Newbie pitch

[Christ]

Originally Posted by jamesqf

I'm more than a little puzzled here. You're talking about adding a Stirling engine to the exhaust of a conventional IC engine, but having that drive a generator which (with batteries) drives the wheel motors (or other electric propulsion). Aren't you getting more than a bit Rube Goldbergish? Why not cut out the IC, and have the Stirling drive a generator which extends the range of your plug-in hybrid?

The battery wouldn't even have to be that large, say enough for 5-10 minutes of driving while the SE comes on-line, so you save on weight & cost.

Mean while, what drives the sterling motor? It has to have a source of energy, unless you're channeling battery heat into it, which would work out great, since you have to vent your lead-acid batteries anyway.

That might be a consideration, to reclaim some battery heat as energy to help recharge the batteries or something, which would still cut out the ICE altogether.

The Mini kit uses an ICE also, just an extremely small one that kicks on when the batteries start to lose power, and it only powers a generator which charges the batteries again while you're driving. In that case, something to reclaim energy from the heat lost through the exhaust would be great.

[theunchosen]

Interesting idea James, but Chris posted first so he gets first response.

I see what you are saying. It's definitely worth testing. It's hard to find numbers on the backpressure inefficiency problem because most people who are putting turbos on don't like to talk about how its a bad idea or even the downsides(read OFF COURSE ITS A GOOD IDEA HOW COULD IT BE STUPID). And venting it off like most Catback systems use dual exhaust to avoid jamming the flow while venting any extra is an even better idea.

It could be done to just have the SE run the gen and bats. The issue with SE's though is they don't respond to changes very quickly at all. An SE capable of 100 hp would take like you said 5-10 minutes to respond to any kind of throttle change. So the engine has two options either output average power for driving(30-40) or output enough power for maximum driving(55-60 mph). If A is chosen I'd be fine in town but as soon as I hit a road thats 55 or the interstate I'm going to be draining those batteries rather quickly and its going to take at least another 5 minutes for the engine to respond to the amped up heat. Any time in either mode you are not doing 40 or 60 it's like holding the clutch in partially and revving it to 3,000 when you only really need 2,500.

That said using the SE in either mode would be more efficient than the ICE(IC=40% on a good day SE=65% on a good day). The reason for using the ICE still is it would allow for a dampening of the SE's problems. During the time the SE is spooling up. . .it's not even moving for about a minute and definitely not generating any power so you have to let the car idle for 5 minutes in the driveway. Any time you change speed you throw away five minutes of heat it won't recognize. It's not being entirely thrown away but for the first 2 1/2 minutes it's just washing across the plate with no effect.

The ICE let's you. . . modulate that a little. So you can change speeds and start off your car without losing as much energy. It's like using a diode in an AC circuit and then throwing in a capacitor. the diode shows a half wave form and you get no energy half the time with the capacitor you get nearly constant voltage. With the ICE you constantly get above 30% and when the SE is in sync you get much better, instead of starting at 0% and climbing to 60%.

And the weight of the engine is not so terrible its heartbreaking. Even if we call it 400 lbs for my 1.5 engine. . .I don't mind considering this would be like doubling your standard gas mileage or better.

[jamesqf]

Originally Posted by Christ

Mean while, what drives the sterling motor? It has to have a source of energy, unless you're channeling battery heat into it

A burner, of course. Something like the burner from a home oil furnace, heating the hot side of the SE directly.

Another point to keep in mind here is that SEs are heat engines, and so the amount of power they can produce is governed by the temperature difference between hot & cold sides. (Basic thermodynamics, IOW.) You're going to be able to get the hot side hotter with a burner than just by running IC exhaust over it.

For the startup time issue, that's why you have the batteries. Your minimum battery requirement is whatever lets you to drive long enough for your SE to get up to speed, though of course if cost & weight allow, you can add more to add more plug-in electric range.

[Christ]

Hm. It seems like a half decent theory, but after re-reading most of this, I don't think it will be very practicable... (note, not practical, practicable... two diff words.)

I'd still like to see OP have a go at it, but I'm not sure what the feasibility will actually be comparing to the work put in, vs the energy actually saved.

Obviously, anything is better than nothing, but if the ROI isn't there, it just can't be justified.

[theunchosen]

Originally Posted by jamesqf

For the startup time issue, that's why you have the batteries. Your minimum battery requirement is whatever lets you to drive long enough for your SE to get up to speed, though of course if cost & weight allow, you can add more to add more plug-in electric range.

You're dead straight on the SE principle, bigger difference more juice. You're still also dead straight that the SE burner alone would be greater(I agreed with you a post back) than the ICE.

The only issue I take with it is with the SE only you burn 5-10 minutes of fuel at 0 mpg. True you are moving but as far as the SE is concerned you are doing 0 mpg.

True also that you would get greater power with an SE burner.

But you still lose out because you either have to produce more energy than you use(to avoid having to heat the engine more to get to a higher speed) or you have to toss another 5 minutes worth of fuel at very low mpg to get up to that speed.

