optimal sfc, rpm, apparent CR, const vol combustion

[schwee]

In SBBlue's thread "Value of Engine Efficiency? (thread71-105903), SBBlue said:
"The smaller the engine, the better the engine efficiency, because the smaller engine operates at a higher rpm to produce the same amount of power as a larger engine."

In considering this, I think I don't at all know what I thought I knew.

I mean, a smaller displacement engine should get better fuel economy than a larger displacement engine for the same load, right? This seems intuitive. But wait – why???

Why can’t I run a bigger engine in bigger gears? Answer: I can. American cars do it. They run bigger, longer stroke engines, and run ‘em slower (and in bigger gears), and this doesn’t murder their fuel economy as much as it should, because, say I:

1) this bigger, slower engine has higher apparent compression ratio than the little one. The throttle is open wider per stroke -- more charge per rev, thus higher compression.

2) the big engine has fewer reciprocating loads and less friction than the little one. Slower revs mean lower recip loads, and fewer strokes per minute means less friction.

3) the big engine closer approximates constant volume combustion. Slower piston speed, so the piston recedes before the flame front more slowly than it does in the higher revving engine. Closer to constant volume combustion equals better fuel efficiency, because relatively higher peak temps and pressures, so more area under the expansion curve, if that's what it's called.

Hmmm. So maybe it’s not so intuitive. It’s pretty much common knowledge that running in gears as big as possible for a given load improves fuel economy for a given engine size and load. So how does this translate when you start shifting engine size around? My buddy gets 30 mpg in his Pontiac Bonneville with an engine bigger than 4 liters, vs. my getting only 40 mpg in a 1.7 liter engine with a significantly smaller car. He really has no right to get mileage that good with such an obscenely huge engine, but he does.

I’m suspecting there must be some kind of continuum. Hmmmmm. And this consideration points to another vexing one:

4) Does running an engine closer to its optimal torque and power (generally toward the higher end of revs) also mean it’s closer to its optimal sfc? How are these related?

I await enlightenment.

 

[GregLocock]

Well, as a professional engineer I'm sure your first instinct was to check with google rather than waste everybody's time, but since you have never heard of google here is its URL,

http://www.google.com

, and here's the very first hit:

http://www.geocities.com/SunsetStrip/4060/consumo.html

The SFC maps there answer your questions.



Cheers

Greg Locock

 

[tbuelna]

schwee,

How's the free piston, turbine thing coming along?

Terry

 

[schwee]

Greg -
Well, I don't think the graphs really answer my questions, but perhaps I haven't expressed them, or formulated them, well enough. I am of course never asking about what is already known - I'm trying to take known observations apart in light of a novel proposition, which in this case is no cranks, HCCI, variable mixture, and variable stroke.

I think I'm basically wondering whether a radically turbocharged HCCI compound free piston-gas turbine engine is going to respond to increased load by simply increasing stroke periodicity or instead by generating more power per stroke, and whether it would make any difference either way in terms of sfc, particularly when slower piston speed would seem to indicate a better approximation of constant volume combustion, and also bearing in mind that an engine at lower periodicity probably has less engine friction than an engine of equivalent output at higher periodicity. On the other hand, running an engine faster at lean mixture may be more efficient than slower and richer. Perhaps I'm wondering whether one should respond to increased power needs by opening the throttle or richening the mixture, or, if both, how much of one vs. the other.

I may also be wondering about bore to stroke issues, since that would seem to me to be the only other way to have much input into stroke periodicity. Or something.

Sometimes I'm not sure what is bothering me, but this seems to describe it more or less. It's not critical to the success or failure of the design, just intriguing -- or at least I thought so. I sort of feel Google wouldn't have much to offer in this arena, since there is no such engine in existence that I'm aware of, but I am of course very likely wrong.

Hi, Terry --

I'm finishing up a 50-pp proposal for the "double-acting HCCI compound free piston-gas turbine engine" as time permits. I've given a presentation of it at my alma mater and have made a few contacts, but nothig firm yet in the way of $$$. I expect to have a patent pending by January, after which time I'll go after a machine shop in earnest for the free-piston part. Once that's complete, I have a contact in Brooklyn with a number of ~100hp gas turbine APUs who has indicated he might donate one to the project and provide help in integrating the free piston and gas turbine components.

I've approached Michigan, Stanford, MIT, and Sandia about it; nothing from Michigan and Stanford (they're both crank-piston HCCI), polite acknowledgement from MIT (David Gordon Wilson, emeritus experimental high-efficiency turbine guy - you may know him). Lots of interest from Hans Aichlmayr at Sandia, who as far as I can discern is the best guy in the world right now on free piston HCCI. I would really love it if Aichlmayr would undertake the linear/rotary alternator aspect of the design, since that's his gig (he uses a linear alternator to pull off all the power of the free piston engine, while I just use it for starting, even running, and synchronization of multiple free pistons).

Once something even sort of kind of working can be cobbled together, it's a road trip to Michigan (Williams International) and Ohio (Honda R&D, I believe). If the road trip is in a car featuring the new engine, so much the better.

Anyway, that's the status at present; let me know if you'd like me to send you some stuff. The proposal features your argument on piston vs. turbine compression -- perhaps you'd like to vet the thing? I'd very much value your input.

--Paul

 

[1Bluegrass]

What efficiency are we talking about?
The conversion of fuel into mechanical power at the flywheel!
The engine volumetric efficiency!
The term can have many meanings until defined.
Bore, stroke and rpm are not all that is involved.