OB_Busadan
Registered
Great looking bike Frank. Let us know how it goes at Bunker Hill.
The Psycho Busa
- Thread starter Frank Adams
- Start date
G
Guest
Motorhead. No, I meant what I said. The stroke of the crank is the distance from the center of the main bearing to the center of the rod journal, TIMES 2. So, to get a 3mm increase in stroke, you need to move the new center of the rod journal 1.5mm (.059055") away from the main. Remember, the crank turns a full 180 degrees to get the piston to travel the full length of the stroke.
Frank Adams
Registered
Tom, are you planning on useing the pw2 on your Busa?
If so, do you have a D-J 250 close to you so you can get a map made for you setup?
If so, do you have a D-J 250 close to you so you can get a map made for you setup?
O
OB_Chanceg
Hey Frank,
I was wanting to know what kind of swingarm is that and how many inches over is it? GREAT looking Busa!!
I was wanting to know what kind of swingarm is that and how many inches over is it? GREAT looking Busa!!
G
Guest
GSXR - The ring stack on the Busa piston is already pretty compact. Probably not worth the cost of full custom pistons for the little bit of room you would gain. I looked at that option but decided that it was to costly for the return.
Frank - No PC yet. And the shop I use has a model 150 (I think). Gonna try a remap with the Yosh box and a small fuel pressure increase. But a PC2 is definitely in the plans.
Frank - No PC yet. And the shop I use has a model 150 (I think). Gonna try a remap with the Yosh box and a small fuel pressure increase. But a PC2 is definitely in the plans.
OB_Johnnycheese
Registered
FRank glad to here you are running.
Tom, I believe you are correct I will find out when I put on later this year,My bottom end is done I just need to figure what to do on Bore and Head.It is harder to deciede than anything else was.
Tom, I believe you are correct I will find out when I put on later this year,My bottom end is done I just need to figure what to do on Bore and Head.It is harder to deciede than anything else was.
OB_Johnnycheese
Registered
tom this is my .02 on the yosh box;
stock pipe gutted air box and no filter at the box set at +5+10+10 on the B setiing,A seting was at about 10:00 to 11:00 oclock postion, plugs still white
even with the fuel pressure rasie I don't think it will be enough,also won't the timing be incorrect due to the stroke.this is what I have been thinking about mine.
stock pipe gutted air box and no filter at the box set at +5+10+10 on the B setiing,A seting was at about 10:00 to 11:00 oclock postion, plugs still white
even with the fuel pressure rasie I don't think it will be enough,also won't the timing be incorrect due to the stroke.this is what I have been thinking about mine.
G
Guest
Johnnycheese,
I had hoped to be able to get close with a simple re-map. From what you are telling me, that doesnt sound like a very good idea. Lean is mean, but I dont want to burn it up. I might as well just get one and have done with it. The ability to alter the timing is a nice plus as well. So who has Power Commanders in stock for a fair price?
I had hoped to be able to get close with a simple re-map. From what you are telling me, that doesnt sound like a very good idea. Lean is mean, but I dont want to burn it up. I might as well just get one and have done with it. The ability to alter the timing is a nice plus as well. So who has Power Commanders in stock for a fair price?
OB_GSXRTURBO1
Registered
Good point Tom, half of that 3mm stroke is at the top of the bore, the other half is at the bottom.
The added height assumes a stock pin location, if custom pistons with different pin locations are used the would not be the case, no? Can this be done on a Busa engine, or is the pin hole already too close to the oil ring grooves?
Thanks for the update Frank, glad your bike is running well.
[This message has been edited by GSXRTURBO1 (edited 12 June 2000).]
The added height assumes a stock pin location, if custom pistons with different pin locations are used the would not be the case, no? Can this be done on a Busa engine, or is the pin hole already too close to the oil ring grooves?
Thanks for the update Frank, glad your bike is running well.
[This message has been edited by GSXRTURBO1 (edited 12 June 2000).]
OB_Johnnycheese
Registered
So far mine is on the big B/O
Jim(Too_Quick Had a good price so did (underdog?) It was a week ago I believe the posted them.
Question: are you useing the head motor mount?
Jim(Too_Quick Had a good price so did (underdog?) It was a week ago I believe the posted them.
Question: are you useing the head motor mount?
G
Guest
Johnny - Yes. Likely have to make some offset bore piece.
Omega - Since you put it that way I cant put it off anymore. Have one coming.
Omega - Since you put it that way I cant put it off anymore. Have one coming.
G
Guest
Honestly Tom, a PC2 for your setup is as essential as condoms in a whorehouse.
OB_THE MOTORHEAD
Registered
hey tom when you smack your head with your 060 base don't say i did not warn you!
G
Guest
Hey Motorhead. Do you really think were not gonna check the deck height? The deck height is one of the most important measurements in the motor. You figure compression based on a specific deck height. Your piston to valve clearance and piston to head clearance are a function of that height. The dech height also determines the quench area, which affects the flame front. If youre building motors and checking the piston to head clearance with the throttle, Im here to tell ya there are better (and cheaper) ways.
OB_Johnnycheese
Registered
Tom I was going to try something different with the hole alginment. You can e-mail me if you want.