Yes the batteries could overcome the problem, but why waste fuel? Running it off ICE exhaust it will generate less overall power (modularly) but the amount its not generating the ICE is off fuel that would have been wasted for the first five minutes.

The reason to employ the ICE-SE combo is to achieve a smooth power delivery curve that's maximized for efficiency

I attached a paint image I sketched very quickly(and horrifically).
Blue is the ICE by itself
Red is the SE by itself
Purple is the ICE-SE.
The initial red trails blue because it takes the ICE less time to achieve 30%(almost instantly, but it takes it time to climb to optimum around 34-35).
The purple matches the blue exactly(it would actually be marginally higher) because at that point its operating just the ICE(the SE's are still warming).
Then purple and Red diverge from Blue because ICE maxes out at 35%.
The dip is presumeably an increase in speed where the stirling loses substantial FE but the ICE-SE still has the ICE dampening the loss. the Blue ICE line does not suffer noteably.

The graph assumes a constant speed with no interruptions otherwise you have more dips for the Red and Purple with the same effects. If the speed is variable above 10 mph the Red(SE) line will tank below the purple(ICE-SE) line.

[theunchosen]

Originally Posted by Christ

Hm. It seems like a half decent theory, but after re-reading most of this, I don't think it will be very practicable... (note, not practical, practicable... two diff words.)

I'd still like to see OP have a go at it, but I'm not sure what the feasibility will actually be comparing to the work put in, vs the energy actually saved.

Obviously, anything is better than nothing, but if the ROI isn't there, it just can't be justified.

Theoretically you could get almost 100% efficiency. But. . .You'd have to have 50-100 SE's to get that last 30%(which is why the best Gas turbine engines weigh several tonnes and still only get 70%).

But Yeah GE does this already. They use the Gas Turbine(GT here forward) to supply immediate demands, like hospital goes down and they need power, but they don't want to have to run the GT full throttle to keep that up so after several minutes the GT spools down as the other systems catch up.

[Christ]

Question:

Given the latest bits of information, I must ask if anyone has considered the RPM at which the sterling would run, and the rpm/work needed to drive the generator circuit?

Are we driving one generator with several SE, or are we driving generator per SE?

I'm thinking that even a 30HP SE is only going 100RPM or so (that might be wrong, probably is, so the math here isn't necessarily correct either, other than analytically.)

If it's generating 30HP of work at 100RPM, and the generator needs to spin at 3000 RPM to make any reasonable amount of power, the reduction would be 30:1 (30 at the generator for every 1 at the SE) Which means that the generator would only see 1/30 of the HP that the SE is giving.

1 HP may not be enough to drive the generator to the 3000 RPM it needs to see with the kind of load it's going to be creating while charging the batteries/electrical system.

Granted, more than one system like this would lessen the load, and technically, you could employ enough systems to lessen the load to the extent that 1HP would be sufficient, but how much weight would you be adding at that point?

It seems kind of counter-intuitive unless the efficiency marks go up significantly for each piece of the puzzle.

That's what I'm thinking, please tell me I'm wrong. I'd hate to see this project wasted over something so consequential.

[theunchosen]

Um, no the engine would not have to run at 3,000. that's around their free-wheel speed. and to counteract the SE having very low torque, we just make the generator inclined for small torque(fewer coils).

Engines of this nature already power the SES Suncatcher in site 1 and site 2(under construction not sure if they have started erecting units yet). They run very low torque and very high RPM at 711 degrees C. This engine would run reasonably high rpm at 1000-1500 and low torque as well providing less HP(less than 33). keep in mind the suncatchers are employed in the desert for maximum sun, but they also have a very high background temperature(well over 100 for the 25 KW tests) and my avg temperature in the summer is less than 80 and the temperature in the winter is 30-40. So at both of those times I start off with an advantage because my engines will be much more capable of heat syncing on open air than theirs as well as a moderate temp difference(theirs is 700-120=580 mine would be ~400-60=340)=(20 hp not including increased heat syncing on cold side due to car speed or lower atmospheric temperatures.)

So roughly 20 HP for 50-60 lbs? What are the 5th wheel guys talking about? increased rolling resistance and huge amounts of weight for how many horsepowers? Also they aren't stripping out trans or several other weight contributors.

[Christ]

So that sterling engine would be outputting 115.5 lbft of torque @ 1500 RPM?

115.5 LBFT x 1500 RPM = 173250

173250/5252 = 32.9 HP

Or for your estimate, about 70 lb ft.

70 LBFT x 1500 RPM = 105000

105000/5252 = 19.9 HP.

Most car alternators, and welding apparatus generators, are built to create max output at a given RPM range, I believe most car alternators peak at about 3000-4000 RPM, which is why I used that for a figure.

[theunchosen]

Ah I understand. I had the total intention of using a car alternator but pulling some winds out of it for lower torque. I would have to adjust it for the lower RPM as well. It might actually serve better to use a Windmill (Wind turbine) alt instead because they are rated for lower RPM and just pop some of the winds out of it instead.