Why does a Diesel need a smaller carburettor?
06-08-2014, 02:31 PM
#127
06-08-2014, 02:56 PM
#129
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A well adjusted engine throttles cleanly, so compression linked to throttle would be unnecessary overkill. I am puzzled by the question although questions are always good. They invite discussion..
06-08-2014, 03:29 PM
#130
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I know that Saab has an adjustable compression diesel that moves the entire block up and down to compensate has differing fuel types and altitude when driving across Europe, similarly model diesels have an advantage with altitude changes.
In another thread there is the MVVS diesel conversion head that a stunt flier used in South Africa going from low coastal altitude right up many thousands of metres where a change of a prop with wider blades to grab the thin air and a lower of the comp dealt with the rarified atmosphere better than simply adding nitro for a glow engine.
In another thread there is the MVVS diesel conversion head that a stunt flier used in South Africa going from low coastal altitude right up many thousands of metres where a change of a prop with wider blades to grab the thin air and a lower of the comp dealt with the rarified atmosphere better than simply adding nitro for a glow engine.
Last edited by Recycled Flyer; 06-08-2014 at 03:31 PM.
06-08-2014, 04:45 PM
#132
?????????????? Mx never heard of that one Jon Garcia --No carb?? how does fuel and air get into engine and how is it metered?? regards martin
Last edited by AMB; 06-08-2014 at 05:17 PM.
06-24-2014, 04:08 PM
#133
Wow! This must be one of the oldest threads in the on-line world of model theory and practice!
I still consider, as I mentioned in other, shorter duration, topics, that the limitation may be - simply? - the combination of a limited capacity "air pump" and a less tolerant 'main combustible' fuel fraction. Years back, I mentioned that kerosene (paraffin) has about a 14.7:1 stoichiometric burn ratio - BY WEIGHT! Air is "somewhat" less dense than any evaporated or atomized fuel fraction. We are not used to thinking in terms of the weight of fuel and air. But gases vary so much in volume in different conditions that only weight allows certain analysis. Chemists also work with the 'weights' of reacting elements/compounds - not the accidental physical volumes they may occupy at a given time...
NOTE: Gasolines (petrols) are more volatile, more easy to ignite, than kerosenes. In the US Navy, the tragic losses aboard fleet aircraft carriers in WW2 from AVGAS fires, resulted, in part, in going to turbo-jet (kerosene fueled) shipboard aircraft. Kerosene, particularly the formulations used shipboard, are much harder to ignite... Which may relate to the narrower f/a ratios that work for us... (See: JP-5, JP-8.)
Methanol (and ethanol) can burn productively, if not ideally, at extremely over-rich f/a conditions. Kerosene cannot. It can burn well in over-lean conditions, but that forfeits the heat yield of matching weights to stoichiometric. Obviously, another "loss." I have no idea what the ratio is, but have observed that compression ignition engines will stop, rich, far sooner than methanol fueled glow engines. Consider: at the compression ratios we need, an excess of atomized fuel may compress out of atomization to liquid. We've all been warned, and some of us have suffered, the dangers of hydraulic lock...
That is, kerosene has only a narrow range of useful mixtures, modified to a degree by the flammability of the fuel's ether fraction. In other words, methanol can yield some additional power regardless of excess richness, while kerosene cannot.
Back to the limit of the weight of air our 'engine as air pump' can provide. Matching the available weight of air to the weight - of essentially the kerosene fraction - of our fuel brings us closer to that double potential of stoichiometric heat release (e.g., in Imperial measure) of, say, BTU per unit weight. If the engine cannot inhale more air due to form and layout, what is possible is defined. Merely building in larger inlets and passages does not allow an engine to pump more than its gross internal volume.
Glow engine RC carbs? Are designed/developed for glow fuel use where methanol adds useful, if grossly inefficient, power at RPM extended beyond the 'pump's' efficient filling of the crankcase. So, they are larger in area than would be optimal for stoichiometric combustion... Our 'diesels' appear amazingly tolerant of prop, fuel and setting variations, so long as they are within the limits.