Johnnycheese@aol.com
Johnnycheese@aol.com
Frank Adams
Registered
Tom, Talked earlier with a guy that has a built 1360 motor and his bike was DIALED with a Yosh box(yes it was tweaked). They dyno'd it with this yosh setting....reset it to "0" and put the pw2 with the 009 map and gained 14hp(fourteen!). He said they ran out of time and couldn't tweak the 009 map out but planned to later on.
Just thought you should know that!
Also talked to a engine builder that said if your putting a spacer plate/ or spacing up your block your in for trouble?(I, Myself don't know anything about it?!)
Chanceg, that is a Mcintosh swingarm. 7 to 9 inches.
Just thought you should know that!
Also talked to a engine builder that said if your putting a spacer plate/ or spacing up your block your in for trouble?(I, Myself don't know anything about it?!)
Chanceg, that is a Mcintosh swingarm. 7 to 9 inches.
Frank Adams
Registered
Omega, I think you would suprise yourself if you just look back at what you wrote?
I can see you Posted without thinking things through first. It's either that or you don't know enough about the game before you commented on this subject?
I took your remarks as a flame....and thats not nessesary!
You will not be investing any money in My prodjects, I'm sure of that.
When you bought your pw2 you where able to recieve a number of different maps.
There is NOTHING to start as a base-line with a setup like mine. There is no Dyno-Jet 250 in My state. The closest one is 7hrs away.
My Busa will go to a 250 soon.
Untill then I needed something(map)....and something is better then nothing.
I totally agree with you about setting each bike up by the dyno.
[This message has been edited by frank adams (edited 14 June 2000).]
I can see you Posted without thinking things through first. It's either that or you don't know enough about the game before you commented on this subject?
I took your remarks as a flame....and thats not nessesary!
You will not be investing any money in My prodjects, I'm sure of that.
When you bought your pw2 you where able to recieve a number of different maps.
There is NOTHING to start as a base-line with a setup like mine. There is no Dyno-Jet 250 in My state. The closest one is 7hrs away.
My Busa will go to a 250 soon.
Untill then I needed something(map)....and something is better then nothing.
I totally agree with you about setting each bike up by the dyno.
[This message has been edited by frank adams (edited 14 June 2000).]
OB_THE MOTORHEAD
Registered
they whatever works frank!
OB_THE MOTORHEAD
Registered
sorry tom but when you said x2 i had to reply and yea i know all about squish area flame propagation etc.
In particular, the discussion focused on the problem of detonation, commonly referred to as "knock," which is a very serious and detrimental problem when it occurs - usually the pressures exerted onto the piston top during detonation are much larger (but of a shorter duration, like a pressure spike) than the mean combustion pressure. Nevertheless they are very detrimental to engine life, as the continual high shock loading of the piston, rod, crankshaft and bearings is quite destructive.
Detonation is the result of an amplification of pressure waves, such as sound waves, occurring during the combustion process when the piston is near top dead center (TDC). The actual "knocking" or "ringing" sound of detonation is due to these pressure waves pounding against the insides of the combustion chamber and the piston top, and is not due to 'colliding flame fronts' or 'flame fronts hitting the piston or combustion chamber walls.'
Let's look in some detail at how detonation can occur during the combustion process: First, a pressure wave, which is generated during the initial ignition at the plug tip, races through the unburned air-fuel mix ahead of the flame front. Typical flame front speeds for a gasoline/air mixture are on the order of 40 to 50 cm/s (centimeters per second), which is very slow compared to the speed of sound, which is on the order of 300 m/s. In actuality, the true speed of the outwards propagating flame front is considerably higher due to the turbulence of the mixture. Basically, the "flame" is carried outwards by all the little eddies, swirls and flow patterns of the turbulence resident in the air-fuel mix. This model of combustion is called the "eddy burning model" (Blizzard & Keck, 1974).
Additionally, the genus of the flame front surface - that is the degree of 'wrinkling' - which usually has a fractal nature (you know, those weird, seemingly random yet oddly patterned computer drawings), is increased greatly by turbulence, which leads to an increased surface area of the flame front. This increase in surface area is then able to burn more mixture since more mixture is exposed to the larger flame front surface. This model of combustion is called the "fractal burning model" (Goudin, F.C. et al. 1987, Abraham et al. 1985). The effects of this are observed in so-called "Schlieren pictures," which are high-speed photographs taken though a quartz window of a specially modified combustion chamber (Fig. 1, above).
Schlieren pictures show the various stages of the combustion process, in particular the highly wrinkled and turbulent nature of the flame front propagation (initially called the flame 'kernel'). A higher degree of turbulence, and hence a higher "effective" flame front propagation velocity can be achieved with a so-called squish band combustion chamber design. Sometimes a swirl-type of induction process, in which the incoming mixture is rotating quickly, will achieve the same goal of increasing the burn rate of the mixture.
As a general rule-of-thumb the pressure rise in the combustion chamber during the combustion phase is typically 20-30 PSI per degree of crankshaft rotation. Once the pressure rises faster than about 35 PSI/degree, the engine will run very roughly due to the mechanical vibration of the engine components caused by too great of a pressure rise. Sometimes, the pressure wave can be strong enough to cause a self ignition of the fuel, where free radicals (e.g. hydroxyl or other molecules with similar open O-H chains) in the fuel promote this self ignition by the pressure wave. However, this can still occur even without the presence of free radicals; it just won't be quite as likely to happen. This is why high octane fuels, with fewer of these active radicals, can resist detonation better. However, even high octane fuel can detonate - not because of too many free radicals - but because the drastic increase in cylinder pressure has increased the local temperature (and molecular speed) so high that it has reached the ignition temperature of the fuel. This ignition temperature is actually somewhat lower than that of the main hydrocarbon chain of the fuel itself because of the creation of additional radicals resulting from the break-up of the fuel's hydrocarbon chains in intermolecular collisions.
