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?

 

[NormPeterson]

Chemistry understood (sort of). But is the mixture sufficiently homogeneous? Mightn't a small but consistently lean region adjacent to something metallic develop a slightly different appetite?

Norm

 

[patprimmer]

Lean mixture will oxidise engine parts, especially pistons even with atmospheric oxygen and normally aspirated.

With more air via supercharging, higher cylinder pressures via high compression and a lot of spark advance, even stoichiometric mixtures can burn holes. In high output engines A:F can be 10 to 15% rich, or even more on alcohol to prevent this.

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.

 

[RETHomer]

>>"Fuel pressures are in excess of 300psi, so the physics of having the front cylinders leaner than the rear cylinders sounds like a bunch of hooey."<<

>300 psi *TO* the injectors doesn't mean anything to the fuel/air mass blasting thru the blower and into the ports. F/R fuel distribution WAS a problem back in the 1970's, but I'd bet it's all been compensated for now.<

Because of the twisted lobes in a Roots supercharger, more air is forced into the front cylinders. Crew chiefs used to compensate for this inherent dynamic by lowering the compresssion ratio in the front cylinders, leaving the middle four alone and raising the CR in the rear cylinders. This was accomplished with con rods about 0.070 shorter in the front than the rear. Some crew chiefs also tried to position air dams in the intake manifold to block air to the front cylinders in the hopes of creating equal distribution. A few years ago, Alan Johnson of the US Army team developed an intake manifold that positions the front of the supercharger over a central plenum. Called the setback supercharger, a longer drive snout is needed but there is more equal distribution to the cylinders. BTW, fuel pressure starts out at about 275psi at the start and increases to around 470psi at the end of the run.

 

[turbocohen]

Dr Webb,

Respectfully I disagree. Admitting O2 in high doses is an experiment I have witnessed. The engine/victim was a small industrial diesel that was used for a nox and particulate study with CNG and other fuels. The piston and exhaust valve and seat were torched minutes after admitting O2. The O2 sensor indicated stoich under load but the failure occurred while lean at part load. Not a surprise.

 

[PeterJesch]

Believe me I know Copressed Oxygen or Cryo-LOX are not things to goof off with. Think about it like this: If only 21% of the air you are shoving in to the engine actually is allowing the fuel to ignite, then what is all the other 79% doing? Well it does help keep from burning the valves and pistons but it also is using up prime real-esate through your ports and in the cylinders. I have built several liquid rocket engines and I know the the better I can control my mix and flows the better my engines will run. So why not loose the blower and pre-presurize the air you are gonna burn, before you even start the engine? If you don't want pure O2, how about 70%O2 and 20%Nitrogen, or whatever combination in between. I am no top fuel expert, I am just currious about what any of you think. I may be attemting this first with maybe a honda or something cheap that I can test my ideas on, without breaking the bank.

 

[PeterJesch]

One more thing about the bomb aspect, to anyone concerned. The Saturn 5 rocket had alot more fuel and oxidizer onboard, thousands of dragsters worth at least. Yes they had their share of emergencies but, done right I realy think it would only be marginally more dangerous that top fuel is now.



Getting the mix to be homogeneous will be the key.



Gasoline = C8H18
100 octane gasoline is mostly C8H18
2(C8H18)+25(O2) => 18(H20) + 16(CO2)
A ratio of 25/2 = 12.5 Oxygen molecules per fuel molecule



Nitromethane is CH3NO2
4(CH3NO2) + 7(O2) => 6(H2O) + 4(CO2) + 4(NO2)
A ratio of 7/4 = 1.75

 

[patprimmer]

Top Fuel has strict rules regarding supercharging, as previously mentioned.

Using pure O2 as an oxidising agent would be considered as chemical supercharging and therefore in breach of the rules.

I think this point alone makes that argument redundant.

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.

 

[Metalguy]

Oxygen *enrichment* is something that can be done with relative safety. Amazing what a tiny 1/8" rubber tube with only a few psi can do. I used to run the carb. secondaries way rich, then slowly increase the oxygen psi until the HP came in. Detonation is a big concern; the spark must be retarded as with nitrous.

Also ran a street Camaro with a big stainless fire ext. full of methanol in the trunk and pressurized with a small 12v tire inflator. Sprayed the meth. down into the carb. and added the oxygen. HP was amazing. But I'd guess I only increased the oxygen level from 21% to maybe 24-5%--things happen in a hurry w/oxygen and even the much milder nitrous.

Running modified car engine on 100% oxygen and no air is NOT going to work, unless you're in the fireworks business-and it will happen in less than a second.

