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How does building one large engine compare to building a multi cylinder engine? What are the pros and cons of each if your goal is power production?

The old engines were generally single cylinder with a large bore, often 10" 12" diameter and more, but my idea is a multi cylinder small bores, reason (my idea) it's easier to heat the air in the smaller cylinder, also the smaller motor will run at higher speed, the big old machines often ran at speeds of 100 / 200 rpm, while my small motors seem to have their best working speed between 600 and 800 rpm.
Ian S C

Ian S C wrote:The old engines were generally single cylinder with a large bore, often 10" 12" diameter and more, but my idea is a multi cylinder small bores, reason (my idea) it's easier to heat the air in the smaller cylinder, also the smaller motor will run at higher speed, the big old machines often ran at speeds of 100 / 200 rpm, while my small motors seem to have their best working speed between 600 and 800 rpm.
Ian S C

Theres no getting around the issue that big magnets equals more power and I will never get any big amount of power with small magnet alternators. I assume that the big machines although they have a low rpm produce enough horse power to turn a fairly heavy low rpm neo magnet alternator about 10 - 20" diameter or am I being a bit too optimistic?
.Wellington.

You have to match the alternator to your motor, you can only get out what you put in minus inefficiencys, so if your motor was able to develop 10 Watts, with a good small alternator you might expect 5 Watts of electrical power. Generators/alternators increase in efficiency as they get bigger, and really become efficient after they get in the Kw's.
Ian S C

Ian S C wrote:You have to match the alternator to your motor, you can only get out what you put in minus inefficiencys, so if your motor was able to develop 10 Watts, with a good small alternator you might expect 5 Watts of electrical power. Generators/alternators increase in efficiency as they get bigger, and really become efficient after they get in the Kw's.
Ian S C

Do you know of any projects that have matched a big engine with a big axial flux alternator?
Wellington

Wellington wrote:How does building one large engine compare to building a multi cylinder engine? What are the pros and cons of each if your goal is power production?

Wellington,
I am not suggesting that multiple cylinder Stirling engines cannot work well and it may be difficult to say with certainty why Stirling engines with multiple cylinders often do not perform as well as one would expect. The books of James Rizzo are very helpful on this point, he gives such a good overview through his experiences with Stirling engines. His experiences may save someone hours and days of frustrating and fruitless effort. And so on this account and others, even at $50 each, his Stirling Engine Manuals are worth the money.

If I were to guess, I think that it has a lot to do with piston speed and the delicate work of capturing those tiny power impulses. There also may be some clue in looking at the indicator diagrams of Stirling engines, although they do not really tell the whole story because in the Stirling engine there is such a paramount relationship between the development of power and frictional losses. These losses are important in other heat engines but they do not present such a singular challenge as they do in the Stirling engine.

With regards to the generator, I know that you are toying with the idea of an axial flux alternator. This should work very well, it can be derated by using air core coils instead of iron, by mechanical gearing, by electronics such as a pulse width modulation controller, or by simply adjusting the distance between the coils and magnets. A simpler alternative might be a shunt wound generator such as an alternator where you can control the current through the field windings but although this will work well if sized and controlled properly, it may be about 10% less efficient than the axial flux alternator because the work of the field windings can be replaced by the permanent magnets.

While I was Googling generators the other day, I came across a funny one about car alternators. The proposal was: got 2 alternators 35W and 70 W, first get the 35W to run as a motor driving the 70W, when the 70W is generating enough power, disconnect the battery and use the power being generated, I think the bloke was serious------ever heard of perpetual motion.
Don't think of car alternators unless you are prepared to strip it out and rewind it, and rebuild the rotor with permanent magnets instead of the wound one.
Look up DIY wind turbines on Google, there are some good designs ranging from a few Watts up to Kw.
Ian S C

Alfista wrote:

Wellington wrote:How does building one large engine compare to building a multi cylinder engine? What are the pros and cons of each if your goal is power production?

With regards to the generator, I know that you are toying with the idea of an axial flux alternator. This should work very well, it can be derated by using air core coils instead of iron, by mechanical gearing, by electronics such as a pulse width modulation controller, or by simply adjusting the distance between the coils and magnets.

Now I'm confused. I thought pulse width modulation improved performance no?
Wellington

Now I'm confused. I thought pulse width modulation improved performance no?

Wellington, I am sorry if the post was confusing. Yes, indeed, some method of regulation may be just the thing to improve performance. PWM controllers are cheap to buy and fairly easy to build, at least in this range of power ! It is an efficient way of limiting the power going into the battery and so it is a good way of matching the available torque from the engine with the load, which in this case may be a battery to be charged. Until the generator is under load, there is virtually no resistance due to current but under load the generator may develop a great deal of physical resistance, for example in the case of a dead short.

I am not an expert on these devices. I mostly build them as DC motor controllers, to control the speed of a DC motor. They are often used in battery charging circuits and I have even read that this kind of charging "can initiate desulfation". I can not say that I know this to be true and personally, I would not depend on it for desulfation.

I hope that my larger point was not lost. The better the match between the available torque from the engine and the power required to drive the generator, the happier you will be ! I am amazed at the efficacy of some systems that are cobbled together through sheer willpower, trial and error, and minimal expense ! Other times, I see so much effort put in to a system with disappointing results.