Detonation usually happens first at the pressure wave's points of amplification, such as at the edges of the piston crown where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine - this is called constructive interference to yield a very high local pressure. If the speed at which this pressure build-up to detonation occurs is greater than the speed at which the mixture burns, the pressure waves from both the initial ignition at the plug and the pressure waves coming from the problem spots (e.g. the edges of the piston crown, etc.) will set off immediate explosions, rather than combustion, of the mixture across the combustion chamber, leading to further pressure waves and even more havoc. Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top. Piston tops, ring lands and rod bearings are especially exposed to damage from detonation. In addition, these pressure fronts (or shock waves) can sweep away the unburned boundary layer (see figure 2 above) of air-fuel mix near the metal surfaces in the combustion chamber.
The boundary layer is a thin layer of fuel-air mix just above the metal surfaces of the combustion chamber (see figure 2, above). Physical principles (aptly called boundary conditions) require that under normal circumstances (i.e. equilibrium combustion, which means "nice, slow and thermally well transmitted") this boundary layer stays close to the metal surfaces. It usually is quite thin, maybe a fraction of a millimeter to a millimeter thick. This boundary layer will not burn even when reached by the flame front because it is in thermal contact with the cool metal, whose temperature is always well below the ignition temperature of the fuel-air mix.
Only under the extreme conditions of detonation can this boundary layer be "swept away" by the high-pressure shock front that occurs during detonation. In that case, during these "far from equilibrium" process of the pressure-induced shock wave entering the boundary layer, the physical principles allured to above (the boundary conditions) will be effectively violated. The degree of violation will depend on (a) the pressure fluctuation caused by the shock front and (b) the adhesive and cohesive strength of the boundary layer. These boundary layers of air-fuel mix remain unburned during the normal combustion process due to their close proximity to the cool metal surfaces and act as an insulating layer and prevent a direct exposure of metal to the flame. Since pressure waves created during detonation can sweep away these unburned boundary layers of air-fuel mix, they leave parts of the piston top and combustion chamber exposed to the flame front. This, in turn, causes an immediate rise in the temperature of these parts, often leading to direct failure or at least to engine overheating.
Scientists and engineers have recently begun to understand combustion in much greater detail thanks to very ambitious computer simulations that model every detail of the combustion process (Chin et al. 1990). Basically, a complete computer model includes a solution to the thermodynamical problem, that is a solution to the conservation equations and equation of state, as well as a mass burning rate and heat transfer model. In addition, a separate code (called a chemical kinetics code) models the chemical processes which occur during combustion and sometimes juggles several thousand different chemical species, some in vanishingly small concentrations! Needless to say these codes require huge amounts of memory and CPU time that only the largest supercomputers in the world can provide. They are far beyond the reach of the private individual and usually only employed by large research institutions or major car manufactures.
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Here's a brief recital of the question we received:
Someone asked:
Rob, I read your "Chemical Soup: The Meaning of Gasoline." Quick question if you have the time... You mentioned that "flame propagation is greatly enhanced by turbulence." Should this be a consideration when an engine is ported? Can turbulence be enhanced by porting without losing the intake flow?
Unless the ports are specifically designed for a strong swirl-type induction process, the turbulence created during the intake process is not very affected by porting. This is true as long as one sticks with the same general port layout. However, drastic porting changes may increase or decrease the turbulence in the combustion chamber, but it is quite difficult to say anything definite. I think that any improvement gained by porting the engine is likely to be far greater than any possibly detrimental effect the porting may have had on turbulence.
As far as I know there is only one motorcycle engine that uses a highly turbulent intake process of the swirl type. It's a "homebuild" single cylinder racing engine from Switzerland that uses cross-scavenging and has two pairs of diagonally opposed intake and exhaust valves. Most conventional ports do induce a very small amount of swirl, but this is not important as far as generating much turbulence. Rather, the biggest benefit is obtained by reducing the squish band to it's safe minimum (about 0.020-0.040 in, depending on the particular engine used). This will have a far greater effect on increasing the turbulence in the combustion chamber than any other modification.
Mike Meagher (meagher@pentec.wa.com) wondered about the effects of the squish band.
It is important to realize the two important functions of reducing the squish band clearance: (a) to enhance turbulence due to rapid ingestion of gas into the combustion chamber, hence increasing the burning rate of the mixture and (b) to reduce the volume of the unburned gas in the boundary layer of cool gas near the piston top and cylinder head surfaces. Typically, gas trapped in the squish area doesn't burn, even if the squish band clearance is relatively large. The cooling effects of the large surface-area-to-volume ratio of this region will prevent any ignition of the fuel-air mix therein, even if the squish band clearance is rather large. Hence any gas caught in the squish band will not be burned near TDC when it does the most good, but later during the combustion process when one cannot extract as much work from the late-burning gases. The amount of gas trapped in the squish band can actually be a substantially greater amount than just the relative volume of the squish band because the pressure wave from the ignition process literally crams a lot of the unburned gas into crevice areas like the squish band. Reducing the squish band clearance will decrease the amount of unburned gas substantially, leading to more complete and faster combustion, lower emissions and improved power. It is one of the few "all gain with no pain" modifications one can carry out on racing or even street motorcycles.
