Diesel Compound Turbocharging Options

[PBB]

Here's some pics of a compound twin turbo setup on a Cummins.





Like the name implies, compound turbocharging compounds the boost. The first turbo compresses the air charge to ~30 psi* into the second turbo which yields ~60* pounds of boost into the intake manifold. (*Arbitrary numbers)
How does the second turbo not surge, spin twice as fast and blow up? Simply, compressor surge and compressor speed are not directly related to the amount of airflow through the turbo but rather the amount of boost the turbo is trying to create. While both turbos are creating ~30 pounds of boost more than the inlet pressure, they are behaving the same.

The above explanation is assuming two equal sized turbos, some of the truly insane compound setups involve two "small" 90mm+ turbos right off the header pressurizing the inlet on a massive 100mm+ single turbo. These are the 100-150psi 2,000-4,000bhp engines used in tractor pulls.

[PBB]

More pics: http://www.texasdieselworks.com/twinspics/

Want serious lungs for your Cummins Dodge? BD’s Super B Twin Turbo is a smart choice from a variety of standpoints. Why two turbos? It’s not just about doubling the amount of boost available by adding a second turbo, though you’ll generate much more boost than a stock single turbo unit. It’s more about broadening the power available over a much wider rpm range. The two turbos have different boost characteristics that compound each other. The smaller turbo spools up faster and gives more boost and power at lower rpms. The second unit is larger and spools up slowly, but has the capacity to offer more boost at higher engine speeds. When combined with increases in fuel rate, rear-wheel-power outputs of 500 horses are possible. What’s more, the increased airflow has a cooling effect if sensible fuel rates are used. For example, one test showed that at a rear-wheel-power output of 450 horses at 51psi boost, a 2002 Cummins developed an EGT of only 1175 degrees and low smoke levels. For more info, contact BD at 800-887-5030 or see www.dieselperformance.com

[bankE46]

that is a cool looking turbo setup. interesting stuff. i always wondered how they got such insane power out of turbo diesels.

[DakianDelomast]

Uh been around for awhile.

Ever hear of sequential turbos?

[fiber optic]

How fast do turbos typically spin? I'm going to guess around 30,000RPM. We have some turbomolecular pumps in the lab that spin at that speed and they sound about the same. We also have a roots booster pump that sounds like a supercharger. It's like working in a performance shop sometimes except much cleaner.

[Mr b00st]

that's an interesting setup.

I am such a diesel nerd, it's not funny.

Has anyone seen the stock Holset turbo that's on the Cummins? That's a big motherfuckin' hairdryer, yo.

that's an interesting setup.

I am such a diesel nerd, it's not funny.

Has anyone seen the stock Holset turbo that's on the Cummins? That's a big motherfuckin' hairdryer, yo.

[PBB]

Uh been around for awhile.

Ever hear of sequential turbos?

Sequential turbo setups are completely different. They use a series of vacumn activated control valves to route the exhaust gases and intake air through a small turbo at low RPM and through a larger turbo at high RPM. Sequential turbo setups were perfected in the early 90's at about the same time single turbos were made efficient enough to render them obsolete. Currently the only production vehicles that use sequential systems are turbodiesel passenger cars, notably BMW.

Full explanation of Sequential Turocharging and diagrams of Supra TT setup.

How fast do turbos typically spin? I'm going to guess around 30,000RPM.

115,000 RPM at full boost, off idle they spin at ~30K so admittedly the sound would be the same.

[clarkma5]

I was just waiting for somebody to confuse it with sequential turbos...

It's a cool idea; seems like it offers a lot of the benefits of sequential turbos without the problems of trying to make the pass off from small to large turbo smooth and predictable.

[DakianDelomast]

Its the exact same concept as a sequential turbo.

[Espen]

Wonderful images

I know diesels are less affected by inlet temperature due to the wasted air fuel ratios - but the inlet temperature at 4 bars must be immense?

A simpler way of looking at turbo sizing in compound applications is looking at pressure ratio and not boost or flow per se. The turbo is only affected by the difference between inlet and outlet pressure (within reasonable levels obviously) - so wether the inlet sees slightly below ambient or 2.0 bars makes little difference. Which is partly why turbocharging is such a benefit for high altitude four stroke engines.

I'm a bit intrigued by the description of different sized turbos in a compound setup. I wathced a two stage compound similar turbo setup on a street driven Merc once, the thing had the most insane boost rush I've ever seen.

When provoked (i.e. driven from idle in gear) it went like lagging.... lagging... wheeeEEEEEEE *surge* *SURGE* *WHOOOOOOOSH* When boost arrived i shot a black jet of compact smoke several meters out the tailpipe and promptly went sideways with crazy wheelspin in third!

Useless but fun to watch

[PBB]

It's a cool idea; seems like it offers a lot of the benefits of sequential turbos without the problems of trying to make the pass off from small to large turbo smooth and predictable.

Definitely, the compound setup provides the holy grail of turbocharging, massive low end power and torque while still maintaining top end boost and efficiency, too bad it only works on diesels. Since cooling the air charge to prevent detonation isn't an issue, there's no intercooler and ancillary piping and no intake piping. This setup is much lighter and more compact than any gasoline multi-turbo setup could ever be. Pretty amazing stuff.

[clarkma5]

Its the exact same concept as a sequential turbo.

It works very differently...and I wouldn't even call it the same concept anyway, it's just trying to solve the same problem that sequential turbos are trying to solve.

But there I go arguing semantics again so nevermind me...

[DakianDelomast]

Why wouldn't you want an intercooler....?

[PBB]

Why wouldn't you want an intercooler....?

The primary porpose of an intercooler on a gasoline engine is to cool the intake charge as a safeguard against detonation. Since diesels are compression fired, every cylinder firing is controlled detonation. An intercooler on a turbodiesel would only yield insignificant power gains.

Most stock turbodiesels use intercoolers for emissions and fuel economy, not specifically for performance.

[DakianDelomast]

Uhm not really. Intercoolers offer a bigger advantage of being able to cool the flow of hot compressed air. Cooler intake air means a denser gas which means better burn.

[PBB]

As if 60-100psi wasn't dense enough?

My point is that, unlike high boost gasoline engines, intercoolers aren't critical equipment.

[clarkma5]

Dayum PBB, that's what we call OWNAGE

[DakianDelomast]

Dude PV=nRT. If you take two gases at the same pressure the cooler one will always be denser...

[clarkma5]

You're talking about changing (delta)p from 85 psi to 86. It's insignificant.

It's not like in a gasoline powered turbocharged engine where a PSI makes for a 5-15% difference in the pressure. This is barely a percent. So why spend the money on an intercooler?

[DakianDelomast]

Wtf? I'm not talking about changing the pressure at all...