28 replies to this topic

[TecnoRacing]

I was watching drag racing over the weekend and was astonished to hear the exhaust pressure alone creates appoximately 2000lbs of downward force, such that when a single cylider is lost, the opposite side of the car will lift visably...

I was curious whether this effect is in play at all with a modern f1 car and whether or not it was a contributing factor in the move away from the rearfacing exhausts...

[ivanalesi]

They were placed there more because of the hot air than this effect, also F1 engines dont use this kind of fuel neither have "economy" any near to them:)

[roadie]

The placement of F1 exhuasts that flow under the rear wing aid downforce by encourgaging the extraction of air from the diffuser. The hot air that comes out of the exhaust is of a lower pressure than the surrounding air and works in tandem with the rear wing in creating a low pressure area under the wing and behind the car.

[DOHC]

The thrust of a jet is massflow*jet velocity. The massflow is the weight of the exhaust volume produced per time unit:

rho*V*revs/2,

where rho is the exhaust density and V the engine capacity. The jet velocity v is the speed by which the exhausts exit the pipe, so the thrust is

rho*V*revs/2*v.

Just to get a rough estimate, use rho = 1.23 kg/^m^3 (density of air, exhaust is heavier, but not by a lot with gas as the fuel), V = 0.003 m^3, revs=300/sec, and v=300 m/s and multiply.

Then you get

1.23*0.003*150*300 = 170 N.

So the peak thrust is on the order of 200 N or so (corresponding to a "20 kg" or "40 lbs" downforce), which is not a lot.

With nitromethane fuel things are a bit different as the mass flow is much larger (the fuel contains extra oxygen for the combustion). Also, the enormous engines increase mass flow as the engine capacity goes up. On the other hand, those engines might not run at 18,000 rpm.

In any case, the claim of "2,000 lbs" downforce seems to be a bit exaggerated. A calculation as rough as the one above would let's say use 5x the density rho (use of compressor and more fuel), 3x engine capacity V, but reduce revs by a factor of 2. I'd be surprised if the downforce vastly exceeded 1-2 kN, which is about "200-400 lbs" of downforce.

[J. Edlund]

Originally posted by DOHC
The thrust of a jet is massflow*jet velocity. The massflow is the weight of the exhaust volume produced per time unit:

rho*V*revs/2,

where rho is the exhaust density and V the engine capacity. The jet velocity v is the speed by which the exhausts exit the pipe, so the thrust is

rho*V*revs/2*v.

Just to get a rough estimate, use rho = 1.23 kg/^m^3 (density of air, exhaust is heavier, but not by a lot with gas as the fuel), V = 0.003 m^3, revs=300/sec, and v=300 m/s and multiply.

Then you get

1.23*0.003*150*300 = 170 N.

So the peak thrust is on the order of 200 N or so (corresponding to a "20 kg" or "40 lbs" downforce), which is not a lot.

With nitromethane fuel things are a bit different as the mass flow is much larger (the fuel contains extra oxygen for the combustion). Also, the enormous engines increase mass flow as the engine capacity goes up. On the other hand, those engines might not run at 18,000 rpm.

In any case, the claim of "2,000 lbs" downforce seems to be a bit exaggerated. A calculation as rough as the one above would let's say use 5x the density rho (use of compressor and more fuel), 3x engine capacity V, but reduce revs by a factor of 2. I'd be surprised if the downforce vastly exceeded 1-2 kN, which is about "200-400 lbs" of downforce.

The massflow through a Top Fuel engine is probably around 6 kg/s or so, 2 kN or 250 N/cyl should be a good estimate. Not much when it comes to downforce, but if one cylinder goes out it is enough to affect the balance of the car.

[voice_of_reason]

170N is not much downforce, but it is 17kW (23hp) of power at 100 m/s (360km/h = Monza straight). No prizes for working out why the exhausts point backwards not upwards!

[Slumberer]

If I recall correctly the positioning of the exhaust is carefully designed not to upset the car's balance when coming on or of the throttle (amongst miriad other considerations).

Exhausts exiting in the diffuser caused problems when the driver lifted for a corner, a substantial amount of downforce was lost and the cars were unsettled.

(I often wondered [in the old days when they could change the car around more] if in a really wet race the exhaust could be configured to blast the standing water from just infront of the rear tyres to aid traction. The reasoning was that in the wet speeds are down, aero is less important so a bulkier exhaust had less effect, and in monsoon conditions it might allow you to out drag other cars from slow corners. Of course if the track dried then you might just melt your tyres...)

