The compression ratio issue (from 9.5:1 or so to about 22:1) could possibly be cured by the new variable compression ratio design engines. That’s where the top half of the engine block pivots away from the bottom half, leaving more space in the top of the cylinder at the top of the stroke – or less when they pivot closer. But the 22:1 pressure would doubtless do major dirt to the pivot mechanism as it did to the 2-bolt mains in the early Cadillac diesels. This is not like the case where your household generator can run on gasoline or propane. In the words of a recent NY Giants football guru, you can’t turn a zebra into a Dalmation because they’re the same color.
Phineas, in these engines is the ignition achieved by spark or by pilot diesel oil injection?
Another issue, if I understand correctly, is that gasoline’s resistance to spontaneous combustion (AKA detonation) is the opposite of what is merited in a diesel engine. Diesel fuel’s cetane rating is inverse to gasoline octane. When diesel injector squirts out fuel into the combustion chamber’s hot air it needs to start burning as soon as possible, before the injector even finishes. Otherwise unburned fuel builds up and can burn all at once, giving results similar to detonation in a spark ignition engine.
A diesel’s higher efficiency is due to a combination of three things: higher compression, higher fuel heat content and lack of throttle losses since a diesel has no intake manifold vacuum.
Maybe gasoline could run in a diesel if the fuel were injected at a spark plug or glow plug that would quickly start the burning process.
“Diesel fuel’s cetane rating is inverse to gasoline octane.”
- True, a higher cetane number indicates easier auto ignition and is better for cold starting.
“Otherwise unburned fuel builds up and can burn all at once, giving results similar to detonation in a spark ignition engine.”
- Actually, that’s unlikely. What usually happens is the unburned fuel goes out the exhaust in the form of bluish smoke. The combustion speed of diesel fuel is really too slow to support that type of “detonation,” so incomplete combustion is more likely.
“A diesel’s higher efficiency is due to a combination of three things: higher compression, higher fuel heat content and lack of throttle losses since a diesel has no intake manifold vacuum.”
- True.
“Maybe gasoline could run in a diesel if the fuel were injected at a spark plug or glow plug that would quickly start the burning process.”
- The problem is that gasoline would auto ignite and burn very quickly (detonation) at those compression ratios essentially what happens in a knocking gasoline engine (but much worse). The fundamental difference between a diesel and a gasoline engine is the slow combustion process in the diesel engine (due to the nature of the fuel).
I am convinced now that detonation is impossible when using direct injection into the combustion chamber. In order to have what we think of as a detonation event, the entire charge has to be there ready to detonate. With an injector, the fuel and therefore the flame is introduce gradually. Mechanically, fuel cannot be introduced fast enough to cause detonation no matter how fast the flame front.
We now know about the Mercedes ?DeisOtto? gasoline burning engine. I recently learned about the (now rather old) SAAB SVC Variable Compression Engine. An excellent article explaining the details is at: http://www.channel4.com/4car/ft/feature/feature/1878/1
On the last page of that article the author writes ?Fuel is then injected directly into the cylinder through a combined spark-plug injector. To make the mixture burn, the ignition timing is advanced a long way and the spark is made to jump a huge 4 mm gap?
First, gasoline combustion is NOT instantaneous! If it were, there’d be no need to advance the spark as RPMs increase. That’s why we run distributors (or more modern tech) and NOT magnetos: a mag’s spark timing is fixed. Airplanes can get away w/ mags because they spend most of their time at cruise, and timing can be optimized for that one condition.
Also, note that redline RPM is inversely related to stroke. High-revving engines tend to be oversquare, with short strokes and wide bores; vice versa for the “stump-pullers.” This is because if the piston outruns flame propogation, much wasted energy results.
And, to the OP’s question of why you can’t “diesel” kerosene or jet fuel: the jet fuel trucks at the airport are diesels, and I’m pretty sure they run on jet-A. Similarly, turbine engines run just fine on diesel (as Jay Leno w/ his turbine motorcycle can attest). Also, certain scofflaws have been known to put kerosene in their diesels at the pump, to aviod paying the “highway taxes” charged to roadable fuels.