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Someone wondered: Is the extra cooling of the squish band less than the added heat?
Basically the mixture in the squish region is in thermal contact with the cylinder wall and piston top and at roughly the same temperature, which is quite lower than the burn temperature. Reducing squish will decrease the amount of the cool gas in the squish region and increase the amount of hot gas in the burn region. A reduced squish clearance will increase temperatures a little even if the compression ratio is held constant. There is no "extra cooling" mechanism if you reduce the squish band clearance. The cooling rate of the gas in the squish zone depends on the thermal conductivity of the gas-metal interface, on the total surface area of this interface and the temperature difference between gas and metal. Note that these factors are all essentially constant at TDC and don't depend on the squish clearance. Hence the cooling rate is the same for large squish clearances and for small squish clearances. Thus there is no "extra cooling" mechanism if you reduce squish band clearance.
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David Goodenough (dg@pallio.sf.ca.us) asked:
Suppose I mix one gallon of 87 octane pump gas, and one gallon of 92 octane pump gas. Are you telling me that instead of two gallons of 89.5 octane gas I have something closer to 92 (like between 90 and 91)?
The mixed gas' octane rating will in general not be a linear function of the original constituents' octane ratings. Neither will it be a simple function in most cases. Rather, the octane rating becomes a quite complicated, non-linear function of some very small amounts of free radicals, such as hydroxyl and hydroxen peroxide, in the fuel. Essentially, there is no simple analytic way to predict the final octane rating of a fuel; rather, extensive tests with a calibrated engine are necessary (see MON and RON explanations in the last article).
David also asked:
While I'm at it, how does the energy per ounce of mixture react?
As mentioned before, the "energy per ounce" (more exactly the Specific Energy for an stoichometric [an ideal] mixture) does not vary much at all between different kinds of pump gasoline or even racing gasoline.
Ramon Hontiveros (r22666@paccvm.sps.mot.com) wrote:
Ok, I got the article and read it, now some questions: Isn't the fuel already in gaseous form due to carburation?
Ramon, the air fuel mix as it flows into the combustion chamber is not perfectly atomized, that is the fuel vapor droplets consist of larger droplets of fuel molecules surrounded by air. It takes additional energy to further atomize this vapor, that is to break the hydrostatic forces (the surface tension of the fuel droplet). This additional energy can be taken from a hot surface (such as the piston crown, etc.), which then leads to a cooling of the piston. The additional energy can also be imparted via large turbulences and pressure waves, as in a squish band-type motor, which will help to further atomize the fuel. Note that the term "atomize" is actually misleading since the molecules are still left intact, that is the hydrocarbon chains (and oxygen bonds for alcohols) are not broken.
Ramon also wondered:
does carburation just "spray" the gas into the air flow as tiny droplets which are thus still in liquified form?
Yes.
Ramon also asks:
Also, if the fuel does evaporate quickly and creates additional pressure - thus reducing the amount of fresh charge - then the engine will produce less horsepower, right?
Correct. The horsepower will depend on the volumetric efficiency of the engine which is a function of the pressure difference between ambient air and cylinder pressures. If additional fuel is vaporized inside the combustion chamber the pressure in the cylinder will rise, and, while the valves/ports are still open, reduce the volumetric efficiency, and thus the power output.
So 2-strokes would benefit from using fuel that has a _lower_ heat of vaporisation rating?
Correct. A fuel with a lower heat of vaporisation will "atomize" easier and thus improve engine cooling, but decrease power somewhat.
So which type of fuel has a lower heat of vaporisation? Leaded or unleaded?
A fuel's heat of vaporisation does not depend on the it's lead content. Rather, it depends on the fuel's main hydrocarbon chains; iso-octane verses n-heptane, for example. Since pump gas can consist of up to 20 different components with a wide range of individual boiling points (We were serious when we called it "chemical soup!") one should look at the specifications sheet for each fuel separately. For racing fuels these are available from the manufacturer.
Lastly, I gather from the article that it's okay if you end up mixing some leaded fuel with the remaining unleaded fuel in the tank?
Most fuels, pump or racing, will give about the same energy release, so when switching from pump to racing fuel (in general) do not expect a drastic increase in power. Mixing leaded fuel with the remaining unleaded fuel in the tank has no advantage and will give inconsistent plug readings; hence I wouldn't do it on a race bike.
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REFERENCES
Abraham, J. et al., 1985, "A Discussion of Turbulent Flame Structure in Premixed Charges", SAE paper 850345
Blizzard, N.C. and Keck, J.C., 1974, "Exp. and Theo. Investigation of Turbulent Burning Model for Internal Combustion Engines", SAE paper 740191
Chin et al., 1990, "Diagnostics and Modeling of Combustion in Internal Combustion Engines," JSME, Tokyo, p. 81-86
In many high-performance situation, riders clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your motorcycle may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indic ation of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)
Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified motorcycles. A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.
The knock tendency (and hence, the Octane rating) of a fuel is a function of the amount of free radicals present in the fuel prior to ignition and can be reduced by the addition of tetra ethyl lead, aromatics and other additives.
Although some racing organizations still use maximum octane number as the discriminating factor for fuel legality, it is really not appropriate for racing purposes.
