Thanks for the very useful info. There is a formulation for figuring out compression ratio and with the above chart will give me a starting point in which to squeeze a little more out of the motor without having to guess. I have always wondered about what the correct fuel octane should be for a given motor without spending a lot of money and time testing.
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Yes. the engine we are developing is for the Navy and it runs on Jet A.
The military has one fuel forward policy. That is they dont want to logistically support multiple fuels. Moreover, the Navy doesnt want gasoline on any ships due to the explosion hazard. We are under contract to develop a small heavy fuel engine for unmanned surface vehicles. This engine would also have application as a generator for armored vehicles and for tactical power in the field. Small diesel engines are heavy and are by the very nature of their combustion process speed limited. A spark ignition engine can advance the spark at high speed and make almost as much power on heavy fuel as it can on gasoline. By using advanced fuel injection we can admit the fuel after the exhaust port is closed and have no wasted fuel (as is typical of a carbureted 2 stroke). Bottom line is that you can get the weight and cost advantages of a 2 stroke with the fuel consumption of a 4 stroke. Not quite as good as a diesel on fuel consumption, but the total package is very light and thats what you need for small unmanned surface vehicles where the propulsion package weight is limited.
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A diesel isn't a high RPM engine, but it's a torque monster. HP is HP, so make up the RPM loss with gearing since you can trade one for the other. Doesn't do anything for the weight though, although I've seen some pretty small (and light) diesels (there's one that will work on model R/C planes).
I'd think a small turbine would work well also, small, light and can run on deisel, kero, and even gasoline if needed....but then there's the economy question.
Dane Lance
700-P
CSH/500Mod
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And there's water injection as a way to kill detonation. But again this is something that does no good if you don't need it . . . and again, lots of guys put water injection systems in cars that didn't need it and still claimed it was hot stuff. And you can never convince them otherwise.
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Smitty, even if you don't need water/methanol injection to cure detonation it will still increase performance in any engine, especially in a diesel. It does this by cooling the intake charge and increasing the air density, both of these things will improve performance.
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My 1st car, a '63 Plymouth Fury with a 318 V8, this would seem to run better in cool misty weather as long as the wires & distributor were fresh enough not to be carbon tracked. Then again the wheels spin up nice on wet pavement so, more power? Maybe a little.
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The following chart posted earlier is generally related to a 4 cycle engine and can be easily misused for a 2 cycle. If it is used a more appropriate value for the compression ratio of the 2 stroke engine is needed. That would be based on the cylinder volume from TDC to the top of the exhaust port. For the Yamato 102 that would be about 6:1 which indicates 81 octane from the chart. The 102 Yamato manual states 86 min so 87 would be a good choice. I only use 87 E free in my stock Yamato 80 (timing at 0.200" = max range in Yamato manual and compression ratio to exhaust port is 6.5:1 higher than 102 or 302). I have tried 91 and more advance carb mixture adjusted as needed, etc- it proved to be not as good by the tach, speed and acceleration, near as I could tell. It certainly didn't run better.
The final octane needed can only be done by testing and that takes time. No easy way out.
#14.1
Ram4x4 commented
Yesterday, 08:08 PM
It's not that easy. There isn't a magic formula you can use. Manufacturers determine the rating after designing and building the engine through testing with decreasing octane until knock is detected. That's why most discussions on octane tend toward "just use what the manufacturer recommends".
However....there are general rules of thumb if you know the compression ratio (carb'd engine, no automatic controls) to get you in the ball park:
Compression Ratio: Octane Number:
5:1 - 72
6:1 - 81
7:1 - 87
8:1 - 92
9:1 - 96
10:1 - 100
11:1 - 104
12:1 - 108
Other than that, there are too many factors involved to use a single formula.
Just changing the timing will affect the octane requirement, etc, so if you are modding the engine, then that will affect the needed octane over what the manufacturer calls for.
p.s. there's no magic equation for converting cylinder pressure in PSI to compression ratio. We know that in terms of cylinder volume we can say that when the piston is at Bottom Dead Center (BDC) the volume is at maximum. When the piston is at Top Dead Center (TDC) it is at minimum, so the comparison of max and min volume can give us the ratio....but, piston rings don't seal perfectly, and 2-strokes have a tendency to shove some of the incoming fuel charge out the exhaust port (i.e. lost compression), also why tuned pipes are great for increasing power on a 2-stroke.
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Originally posted by Ram4x4 View PostA diesel isn't a high RPM engine, but it's a torque monster. HP is HP, so make up the RPM loss with gearing since you can trade one for the other. Doesn't do anything for the weight though, although I've seen some pretty small (and light) diesels (there's one that will work on model R/C planes).
