Why don’t they put turbos on top fuel dragsters?
I know the engines already have a belt driven supercharger, which gives a linear amount of boost and, of course, eliminates turbo lag, but surely someone can take advantage of the free energy being shot out of the exhaust pipes.
Does the NHRA or IHRA have rules against this?
There is really no reason to have both a turbo and a super charger. Both devices make the engine work a bit harder to force air into the engine and produce more power.
Instead of adding a turbo to a supercharged engine, you’re better off to increase how much air the supercharger forces into the engine.
The limiting factor in a top fuel dragster is not how much air that can be forced through the engine. With a supercharger, they are already pushing more air and fuel into the engine than the engine can use.
Actually, depending on how you look at it, the limiting factor IS how much air can be forced in. Top fuel dragsters use 85% nitromethane for fuel (it’s mixed with methanol to make it more stable and safer to handle). Nitromethane actually produces LESS energy per unit volume than gasoline, but it is used because it requires less air to burn, and they can cram much more fuel through the engine given the amount of air the supercharger is forcing in, resulting in a net increase in output. A top fuel dragster uses an astonishing 10 to 12 gallons of fuel on a pass.
Good info. I think that a TFD motor is typically at it’s physical limits to begin with so adding more pressure to the engine via turbos would just turn the whole engine into an 800lb handgrenade.
OP;
Plus they have to keep these engines relatively simple as they rebuild the motors between runs. All the tubing and extra machanical connections of a bunch of turbos would just slow them down and drive the price of the motors way up.
Plus, I think they’re already putting out several thousand HP, (7,000?) and they need those tires to stick a little, no?
Turbos are tricky to optimize because they comprise a positive feedback system…the more air you put into the engine, the more exhaust comes out, which drives the turbine harder, which puts more air into the engine which makes more exhaust…etc.
Top teams tune the induction system for extant conditions…altitude, humidity, temperature. With the roots blower, this is a matter of swapping in the appropriate pulleys and re-tensioning the drive belt. With a turbo, this might require different nozzled volutes, compressor trims, turbine trims, etc. Due to the feedback, there is no simple formula for getting it right.
What might be more practical would be turbo compounding…where the turbine drives not a compressor, but is instead geared to the crankshaft. Such was done on the last of the big radial aircraft engines.
I may picking nits here, but the limiting factor is not how much air can be forced in, it’s how much air the engine can process. The engines are limited to 500 CI in the NHRA, so the only way to process more air is to spin the engine faster.
The engines are already turning enough RPM’s to destroy themselves in a 1/4 mile.
They don’t turn that many RPM, and are fitted with a rev limiter (an F1 engine turns 10K more RPM and will hopefully last a few hours). It’s just that at those power levels, the engines can’t hold out for much longer than 5 seconds or so. FWIW, the engines only complete about 650~700 revolutions in a pass.
Well, you can either increase the RPMs or increase the atmospheric pressure, which is where the charging (turbo or super) comes in. NHRA limits the amount of boost pressure that can be generated, and I think this is the real answer to the OP’s question. If you’re restricted to 3.45 bar (or whatever it is), then you want to be making 3.45 bar throughout the run. Turbos build pressure as the engine speed increases, so if you want to make 3.45 bar max pressure at 9000 rpms, you’re only going to be making some fraction of that at 5000 rpms. A roots blower is going to behave much more linearly, providing a set amount of air pressure regardless of engine speed.
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I don’t think the NHRA limits boost, Wikipedia says that some teams will run up to 5.0 .
And they have the engine rev limiters set to 8,400 rpms, so if you divide that by 60 and then multiply by about 5 seconds, you’ll get the amount of revolutions the engine actually makes - about 700. They rely on the clutch to adjust traction to the tires whilst the engine is pretty much at full throttle all the way down the track.
I’m going to poke around a little, but I’m fairly sure the NHRA rules disallow turbochargers. A Turbocharger is much more efficient than a supercharger. At peak RPM, those superchargers are actually using 800 - 1000 HP due to parasitic loss (how much energy the engine has to put into spinning the supercharger).
The 2007 NHRA rule book says that for Top Fuel dragsters, the forced induction engine is restricted to Roots-type supercharger, rotor helix angle not to exceed that of standard 71-series GM-type rotor. Turbocharger and/or centrifugal supercharger are prohibited.