Car motor heat to drive a steam engine Why wouldn't be able to place a car motor inside a water tank or another liquid to be heated by the motor, instead of wasting the heat and then use the steam generated, to drive a steam engine, which itself would be submerged in the same liquid to not let any heat escape from the system?
 A: BMW is actually working on something similar - they call it the turbosteamer concept.
It is a combined cycle where a heat exchanger captures heat from the engine and uses it to drive a steam circuit, recovering some mechanical energy.
There are more details at this SAE article. There are apparently issues to be worked out in getting the system small enough to fit in a passenger vehicle before it will reach the market.
Clearly, surrounding a car engine with a fluid that could be boiled and have the enthalpy to do useful work would be prohibitive in size and weight. Thus a practical system would have to be based on capturing heat from, say, exhaust gases.
Similar combined cycle techniques have been tried at power stations and in shipboard engines, with some success. Examples include combined cycle power plants with gas turbines fired by natural gas, with some of the waste heat being captured by heat exchangers and driving a steam turbine to generate additional power. 
You can do a search on Google for "combined cycle power generation" and find much material, although I can't post it here as it is probably copyrighted.
As has been pointed out, there is a definitive limit to the amount of heat energy that can be recovered to do useful work, the Carnot efficiency. It will never be 100% of the heat energy. This is an inevitable consequence of the second law of thermodynamics.
A: Yes definitively.
Is it practical?Maybe.
Any temperature difference can be used to run an engine in principle.
However that does imply that heat will not escape the system.
If you can convert all the Heat into work then you violate the second law.
The limitation imposed by Carnot is still valid.
