Thermal Efficiency Discussion

[ericjon262]

the vapor displacement concept Robertisaar mentioned had me thinking, how much air can be displaced by the expanding fuel? it makes me think TBI would probably be a better route, because then the fuel and air are drawn in together. more on this over the weekend. but for now, I need some sleep.

[RobertISaar]

how about a 2" wide check valve?

[1project2many]

how much air can be displaced by the expanding fuel?

All of it. Ever wonder why an electric fuel pump, essentially an electric motor, placed into a tank full of fuel doesn't cause the tank to explode?

TBI would probably be a better route, because then the fuel and air are drawn in together.

Actually, more power is made when fuel vaporizes in the cylinder and air is drawn in cold. This is because both the fuel and air enter the cylinder at greater density than if they were heated first. The downside is it's harder to get complete combustion this way.

Pressurize the fuel / air mix then heat it to vaporizing temperatures and you'll get rapid and thorough combustion without loss of power or air.

[ericjon262]

All of it. Ever wonder why an electric fuel pump, essentially an electric motor, placed into a tank full of fuel doesn't cause the tank to explode?

mixture is too rich. you can put a cig out in a gas tank if you want(please don't).

I think you missed my thought. I was more curious how much expansion would occur, and if it would be violent enough to starve the engine of air, remember, we aren't just hosing fuel down into the engine, we are precisely metering it in.

Actually, more power is made when fuel vaporizes in the cylinder and air is drawn in cold. This is because both the fuel and air enter the cylinder at greater density than if they were heated first. The downside is it's harder to get complete combustion this way.

this makes sense because the vapor expanding would help drive the piston down, almost like a supercharging effect, but I only really see this helping at lower RPM. keep in mind to, that we aren't talking 14.7:1 here, way leaner, way hotter, also, we aren't necessarily talking max power either, but instead efficiency. remember, heat is energy based on a difference in temperature, and only temperature, higher temperature difference, more heat, thus, more heat transfer. If everything is the same temperature, energy can't be lost to heat. cold A/F charge means more energy has to be put in to make it burn, lowering efficiency.

Pressurize the fuel / air mix then heat it to vaporizing temperatures and you'll get rapid and thorough combustion without loss of power or air.

basically, a draw through supercharger or turbocharger, this is similar to how smokey did it. he referred to the turbo as a "homogenizer"

[EagleMark]

All of it. Ever wonder why an electric fuel pump, essentially an electric motor, placed into a tank full of fuel doesn't cause the tank to explode?

Gasoline in liquid form is not flammable. The fumes that come of it are when mixed with air.

mixture is too rich. you can put a cig out in a gas tank if you want(please don't).

High school auto shop our teacher had a little cabinet and placed a small can/cap of gasoline in there the night before. The next morning everyone was asked to be quite and no banging. We had a bet. He/teacher said gasoline is not flammable, we students knowing it is and most of us have caused many fires as well so we had proof disagreed. Loosers would be acid cleaning the floors in shop during lunch... This was going to be great watching him scrub floors!

So he lights a match with a arm length welding glove and other protection on, so we were also going to get to watch our teacher blow up! Cool bonus but we all liked him! He put the match out in the gasoline!

Cool! But we disagreed and said it must not be gasoline? He assured us it is gasoline and would prove it, he tapped the shelf a few times with his magic wrench and asked if we would like to smell? Dam, that's gas? So on a long stick he had a lit match and got it close and POOF, the fire we are all used to!

This was the beginning of our lessons of Air Fuel Ratios... he did buy pizza as we scrubbed floors during lunch!

[RobertISaar]

i'd be curious to see what all methods cylinder pressure could be increased without increasing cylinder temperature(or even lowering it).... lower temps will automagically cause less heat losses, and higher pressure means more output.

[ericjon262]

i'd be curious to see what all methods cylinder pressure could be increased without increasing cylinder temperature(or even lowering it).... lower temps will automagically cause less heat losses, and higher pressure means more output.

I'd like to hear the magic behind this, combustion is really hot.

[RobertISaar]

well, current tech basically boils down to:

take a mixture of fuel and air and compress it(though external combustion is still around and doesn't compress the mix)
ignite it, creating pressure due to heating the air
get rid of it and repeat

now, the magic is all in step 2, since that is where the air and fuel start turning into something other than air or fuel. given the octane levels most people have to work with, what can be done to maximize the amount of pressure created by combustion, without causing detonation?

heat will obviously cause detonation, but it's a necessary evil, since heat(from the spark plug's arc) is what causes combustion. pressure alone doesn't cause detonation, but the heat created by pressurizing air/fuel can.

http://en.wikipedia.org/wiki/Atkinson_cycle

atkinson cycle engines seems to circumvent some of the negative work portions of the 4-stroke cycle. a higher static compression ratio offsets most/all/more(???) of the power lost due to the air charge getting taken out of the cylinder at the beginning of the upward stroke of the piston. without direct injection, it might take fuel with it? with a tighter chamber, a better chamber design in general would be useful.

this type of engine..... would probably sound a bit odd. i imagine any form of boost would be appreciated and would make the losses caused by being an atkinson design completely irrelevant or absolutely minimal since positive pressure in the manifold will keep air in the cylinder when boost is present.



i may or may not have gotten off-track from beginning to end.

[RobertISaar]

some tech aspects.

http://pressroom.toyota.com/article_...rticle_id=2722

there is probably a better link to be had, since this one tries to print, but it also doesn't load a whole bunch of other crap.