Years ago, I did find an SAE paper studying flame-propagation rates of different fuels. Admittedly, we don't have highly similar conditions to the purely 'flammables plus ambient air' SAE test cylinder. We have other fractions present - oil, damping temperature and reaction by - if nothing else - dispersing flammables from the oxygen. But, they also draw off heat into non-flammable oil present, reducing output yield. In "CI" combustion, the initiating ether fraction produces less heat than the kerosene. (I have no number for this but presume it lower than for kerosene.)
The SAE paper found finite flame propagation velocities for "diesel" (I.e., kerosenes ) fuels and petrol (gasoline) fuels, but found methanol/ethanol rates too rapid to measure. This suggests other ideas, which do not relate to our OP topic...
So, in conclusion, IF an engine can inhale sufficient air for a stoichiometric burn at partial throttle, admitting more air, which affects the throttle fuel metering balance more than the weight of air ingested, is wasted effort. "The water overflows the pail and does nothing usable." An optimum has been achieved - anything below or beyond the optimum is LESS than optimal, no?
I still consider, as I mentioned in other, shorter duration, topics, that the limitation may be - simply? - the combination of a limited capacity "air pump" and a less tolerant 'main combustible' fuel fraction. Years back, I mentioned that kerosene (paraffin) has about a 14.7:1 stoichiometric burn ratio - BY WEIGHT! Air is "somewhat" less dense than any evaporated or atomized fuel fraction. We are not used to thinking in terms of the weight of fuel and air. But gases vary so much in volume in different conditions that only weight allows certain analysis. Chemists also work with the 'weights' of reacting elements/compounds - not the accidental physical volumes they may occupy at a given time...
NOTE: Gasolines (petrols) are more volatile, more easy to ignite, than kerosenes. In the US Navy, the tragic losses aboard fleet aircraft carriers in WW2 from AVGAS fires, resulted, in part, in going to turbo-jet (kerosene fueled) shipboard aircraft. Kerosene, particularly the formulations used shipboard, are much harder to ignite... Which may relate to the narrower f/a ratios that work for us... (See: JP-5, JP-8.)
Methanol (and ethanol) can burn productively, if not ideally, at extremely over-rich f/a conditions. Kerosene cannot. It can burn well in over-lean conditions, but that forfeits the heat yield of matching weights to stoichiometric. Obviously, another "loss." I have no idea what the ratio is, but have observed that compression ignition engines will stop, rich, far sooner than methanol fueled glow engines. Consider: at the compression ratios we need, an excess of atomized fuel may compress out of atomization to liquid. We've all been warned, and some of us have suffered, the dangers of hydraulic lock...
That is, kerosene has only a narrow range of useful mixtures, modified to a degree by the flammability of the fuel's ether fraction. In other words, methanol can yield some additional power regardless of excess richness, while kerosene cannot.
Back to the limit of the weight of air our 'engine as air pump' can provide. Matching the available weight of air to the weight - of essentially the kerosene fraction - of our fuel brings us closer to that double potential of stoichiometric heat release (e.g., in Imperial measure) of, say, BTU per unit weight. If the engine cannot inhale more air due to form and layout, what is possible is defined. Merely building in larger inlets and passages does not allow an engine to pump more than its gross internal volume.
Glow engine RC carbs? Are designed/developed for glow fuel use where methanol adds useful, if grossly inefficient, power at RPM extended beyond the 'pump's' efficient filling of the crankcase. So, they are larger in area than would be optimal for stoichiometric combustion... Our 'diesels' appear amazingly tolerant of prop, fuel and setting variations, so long as they are within the limits.
Years ago, I did find an SAE paper studying flame-propagation rates of different fuels. Admittedly, we don't have highly similar conditions to the purely 'flammables plus ambient air' SAE test cylinder. We have other fractions present - oil, damping temperature and reaction by - if nothing else - dispersing flammables from the oxygen. But, they also draw off heat into non-flammable oil present, reducing output yield. In "CI" combustion, the initiating ether fraction produces less heat than the kerosene. (I have no number for this but presume it lower than for kerosene.)
The SAE paper found finite flame propagation velocities for "diesel" (I.e., kerosenes ) fuels and petrol (gasoline) fuels, but found methanol/ethanol rates too rapid to measure. This suggests other ideas, which do not relate to our OP topic...
So, in conclusion, IF an engine can inhale sufficient air for a stoichiometric burn at partial throttle, admitting more air, which affects the throttle fuel metering balance more than the weight of air ingested, is wasted effort. "The water overflows the pail and does nothing usable." An optimum has been achieved - anything below or beyond the optimum is LESS than optimal, no?
