# Hot Air engines

• posted

Nick,

As you appear to be our Hot Air Engines expert you may be able to answer some questions please.

1. How does the engine cycle work
2. How does pressurizing the engine make it perform better.

Martin P

• posted

==== I read somewhere (perhaps in Rizzo's "The Stirling Engine Manual") that explaining how a Stirling engine works is like trying to describe the simultaneous events in a 3-ring circus! - or words to that effect!

JW² ====

• posted

The short answer is that a charge of air is heated causing its pressure and volume to increase, performing useful work on a piston, the same charge of air is then cooled and the cycle repeated ad infinitum. Raising the starting pressure gives a greater pressure increase for the same temperature change and thus more power!

A little more detail? (but still hugely over simplified). Picture an indicator diagram of the usual pressure/volume type. Process A-B; heat is added to a fixed mass of gas at constant temperature Te, its volume and pressure thus increase performing useful work on a piston. B-C; the temperature of the gas is reduced at constant volume to Tc. C-D; the gas is compressed to its original volume at constant temperature Tc, work must be done on the gas during this phase and heat is rejected. D-A; the temperature of the gas is increased at constant volume to Te. Fortunately for a power cycle, the work done on the piston during A-B is greater than the work done on the gas during C-D! The net work is of course represented by the area of the indicator diagram and will rise with an increase in mean pressure.

This is usually referred to as the 'text book' cycle and in an actual engine, all the above processes merge into a broadly oval indicator diagram.

The clever bit, wot that nice Mr Stirling invented, is recycling the 'non productive' heat exchanged during the constant volume temperature change processes B-C and D-A by holding it in a temporary store called a regenerator. Thus making the cycle reversible (in the thermodynamic sense) and the cycle efficiency equal to the Carnot (maximum possible) efficiency between Te and Tc.

Well you did ask!

• posted

I'm beginning to wish I hadn't :-))

Thanks Nick and Dave.

My brain hurts now :-((

Martin P

• posted

"campingstoveman" wrote

Infinitely more brain pain can be yours:-

• posted

Martin P

• posted

"campingstoveman" wrote

And so much of it!

• posted

Found this with pretty pictures and everything. You will immediately note that, Shame of shames, I got my isothermals the wrong way up in the above description - shows how long it is since I read a thermo text book!

• posted

"Nick H" wrote

And - wait for it, wait for it - here's the link!

Can I go home now?

• posted

Good evening,

I am looking for a "simple explanation", an easy understandable description of the phenomenon that explains that if the same amount of heat is added to a fixed mass of low compressed gaz then the yield in mechanical power will be fairly low and if the same heat is added to the same fixed mass of highly compressed gaz then the yield in power will be much bigger...

Of course I read P-V diagram but how would you keep it simple?

The only modern "cycled engine" without much compression phase is the V1 pulsoreactor of the bad days and, as far as I know, this Kiwi autopropelled Lawnmover:

Have you some of those in UK?

Bonsoir à toutes et tous, Yves

• posted

"ylc" wrote (snip):-

Assuming we are still talking about hot air (stirling) engines, pressurisation increases the mass of air in the system and thus the potential specific power output - ie more power for the same size of engine. It does not change the potential efficiency (which remains the Carnot figure), so more heat must be added (and rejected) to realise this higher power.

• posted

That URL was fine, but unless you want to read a lot of on line betting ads & face a surge in your spam filter's activity, avoid the good-sounding