Okay here's something I learned very recently. The word "Diesel" does not refer to a specific blend of fuel, but rather to a specific type of engine that does not require spark plugs. A Diesel Engine is one where the fuel is auto-ignited simply by the large compression ratio. Air is squeezed and compressed to such a small volume that the temperature increases and increases until finally reaching the auto-ignition point of the air-fuel blend. This was pretty revolutional for me because "Diesel" is something I think of as a fuel you buy at the gas pump for diesel engines. So i always thought that diesel engines were just engines tuned for a premium fuel. But it turns out that you can make a diesel engine for any fuel as long as it's auto-ignited by compression. Anyway, what I want to know now: What compression ratio is needed to auto-ignite anhydrous ethanol? This might depend on the ratio of air-fuel---so is there a formula that takes into account the oxygen content, nitrogen content, vaporized ethanol content, and ambient pressure, and then yield the compression ratio needed for auto-ignition? BTW I don't necessarily believe in ethanol fuel for the future. I just want some solid numbers for comparison purposes, which seem really hard to find these days even on wikipedia.
I found the following, which indicates that ethanol has such a high octane rating that an ignition promoter is required in order to get it to ignite in the diesel cycle, presumably regardless of compression ratio: http://en.wikipedia.org/wiki/Ethanol_fuel I also found this, on the subject of ethanol blends: http://www.sciencedirect.com/science/article/pii/S1743967114000646 which does give some compression ratio information. But, like you, I did not find anything directly confirming what compression ratio is typically used with pure ethanol fuel in the diesel cycle. P.S. In my former job developing industrial lubricants for large diesel engines, I used to visit the factory in Augsburg where Rudolf Diesel made the first Diesel engine. It is still in use for assembling and testing the large marine diesels made by MAN (Maschinenfabrik Augsburg-Nurnburg). The engines stand about 15ft high, and have 6-16 cylinders, turbocharged of course, with power outputs of 10-40 000 hp. You need earplugs! There is a brass plaque on the wall and beside it they always keep a small bunch of fresh flowers. Rather a nice touch, by these unemotional Germans. But these engines burn heavy fuel oil ("furnace oil"), not truck diesel fuel.
compression ratio and engine heat both play a role in dieseling. the hotter the engine, the less ratio is needed. a "regular" engine will start dieseling if the engine is hot enough.
Yes, but this is not what this thread is about. It is about making a diesel engine run on ethanol, whereas your remarks appear to address the issue of unintentional knocking or "dieseling" in a spark ignition engine. Nobody makes a diesel engine by overheating a gasoline engine: you have to do it properly.
actually i was pointing out that compression ratio isn't the only variable. there are 3. one is given; ethanol. the others are engine heat and compression ratio. it applies to all engines of this type. edit: increasing the air/fuel mixture temp will have the same effect. this would require some sort of heater in cold weather.
http://www.me.umn.edu/centers/cdr/reports/agexpoposter.pdf http://www.iaeng.org/publication/WCE2010/WCE2010_pp1602-1606.pdf diesel has a high flash point- 126F, and low autoignition temperature- 493F ethanol has a low flash point- 70F and a high autoignition- 685F temperature http://www.mncorn.org/sites/mncorn....ts/201206/EthanolUtilization-DieselEngine.pdf Perhaps, it would be better to design an engine to run on ethanol rather than using a diesel engine which was designed to run on diesel?
Ah, yes to clarify I'm only talking about pure ethanol, or at least as reasonably pure as an ethanol/water mix can be, which I think is somewhere around 95% by volume. I'm not interested in any blend of ethanol and another fuel, like gasoline. I know the autoignition temperature of ethanol is significantly higher, but will it be a linear relationship between that and compression ratio? In other words, if we translate the auto-ignition temps to kelvin, then take the ratio of ethanol's auto-ignition temp to diesel fuel's auto-ignition temp, can we assume that the compression ratio needed for ethanol is the *same ratio* over diesel fuel? This is a great point. Thinking about this, I believe the best engineering process is to, of course, cool the engine with air or water, and pick a cooling point where the engine temperature is not supposed to exceed. Then choose a compression ratio that involves this nominal heat. Nevertheless, to start the engine, it may or may not require heating to this nominal point. You know, isn't it possible to have variable compression with piston engines? the arm of the piston can have some kind of screw that telescopes in or out, varying the arm length a little. This would allow different compression ratios to compensate for different nominal temperatures. Another thing I thought of was some kind of super thermal-resistant interior coating of the cylinders, like maybe ceramics or aerogel. But the problem is, the cooling process actually needs thermally conductive materials in order to transfer heat to the coolant. Of course theoretically, you could have a material that's so thermally insulating that the engine would never heat up to a dangerous temperature, but it would depend on how long you need to run the engine. Any thoughts on this?
Intuition tells me it seems possible. A diesel engine running off of ethanol reminds me of Tester and Cox model engines. They run off of a mixture of another alcohol, methanol, and nitromethane, with oil added to the mixture for lubrication. Those small engines use a glow plug that gets hooked to a battery when starting, and thereafter keep self-igniting at the peak of each compression stroke.
