# Why is it necessary for a heat engine to work in cycles?

We can make a steam engine just by putting huge amount of water in large tank and heat it and then use the steam to run the wheel. We just have to put huge amount of water and heat then engine will work for days.

But in my books, all engines are based on cyclic process. Why?

• What happens when you run out of water/heat? You will refill everything. And then... hey look a cycle. I am not sure I am understanding how what you are proposing will not result in a cycle. Eventually if you want to keep running everything you will need to reset things back to where they were. Can you expand on that more? Jun 5, 2021 at 4:54
• What I want to say is that in carnot engine we have to provide heat and fuel (to maintain the temperature of hot reservoir) and since we have to make a cycle, we use some little amount of energy to push the piston back to its initial position. But in the engine I referred in my question we don't need to put some energy back to the engine to make it work in cycle. We just have to provide heat and water. Then my engine will be more efficient then carnot' one. Jun 5, 2021 at 5:22
• You are neglecting the energy you will need to use to actually heat up the water. And then what happens when you need to get the engine up and running again? You have to reset everything, and hence you have a cycle. I still don't see how 1) You are proposing something that is not a cycle, and 2) how it is any more efficient than typical cycles. Jun 5, 2021 at 5:35
• I assume my engine to be ideal engine 1) My engine is actually cyclic but if I put a huge amount of water in the tank then it can work for days and I want to know whether or not it is more efficient ,in those days, than carnot' one. 2)Because in carnot engine some energy was dumped or wasted to make the engine cyclic( in isothermal and adiabatic compression some energy is always used or wasted),but that is not the case in my engine. Actually, I think that we can not make a CYCLIC engine better than carnot' one but can make a non cyclic(temporary engine) engine better than carnot's one. Jun 5, 2021 at 6:11
• In other words, the question(s) is(are) legitimate, but I don't think the post itself is. And that's a valid closure reason. Closed $\neq$ invalid question, generally speaking. Jun 5, 2021 at 14:35

We can make a steam engine just by putting huge amount of water in large tank and heat it and then use the steam to run the wheel. We just have to put huge amount of water and heat then engine will work for days. But in my books, all engines are based on cyclic process. Why?

This is essentially a question of definition. You can indeed extract work from a non-cyclical thermodynamic process, but such processes are not considered to be heat engines. Heat engines are defined by returning to the initial state with the exception of a transfer of heat from a hot reservoir to a cold reservoir. If a device truly does not have a cycle then it is not a heat engine by definition.

So the real question is not whether all heat engines are based on a cycle, they are by definition. The real question is whether or not your example qualifies as a heat engine.

Your example is a standard steam turbine which is based on the Rankine cycle: https://en.wikipedia.org/wiki/Rankine_cycle

At some point you will need to refill the water. So the complete cycle includes condensing the water from steam back to a liquid. It doesn’t matter if that condensation is done in a closed chamber or in the open atmosphere. Either way the condensation and refilling is considered part of the cycle. So your device does have a cycle (the Rankine cycle) and is indeed a heat engine (a steam turbine).

Regarding efficiency: if you replenish the water then you have a legitimate heat engine with a cycle and the efficiency is less than the Carnot efficiency. If you do not replenish the water then you do not have a heat engine at all and the concept of efficiency doesn’t make sense.

All heat engines operate between a hot temperature source and a low temperature exhaust, where useful work gets extracted from the heat source and whatever remains is dumped out as exhaust. The operating principle of a particular heat engine can be plotted on a certain kind of chart called a P-V Diagram, and when you go through each step of the heat engine process in order on that chart, the path forms a closed loop called a cycle.

So in the case of your steam engine, 1) heat is added to water to boil it into steam, 2) hot steam is shot into a cylinder where it presses on a piston, causing it to move, 3) the piston turns a crankshaft, performing useful work, and 4) the leftover steam gets blown out the cylinder. All those cycle steps are being performed over and over again as the steam engine "works for days".

• No. There is no piston. The steam is directly rotating the wheel. We just need to add water and heat. And in case of carnot engine, we have to give some energy back to the piston to make it work in a cycle but in my engine there is no need for that. So my engine can be more efficient then carnot' one, I think. Jun 5, 2021 at 5:30
• you have a turbine, then? exactly the same rules apply. And do not delude yourself that you can beat Carnot because you cannot. Have you studied thermodynamics? Jun 5, 2021 at 5:36
• My engine does not work in cycles. So there will be no dumping of energy to make it cyclic. In carnot engine, some energy is always dumped(by isothermal and adiabatic compression of gas) to surroundings. But in my engine, there is no need for that. The steam directly rotates the propeller joined to the turbine. We just need to add water and heat( just like we add fuel and heat in carnot engine). Hope I made myself clear this time. Thanks. Jun 5, 2021 at 5:52

Question does not make any sense. An engine has to operate on a cycle, otherwise you will only get a small amount of work in the beginning as your source and your sink thermalize, and that is it. The moment you reheat stuff, you are re-doing the cycle, making an engine.

• I assume my engine to be ideal engine 1) My engine is actually cyclic and I am providing water and heat to it( just like we provide heat and fuel to carnot engine). 2)In carnot engine some energy is always dumped or wasted to make the engine cyclic( in isothermal and adiabatic compression some energy is always wasted),but that is not the case in my engine. Actually, there is no adiabatic and isothermal compression in my engine so no heat is dumped at all which makes my engine more efficient than carnot' one. Plz tell me if I am wrong. Jun 5, 2021 at 6:24

We can make a steam engine just by putting huge amount of water in large tank and heat it and then use the steam to run the wheel. We just have to put huge amount of water and heat then engine will work for days. But in my books, all engines are based on cyclic process. Why?

Yes, early steam engines were like that.

https://en.wikipedia.org/wiki/Steam_engine#Cold_sink

Those are not very efficient on the surface of the earth, where the steam stops expanding when it still has one atm pressure left.

A modern steam engine is equipped with a condenser. A condenser is basically a device that sucks like million vacuum cleaners.

If we were to put a steam engine and a large water tank on the top of a 100 km tall pole, then efficiency of the engine would be over 90%, and no condenser would be required.

The first documented steam engine was built by hero of Alexandria in the first century AD. It worked much as you describe, except that it was a rotary engine and the whole boiler rotated. In the last century, ships and power stations increasingly came to employ continuous-flow steam turbines. Many, including all coal and nuclear fuelled power stations, still do.

Turbojets and turbofans are also examples of heat engines which run continuously. Fuel-burning rockets provide an example which does not even need to turn a wheel.

You are thinking of the piston engine, which chuffs cyclically in and out of its cylinder to turn a crank shaft round. Pistons have many advantages over continuous-cycle operation. They are more flexible in operation, able to deliver wider combinations of power vs. speed according to the control settings, and all with near-instantaneous response and reasonably good efficiency - i.e. low fuel consumption - throughout. They are cheaper and easier to make, and especially to repair. The internal combustion variety can be tailored to burn a wider range of fuels. Altogether far more practical for most road vehicles and general stationary duties.

However the piston engine has its limitations. For near-constant-speed, near-constant-output regimes the piston is indeed less efficient than the turbine. Also its power-to-weight ratio is not as good and gas flow rates are more limited, making it unsuitable for high-speed or very-high-altitude aircraft. And of course it cannot drive a space rocket.