Using helium as a car fuel is considered "thermodynamically unfavorable" because it does not efficiently store or release energy for propulsion in the context of internal combustion engines. This statement is based on several factors:
Energy density: Helium has a very low energy density compared to conventional fuels like gasoline or diesel. Energy density refers to the amount of energy that can be stored in a given volume or mass of fuel. Helium has a significantly lower energy content per unit volume compared to hydrocarbon fuels, which means it would require a much larger volume of helium to store the same amount of energy.
Combustion properties: Helium is an inert gas and does not readily participate in combustion reactions. In internal combustion engines, the fuel needs to combust in the presence of oxygen to release energy. Since helium is chemically stable and does not readily react with oxygen, it cannot be used as a fuel in the conventional sense.
Heat transfer properties: Helium has low thermal conductivity, which means it does not transfer heat efficiently. In internal combustion engines, efficient heat transfer is crucial for the proper functioning and performance of the engine. The low thermal conductivity of helium would impede the effective transfer of heat, leading to reduced engine efficiency.
Availability and cost: Helium is relatively scarce on Earth and expensive to produce and store. It is primarily obtained as a byproduct of natural gas extraction. The limited availability and high cost make helium an impractical choice as a widespread vehicle fuel.
Considering these factors, using helium as a primary fuel for cars or other combustion engines is not practical from a thermodynamic standpoint. Other fuels, such as gasoline, diesel, or even alternative fuels like hydrogen, offer higher energy densities, better combustion properties, and more efficient heat transfer, making them more suitable for use in internal combustion engines.