[Canuck]

The downforce attributed to Top Fuel engine exhaust has little to do with actual exhaust gas flow.

Nitromethane burns rather slowly so Top Fuel engines typically run less than 9,000rpm and somewhere in the order of 600 total revolutions per race (not including burnout and staging). In order to make the estimated 7-8000hp from their capped V8 displacement of 500 CID, they're consuming tremendous quantities of fuel (roughly 1.5 gallons per second).

The other effect of nitromethane's slow burn is that it's still burning (and expanding) in the exhaust, hence the yellow flame visible in low light conditions (the white flame is burning hydrogen that's been separated from the air by the exhaust gas). The downforce is due to the expansion, not just flow. That said, it's more like 800 lbs. The wing on the other hand produces over 12,000lbs.

If you've never watched one of these cars accellerate past 500km/h in under 5 seconds, you really ought to.

[ciaoduc1]

How do teams test the effects of the exhaust flow? Is it trial-by-error, computer modeling or do they actually fire the engines up in the wind tunnel?

[J. Edlund]

Originally posted by Canuck
The downforce attributed to Top Fuel engine exhaust has little to do with actual exhaust gas flow.

Nitromethane burns rather slowly so Top Fuel engines typically run less than 9,000rpm and somewhere in the order of 600 total revolutions per race (not including burnout and staging). In order to make the estimated 7-8000hp from their capped V8 displacement of 500 CID, they're consuming tremendous quantities of fuel (roughly 1.5 gallons per second).

The other effect of nitromethane's slow burn is that it's still burning (and expanding) in the exhaust, hence the yellow flame visible in low light conditions (the white flame is burning hydrogen that's been separated from the air by the exhaust gas). The downforce is due to the expansion, not just flow. That said, it's more like 800 lbs. The wing on the other hand produces over 12,000lbs.

If you've never watched one of these cars accellerate past 500km/h in under 5 seconds, you really ought to.

Nitromethane does actually burn faster than gasoline with a laminar flame velocity of 0.5 m/s instead of a typical 0.35 m/s for gasoline. It also burns hotter, with a flame temperature of 2400 degC instead of around 2000 degC.

The reason for the long burns in Top Fuel is because of the rich mixtures used. Since nitromethane can serve as a mono fuel you can basicly increase power by increasing fuel flow alone, that is until the ignition system sets the limit. The fuel pump also reaches the limit at around 1.5 gallons per second at 8000 rpm. Around 5 gallons is used for a run.

The story about burning hydrogen that have been separated is just a myth. Infact, the exhaust gas temperature of a Top Fuel engine is just 500 degF at idle and 1800 degF at the end of the run. The rich mixture of nitromethane has very cood cooling properties.

The truth about the white flames you can probably find if you take a look how nitromethane burn when used as a mono fuel.

4CH3NO2 - > 2N2 + CO2 + 3CO + 3H2O + 3H2

This would create free hydrogen which can then react with the oxygen at the end of the exhaust pipes.

You can't create thrust from expansion. Thrust is created from massflow and velocity. Expansion can only be used to increase the velocity in a nozzle which then create thrust. But unlike a jet engine the velocity of the flow is not as high, and the mass flow is also low in comparison which means you won't get that much thrust as downforce.

My estimate is that the massflow through a Top Fuel engine is somewhere around 6-8 kg/s at 8000 rpm, this means if you want to reach 800 lbs (3530 N) of downforce the exhaust velocity must be 440 m/s (supersonic).

EDIT: Corrected the formula for how nitromethane burn as a mono fuel.

[Powersteer]

Top fuel dragsters are designed narrow so if one wheel has more grip than the other it won't effect its straight line ability too much. With this in mind the car is pretty much sensitive towards inbalanced vertical forces. On the exhaust, thier engines are massive (supercharged) and running on nearly warhead fuel. I don't know how many cubic inchs but having that size of an engine running those rpms and having the exhaust pointing upwards would certainly generate something.

[Canuck]

I stand corrected - learn something new every day. I got the 'nitro burns slow' from a Fuel Bike crew chief. Course, they aren't exactly winning either...

What I meant to convey was that calculations of exhaust flow that fail to take into account the increased velocity due to still-burning nitromethane in the exhaust were likely to be shy of actual pressures produced.

[Slumberer]

How do teams test the effects of the exhaust flow? Is it trial-by-error, computer modeling or do they actually fire the engines up in the wind tunnel?

They pipe compressed air through the model to simulate exhaust gases.

