How do humans and Carnot engines differ?

A thin line exists.

Fundamentals

Energy

The?foods?and?beverages?we?consume?provide?the?body?with?energy. Foods?have?a?lot?of?chemical?energy?stored?in?them,?which?your?body?absorbs?and?breaks?down?into?smaller?parts?to?use?as?fuel. Carbohydrates?are?the?primary?energy?source,?while?protein?and?fats?are?the?other?two?main?sources?of?energy. When?the?body?runs?out?of?carbohydrates,?it?can?use?fats?and?proteins?as?an?energy?source. The?chemical?processes?that?occur?in?your?body's?cells?to?convert?this?food?into?energy?are?known?as?metabolism.

Temperature Humans?adapt?to?their?surroundings?and?work?to?keep?their?bodies?at?a?constant?temperature?of?37?°C.?This?applies?to?extreme?cold,?intense?heat,?the?humidity?of?Amazonian?forests,?and?mountainous?Himalayan?ranges. Because?its?muscles?and?cells?function?at?their?peak?efficiency?at?37?°C. In?contrast?to?"cold-blooded"?species,?which?are?defined?as?poikilotherms?whose?internal?temperature?follows?that?of?the?surrounding?air,?this?"warm-blooded"?species?is?said?to?be?a?"homeotherm."Where is the difference between a Carnot engine and a human?

The distinction is in the way chemical energy is generally utilized by humans and internal combustion engines. The way this chemical energy is used makes a difference. When using an internal combustion engine, the process involves two stages: first, the fuel and oxygen are burned to produce heat, which is subsequently transformed into work. This approach puts you in violation of the Carnot limit, which prohibits your efficiency from rising above 1?TC/TH, where TH is the maximum temperature that can be reached by burning the fuel and TC is the ambient temperature.

A?human?living?cell?with?an?aerobic?metabolism?combines?chemical?fuel?with?oxygen?in?order?to?release?energy. However,?before?it?can?convert?to?heat,?the?majority?of?this?energy?is?directly?absorbed?by?molecular?processes.

The?Carnot?limit?is?avoided?by?transforming?energy?straight?from?one?form?into?another?without?first?allowing?it?to?become?heat.

You?cannot?create?a?perpetual?motion?engine?or?anything?else?in?this?situation?because?there?is?a?different?thermodynamic?limit?(given?by?μ?reactants?μ?products,?where?μ?is?the?chemical?potential),?but?this?chemical?energy?limit?can?be?significantly?higher?than?the?Carnot?one.

When?sugar?is?oxidized,?muscles?release?energy?that?is?transformed?into?a?small?number?of?molecules'?kinetic?energy. Molecular?kinetic?energy?is?normally?quickly?transferred?to?other?molecules?and?converted?to?heat,?but?the?muscle?fibres?use?this?energy?to?contract?the?muscle?before?it?can?do?so. Without?ever?being?transformed?into?heat,?the?energy?moves?directly?from?chemical?energy?to?mechanical?work. Human?efficiency?is,?therefore,?not?constrained?by?Carnot's?efficiency.