Instead one should look at the amount of energy (heat) released in the burning of a particular fuel. This is described by the specific energy of the fuel. This quantity describes the amount of power one can obtain from the fuel much more accurately. The specific energy of the fuel is the product of the lower heating value (LHV) of the fuel and molecular weight of air (MW) divided by the air-fuel ratio (AF):
Specific Energy = LHV*MW/AF
For example, for gasoline LHV= 43 MJ/kg and AF=14.6, while for methanol LHV= 21.1MJ/kg (less "heat" than gasoline) and AF=6.46 (much richer jetting than gasoline). Using the above formula we see that methanol only has a 10% higher specific energy than g asoline! This means that the power increase obtained by running methanol, with no other changes except jetting, is only 10%. Comparing the specific energy of racing and premium pump gas you can see that there is not much, if any, difference. Only alcohol s (such as methanol or ethanol) have a slightly higher specific energy than racing or pump gas.
Other oxygen-bearing fuels, besides the alcohols and nitromethanes, such as the new ELF fuel, will also produce slightly more power once the bike is rejetted. However, at $15.00 to $20.00 at gallon for the fuel the reportedly minor (1% - 2%) improvement is hardly worth the cost for the average racer.
The real advantage of racing gasolines comes from the fact that they will tolerate higher compression ratios (due to their higher octane rating) and thus indirectly will produce more power since you can now build an engine with a higher compression rati o. Also, alcohols burn cooler than gasoline, meaning even higher compression ratios are possible with them, for even more power.
The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.
However, it sometimes happens that when you use another fuel, the engine suddenly seems to run better. The reasons for this are indirect: First, the jetting may be more closely matched to the new fuel. Secondly, the new fuel may improve the volumetric e fficiency (that is, the "breathing") of the motor. This happens as follows: Basically a fuel that quickly evaporates upon contact with the hot cylinder wall and piston crown will create additional pressure inside the cylinder, which will reduce the amount of fresh air/fuel mix taken in. This important--but often overlooked--factor is described by the amount of heat required to vaporize the fuel, described by the 'enthalpy of vaporization' (H), or 'heat of vaporization' of the fuel.
A high value of H will improve engine breathing, but the catch is that it leads to a different operating temperature within the engine. This is most important with two-strokes, which rely on the incoming fuel/air mix to do much of the cooling--even mode rn water-cooled two-strokes rely on incoming charge to cool the piston. For two-strokes a fuel that vaporizes, drawing a maximum amount of heat from the engine, is essential--the small variations in horsepower produced by different fuels is only of second ary concern.
Also important is the flame speed: Power is maximized the faster the fuel burns because the combustion pressure rises more quickly and can do more useful work on the piston. Flame speed is typically between 35 and 50 cm/sec. This is rather low compared to the speed of sound, at which pressure waves travel, or even the average piston speed. It is important to note that the flame propagation is greatly enhanced by turbulence (as in a motor with a squish band combustion chamber).
The most amazing thing about all this is that you can get the relevant information from most racing gasoline manufacturers. Then, just look at the specification sheet to see what fuel suits you best: Hot running motors and 2-strokes should use fuels wit h a value of "H" that improves their cooling, while more power (and more heat) is obtained from fuels with a high specific energy.
By the way, pump gas has specific energies which are no better or worse than most racing gasolines. The power obtained from pump gas is therefore often identical to that of racing fuels, and the only reason to run racing fuels would be detonation probl ems, or, since racing fuels are often more consistent than pump gas--which racers call "chemical soup"--a consistent reading of the spark plugs and exhaust pipe.
Goudin, et al., 1987, "An Application of Fractals to Modeling of Premixed Turbulent Flames", Combustion and Flame 68, p.249-266
[This message has been edited by THE MOTORHEAD (edited 14 June 2000).]
In particular, the discussion focused on the problem of detonation, commonly referred to as "knock," which is a very serious and detrimental problem when it occurs - usually the pressures exerted onto the piston top during detonation are much larger (but of a shorter duration, like a pressure spike) than the mean combustion pressure. Nevertheless they are very detrimental to engine life, as the continual high shock loading of the piston, rod, crankshaft and bearings is quite destructive.
Detonation is the result of an amplification of pressure waves, such as sound waves, occurring during the combustion process when the piston is near top dead center (TDC). The actual "knocking" or "ringing" sound of detonation is due to these pressure waves pounding against the insides of the combustion chamber and the piston top, and is not due to 'colliding flame fronts' or 'flame fronts hitting the piston or combustion chamber walls.'
Let's look in some detail at how detonation can occur during the combustion process: First, a pressure wave, which is generated during the initial ignition at the plug tip, races through the unburned air-fuel mix ahead of the flame front. Typical flame front speeds for a gasoline/air mixture are on the order of 40 to 50 cm/s (centimeters per second), which is very slow compared to the speed of sound, which is on the order of 300 m/s. In actuality, the true speed of the outwards propagating flame front is considerably higher due to the turbulence of the mixture. Basically, the "flame" is carried outwards by all the little eddies, swirls and flow patterns of the turbulence resident in the air-fuel mix. This model of combustion is called the "eddy burning model" (Blizzard & Keck, 1974).