In the case of the Prius engine , the effective compression ratio is about 8:1, while the expansion ratio is about 13:1. As a result, it is 12% to 14% more efficient, in terms of power output per fuel consumed, than the non-Atkinson engine upon which it is based.

so, static compression is 13:1, while the engine only has to drive the load equivalent of 8:1 compression. 61.5% difference in compression ratios. from the way it is operated(pushing the air back through the intake rather than through the exhaust), pumping losses are also cut down as well. and the result is a ~13% improvement in BSFC. certainly something interesting to look into.....

[RobertISaar]

had an interesting thought a couple of days ago, still seems to have some merit, so i'll throw it out there:

with sequential PFI: as soon as the intake valve closes, inject a calculated amount of fuel, wait, then inject the rest later.

thought process behind this requires a fair bit of math, but: for the first injection, inject only the amount of fuel that will vaporize by the time the rest of the fuel charge needs to be delivered. due to a smaller mass of fuel sitting on the back of the valve, the average temperature of the fuel mass should be noticably higher than a larger mass in the same situation.

this SHOULD result in a better percentage of the fuel being vaporized before being drawn into the cylinder. because of this, i imagine mixing will improve at least slightly, since fuel vapor is going to move and combine with oxygen a lot easier than a smaller number of larger droplets will.



if this actually does anything beneficial, or is even harmful, i don't know. seems like the kind of thing a manufacturer would patent if it worked well(or at all).

[1project2many]

Robert, you don't want to maximize pressure in the chamber. You want to apply the most pressure where it does the most good. With the crank arm at or near TDC, pressure in the cylinder is nearly useless. When the crank arm is 90d ATDC, pressure in the cylinder does the most work. Ideally you'd create pressure quickly as the piston approaches 90 deg ATDC. But with cylinder volume increasing (at an increasing rate), this is tough to do. The best method would use a high octane, fast burning fuel and start ignition ATDC to allow maximum compression ratio while resisting detonation and eliminating negative work.

I'd love to see the timing curve on one of Smokey's hot vapor engines.

[RobertISaar]

thinking about burn speed and octane, ignore gasoline's burn speed and let's say combustion happens instantly after being sparked. or at least, some yet unknown fuel is being used that has an instantaneous burn.

would octane rating matter if all of the fuel that was going to be needed for that cycle not get pumped in until the very last moment before ignition? there wouldn't be the opportunity for pre-ignition/detonation if no fuel was present. the flowrate of the injectors needed to do this type of event would have to be monsterous though.

i seem to remember this being very similar to how diesels can get away with very low octane ratings, when they inject, the fuel is instantly being consumed.

[1project2many]

I'd say octane probably wouldn't matter in that case. But you still need resistance to pre-ignition as the chamber and the air within is hot.

I thought part of the attraction of gasoline direct injection is that fuel can be injected at or near the combustion event?

Diesel fuel is rated by cetane, which is a measure of how well it self ignites. Practically it's the opposite of octane.

Microwave ignition has been tried to get gasoline to ignite "everywhere, all at once." I'd have to dig around to find the expected decrease in time required for start to completion of reaction if this type of ignition would work.

FWIW, cetane improvers for diesel can be added to gas to encourage combustion. I do this frequently in my own cars. The results are obvious at the tailpipe and up to one ounce per gallon seems to work for most NA cars for an up to 10% increase in economy. But don't add too much as you'll get pinging.

[EagleMark]

I thought part of the attraction of gasoline direct injection is that fuel can be injected at or near the combustion event?

Been awhile since I read up on it but yes, compression of air is easier without the fuel, then injection before ignition. Also eliminates possibility of detonation with more spark advace. But a second injection of fuel during the combustion event, no ignition needed extends the power stroke. Or FWIR it took less fuel in two injection events to make same power...

[RobertISaar]

from what i understand with diesel, there is a very short delay after the injectors open and start flowing fuel before the fuel ignites. being compression ignition, they have a comparatively small window that they can start injecting in without risking there being enough heat in the cylinder to ignite the fuel. this window would be pretty centered around TDC with maybe some slight skewing towards the ATDC side. i would assume compression ratio dictates window size to a large degree. in a simplified and slightly inaccurate sense, the injectors inject fire. i have no idea how long combustion would still be taking place after the injector quits flowing fuel, probably dependant upon AFR? in any case, fuel that combusts(and gives pressure rise) after 90* ATDC would give diminishing returns, though would still give more output as long as there is still oxygen left in the cylinder to combust with.

that, from my limited understanding of diesel tech, is how diesels are so easily manipulated to increase output at the expense of fuel usage. it would seem you simply add more and more fuel until you end up with a rolling smokestack or quit gaining output. it actually seems easier on paper than managing a gasoline engine since you only really control when to start injecting and when to stop. with gas, it's when to start fuel, when to stop fuel, when to start charging the coil(s) and when to stop charging the coil(s).

if you were using a spark ignition, direct injection engine(note no mention of gasoline), you don't have to rely upon compression for the heat necessary to start fuel combustion. with a fast enough flame speed and an injector that was capable of flowing huge amounts of fuel in a very short amount of time, if the injection event started(and having a spark event VERY soon after injection begins) say at 75* ATDC and was completely done by 85*(and combustion stopped shortly after 90*), i don't think it would be of consequence if the fuel auto-ignited? this isn't really a good example due to completely ignoring engine speed, which is probably its downfall, along with unrealistic flame speed with conventional ignition systems, common fuels and the injector flowrate required.

microwave ignition, that seems like it would be....... electrically noisy. i'm sure that could be worked around at some cost, but otherwise interesting. i've seen mention of laser ignition for quite some time, but nobody actually implimenting it.

cetane as a gasoline additive, i can't say i've heard of doing that before(and am kind of curious to try out now). any known emissions issues, or is that actually an alternative use of it(ala "guaranteed to pass" among other products)?