Assuming you have full injector control (timing, pulsewidth, pressure, etc.), and a custom injector nozel with an optimized orifice diameter and inlet radius, how well could this perform in comparison to a traditional compression ignition engine. There is ALOT to this and I am verifying this analytically, computationally (CFD), and experimentally (high speed imaging). But I am in the beginning stages of literature review currently.
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Spark Ignited Heavy Fuel Direct Injection 1
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JayMaechtlen
Industrial
this sounds like 'what happens if I drop the CR of a Diesel engine to where it won't run, then add spark plugs so it will run.
You give up the efficiency of the high compression, and maybe give up the ability to run at very lean mixtures.
overall, sounds like a bad deal. High CR is a big piece of Diesel (CI) efficiency, right?
Jay Maechtlen
You give up the efficiency of the high compression, and maybe give up the ability to run at very lean mixtures.
overall, sounds like a bad deal. High CR is a big piece of Diesel (CI) efficiency, right?
Jay Maechtlen
Lou Scannon
Automotive
On paper, yes, but in practise, not so much. My understanding is that diesel engine compression ratios today are driven by startability and ignitability (of marginal fuels, at low load), not so much by thermal efficiency any more. This is especially true of supercharged engines (and which diesel engines today aren't?), where the thermodynamic internal CR requirement is moderate, since so much compression is taking place outside the cylinder.JayMaechtlen said:
I believe the other drivers limiting optimum diesel CR are mechanical and thermal losses due to friction and heat transfer resulting from high CR, and perhaps, at a system level, the trade-off between thermal efficiency, and total mass (driven by stress... respectively driven by CR).
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
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BrianPetersen
Mechanical
The compression ratio may be driven by cold starting requirements but still, an engine with a 17:1 compression ratio is going to trend towards having higher thermal efficiency than one with 9:1 compression. Supposedly there is an optimum balance between heat losses (gets worse with high compression) and theoretical cycle efficiency (gets better with high compression) somewhere near 14:1 but the drop-off with increasing compression beyond that is nowhere near the drop-off with lowering compression below that.
Anyhow, there is still a fundamental problem. Sparks are used for igniting mixtures of vaporized fuel and air (within a certain range of lambda that is ignitable by spark). Heavy (low volatility and high viscosity) fuels are resistant to vaporizing. In a diesel engine, they tend to form a mist of fine droplets which, when mixed in air that is above the ignition temperature, each individually burn on the surface of the droplet until it is gone. There is a trend nowadays towards intentionally delaying ignition until after a certain fraction of the droplets have actually vaporized (and this requires compression ratios lower than the historical norm - and closer to that 14:1 apparent optimum range). But these are still compression-ignition engines.
Compression ignition engines use one of two methods of ignition when the fuel doesn't want to ignite (cold starting): Glow plugs, or pre-heating the intake air. The function of pre-heating the intake air should be obvious. Glow plugs work by locally raising the temperature above the ignition temperature so that any fuel droplets that come near them have a chance to ignite. Glow plugs are glowing throughout the whole combustion process (and in fact, through the whole engine cycle, in which they also serve to preheat the intake air). Sparks only are present for microseconds. That's enough to ignite intimately-mixed fuel vapor, but unreliable for droplet mists.
Basically, spark ignition of diesel fuel doesn't work very well (if at all), and glow-plug ignition of gasoline doesn't work very well (if at all).
If you are building a compression-ignition engine, what's wrong with using glow plugs (or intake air preheaters)? Why do you want to do it the hard (if not impossible) way by using sparks? What's the proposed advantage? Glow plugs work. Intake air preheaters also work.
Anyhow, there is still a fundamental problem. Sparks are used for igniting mixtures of vaporized fuel and air (within a certain range of lambda that is ignitable by spark). Heavy (low volatility and high viscosity) fuels are resistant to vaporizing. In a diesel engine, they tend to form a mist of fine droplets which, when mixed in air that is above the ignition temperature, each individually burn on the surface of the droplet until it is gone. There is a trend nowadays towards intentionally delaying ignition until after a certain fraction of the droplets have actually vaporized (and this requires compression ratios lower than the historical norm - and closer to that 14:1 apparent optimum range). But these are still compression-ignition engines.
Compression ignition engines use one of two methods of ignition when the fuel doesn't want to ignite (cold starting): Glow plugs, or pre-heating the intake air. The function of pre-heating the intake air should be obvious. Glow plugs work by locally raising the temperature above the ignition temperature so that any fuel droplets that come near them have a chance to ignite. Glow plugs are glowing throughout the whole combustion process (and in fact, through the whole engine cycle, in which they also serve to preheat the intake air). Sparks only are present for microseconds. That's enough to ignite intimately-mixed fuel vapor, but unreliable for droplet mists.
Basically, spark ignition of diesel fuel doesn't work very well (if at all), and glow-plug ignition of gasoline doesn't work very well (if at all).
If you are building a compression-ignition engine, what's wrong with using glow plugs (or intake air preheaters)? Why do you want to do it the hard (if not impossible) way by using sparks? What's the proposed advantage? Glow plugs work. Intake air preheaters also work.
a short comment on compression ratios - it seems that smaller diesels tend toward higher ones, while larger diesels tend toward smaller ones. The diesels I'm most familiar with in the 7-18L range have compression ratios between 16 and 17.5, and the one I know best at 296L has a compression ratio of 11 to 13 depending on the rating. The big one can be used to burn heavy fuel oil or distillate fuel.