Last edited by Lou Crane; 06-24-2014 at 06:11 PM.
06-26-2014, 04:55 PM
#134
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QUOTE=AMB;11819978]?????????????? Mx never heard of that one Jon Garcia --No carb?? how does fuel and air get into engine and how is it metered?? regards martin[/QUOTE]
I think Max meant no intake throttle as opposed to no carb. He's quite right too-the Jan Garcic R/C version does use an R/C adjustable compression setup for throttling. See attached pics. The same approach was taken early on by Bob Davis for his 049 and 09 Cox conversion heads-there was an adjustable compression R/C head option....
ChrisM
I think Max meant no intake throttle as opposed to no carb. He's quite right too-the Jan Garcic R/C version does use an R/C adjustable compression setup for throttling. See attached pics. The same approach was taken early on by Bob Davis for his 049 and 09 Cox conversion heads-there was an adjustable compression R/C head option....
ChrisM
Last edited by ffkiwi; 06-26-2014 at 04:59 PM.
06-26-2014, 09:48 PM
#135
I have a book published in about 1948 by Air Age (parent of Model Airplane News, at least at that time) in which one 'diesel' engine changed compression by means of an eccentric crankshaft bearing. A lever looking like the spark advance lever on a spark-fired engine rotated the bearing to move the shaft up or down in the 'case. I'll have to dig that out for the details... Could that have been an Atom?
What that did to the 'sleeve' timing and geometry doesn't bear thinking about... But it did work. With an adjustable contra-piston, all that remains the same, except for the variable combustion chamber volume.
Back to the throttle top range and lack of apparent matching power in our converted diesels... It occurred to me since last post that the metering progression for a glow-fuel engine may be skewed to go 'relatively' richer as WOT approaches, to provide more oil, and more evaporative cooling from the methanol. -IF- the metering progression stayed to the lean, or even perfect, side instead, AND the engine maintains excellent volumetric efficiency at the higher power end, we might not get this oddity of glow carbs not benefitting over the top 20%-25% of the range... And/or we might be entering a range where a compression tweak would help... Can't try either with of the shelf available pieces...
What that did to the 'sleeve' timing and geometry doesn't bear thinking about... But it did work. With an adjustable contra-piston, all that remains the same, except for the variable combustion chamber volume.
Back to the throttle top range and lack of apparent matching power in our converted diesels... It occurred to me since last post that the metering progression for a glow-fuel engine may be skewed to go 'relatively' richer as WOT approaches, to provide more oil, and more evaporative cooling from the methanol. -IF- the metering progression stayed to the lean, or even perfect, side instead, AND the engine maintains excellent volumetric efficiency at the higher power end, we might not get this oddity of glow carbs not benefitting over the top 20%-25% of the range... And/or we might be entering a range where a compression tweak would help... Can't try either with of the shelf available pieces...
Last edited by Lou Crane; 06-26-2014 at 10:06 PM.
06-26-2014, 09:58 PM
#136
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I have a book published in about 1948 by Air Age (parent of Model Airplane News, at least at that time) in which one 'diesel' engine changed compression by means of an eccentric crankshaft bearing. A lever looking like the spark advance lever on a spark-fired engine rotated the bearing to move the shaft up or down in the 'case. I'll have to dig that out for the details... Could that have been an Atom?
What that did to the 'sleeve' timing and geometry doesn't bear thinking about... But it did work. With an adjustable contra-piston, all that remains the same, except for the variable combustion chamber volume.
What that did to the 'sleeve' timing and geometry doesn't bear thinking about... But it did work. With an adjustable contra-piston, all that remains the same, except for the variable combustion chamber volume.
As regards timing well there was/is a John 0.35cc where the entire cylinder screws up or down in the case to adjust compression-that too must affect the timing-but it doesn't seem to have any deleterious effect on running.....quite distinctive looking it has vertical fins on the head...
ChrisM
06-28-2014, 10:08 AM
#137
Senior Member
I had a Tower 40 with a custom venturi for control line stunt. I flew it with muffler pressure and uniflow tank. I flew it diesel with a 12 x 6 and glow with an 11 x 5. Larger tank for glow. It flew the airplane about the same in both configurations.