 

[bentwings1]

Being old as dirt and growing up in the early days of drag racing, I got to see and experience a lot of these things first hand both in dragsters and funny cars.
Exhaust....Garlits was one of the first to go from weedburners to zoomies and entered the 200 mph +. It was due to streamlining the air flow over the tires at first but today the down force is quite large and has been measured.
Adding nitrous to nitro is not allowed in competition. Not even to alcohol.... not that it hasn't been done. It's not needed as noted in previous posts. There is still plenty of hp available when needed.
only 85% nito is allowed in TF and FC (last I heard) and blower OD is limited. I ran 60% OD on an 8-71 back in the mid 70's and it worked great for the launch but hurt the top end. It's still who gets there first that counts not how fast you run so it worked for us.
spark plugs... nitro cars burn the ends off by half track. yes they have recessed electrodes but the guts are toast quickly. The motor runs on the ragged edge of detonation all the time. We even burned the ends on alcohol, however 3-4 threads of heat is the preferred measurement. Alcohol likes to have spark all the time. In the old days there wasn't enough juice from the magneto to light the fuel that could be pumped so it was by-passed to the tank. It wasn't untill the dual mags and rare earth magnets that the fuel cars made leaps and bounds in speed and E.T. You don't ever want to grab todays mag and give it a twist. You will probably still have a good "buzz" on when you are talking to St.Peter. haha something like 40 amps or better.
fuel distribution...early on everyone noted differences in fuel burn patterns. Nobody really knew what caused it. Some leaned the front cyl, some richened them, some just ran the blower faster,put more fuel in it and advanced the mag...and paid the parts bills. Lot's of things were tried before realtime testing took place. Somebody came up with the "delta plate" which was a triangular opening plate under the blower to redirect the flow. Some ran it forward some ran it backward..some didn't run it at all. Now the blowers come with it built in. Now some are moving the blower forward and backwards the rules are limiting it.
Racers will do about anything to gain an edge...including BS.ing. haha don't ask how I know.
Drag racing today is very technical. Many teams employ engineers who do nothing but investigate how to go faster and quicker. It's come a long ways from blue jeans and grimy tee shirts of the old days....but it was sure fun back then.

 

[CapriRS]

Interesting thread. I'm an NHRA fan and a friend was the clutch guy for an alcohol funny car team (Steven Gasparrelli) while Randy Anderson was the crew chief. I got to learn a ton by hanging out in the pits with these guys.

There is massive thrust from the headers. There is empirical evidence for even casual observers by watching a fuel car make a pass. When you see a cylinder go out on one side of the car (exhaust plume goes white/cloudy), the resulting thrust differential will push the car towards the side with the dead cylinder. This happens quite often.

Beyond the rules banning the use of nitrous, as stated the engines make so much power that they can spin/smoke the tires at 300mph even with all the downforce so the focus is on traction instead of power. Typically, the fuel cars run less fuel at the beginning of the pass where traction is limited and build heat in the heads. Then the fuel is increased where the traction is better and the hotter heads help burn the increased fuel. That was a trick implemented by Bernstien's team to got him so many speed records at the time.

There have been a few turbocharged nitro dragsters but they have not been legalized by the sanctioning bodies.

 

[TFHydro]

I just stumbled accross this forum. Interesting and very accurate. Nitrous has been used in the NHRA. Don The Snake Prudome used it in his Funny Car and went 2 tenths faster than anyone before- blew up- and Nhra banned it. Mickey Thompson built a funny car with compressed o2 cylinders alongside each frame rail and called it his bottle baby. Im not sure how it worked. Other additives have been tried and some work well. They are very caustic and unstable. All are banned. Nitro is very trickey to get right. We do lean our engine on the top end close to the 1.7 fuel to air ratio. ( Gasoline 1 lb. gas to 15lbs. air--Nitro 1 lb. to 1.7 lbs. air) This is where we burn pistons if we miss the corrected altitude/air density and load on the engine from nitro percentage, mag/ignition lead, prop selection, gearing and water density. The combinations are endless. One last point the NHRA is retricted to 85% nitro and run a spec MSD Mag that limits RPM at 8400 by a rapid retard if it is reached.

http://dragboats.com/images/gallery/large/04_11_IHBA_6360_692.jpg

 

[fasteddiesrat]

hey! new here, but i'd like to say i have 1st hand knowlage of the amount and force of the raw unburnt fuel exiting the pipes from a top fueler. picture if you will, three boys standing in direct line of fire from, Don "the snake" Prudomes ride. they load her up, and hit the starter motor on the blower and we are mezmerised... BAMM!
we caught the full force of them pipes smack in the face! it felt like a hammer to the face, with fumes... instantly blinded and slammed in the face, i did an immediate about face only to slam head first into a goose neck trailer directly behind me. almost broke my damn neck in the proccess. there are few things in this world that can truly drag your ass into existance like a face full of hot unburnt nitro!
ed