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This is not a matter of trying to do something different. This is a contract requirement for conversion of spark ignited gas engines to spark ignited heavy fuel as the military would like to use only heavy fuels (JP5, JP8, etc) rather then having to suppliment multiple fuels. The droplet size of a heavy fuel can be significantly reduced with an air assisted injector.. Apparantly other research has shown ~20% decrease in power at full load compared to gasoline, but at lower loads/rpm's, it is compareable. Take a look at this company Hirth-Engines:
JP5 and JP8 are not what I'd call "heavy fuels" - they're distillate aviation fuels closer to regular diesel than to heavy fuel oil or marine diesel oil.
...so you're trying to convert gasoline engines so that they run on diesel fuel. You've mentioned (adding?) direct injection, which I would assume means a change of cylinder heads to accomodate the injectors, but you're hoping to retain spark ignition. Is changing the pistons over to conventional diesel ones out of scope? Are these "gas" (natural gas, LPG, propane, etc) or "gasoline" engines you're talking about?
...so you're trying to convert gasoline engines so that they run on diesel fuel. You've mentioned (adding?) direct injection, which I would assume means a change of cylinder heads to accomodate the injectors, but you're hoping to retain spark ignition. Is changing the pistons over to conventional diesel ones out of scope? Are these "gas" (natural gas, LPG, propane, etc) or "gasoline" engines you're talking about?
BrianPetersen
Mechanical
If this is a contract requirement then whoever wrote the contract is the one to point the finger at. That does not make the concept work any better.
The combustion chamber shape requirements for a direct-injection engine - spark or compression ignition - are very different from those of a spark-ignited gas (natural gas or gasoline) engine and will require different cylinder heads and piston designs anyhow, in order to function correctly. At that point, it is largely a new engine.
The conceptually best thing to do is simply to screw glow plugs into the head rather than spark plugs, but remember that they have to be positioned correctly in order to work properly.
You can get a compression-ignition engine to start in cold weather by heating the intake air and not using glow plugs.
If you are stuck with contractual requirements to have spark plugs in the cylinder head ... then keep them in the engine as required, but also fit an electric intake air preheater. The engine will start and run as a compression-ignition engine. The spark plugs will not do anything. It will not even matter if you connect them to ignition coils and it will not matter if you spark them at the right time or not. But they will be there, to satisfy the contract requirements, even if they accomplish nothing!
The combustion chamber shape requirements for a direct-injection engine - spark or compression ignition - are very different from those of a spark-ignited gas (natural gas or gasoline) engine and will require different cylinder heads and piston designs anyhow, in order to function correctly. At that point, it is largely a new engine.
The conceptually best thing to do is simply to screw glow plugs into the head rather than spark plugs, but remember that they have to be positioned correctly in order to work properly.
You can get a compression-ignition engine to start in cold weather by heating the intake air and not using glow plugs.
If you are stuck with contractual requirements to have spark plugs in the cylinder head ... then keep them in the engine as required, but also fit an electric intake air preheater. The engine will start and run as a compression-ignition engine. The spark plugs will not do anything. It will not even matter if you connect them to ignition coils and it will not matter if you spark them at the right time or not. But they will be there, to satisfy the contract requirements, even if they accomplish nothing!
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- #10
These are gas engines, both 2 and 4 stroke, piston and rotary. And yes it is well understood the cylinder head would need to be replaced to integrate direct injection. This is not an immediate concern. Obviously this is not economoically feasible, I am just proving the technical feasibility. Its the military, they fund very exotic and impractical research to promote the avenues less taken. Obviously a compression-ignition diesel or spark ignition gas engine is optimized beyond what I am proposing, hence the reason we have them in everything......otherwise I wouldnt be asked to pursue this research. If it was well understood, it would be in everything already as well. Keep that in mind.
My current task is to develop an injector that can produce a droplet size of XXXX size to promote atomization similar to GDI, and improve combustion. Multipulse control will be used as well to control the combustion characteristics. Not sure if I will be going with one of our patent pending injector designs (possibly tweaked even further) or designing a new air assisted injector.
My current task is to develop an injector that can produce a droplet size of XXXX size to promote atomization similar to GDI, and improve combustion. Multipulse control will be used as well to control the combustion characteristics. Not sure if I will be going with one of our patent pending injector designs (possibly tweaked even further) or designing a new air assisted injector.
Sounds like reinventing the wheel. In the past there were many multifuel diesel engines that were offered by
various manufactures. 1) They did not use spark plugs. 2) To run on high octane gasoline they could not have a very low CR. If these said engines are a diesel that was converted to gas, then convert them back. I'd like to hear more details.
various manufactures. 1) They did not use spark plugs. 2) To run on high octane gasoline they could not have a very low CR. If these said engines are a diesel that was converted to gas, then convert them back. I'd like to hear more details.
SomptingGuy
Automotive
Military is the key word here. I fuel for all. Expense no barrier.
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turbocohen
Automotive
Atchooly this is in the works. DI multi fuel and "hot electrode/spark plug" combined can accomplish much that could not be accomplished in the past.
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