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Supercritical-Fluid Common-Rail Direct-Injection

energy recycled back into the system through a heat exchange between exhaust manifold and high-pressure-accumulator (fuel distribution rail)
  (+3, -1)
(+3, -1)
  [vote for,

Even at just below their critical points, preheating gasoline and diesel will yield a 1% and 3% respectively increase in energy in the cylinder. Both will vapourize and both will autoignite, for whatever savings/efficiencies that's worth.

But exhaust system temperatures (550-800ºC) are well in excess of the critical points for either fuel (gasoline 296ºC, diesel 446ºC), lending the possibility of even greater system efficiency.

Above the critical point, where the definition of "liquid" becomes so shaky they call it a "supercritical fluid", too much weird stuff happens, (for me) to easily put out some solid figures on density, viscosity and heat capacity (ie: I can't find the math. I have tried - I know it's out there somewhere)

So call it, very conservatively, 3% for gas and 4% for diesel give or take, under normal running conditions.

The engine design is still the same, save metalurgically beefing up the rail system to handle higher temperatures, and using injector types more suitable for gases than liquids.

I don't expect NOx to be a problem, rather the opposite since the fuel is coming in with more thermal energy than the air charge.


note: there's a process called "hot fuel injection" in which the injector adds compressed air to the spray, heating up the fuel a bit and atomizing it better. Unrelated.

FlyingToaster, Oct 05 2012

Common rail http://en.wikipedia.org/wiki/Common_rail
[normzone, Oct 05 2012]

Common rail http://www.google.c...:429,r:1,s:37,i:199
[normzone, Oct 05 2012]

"It is our most common rail..." http://www.birdingi...allinule/sora-rail/
[normzone, Oct 05 2012]

Heated intake air and fuel
[xaviergisz, Oct 05 2012]

Smokey Yunick's Hot Vapour Engine http://www.legendar...ur-exclusive-video/
heats both air and fuel in a carburetted system, ending up with a mixture just short of autoignition. [FlyingToaster, Oct 05 2012]

Supercritical fluid http://en.wikipedia...Supercritical_fluid
Oddly no mention of its use in fuel delivery. [FlyingToaster, Oct 05 2012]

Transonic Combustion Inc. http://www.tscombustion.com/
This company's fuel injectors electrically heat the fuel to the SCF phase. [goldbb, Mar 06 2013]


       [bella] huh ? [edit: <facepalm> ]   

       [xg] This idea is specific to a common-rail fuel delivery system, taking advantage of the relatively large heat transfer area of the rail's exterior and the high pressure of its contents.   

       Unlike Smokey Yunick's carburetted hot-vapour engine, this system isn't restricted to keeping the fuel beneath its ignition temperature: in fact the system promotes autoignition.
FlyingToaster, Oct 05 2012

       Ha - I thought Common Rail was the Bakerloo line at rush hour.
xenzag, Oct 05 2012

       Right, let's try again. Retexted; all done up with a pink bow.
FlyingToaster, Oct 06 2012

       umm, why the bone ?
FlyingToaster, Oct 10 2012

       I like this, but a few things would need to be worked through before I'd be entirely happy...   

       How is the engine started? if the injectors are more gas compatible than liquid, will a second set of injectors or spark plug be necessary for diesel gasoline respectively? Or perhaps you are thinking of electrical heating.... but this will be prohibitively time consuming for a car.   

       How will a rail, full of fuel pressurized to a temperature where it autoignites on contact with oxygen, react to a crash? Are we talking an amusing "pop" or a real hazard?
bs0u0155, Mar 06 2013

       [+] I like the idea, but a better (cheaper, safer) version of it has been baked.   

       Transonic's fuel injection system does something similar (but not identical) to this idea.   

       Instead of using a very high pressure pump and very high pressure common rail system, they use an off-the- shelf fuel pump, off-the-shelf common rail system, and pressures typical of other common rail fuel systems.   

       Instead of using exhaust gas to heat the fuel to SCF temperatures, they merely use exhaust gas heat the (moderately pressurized) fuel to just shy of boiling.   

       The almost-boiling fuel passes through the common rail to the injectors, which then use electrical resistance to heat the fuel the rest of the way to SCF temperature.   

       This heating is done with the (initially liquid) fuel in a small, fixed volume chamber... just enough volume for one cycle's worth of fuel. Adding heat naturally raises the pressure, so no high pressure pump is needed.   

       Not as much exhaust heat is recycled as in your idea, but it's still more thermally efficient than a spark ignited or diesel engine.   

       Regardless, their idea isn't the same as your idea, and even if it was, it's not widely known to exist.
goldbb, Mar 06 2013

       [bs0u] I did some math when I first posted this... then of course promptly forgot about it, but IIRC there's only a few cc's in a rail at any given time. This rail would be longer and hold more fuel, to accomodate heat-exchange with the exhaust manifold, but the resultant FOOM! wouldn't be that much more impressive. If you're in a crash serious enough to break a fuel rail you probably won't notice if your muffler falls off. [goldbb] has provided a possible starting process: use electrically heated fuel-injectors, tapering off while warming up.   

       I'm not too sure about how to class the injectors except to say that they're passive: on/off, they don't add to the pressure or temperature: the fuel is going to be SCF on the rail side with a lower sp.gr. than a fluid but a greater heat-capacity than a gas, and I imagine it's going to be a gas when it hits the relatively low pressure of the combustion chamber.   

       It's all predicated on "what is the best injection pressure for supercritical-fluidic fuel ?".   

       [gold] I like mine better of course :), I don't believe it's any more dangerous (okay, not _much_ more dangerous), nor expensive.   

       From a thermal efficiency point of view the only gain they have is that of raising the temperature of the fuel to just below boiling at ambient'ish pressure. The amount of energy required to flash the fuel electrically in every fuel injector to injection pressure is going to be the same as that required by the one high-pressure fuel pump in my system, pre-rail.   

       On the other hand my system is going to sop up as much heat from the exhaust system as possible, transferring that into the chamber (which means less fuel used).   

       To compare, in a simple form, my system requires a high-pressure fuel pump (as found in existing high-pressure rails) and a larger rail to enable heat exchange, and theirs requires a low pressure fuel pump and <x> amount of specialized injectors.   

       So what is the //better (cheaper, safer)// system you mentioned ?
FlyingToaster, Mar 06 2013

       So where's the heat exchange going to take place? Are you gonna stick the rail directly on top of the exhaust manifold? I'd have thought pulling the energy out directly after the turbo would be best for efficiency. Also, 25-500C 2-3 times a day for 10 years, I'm seeing thermal failure as a major issue.
bs0u0155, Mar 07 2013

       //thermal failure// Pistons, cylinders, valves, exhaust manifold, etc. all operate at those temperatures and greater.   

       The heat exchange would best, I imagine, take place within the exhaust manifold, that being where the highest accessible temperature is.   

       Picture the rail, which is just a thick pipe with a pump on one end and feed lines to the cylinder heads (with tiny valves) on the other, as being placed within the exhaust manifold, extending up from say where the catalytic converter would be. Fuel flows in at the bottom end and gradually works its way up getting hotter and hotter until, just before injection, it's at the hottest place in the engine where a heat transfer could take place.
FlyingToaster, Mar 07 2013


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