I'd think a small turbine would work well also, small, light and can run on deisel, kero, and even gasoline if needed....but then there's the economy question.
Turbines as they get real small have issues. You need to keep the tip speed the same to do the compression and turbine work. So as the tip speed stays the same and the diamter goes down the speed has to go up. To get down in the 5 kw class you need to be running at speeds of around 200,000 rpm. That means you need air bearings and other things to try to keep the rotors supported at these high speeds. I've been desiging turbines for just about 40 years and as a practical matter you really aren't going to have a workable efficient turbine much below 20kw, but that's just my personal opinion. You can do it, but as you noted the efficiency is going to suck. It's kinda natures way of saying don't touch... (with apoligies to Gary Larson)
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The engine in the first STP car was a PT-6, which have ratings of up to 550 hp. Don't know if there was a restrictor on it to choke down the inlet like there was on later cars, but they had lots of power coming off of corners. The reason for that is that free turbine engines (like this one) spool up the gas generator and make full power as soon as it spools up and then the turbine acts like a torque converter, so if you're in a normal engine and you hit the throttle it takes time to make power the turbine, if you power brake it a bit will spool up and take off like a rocket.
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Originally posted by Smitty View PostAnd there's water injection as a way to kill detonation. But again this is something that does no good if you don't need it . . . and again, lots of guys put water injection systems in cars that didn't need it and still claimed it was hot stuff. And you can never convince them otherwise.
What's surprising is how little water it takes to prevent detonation. You want the water in there when you need it (not very often) and the rest of the time you can enjoy the power and fuel economy that comes from a high compression ratio.
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Wikipedia, for what its worth;
"The 1962 Oldsmobile F85 was delivered with the Fluid-Injection Jetfire engine and referred to the water/alcohol mixture as 'Turbo-Rocket Fluid'. . With the introduction of the intercooler the interest in water injection almost disappeared, but recently water injection has also been of interest because it can potentially decrease nitrogen oxide (NO) and carbon monoxide (CO) emissions in exhaust"
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The small boat we are doing is a surveillance platform for the littoral environment, rivers and lakes, the boat is 66 inches long and weighs 84 pounds fully laden. That's light enough for two people to schlep... We have an advanced fuel injection system developed by Orbital Engines and it is very similar to the system used in the Mercury Optimax heavy fuel motors. This system injects fuel after the ports are closed so there is no wasted fuel like in 2 strokes with a carb. The system injects a mixture of fuel and air and it runs way past the normal lean limit. That is the cloud of fuel and air is ignited as it passes the spark plug near the end of the injection process, and combustion starts in this stratified mixture. By running so lean you can get pretty good fuel consumption. The only time we get near stoichiometric fuel air ratio is when we are running at wot. Specific fuel consumption isn't as good as a diesel, but by running over lean we can make up for the lower CR. I can't say what the trapped CR is, and it's lower than on gasoline but the trick is to pull timing advance at lower engine speeds and then as you get to high speed you can put it back in and get good power and not bad specific fuel consumption.
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With this kind of fuel spark plug fouling is always an issue. You need to keep the plug hot and keep the combustion lean. We are using the Mercury oil that is used in their heavy fuel outboards for the special forces. It is full of stuff to keep the rings clean and improve lubricity and ring life. Ring life is an issue because the fuel gets on the walls and it doesn't evaporate like light fuel so there is a lot of dilution of the oil on the walls. This oil is also full of heavy metals and it is hard on spark plugs if you don't keep them hot enough. Above idle we are doing well but have work to do in the next month to improve plug life at idle. We have a special cylinder head that we have developed that improved combustion at idle and low power and this has worked pretty well for us. Burning efficiently at idle is the key, that way the combustion temp increases and things get better in a hurry. If you put a load on the engine at idle and drag the engine down and it starts to run rich the temps drop and it all can go to hell in a handbasket. Lots of tuning and working with advance curves, fuel injection timing and combustion chamber shaping to get it to all work together. As an aside, the Mercury Multifuel oill is really good for racing outboards and Mercury used to recommend it for their big racing motors, but it's really expensive and you have to buy 5 gallons of it at a time.. $25.00 a quart is pretty dear already, but when you have to buy 5 gallons you're shelling out $500... Ouch...
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Hah, Worm, you can't shut it off now, even though you are the O.P.!! This is one of those threads that takes on a life of its own, and goes meandering far from the original question.
