For automobile vehicles, even though thermal efficiency is important, mechanical efficiency is taken priority and given weightage since life of engine is more and more important.

Mechanical efficiency should and be given prime importance in design of thermal equipment. Life of the equipment is important. The more the sensitive the equipment, fastly the equipment goes down and spoils.

Mechanical efficiency is to be given first and top priority in thermal equipment design then comes thermal efficiency. This is to see that equipment once designed or installed with lots and lots of money should satisfy to work efficiently without sudden breakdowns especially when used in industries.

Mechanical efficiency of thermal equipment is considered and given weightage. Thermal efficiency is given second priority.

Engine or any thermal equipment should work withstanding various inputs like flows, flow rates, pressures, temperatures etc. The materials are also selected in this view. Hence, the top most priority is given to mechanical working hence mechanical efficiency and later thermal efficiency.

During any kind of Heat Exchanger there are two important aspects mass and enthalpy balance dependent on

Thermal Efficiancy and also

Mechanical Design ie gasket ,

Shell and Tube thickness etc.

Thank you @Jaydip Datta for your answer.

Mechanical and thermal efficiency is different thing. We should use mechanical efficiency for mechanical process, while the thermal efficiency for thermodynamic process. For thermal equipments, such as heat exchangers, it is better to use thermal efficiency or effectivity. And for automobiles, there are mechanical efficiency, transmission efficiency, and thermal efficiency, such as brake thermal efficiency. We use these terms in different equipment/process. So, for me, it is not important to choose the priority.

Thank you @Nazaruddin Sinaga for your answer.

I think both types of efficiencies should be given importance. Unless the thermal parameters are chosen correctly then one is not getting the most out of the fuel using that process. For this enthalpy-temp-entropy diagrams must be consulted carefully along with other factors such as air/fuel ratio and combustion chemistry. Once the thermal efficiency is optimized to the extent possible then the mechanical efficiency also must be optimized since this tells you how efficiently you transmit the gains from the combustion process to mechanical (kinetic) energy. This would encompass looking at the transmission system (the shafts, pulley, wheels, lubrication, etc.)

Following answers

Thank you @Hazim Al Dilaimy for following.

Thank you @Srini Vasan for your answer.

Both types are important in their respective fields

Thank you Nirmala S.V.S.G for your answer.

In any equipment design, mechanical efficiency is most preferred and considered. The reason being after taking a long time for equipment erection, one has to see that it should work for a longer time. Hence thermal efficiency of thermal equipment comes under second priority.

Thermal equipment works based on temperature or gives energy or heat transfer. The construction of this equipment is done based on several critical aspects taken under consideration. Since, this involves several things, hence mechanical efficiency is given top priority compared to thermal efficiency.

Most of the thermal equipments should and are designed to run for a longer duration. Several parameters are taken into account for successful functioning. This involves man power, cost and hard work. Hence, mechanical efficiency is given top priority and next comes thermal efficiency.

In thermal equipment design, mechanical efficiency is given importance than thermal efficiency. The reason being, the design of the thermal equipment is to be suited and erected in a particular place where it functions well with safe performance. Eventhough, the thermal efficiency leads to good performance when compared, mechanical efficiency is given top priority.

Design is considered for mechanical equipment. A proper design is to be made for the equipment to function for a longer duration. Hence, even though thermal equipment designed has high temperatures and pressures which are taken into account, mechanical efficiency is taken prime mportant than thermal efficiency.

In thermal equipment, one would be interested to increase the thermal efficiency by considering several aspects but as far as thermal equipment in practical, it is preferred to have high mechanical efficiency, since, the equipment has to run for several years without major troubles and hurdles. Hence, the thermal equipments are to be designed firstly taking priority to mechanical efficiency and next thermal efficiency.

Thermal efficiency depends upon the mechanical design

Every effort is to be made to increase the thermal efficiency in every engine

Temperature rise upto maximum and minimum loss of thermal energy in system is designed