[DOHC]

The basic principle of jet/rocket propulsion is to create a unidirectional mass flow (a jet). According to Newton's 2nd law the thrust is simply the net momentum per time unit ejected by the nozzle. Newton's 3rd law then provides the reaction force on the device that creates the jet.

This is the case if you take a toy balloon and release it, or if you take a water-jet propulsion device for a boat, but also if you take a jet engine with or without afterburner, or a rocket motor. This means that thermodynamic expansion per se is not a different or "added" principle, although the added thermal energy may be used to increase nozzle velocity (see J. Edlund's post).

The idea beind chemical/combustion jet propulsion is to produce high pressure, high temperature exhaust through exothermal reactions. Thermodynamic expansion is used to vent the exhaust gasses into a low pressure ambience. With a suitable nozzle shape the exhaust expansion partly converts thermal energy to kinetic energy in the form of momentum carried by the jet. This is what a turbojet engine does. Switch on the afterburner, pump more fuel into the hot exhaust, and you further increase nozzle jet velocity. Still, the thrust cannot exceed the product of nozzle velocity and mass flow (total mass throughput per second).

As for exhaust thrust from internal combustion engines, the mass flow can be fairly accurately estimated. The very large downforce numbers claimed do imply -- as J. Edlund remarks -- that supersonic nozzle velocities are needed. The 2,000 lbs figure seems to require exhaust pipe velocities well above 1,000 m/s. That's quite a rocket motor. If directed to the rear it would offer an added 1G acceleration without requiring traction.

[Greg Locock]

You'd get supersonic flow, you'd expect to see shock diamonds, so I can only assume these estimates of velocity are wrong, or the mass flow rate is wrong, or the downforce estimate is wrong.

http://www.aerospace...ion/q0224.shtml

[-RM-]

Would visable shock diamonds have time to form in the exhaust pulse?

Just as a reference, nitromethane used as a monopropellant in rocket motors has a theoretical exhaust velocity of around 1800 m/s at sea level.

[Greg Locock]

To a first approximation the speed of sound is proportional to the square root of the absolute temperature, so it wouldn't change much as a function of the combustion process and exhaust temperature.

[McGuire]

Originally posted by J. Edlund


The massflow through a Top Fuel engine is probably around 6 kg/s or so, 2 kN or 250 N/cyl should be a good estimate. Not much when it comes to downforce, but if one cylinder goes out it is enough to affect the balance of the car.

That could be a little high even...on paper the ariflow looks like around 4.5 kg/s to me. But I do know the vehicle telemetry indicates the download or "thrust" is in the range of 600-800 lbs so there is something going on there. It's an interesting question. When one candle goes out it will definitely steer the car, that's for sure.

Nitromethane is an interesting substance. If you pour a small puddle of it on the floor and throw a lighted match in it, it will snuff out the match. But if you hit the puddle with a hammer it will go off. In text books its flame speed is supposedly slower than gasoline...but in stoichiometric combustion under pressure it approaches that of dynamite. The extreme spark advance required is simply a result of the very large fuel volume that can be burned per cycle... A/F ratios are in the range of 1.5:1 by weight, which is where all the power comes from. This is not an engine so much as a big old "flame pump" if you will... for lack of a better term. A reciprocating rocket engine.

These engines can do all kinds of crazy things that are hard to validate or even understand. They have been known to "two-stroke" or "diesel" and still make significant power even with the ignition disabled. Often when they are torn down these engines will show "back-siding" on the pistons...the drag racer's term for when there is more detonation and flame damage on the underside of the piston than on the top! It's a different world...and because these engines are too fragile and unstable to run for more than a few seconds, and can't be dyno'ed or quantified in any meaningful way, it will probably remain more of an art than a science. I believe some of the lore is more myth than fact but I am in no position to dispute any of it.

Reminds me of one of my all-time favorite racing quotes... Keith Black was one of the sharpest fuel racers ever, and manufactured most of the blocks and heads used in these engines. A friend down the street phoned him up, said they had reconfigured an enormous old marine dyno, they had a Top Fuel engine all hooked up and ready to go; would he like to come over and watch the first pull? KB said, "Hell no, I'm too close now."

[DOHC]

But that's in perfect agreement with J. Edlund's estimates, I think.

If we use the known 0.6 kg/s airflow in an F1 engine, but use 3x the engine capacity and 5x supercharging with 1/2 the revs, we get 3*5*0.6/2 kg/s = 4.5 kg/s.

With an A/F ratio of 1.5:1 by weight, you have 3 kg/s fuel added, for a total massflow of 7.5 kg/s (within Edlund's 6-8 kg/s estimate).