Doc, don't know how old you are, but in the late-'70s/early-'80s several organizations from the State of California to various consumerist outfits did controlled testing of the purported fuel-saving devices of those days following the OPEC rise in oil and fuel prices. These included various water and "vapor" injection systems. In the studies I saw, no significant benefit was shown for these injections systems. Yes, depending on design they could lower inlet charge temperature, but they also cool the burn, which is what they are supposed to do to quell detonation. If you cool the burn in a heat engine, one that's not detonating, that doesn't help performance (though it is another way, besides EGR, to reduce nitrogen oxides that promote smog). Naturally these testing agencies all concluded that water injection had no value. And it did not . . . not in the low-compression smoggers sold by car companies in those years. But they could have said it would have great value in allowing car companies to build lightweight high-compression engines with lots of practical street torque and therefore better fuel-efficiency.
I'm with Yellowjacket in thinking that a new generation of vehicle engines designed by the factory WITH sophisticated A.D.I. systems, computer actuated to turn on and off with the usual sensor inputs, could save a lot of barrels of oil. Additionally, they could use the A.D.I. to replace the whole EGR system as well (this has been done, a long time ago, using a special injector that would spray very small amounts of water). If the A.D.I. tank carried 50/50 water/methanol as the airplanes did, freezing of the fluid would not be a problem except in cars used in the far north, and there the tank could be warmed by the engine exhaust (via those flapper-valves that have been in exhaust systems since the '40s), and most people up there use plug-in engine heaters as well. As to the water/methanol, that would require gas stations to add tanks and pumps specifically for that, a major hurdle and maybe a deal-killer for the whole idea. But consider that methanol can be made from garbage, from logging slash, and other stuff that we have to get rid of in any case; why not get something useful out of it?
Well anyway, it's an idea . . .Last edited by Smitty; 09-05-2015, 11:14 AM.
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Smitty - BMW is doing exactly what you state in the M series racing engines with regard to water injection to cool the air/fuel mixture in engines. Article for those who are interested in how BMW does it is at: http://www.autoblog.com/2015/02/13/b...ection-system/
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Interesting article. It sounds like they are using a high boost engine with high compression and if you don't get some help from water injection it wouldn't be pretty. My water injection system wasn't sophisticated at all. It was just a vacuum switch that turned on a windshield washer pump that sprayed water into the air box upstream of the carbs. I had a small nozzle that I got from a "nebulizer" and that gave me a mist and that was it. With today's technology you could tune the amount of water you admitted and as they are doing, install interlocks that make sure you don't slag down the engine if you run it out of water or something fails. Considering all of the other very complex systems that we have in cars today, putting in system to suppress detonation would be relatively easy and not that expensive. With all the push for CAFE regulations I'm surprised that they haven't done it by now on a major scale.
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I have used water injection many times in outboard racing. When I get a bad start I sometimes attempt an inside pass up the back stretch, this often results in a powerful water injection system being applied in turn two. Sometimes this works great cooling both the motor and the driver but, more often than not an overall gain is not achieved. I would recommend avoiding this kind of water injection in outboard racing as its use could result in the even less desirable water immersion.
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Water injection was used on a FEH, and FER in here Ca for years. Used it on runabouts coming out of the corner and on the straight once in awhile. Was on a 3cly motor. Worked very well and out of the turn it was great.
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-are you not talking about water injection into the exhaust manifold to cool the exhaust gases, cooling and slowing down the sound waves, for more torque at lower rpms ?
Most found sliding the pipe was more effective
Early Quincys water injected ther exhausts to some effect.Last edited by bh/; 09-06-2015, 10:02 AM.
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No, bh/ and 1100-one, we're not talking about that, which is a whole different thing. Spraying water into the pipes cools the medium in which the acoustic waves travel, slowing them and making the pipe work like it was longer and therefore tuned for a lower rpm powerband, what you want while planing-off or accelerating out of a slow corner.
What we were talking about is spraying a fine mist of water or water/alcohol into the induction side of the engine, along with the fuel/air mixture, for the sole purpose of stopping detonation. In Unlimited hydros in the piston engine days, when the regular racing fuel was nearly always 115/145 av-gas (doped-up with additives for higher octane yet) the water/alky was not started until the engine was making 60" manifold pressure, which in an airplane would have been a "5-minute war emergency" throttle setting (the raceboats, then, and the Reno Air racers now, pull as much as 130" M.P., which is why the engines have to be rebuilt after a few hard heats. Doc, in some of the later U-boats, this water injection did by intention serve a second purpose exactly as you described, as a charge-cooling agent, and the aftercooler behind the blower was eliminated. Shane's link shows that BMW is doing this as well, in the engine described.
In a car, water injection might be controlled with a push-button on the steering wheel, or a manifold vacuum switch, or a knock sensor, or nowdays with a combination of sensor inputs via the computer. Has anybody tried such a thing on a PRO outfit, in conjunction with a high-compression head that would detonate without the water?Last edited by Smitty; 09-05-2015, 11:03 AM.
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