Additionally, the genus of the flame front surface - that is the degree of 'wrinkling' - which usually has a fractal nature (you know, those weird, seemingly random yet oddly patterned computer drawings), is increased greatly by turbulence, which leads to an increased surface area of the flame front. This increase in surface area is then able to burn more mixture since more mixture is exposed to the larger flame front surface. This model of combustion is called the "fractal burning model" (Goudin, F.C. et al. 1987, Abraham et al. 1985). The effects of this are observed in so-called "Schlieren pictures," which are high-speed photographs taken though a quartz window of a specially modified combustion chamber (Fig. 1, above).
Schlieren pictures show the various stages of the combustion process, in particular the highly wrinkled and turbulent nature of the flame front propagation (initially called the flame 'kernel'). A higher degree of turbulence, and hence a higher "effective" flame front propagation velocity can be achieved with a so-called squish band combustion chamber design. Sometimes a swirl-type of induction process, in which the incoming mixture is rotating quickly, will achieve the same goal of increasing the burn rate of the mixture.
As a general rule-of-thumb the pressure rise in the combustion chamber during the combustion phase is typically 20-30 PSI per degree of crankshaft rotation. Once the pressure rises faster than about 35 PSI/degree, the engine will run very roughly due to the mechanical vibration of the engine components caused by too great of a pressure rise. Sometimes, the pressure wave can be strong enough to cause a self ignition of the fuel, where free radicals (e.g. hydroxyl or other molecules with similar open O-H chains) in the fuel promote this self ignition by the pressure wave. However, this can still occur even without the presence of free radicals; it just won't be quite as likely to happen. This is why high octane fuels, with fewer of these active radicals, can resist detonation better. However, even high octane fuel can detonate - not because of too many free radicals - but because the drastic increase in cylinder pressure has increased the local temperature (and molecular speed) so high that it has reached the ignition temperature of the fuel. This ignition temperature is actually somewhat lower than that of the main hydrocarbon chain of the fuel itself because of the creation of additional radicals resulting from the break-up of the fuel's hydrocarbon chains in intermolecular collisions.
Detonation usually happens first at the pressure wave's points of amplification, such as at the edges of the piston crown where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine - this is called constructive interference to yield a very high local pressure. If the speed at which this pressure build-up to detonation occurs is greater than the speed at which the mixture burns, the pressure waves from both the initial ignition at the plug and the pressure waves coming from the problem spots (e.g. the edges of the piston crown, etc.) will set off immediate explosions, rather than combustion, of the mixture across the combustion chamber, leading to further pressure waves and even more havoc. Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top. Piston tops, ring lands and rod bearings are especially exposed to damage from detonation. In addition, these pressure fronts (or shock waves) can sweep away the unburned boundary layer (see figure 2 above) of air-fuel mix near the metal surfaces in the combustion chamber.
The boundary layer is a thin layer of fuel-air mix just above the metal surfaces of the combustion chamber (see figure 2, above). Physical principles (aptly called boundary conditions) require that under normal circumstances (i.e. equilibrium combustion, which means "nice, slow and thermally well transmitted") this boundary layer stays close to the metal surfaces. It usually is quite thin, maybe a fraction of a millimeter to a millimeter thick. This boundary layer will not burn even when reached by the flame front because it is in thermal contact with the cool metal, whose temperature is always well below the ignition temperature of the fuel-air mix.
Only under the extreme conditions of detonation can this boundary layer be "swept away" by the high-pressure shock front that occurs during detonation. In that case, during these "far from equilibrium" process of the pressure-induced shock wave entering the boundary layer, the physical principles allured to above (the boundary conditions) will be effectively violated. The degree of violation will depend on (a) the pressure fluctuation caused by the shock front and (b) the adhesive and cohesive strength of the boundary layer. These boundary layers of air-fuel mix remain unburned during the normal combustion process due to their close proximity to the cool metal surfaces and act as an insulating layer and prevent a direct exposure of metal to the flame. Since pressure waves created during detonation can sweep away these unburned boundary layers of air-fuel mix, they leave parts of the piston top and combustion chamber exposed to the flame front. This, in turn, causes an immediate rise in the temperature of these parts, often leading to direct failure or at least to engine overheating.
Scientists and engineers have recently begun to understand combustion in much greater detail thanks to very ambitious computer simulations that model every detail of the combustion process (Chin et al. 1990). Basically, a complete computer model includes a solution to the thermodynamical problem, that is a solution to the conservation equations and equation of state, as well as a mass burning rate and heat transfer model. In addition, a separate code (called a chemical kinetics code) models the chemical processes which occur during combustion and sometimes juggles several thousand different chemical species, some in vanishingly small concentrations! Needless to say these codes require huge amounts of memory and CPU time that only the largest supercomputers in the world can provide. They are far beyond the reach of the private individual and usually only employed by large research institutions or major car manufactures.
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Here's a brief recital of the question we received:
Someone asked:
Rob, I read your "Chemical Soup: The Meaning of Gasoline." Quick question if you have the time... You mentioned that "flame propagation is greatly enhanced by turbulence." Should this be a consideration when an engine is ported? Can turbulence be enhanced by porting without losing the intake flow?
Unless the ports are specifically designed for a strong swirl-type induction process, the turbulence created during the intake process is not very affected by porting. This is true as long as one sticks with the same general port layout. However, drastic porting changes may increase or decrease the turbulence in the combustion chamber, but it is quite difficult to say anything definite. I think that any improvement gained by porting the engine is likely to be far greater than any possibly detrimental effect the porting may have had on turbulence.
