Human as a heat engine humans are heat engines. heat engines can't operate when the source and the sink are at the same temperatures and also if the temperature of the sink is higher than that of the source. in humans' case, the source is our body itself (technically the mitochondria) and the sink is the surrounding. so how can people keep on working when the temperature >= body temperature. ??
 A: This is actually a very interesting question. Peter Shor's answer that humans aren't heat engines, but are instead powered by chemical energy, is entirely correct. However, in a comment, you (kiranadhikari) mention that petrol and diesel are also chemicals, and you are also entirely correct in inferring that internal combustions engines are also driven ultimately by chemical energy. In this answer I will try to clarify why humans are much more efficient engines than internal combustion engines, given that this is the case, and why it is that muscles are able to convert chemical energy into work without being much hotter than their environment.
The difference is in how this chemical energy is used. In the case of an internal combustion engine, it's done in two steps: first, the fuel is burnt with oxygen to produce heat, and then this heat is converted into work. If you do it this way, you're bound by the Carnot limit, which prevents your efficiency from being greater than $1-T_C/T_H$, where $T_C$ is the temperature of the environment and $T_H$ the temperature you can attain by burning the fuel.
In the case of a living cell with an aerobic metabolism, the chemical fuel is also combined with oxygen to release energy. But most of this energy is captured directly by molecular processes before it has a chance to become heat. By converting energy directly from one form into another instead of letting it become heat first, the Carnot limit is avoided. In this case there is a different thermodynamic limit (given by $\mu_\text{reactants}-\mu_\text{products}$, where $\mu$ is the chemical potential), so you can't make a perpetual motion engine or anything, but this chemical energy limit can be much higher than the Carnot one.
In the case of muscles, the energy released by oxidising sugar is converted (via a complex series of reactions involving ATP) into the kinetic energy of a few molecules. Ordinarily molecular kinetic energy is rapidly transferred to other molecules and becomes heat, but before this can happen, the muscle fibers divert this kinetic energy into work, contracting the muscle. The energy goes straight from chemical energy to mechanical work without being converted to heat at any point. As pointed out by Edwin Jaynes (p. 16 onwards), if you define temperature as energy per degree of freedom, this amounts to a temperature of around $5000 \,\mathrm{K}$. You won't measure such a high temperature with a thermometer because it's concentrated in only a few molecular degrees of freedom rather than being spread throughout the whole cell.
If you're willing to think of this concentrated molecular motion as a kind of heat then you can apply the Carnot limit after all - it's just that $T_H$ is nearly the temperature of the Sun, much hotter than the temperatures you can find inside an internal combustion engine.
You might be thinking that it's very inefficient of us to be using internal combustion engines in our cars instead of converting the energy in hydrocarbons directly into work. This is quite right - if our car engines contained muscles rather than combustion chambers they could in principle be enormously more fuel-efficient. The problem is just that the technology to create an artificial muscle that runs on petrol (and doesn't waste a lot of energy maintaining its own metabolism) is presently some distance beyond our grasp.
A: Humans aren't heat engines; the source for our energy isn't heat; it's chemical energy derived from food. 
A: Humans are not heat engines as described above but the heat in our body comes from the blood which comes from many other processes. Humans need food for energy which in turn needs energy from sun.
If you mean't our heart as an engine then yes, heart is a constantly pumping engine that keeps us alive in cold by means of pumping more blood and with other functions of the body that help bring our temperature at a reasonable stage.
A: human being is heat engine as chemical reactions take place and some of energy is utilized in working of body rest is released in atmosphere. when temperature of sourrounding is higher then human body still it releases heat in form of sweting.
