Top Fuel Dragster 6

[HydroScope]

Hi,

I recently discovered the world of drag racing and wondered why top fuel dragsters dont combine their nitromethane with nitrous? I understand that nitromethane is already a combination of methane ~ CH4 and nitrous ~ N2O that is a liquid ~ CH3NO2, but beleive it would still burn in nitous atmosphere, Am I correct?. Is it because nitromethane already develops a high enough power density no need for more? at limit of engine compents? or are these chemicals a bad combo for some reason?

 

[globi5]

Greg you make a good point that the exhaust velocity cannot exceed sonic speed as the only way to reach supersonic speed is apparently by using a converging-diverging nozzles.
(I only read this as a fact but haven't read an explanation why this is so. So please post if anybody knows more about this.)
However the sonic speed of any gas is (among other factors)proportional to the squareroot of the temperature. Since the temperature in the exhaust gas is extremely high it can reach double the sonic speed of air (at 'normal' pressure and temperature) without exceeding the sonic speed limit of the exhaust gas.

Besides even if the velocity is lower, the pressure of the exhaust before leaving the pipes also adds to the downforce.

http://exploration.grc.nasa.gov/education/rocket/rockth.html

So if you combine the massflow and the pressure difference, 2000 lbs. doesn't sound unreasonable at all.

If you can increase the exhaust velocity by using converging diverging nozzle and if horsepower is less of an issue than tire friction (if the clutch is such a crucial element this only means that there's a lack of tire friction or downforce), why aren't there converging diverging nozzles mounted to the exhaust pipes? (Even if they increase backpressure and therefore reduce horsepower, they could significantly increase downforce).

 

[sbmar]

This may not be "Top Fuel", but it is certainly "Top Diesel"..

The guys on this site

http://www.cumminsracing.com/

have taken the 5.9 L Cummins to level that I find hard to believe..

I know this engine well and knew was was tough, but I am now even more of a Cummins fan.............

Under 8 seconds, over 160 MPH, and 100lbs of boost--all on bio-diesel.





Tony Athens

http://www.sbmar.com

 

[JWaterstreet]

Uh, if the exhaust pressure was really 2K lbs total as several posters have surmised, why wouldn't they all be pointing the pipes towards the rear of the car?

2K lbs of "jet thrust" surely would shave a few hundreds (or tenths) off the time, no?

Jeff

 

[kenre]

Uh, if the exhaust pressure was really 2K lbs total as several posters have surmised, why wouldn't they all be pointing the pipes towards the rear of the car?

2K lbs of "jet thrust" surely would shave a few hundreds (or tenths) off the time, no?


Jwaterstreet,

I see where your coming from, but forward thrust isnt where a top fuel is lacking, its tyre traction. More traction means more power can be applied to the tyres. Watch any jet verses car drag race, tyres always accelerate off the line faster than jets, Being a direct friction contact, compared to reaction/opposite reaction.

Ken

 

[GregLocock]

Also, of course, the downward thrust due to exhaust is high at low speed when the aero downforce is minimal, but when grip is most important.

But that was a good point.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

I do think that the pipes lean back a few degrees from the vertical, so there would be some forward thrust as a result.

Norm

 

[globi5]

I guess the pipes should have movable deflectors, such that the downward thrust is 100% below 100 mph when the wing is not or less effective and 100% rearward thrust at top speed (the downforce and drag of the wing at 300 mph is almost 10 times higher than at 100 mph).

Since these car don't have gears or any kind of torque converters also indicates that they can't utilize the engine power at lower speeds (and confirms that there's a lack of tire friction).

 

[SomptingGuy]

The exhaust/rocket/downforce debate...

One thing people seem to have missed is that the speed of sound of hot exhaust gases is a whole lot higher than that of plain old cold air. Remember the sqrt(gamma.R.T) formula? How hot are the exhaust gasses of a dragster I wonder? Even at a conservative 1200K that gives about 700 m/s.

