for passengers during both hot and cold weather conditions. The system typically consists of several major components: the compressor, condenser, evaporator, expansion valve or orifice tube, refrigerant, and electrical controls.

The Compressor

The compressor is the heart of the air conditioning system. It&#;s responsible for circulating the refrigerant throughout the entire system. When the thermostat signals that cooling is required, the compressor pressurizes and pumps low-pressure refrigerant vapor into the condenser. This component is usually driven by a belt connected to the engine, but some newer systems use electrically powered compressors.

The Condenser

After leaving the compressor, high-pressure, high-temperature refrigerant gas enters the condenser. Here, heat from the refrigerant is transferred to the outside environment with help from ambient air passing through fins surrounding tubes carrying the heated refrigerant. As this process continues, the refrigerant changes state from a gas to a liquid due to decreased temperature and increased pressure. Once cooled sufficiently, it moves on to the next stage.

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The Evaporator

Following the condenser, the now cooler liquid refrigerant travels through an expansion device &#; either an expansion valve or orifice tube &#; which regulates the flow of refrigerant into the evaporator coil. Inside the evaporator, warm cabin air comes into contact with the chilled refrigerant causing it to absorb heat energy. Simultaneously, moisture from the humid air condenses onto the cold surface of the evaporator creating a dehumidifying effect inside the vehicle. After absorbing enough heat, the refrigerant turns back into a low-pressure vapor, ready to return to the compressor to repeat the cycle.

Expansion Valve vs Orifice Tube

Both devices serve similar purposes; they control the amount of liquid refrigerant entering the evaporator. However, their operation methods differ slightly. An expansion valve uses a diaphragm controlled by a bulb filled with refrigerant from the outlet of the evaporator. As the temperature drops at the end of the evaporator, less refrigerant flows through the valve, maintaining optimal superheat levels within the evaporator. On the other hand, an orifice tube maintains a constant restriction regardless of changing operating conditions, relying on a metering rod to adjust flow based on pressure difference across the two sides of the orrifice. While simpler in design, orifice tubes can be less efficient than expansion valves under varying load conditions.

Refrigerants

Refrigerants are critical to the functioning of any AC system. They have unique properties allowing them to change phase easily when subjected to different temperatures and pressures. Older systems used R-12 (Freon), but environmental concerns led to its phasing out. Modern systems primarily utilize R-134a, although new generations of lower global warming potential refrigerants like R-407C and R-410A are becoming more common.

Electrical Controls

Various sensors and switches monitor system performance and ensure proper functionality. Key among these are the temperature sensor, which measures interior temperature and sends feedback to the control module, and the pressure switch, which safeguards against excessive system pressure. Additionally, climate control panels allow occupants to select desired settings such as fan speed, mode (e.g., floor, dashboard, defrost), and temperature. These inputs are processed by the electronic control unit (ECU), which manages operations accordingly.

Conclusion

In summary, a bus air conditioning system relies on multiple interconnected components working together harmoniously to deliver comfortable temperatures for passengers despite external weather conditions. Through continuous circulation, compression, condensation, evaporation, and expansion of refrigerant, along with precise regulation via various sensors and controls, these sophisticated systems significantly enhance travel experiences.
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Current electric school bus battery ranges can cover most bus routes

One of the most common questions about electric school buses is about their range: how far can they go on a single charge? Electric school buses are becoming more advanced, with longer ranges, with each new generation of bus. Many manufacturers are on their second or third iteration &#; and some even further along. Electric school buses currently have ranges of up to 210 miles for Type C buses; and all Type A, C and D buses listed offer over 100 miles of range, enough to cover most bus routes.

When examining the range of the bus, it is important to be aware of the advertised or &#;nameplate&#; battery capacity compared to its &#;usable&#; capacity. Many manufacturers will advertise the actual battery size (kWh) or nameplate capacity of a bus, but in reality, around 80% - 90% of that advertised capacity will be &#;usable.&#; Manufacturers reserve about 10-20% of the battery to maintain the battery state of health over the long run. Reserving some of the battery power also ensures that critical functionality will work and that the vehicle will not suddenly shut-off.

Should the charge level reach less than 10-20% (the bus will warn a driver when it approaches this range), the bus may eliminate power to external systems to conserve it for essential functions. This function is similar to fossil fuel vehicles that are low on fuel.

The range listed is typically based on simply running the bus only; however, other systems like heating and air conditioning would also use the battery and reduce mileage range. Notably, a wheelchair lift or ramp does not draw power from the high voltage battery that is responsible for propulsion &#; so there&#;s no impact on range. Instead, a separate 12-volt battery is used to power the lift and ramp.

In contrast, regenerative braking can capture extra energy, which could extend the range of the bus. Regenerative braking occurs on downhill rides or during slow stopping like at stop signs, traffic lights and bus stops, or while in traffic. It has an added benefit of lower wear and tear on braking systems.

Over the lifetime of an electric school bus, the battery will naturally degrade by around 20% after several years of use &#; similar to how a smart battery doesn&#;t last as long after a few years. If an electric school bus operator participates in vehicle-to-grid (V2G) or vehicle-to-anything (V2X) services, the added battery cycles may degrade the battery more quickly. 

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