As far as I know there is only one motorcycle engine that uses a highly turbulent intake process of the swirl type. It's a "homebuild" single cylinder racing engine from Switzerland that uses cross-scavenging and has two pairs of diagonally opposed intake and exhaust valves. Most conventional ports do induce a very small amount of swirl, but this is not important as far as generating much turbulence. Rather, the biggest benefit is obtained by reducing the squish band to it's safe minimum (about 0.020-0.040 in, depending on the particular engine used). This will have a far greater effect on increasing the turbulence in the combustion chamber than any other modification.
Mike Meagher (meagher@pentec.wa.com) wondered about the effects of the squish band.
It is important to realize the two important functions of reducing the squish band clearance: (a) to enhance turbulence due to rapid ingestion of gas into the combustion chamber, hence increasing the burning rate of the mixture and (b) to reduce the volume of the unburned gas in the boundary layer of cool gas near the piston top and cylinder head surfaces. Typically, gas trapped in the squish area doesn't burn, even if the squish band clearance is relatively large. The cooling effects of the large surface-area-to-volume ratio of this region will prevent any ignition of the fuel-air mix therein, even if the squish band clearance is rather large. Hence any gas caught in the squish band will not be burned near TDC when it does the most good, but later during the combustion process when one cannot extract as much work from the late-burning gases. The amount of gas trapped in the squish band can actually be a substantially greater amount than just the relative volume of the squish band because the pressure wave from the ignition process literally crams a lot of the unburned gas into crevice areas like the squish band. Reducing the squish band clearance will decrease the amount of unburned gas substantially, leading to more complete and faster combustion, lower emissions and improved power. It is one of the few "all gain with no pain" modifications one can carry out on racing or even street motorcycles.
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Someone wondered: Is the extra cooling of the squish band less than the added heat?
Basically the mixture in the squish region is in thermal contact with the cylinder wall and piston top and at roughly the same temperature, which is quite lower than the burn temperature. Reducing squish will decrease the amount of the cool gas in the squish region and increase the amount of hot gas in the burn region. A reduced squish clearance will increase temperatures a little even if the compression ratio is held constant. There is no "extra cooling" mechanism if you reduce the squish band clearance. The cooling rate of the gas in the squish zone depends on the thermal conductivity of the gas-metal interface, on the total surface area of this interface and the temperature difference between gas and metal. Note that these factors are all essentially constant at TDC and don't depend on the squish clearance. Hence the cooling rate is the same for large squish clearances and for small squish clearances. Thus there is no "extra cooling" mechanism if you reduce squish band clearance.
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David Goodenough (dg@pallio.sf.ca.us) asked:
Suppose I mix one gallon of 87 octane pump gas, and one gallon of 92 octane pump gas. Are you telling me that instead of two gallons of 89.5 octane gas I have something closer to 92 (like between 90 and 91)?
The mixed gas' octane rating will in general not be a linear function of the original constituents' octane ratings. Neither will it be a simple function in most cases. Rather, the octane rating becomes a quite complicated, non-linear function of some very small amounts of free radicals, such as hydroxyl and hydroxen peroxide, in the fuel. Essentially, there is no simple analytic way to predict the final octane rating of a fuel; rather, extensive tests with a calibrated engine are necessary (see MON and RON explanations in the last article).
David also asked:
While I'm at it, how does the energy per ounce of mixture react?
As mentioned before, the "energy per ounce" (more exactly the Specific Energy for an stoichometric [an ideal] mixture) does not vary much at all between different kinds of pump gasoline or even racing gasoline.
Ramon Hontiveros (r22666@paccvm.sps.mot.com) wrote:
Ok, I got the article and read it, now some questions: Isn't the fuel already in gaseous form due to carburation?
Ramon, the air fuel mix as it flows into the combustion chamber is not perfectly atomized, that is the fuel vapor droplets consist of larger droplets of fuel molecules surrounded by air. It takes additional energy to further atomize this vapor, that is to break the hydrostatic forces (the surface tension of the fuel droplet). This additional energy can be taken from a hot surface (such as the piston crown, etc.), which then leads to a cooling of the piston. The additional energy can also be imparted via large turbulences and pressure waves, as in a squish band-type motor, which will help to further atomize the fuel. Note that the term "atomize" is actually misleading since the molecules are still left intact, that is the hydrocarbon chains (and oxygen bonds for alcohols) are not broken.
Ramon also wondered:
does carburation just "spray" the gas into the air flow as tiny droplets which are thus still in liquified form?
Yes.
Ramon also asks:
Also, if the fuel does evaporate quickly and creates additional pressure - thus reducing the amount of fresh charge - then the engine will produce less horsepower, right?
Correct. The horsepower will depend on the volumetric efficiency of the engine which is a function of the pressure difference between ambient air and cylinder pressures. If additional fuel is vaporized inside the combustion chamber the pressure in the cylinder will rise, and, while the valves/ports are still open, reduce the volumetric efficiency, and thus the power output.
So 2-strokes would benefit from using fuel that has a _lower_ heat of vaporisation rating?
Correct. A fuel with a lower heat of vaporisation will "atomize" easier and thus improve engine cooling, but decrease power somewhat.
So which type of fuel has a lower heat of vaporisation? Leaded or unleaded?