 

[NickE]

http://motortrend.com/features/consumer/112_0502_numbers/index1.html

http://motortrend.com/features/consumer/112_0502_numbers/index2.html

Thats what they published.




Nick
I love materials science!

 

[Fabrico]

One source suporting the claim of up to 2,000 pounds of downforce from exhaust was from tests done by Paul Van Valkenburgh and published in Race Car Engineering magazine.

 

[PeterJesch]

Let me know if you think this would work, or if not please explain.

What about the idea of dropping the super-charger which I know consumes a Significant amount of hp, and instead having a small
size, say 10 Liter tank of liquid O2 Regulated to the same PSI and flow rate that the super-charger adds to the engine. Also remember that normal air is only 21% O2 and that the tank is at 900psi and also the fact that liquid O2 expands 800 times in volume when vaporizing. You could even preheat it through the intake manifold or headers, you could regulate it to whatever atmosphere you needed at aprox. 21% 0f 3000 cfm. Any input. Could this work? Any benifit? 10 Liter tank only weighs about 65 pounds full. The engine would then be running on pure oxygen.

 

[waross]

Quite a few metals are flammable in pure oxygen. This may be a problem with hot engine parts.

 

[GregLocock]

Peter Jensch. What you propose has been tried by at least one of the more adventurous souls on this board. You are basically building a bomb, or a rocket.

Your logic is fine, but the practicalities are lethal.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[turbocohen]

Pure O2 would produce lots of heat but the metal engine parts would not tolerate it and the risks are way too great for the driver. Everything burns in the presence of O2

 

[globi5]

If I'm not mistaken, they're only allowed to use roots blower as superchargers (which aren't really designed to generate pressure ratios at that scale). If they were to use more efficient twin scroll superchargers, they could probably gain a significant amount of power as well.

 

[MikeHalloran]

Peter Jesch,
read up on the Purdue University Charcoal Lighting Contest.

The record is something like three seconds to ignite 65 pounds of charcoal, by dumping LOX on it. Not recommended. The barbecue itself is also a consumable.

The actual Purdue site has been removed, but you can find stories about it. The videos were most impressive. Maybe there's an echo somewhere.

The important thing to remember is that with oxygen, any container you can put it in will serve nicely as fuel. Industrial gas producers will provide sobering, if unspectacular, information.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

The whole point in limiting Top Fuel to a Roots blower (14:71 max I think) is because they are inefficient and they thereby limit the power to something like 6000 HP.

I believe they also limit OD ratio and the amount of nitro methane in the fuel to try to keep speeds down to a level that does not exceed their tyre ratings.

I am not fully up to date and not totally familiar with Top Fuel regs, but from anecdotal evidence, they seem a bit reluctant to also reduce wing substantially, and therefore reduce down force to a level where the tyres and chassis are relatively safe. I believe that they have had several fatalities in recent years due to chassis, and/or wing strut and/or tyre failures at the top end.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[globi5]

Apparently they used to bombard spectators with blowers, because the mixture would pre-ignite before the intake valve closed. That's why they use straps to prevent this from happening.

If they had used more efficient superchargers (reduce the size of the engine instead), the temperature after the supercharger would have been lower and reduce the probability of pre-ignition before the intake valve closed.

Also if they used centrifugal superchargers they would have a more favourable torque curve. Less torque at low engine speeds and more torque at high engine speeds when more downforce is available. (The clutch could engage at a lower speed and generate less overall waste heat).

Or what's missing?

 

[drwebb]

Implications that pure O2 will set the metal of the engine on fire are sensational. When metered into the combustion chamber the O2 (whether dilute or concentrated) will always react first with whatever has the lower energy of activation. At rich to stoichiometric ratios used for maximum power this will be the fuel and not the engine parts. There may be lots of reasons why it won't work, but that difficulty isn't one.

 

[WilliamH]

A star for you DRWEBB. I was hoping someone would step in and define the chemistry in this discussion.

Will