Finally, to have a 9 kN thrust (equiv to the 2000 lbs), you need at least 1,200 m/s (almost Mach 4) nozzle velocity. The more modest 800 lbs claim requires about 500 m/s, still supersonic at M = 1.5.

Whether one would be able to see Mach discs is not clear to me, as the flow through an exhaust pipe is a pulsed flow rather than a steady flow. Would Mach discs be stationary under such circumstances?

But I'm still wondering about why one would use the exhaust pipes as afterburners (unless it's done for the spectacular looks). First, nozzle shape design ususally tries to optimize thrust, which you do by having exhaust pressure = ambient pressure at the nozzle's open end. If you have underexpanded exhaust (higher than ambient pressure), you have less than max possible thrust (you waste energy), and conversely, if it is overexpanded, you waste energy because you have drag in the exhaust's passage through the nozzle.

If one really had the enormous thrust claimed, it would make a lot of sense to put real rocket nozzles at the end of the exhaust pipes, because you'd be able to increase thrust, as well as use that thrust for acceleration.

[Gear Jammer]

Hello all, I'm a first post rookie and I certainly don't have the smarts of the experts here but I wanted to bring to your attention an incident that happened at last weekends US National when Tony Pedregons Funny Car was involved in a start line incident as a result of the body fouling the throttle linkage.

A crew member was lowering or raising the body on to the chassis when it snagged the throttle causing the car to lurch forwards, slightly injuring 2 crew members.

Now to the point of the post, the crew chief Dick Venerables was standing close to the right side of the car and literally had his pants blown clear off by those exhaust gasses and suffered some slight burns due to the heat and flames coming out of the pipes.

I think that is a good example of the ferocity of the exhaust pulses a 500ci Nitro engine produces.

PS both crewmen where back at the track the next day.

GJ

[McGuire]

Originally posted by DOHC
But I'm still wondering about why one would use the exhaust pipes as afterburners (unless it's done for the spectacular looks).

This forum cracks me up sometimes.

[DOHC]

;)

[-RM-]

Afterburner? Why not go all the way and use the exhaust as a gas generator for a pump fed rocket engine? If anyone asks just say that it is for emission control.

[J. Edlund]

The massflow through a Top Fuel engine is a little hard to estimate, but on an 8.2 litre engine operating with a boost of 4 bar absolute the flow should be around 2.5 kg/s, but it's hard to guess how the volumetric efficiency is affected by the high fuel flow.

For the fuel flow we know that the pump sets the limit at around 6 kg/s at 8000 rpm, but maximum fuel flow are probably not used at maximum speed.

Exhaust flow is then air + fuel flow.

As McGuire mentioned nitromethane is also pressure sensitive, infact even a high fuel pressure can cause a certain danger. I assume that this is also the reason why no octane number is given to it. It's also possible to make the mixture even more "explosive" by adding a small amount of hydrazine to the nitromethane. 1% by volume should be enough to give a significant power increase, but the mixture is very unstable.

[DOHC]

Originally posted by Gear Jammer
pants blown clear off

If we consider skydiving, you reach a maximum velocity of some 50 m/s, which, although a low speed in the present context, is likely to tear your average Armani suit to shreds. This shows that it's difficult to remain properly dressed already at Mach 0.15.

[Patrice L'Rodent]

Quote:
Originally posted by Gear Jammer
pants blown clear off

I have heard that if you take a young woman for a quick trip on the back of a motorbike.......Nah, forget it!
PDR

[McGuire]

Yep, hydrazine was reportedly the secret behind Chris Karamasines running 200 mph at Alton, IL in 1960. The speed was highly disputed and would not be matched until Garlits officially ran 200 in 1964. However, hydrazine is extremely dangerous, not to mention hard to manage, and is way illegal in NHRA. Propylene Oxide is somewhat more reasonable but it's illegal too. The fuel rules are pretty strict in NHRA these days...basically spec fuel.

[Gear Jammer]

If we consider skydiving, you reach a maximum velocity of some 50 m/s, which, although a low speed in the present context, is likely to tear your average Armani suit to shreds. This shows that it's difficult to remain properly dressed already at Mach 0.15.

I have heard that if you take a young woman for a quick trip on the back of a motorbike.......Nah, forget it!
PDR

Urban myth, I ride a bike and its never worked for me
;)

[phantom II]

Filthy swine.

Originally posted by Patrice L'Rodent
Quote:
Originally posted by Gear Jammer
pants blown clear off

I have heard that if you take a young woman for a quick trip on the back of a motorbike.......Nah, forget it!
PDR