Everything you have said is correct, but it has nothing to do with the point I was trying to make.
My only point was that; in an ideal otto cycle the combustion takes place under a constant volume condition, and in a dual standard air cycle (diesel engine) a portion of the combustion takes place under a constant pressure condition (during the power stroke). That is the fundamental difference between these two cycles. It’s been a while since I’ve taught undergrad thermodynamics, but that is a very basic fact you can look up for yourself if you don’t believe me.
Also, as I said in my previous post, “the combustion in a real engine is not instantaneous, it actually takes place from some point BTDC to some point ATDC, but there is very little meaningful contribution to the pressure during the power stroke (i.e., it is close to adiabatic).”
To be clear, the power stroke in an ideal otto cycle is adiabatic, meaning there is no heat addition due to combustion during the power stroke. Everyone understands that there will actually be some residual combustion during the power stroke (adding heat) as well as some thermal losses (removing heat), but these are not significant to the cycle. As a result, an otto cycle is a decent model of a modern gasoline engine. I don’t know how to make that any clearer.
All the design details of an actual engine (ignition timing, maintaining the appropriate piston speed by changing the bore/stroke ratio, etc.) are required to make the actual combustion occur at the appropriate point in the stroke, and to ensure the mean piston speed is not too high for the actual combustion speed of the fuel. These design provisions are intended to make the actual engine behavior close to the ideal otto cycle. Fortunately, the combustion characteristic of gasoline (much faster than diesel fuel) allow engines to be designed for reasonably high speeds while still having “essentially instantaneous” combustion (compared to the time of the power stroke). That does not mean the combustion takes place instantly, it simply means that the energy is released in a very short time relative to the piston movement (modeled as constant volume in the ideal cycle). When/if the piston speed becomes too high and “outruns flame propagation,” the engine is no longer acting like an otto cycle and efficiency is lost rapidly.
I agree that #1 fuel oil (kerosine or jet fuel) will burn just fine in a diesel engine, the combustion characteristics are similar to #2 diesel but the volumetric energy content is lower (also the lubricity is lower, so it’s a little tough on injector pumps).
OK I think the real issue here is the speed of propagation. You believe that the the combustion process is virtually instantaneous, I don’t. The only way to have instantaneous combustion would be to atomize the fuel down to the molecular level, surround each molecule with oxygen and concentrate it all into one infinitely small area. In a diesel, the fuel droplets ignite as injected, but they still burn from the outside in. In a gas engine, the fuel droplets not only burn from the outside in, but the flame must jump from droplet to droplet from the source of the ignition to the walls, roof and floor of the cylinder.
Your diagrams also do not take into consideration that the portion of the total cycle that develops power is only 120? and not 180?. This is where, ideally, you want the maximum expansion of the gasses to occur, regardless of the type of fuel used. This even applies when electricity is your “fuel”. That is why all modern generators and all high powered electrical motors are three phase and not one or two.
For the third time, the combustion process is not really instantaneous is an actual engine, but it is modeled as such in the ideal otto cycle. The actual flame propagation speed is very fast compared to the piston speed near TDC, as such it does not provide meaningful heat addition during the power stroke. That is the fundamental difference between an otto cycle and a duel air standard (modern diesel) cycle.
Maybe you are confusing the expansion of the hot gas during the power stroke (a nearly adiabatic process) with the heat addition during combustion (definitely not adiabatic) near TDC. Sorry, but I don’t know how to explain this concept any better. Try doing a search on “otto cycle,” of look it up in any undergrad thermodynamics book. This is not just my opinion (and these are not “my diagrams”), this is basic thermodynamics that has been around for about a century now and forms the basis of gasoline engine design. I am not debating this, I’m just trying to explain it.
The details of flame propagation speed in an actual engine are really too complex to explain in any more detail here. There are just to many variables, and there is not a single number. If you are really interested, find yourself a copy of a book by Charles Taylor called, “The Internal Combustion Engine in Theory and Practice.” IMHO, it is about the best basic reference for engine design. It’s two volumes (about 1200 pages) and it does a pretty good job of explaining both the theory (volume 1) and the actual design issues (volume 2). It includes lots of info on flame propagation speed.