Homebuilt Air Compressor - RX7 Engine?

[jamie1974]

   I had a crazy idea and I'm not sure if this would work or not. I would like to build an air compressor for my garage, and I was looking at using an old RX7 engine to serve as the air pump, and hooking up a 3 horse electric motor to spin it over. I can see some advantages to the design, and some potential problems.

  Advantages:
  Water cooled - I've had a number of compressors in the past that got too hot when I'm doing big sandblasting projects. With the RX7 engine, the duty cycle would be a lot longer because it would stay cool with the water pump and oil.

  Relatively inexpensive - I could pick up an RX7 core for a couple hundred bucks.

  Possible disadvantages -

  I was trying to figure out if the peak PSI would be a problem. I did some googling, and it sounds like the average compression on an RX7 would be around 85 psi for a used motor. Now, the question is, that psi rating could go up if the output of the engine was hooked up to an air tank. That psi rating could be limited simply because there is only so much air it can take in and compress in one cycle in a closed chamber. If multiple engine cycles are pumping the air into a chamber, does anyone know if the top psi in the air chamber would be higher than 85 psi? I'd like to see around 120 psi or higher if possible.

  If anyone else can think of any pros/cons, please let me know. Thanks!

[bordin34]

Isn't it a Rotary Engine?

[Khyron]

not to hijack but I almost bought an RX7 with a V8 conversion in it. That car with a 355 


pure evil 

for your question. I have no Idea 

[jamie1974]

Isn't it a Rotary Engine?

   Yeah, it is a rotary. I thought it might be possible if it generates compression, then it should be able to compress air into a tank.

[Charger_Fan]

If it's just an old engine you have laying around, seems to me that you have nothing to lose. Give it a shot & report back!

My bet is that it won't build up enough pressure to be of much use, but who knows?

[last426]

Gee, I normally go for wacky ideas but not this one.  First, it would take a fairly large electric motor to spin that thing around.  Second, it would be a mess trying to set some cooling thing up.  But the biggest strike is the oil.  I was lucky and bought a brand new rx7 in 79, their first year. Had it for maybe fifteen years.  Those engines are designed to use oil to lube the rotary seal (I can't remember what it is called).  Anyway, that oil is normally burned by the engine -- in your case you would need to collect it to prevent it from going into the air (unless you are using the thing only for air tools).,  They make three types compressors that I know about, a screw, a vaned, and a piston.  It would be a better use of money to buy one instead of trying to convert a Wankel, at least in my opinion.  Kim

[73dodge]

If you really want to experiment with this why not find a old lawn mower grab the engine and use that as a compressor motor.

But the draw back to this whole thing is what are you going to do with the exhaust gases? If you use a conventional engine you will either need to pipe the exhaust gases out of the garage or run the thing outside.

[Troy]

Those engines are designed to use oil to lube the rotary seal (I can't remember what it is called).

I believe it's called an apex seal and that's the part that also holds the compression. When it goes the engine is mostly useless so if it's already a junk motor...

Troy

[dkn1997]

lets' not forget the fact that the rotary was pretty unreliable back in the day.  my friends had a total of 4 between them. all the way from 86 up until the last really fast one in what, 93?  the twin turbo one.  all had the same problem.  if you drove somewhere and were not fully warmed up and shut her down for any reason(leave house in morning for work, stop for coffee...)   when you went to start it.....flooded.  solution: yank the plugs and let her air out for a day.  at least that's what the dealer did when they would take them in for the crank/no start.

[jamie1974]

If you really want to experiment with this why not find a old lawn mower grab the engine and use that as a compressor motor.

But the draw back to this whole thing is what are you going to do with the exhaust gases? If you use a conventional engine you will either need to pipe the exhaust gases out of the garage or run the thing outside.

  Well, there wouldn't be any exhaust gases because there wouldn't be any combustion. The motor would be turned by an electric motor and the engine would act as an air pump.

  The lawn mower engine is a good idea. I should try running a compression test on the lawnmower to see how much psi it will generate.

[jamie1974]

Gee, I normally go for wacky ideas but not this one.  First, it would take a fairly large electric motor to spin that thing around.  Second, it would be a mess trying to set some cooling thing up.  But the biggest strike is the oil.  I was lucky and bought a brand new rx7 in 79, their first year. Had it for maybe fifteen years.  Those engines are designed to use oil to lube the rotary seal (I can't remember what it is called).  Anyway, that oil is normally burned by the engine -- in your case you would need to collect it to prevent it from going into the air (unless you are using the thing only for air tools).,  They make three types compressors that I know about, a screw, a vaned, and a piston.  It would be a better use of money to buy one instead of trying to convert a Wankel, at least in my opinion.  Kim

   Yes, that is a really good point...the oil. I had left that out of the equation entirely. I would imagine this would really be a problem that would be hard to get around, unless you had some kind of oil seperator on the output.

   Actually, without spark plugs in the engine, it is easy to spin an rx7 motor. The problem would come in once the air pressure starts to build up. At peak, we would have 120 psi pushing back, and at that point there would be an issue with power. But for the first half of the tank fillup cycle, there would be hardly no resistance at all.

  The more I think about the lubrication problem, the bigger the problem seems. I imagine there would be a lot of oil to seperate, and probably not worth the trouble.

[SirNik73]

I went to the Towe Auto Museum here in Sacramento last weekend. they had an old V8, for the early 30s, not a Ford, some other brand, can remember, anyway... it was converted to an air compressor. 4 of the 8 cylinders had spark plugs and powered the engine, the other 4 cylinders had piping to the tank. it was an all in one unit that was on a hand cart. I'm assuming it was used in construction. i thought that was very cool.