Talk:External combustion engine
Nuclear fission; non-related?
In the article it mentions steam turbines, which I assume are widely regarded as external combustion engines, also often run on the heat provided by nuclear fission. Nuclear fission, then, must be a form of combustion - which it isn't. Can somebody explain this?
- Not qualified to discuss nuclear fission. However I think that as much as anything, your point is a semantic one and as such I have to say that I have long had problems with the whole term External Combustion - not just combustion. I think that the root of the problem is historical. In the early days of coal or wood firing, this was used to heat a boiler to provide steam pressure and that and/or condensation of it did the work. This immediately brings up another problem I have with the term heat engine, IMO heat in itself never drove an engine. What drives the engine is the effect of heat on a gas or liquid; if the expanded fluid is constrained in a restricted space such as a boiler or a cylinder, pressure is created which can be made to act upon a moveable piston or rotor. If we restrict the term "heat engine" to disinguish its working principle from that of a "gravity engine" (water turbine) or an "eolian" engine (windmill), for instance, where the effect of heat is far less direct (sun on the atmosphere), then the situation is clearer. Consequently if you look on nuclear fission as a component of a heat engine or more specifically of a steam engine, its role is no different from that of, say, coal burning in a firebox, which answers your objection. It is when we develop the heat engine idea further that we run into problems. The term heat engine goes back to the very early days of industrial steam. Late 18th Century scientists were fascinated by heat: was it a substance (caloric, phlogiston) or a phenomenon (kinetic theory)? Today the latter theory has prevailed, but a man like Sadi Carnot who laid the foundations of thermodynamics saw heat in terms of the substance, "caloric". (By the way, the Carnot heat engine article is much improved lately). In the mid-19th Century the internal combustion engine comes along with the invention of gas (coal-gas, not gasoline) engines and what is the process there? The same as before only faster: compression or an electric spark or a glowing coil ignites a volatile gas or air/liquid mixture creating a rapid pressure increase which moves the piston. As this all takes place inside the power cylinder and eliminates the requirement for a separate boiler, the process is called internal combustion. The point is that before the invention of internal combustion, there was no need for an opposite term and one wonders when external combustion was coined. I suspect it was relatively recently and what seems likely is that it was coined simply as the opposite of internal combustion. Today when most heat engines you see working every day are of the internal combustion type, that is the term of reference. Being a defensor of of steam technology, I can tell you that the level of ignorance regarding steam, even on the part of many engineers is unbelievable. One of Ted Pritchard's [http//www.pritchardpower.com] quips is something like "we never show the cylinders what we are burning", which aptly sums up the situation. External combustion may be semanically correct and just about applicable to a Stirling cycle engine, but regarding steam engines it represents absolutedly nothing and just adds to the general confusion. I have suggested that we should speak rather of separate combustion. It is interesting in this context to quote the opening sentence in the Wikipedia article: "An external combustion engine is a heat engine which burns fuel to heat a separate working fluid". The difference is that I visualise it in terms of the heat source being separate from power delivery. Of course I am on a personal hobby-horse here (POV) and as such my suggestion has no place an encyclopedia - or Wikipedia. No problem with that, but it does mean that when writing for Wikipedia or any encyclopedia we sometimes have to compromise and accept things we don't completely agree with or cope with inadequate terminology that nevertheless allows clarification of some essential point. I haven't answered your question but perhaps I have shown that it is part of a wider, fundamental one that nobody seems willing or able to tackle - the weight of history! The proof is that nobody has taken me up here on this yet, even though my point of view is very controversial. --John of Paris 11:58, 25 November 2006 (UTC) --John of Paris 09:22, 18 December 2006 (UTC)
Rewrite 11/03/07
Just a couple of points for Eric Norby: Re "It is then cooled or exchanged for cooler fluid (open or closed cycle"). Can't think off-hand of a case where an open cycle application needs to be cooled. Usually the issue is to prevent the steam from cooling throughout the cycle by avoiding wall effects etc. What often happens is heat recuperation at the end of the cycle, usually for feedwater heating etc. Feed can be heated in two ways: i) by waste combustion gases (economiser) and ii) by exhaust steam in many ways. The other point is that, at least in the case of steam what differentiates internal from external combustion is that a variety of radically different types of "heat exchanger"/"generator"/boiler can be married to an equally large variety of radically different types of "engine unit", "energy convertor", "expander" "mechanism" or what you will. Finally why, when discussing heat engine technology are we only allowed to envisage this question in terms of heat. Heat in itself never produced power; what produces power in any "heat engine" (EC, IC or even a Stirling) is surely the effect of heat on a fluid. In such a case pressure surely has to be an important consideration. Sorry to get back on my old hobby horse again!--John of Paris 17:21, 12 March 2007 (UTC)
John of Paris,
You are correct. Open cycle removes the necessity of "cooling" the fluid. That is true because the exchanged fluid is cool when entering and can cool the cold side. I suppose it is possible to evict the hot fluid from the cold side and pull cold fluid in the hot side, thus potentially needing a cooling system to cool one side just to eliminate damage to the mechanism, but that might likely be a result of poor engineering.
The sentence is vague and needs to be improved to the following. "It is then cooled (closed cycle) or exchanged for cool fluid (open cycle)". Sorry, I hashed it up when I was striving to be brief.
The heat exchanger, recuperator or economizer, in a steam plant are not necessarily just used just for open cycles, although I don't personally know that. I assume they can be used to heat either feed water, fuel or even oxidizer (air) to economize the plant. I don't know how much that is actually done. In a closed cycle, the feed-water can be used to cool the condenser.
This paragraph is in regard to the convertibility of moving a steam engine to any boiler system. Think about this. If you inject into the input of a steam engine liquid oxygen and hydrogen, slight modification of adding an input "burner", you could run steam engines off an internal combustion that generates "steam". Making any steam engine an internal combustion engine. Some rockets have used that process. Thus the terms are very general and not really used for or in engineering equations.
As far as the "heat" doing the work or not, think about the "fluid" as an integral part of the engine. The heat is the one consumable part, i.e., the input. Output is both work (motion) and waste heat. In addition, pressure doesn't produce work unless it is allowed to expand/escape. So the point is that no one part of the machine is sufficient to produce "work". However, heat applied to an open body of air produces motion/work without any other parts. They are called heat engines because you input heat, waste heat and get useful work/engine motion out of them.
Furthermore, one type of heat engine is the thermocouple. No gas. No pressure effects. Just electron flow/work as a reaction to heat. There is at least one liquid engine, some liquid to solid engines, and at least one solid to solid engine. Also Nitinol memory metal, as well as a whole host of other heat engines not using gases or fluids, such as a solar cell.
It all depends on how much you believe. If you believe definition must be true, you will never get past the semantics. If you see a clearer way of describing the situation, by all means, please type forth!
Just to add confusion to the issue someone should add that the external heat source could also be internal to the engine surrounded by the working fluid. An example is the case of the nuclear-cell fueled Stirling Engine where a small chunk of hot fission material is placed inside the hot section. Imagine being able to completely seal an engine and insulate everything hot, everything but the side that is supposed to be cold anyway. Excellent!
Wow! This soapbox I'm on is extremely high! ;)