A fuel's heat of vaporisation does not depend on the it's lead content. Rather, it depends on the fuel's main hydrocarbon chains; iso-octane verses n-heptane, for example. Since pump gas can consist of up to 20 different components with a wide range of individual boiling points (We were serious when we called it "chemical soup!") one should look at the specifications sheet for each fuel separately. For racing fuels these are available from the manufacturer.
Lastly, I gather from the article that it's okay if you end up mixing some leaded fuel with the remaining unleaded fuel in the tank?
Most fuels, pump or racing, will give about the same energy release, so when switching from pump to racing fuel (in general) do not expect a drastic increase in power. Mixing leaded fuel with the remaining unleaded fuel in the tank has no advantage and will give inconsistent plug readings; hence I wouldn't do it on a race bike.
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REFERENCES
Abraham, J. et al., 1985, "A Discussion of Turbulent Flame Structure in Premixed Charges", SAE paper 850345
Blizzard, N.C. and Keck, J.C., 1974, "Exp. and Theo. Investigation of Turbulent Burning Model for Internal Combustion Engines", SAE paper 740191
Chin et al., 1990, "Diagnostics and Modeling of Combustion in Internal Combustion Engines," JSME, Tokyo, p. 81-86
In many high-performance situation, riders clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your motorcycle may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indic ation of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)
Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified motorcycles. A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.
The knock tendency (and hence, the Octane rating) of a fuel is a function of the amount of free radicals present in the fuel prior to ignition and can be reduced by the addition of tetra ethyl lead, aromatics and other additives.
Although some racing organizations still use maximum octane number as the discriminating factor for fuel legality, it is really not appropriate for racing purposes.
Instead one should look at the amount of energy (heat) released in the burning of a particular fuel. This is described by the specific energy of the fuel. This quantity describes the amount of power one can obtain from the fuel much more accurately. The specific energy of the fuel is the product of the lower heating value (LHV) of the fuel and molecular weight of air (MW) divided by the air-fuel ratio (AF):
Specific Energy = LHV*MW/AF
For example, for gasoline LHV= 43 MJ/kg and AF=14.6, while for methanol LHV= 21.1MJ/kg (less "heat" than gasoline) and AF=6.46 (much richer jetting than gasoline). Using the above formula we see that methanol only has a 10% higher specific energy than g asoline! This means that the power increase obtained by running methanol, with no other changes except jetting, is only 10%. Comparing the specific energy of racing and premium pump gas you can see that there is not much, if any, difference. Only alcohol s (such as methanol or ethanol) have a slightly higher specific energy than racing or pump gas.
Other oxygen-bearing fuels, besides the alcohols and nitromethanes, such as the new ELF fuel, will also produce slightly more power once the bike is rejetted. However, at $15.00 to $20.00 at gallon for the fuel the reportedly minor (1% - 2%) improvement is hardly worth the cost for the average racer.
The real advantage of racing gasolines comes from the fact that they will tolerate higher compression ratios (due to their higher octane rating) and thus indirectly will produce more power since you can now build an engine with a higher compression rati o. Also, alcohols burn cooler than gasoline, meaning even higher compression ratios are possible with them, for even more power.
The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.
However, it sometimes happens that when you use another fuel, the engine suddenly seems to run better. The reasons for this are indirect: First, the jetting may be more closely matched to the new fuel. Secondly, the new fuel may improve the volumetric e fficiency (that is, the "breathing") of the motor. This happens as follows: Basically a fuel that quickly evaporates upon contact with the hot cylinder wall and piston crown will create additional pressure inside the cylinder, which will reduce the amount of fresh air/fuel mix taken in. This important--but often overlooked--factor is described by the amount of heat required to vaporize the fuel, described by the 'enthalpy of vaporization' (H), or 'heat of vaporization' of the fuel.
A high value of H will improve engine breathing, but the catch is that it leads to a different operating temperature within the engine. This is most important with two-strokes, which rely on the incoming fuel/air mix to do much of the cooling--even mode rn water-cooled two-strokes rely on incoming charge to cool the piston. For two-strokes a fuel that vaporizes, drawing a maximum amount of heat from the engine, is essential--the small variations in horsepower produced by different fuels is only of second ary concern.
Also important is the flame speed: Power is maximized the faster the fuel burns because the combustion pressure rises more quickly and can do more useful work on the piston. Flame speed is typically between 35 and 50 cm/sec. This is rather low compared to the speed of sound, at which pressure waves travel, or even the average piston speed. It is important to note that the flame propagation is greatly enhanced by turbulence (as in a motor with a squish band combustion chamber).
The most amazing thing about all this is that you can get the relevant information from most racing gasoline manufacturers. Then, just look at the specification sheet to see what fuel suits you best: Hot running motors and 2-strokes should use fuels wit h a value of "H" that improves their cooling, while more power (and more heat) is obtained from fuels with a high specific energy.
By the way, pump gas has specific energies which are no better or worse than most racing gasolines. The power obtained from pump gas is therefore often identical to that of racing fuels, and the only reason to run racing fuels would be detonation probl ems, or, since racing fuels are often more consistent than pump gas--which racers call "chemical soup"--a consistent reading of the spark plugs and exhaust pipe.
Goudin, et al., 1987, "An Application of Fractals to Modeling of Premixed Turbulent Flames", Combustion and Flame 68, p.249-266
[This message has been edited by THE MOTORHEAD (edited 14 June 2000).]
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