Ive been reading about compound turbo charging but cant find any math on the subject to help me understand

my basic knowledge of it so far is you run a small turbo from the exhaust as normal, this then spools a larger turbo to the intake manifold thus giving more boost quicker. it was developed in WW2 by the looks and development stopped because they where producing too much power and jet engines became easier.

it seems they do it on big power conversions in the US (think 1200bhp+) but i cant find any mention of here in the UK, is this because its called something else here, or just because we dont do it? It also appears to be a bit of a 'dark art', there must be information to help the noob

basically i was looking for some calculations to work out the matching of the turbos to see what was possible, the benefits seem to be immense but i might be misunderstanding.

heres a sexy picture i found





I dont have an application for it (yet), just interested

I realise it may be just my eyes - age you know - but in that photo the primary turbo seems largere than the secondary turbo, opposite to your descrition.

its definatley smaller to larger (from what ive read)

that pic just turned up on google with me doing a search for compound turbo, it might well not be

it seems a really easy way to make big power, providing the turbos can be matched, i imagine heat to be a problem and Im not sure I understand pressure and boost when it enters the cylinder from the manifold. as in can i put too much air in?

I like the idea of that but I think the recent modern turbos spool up so quick and will boost to 30psi so easily it’s not worth the trouble.

But the idea is very good and probably when feeding a large pot V8 or a V12 Lambo it may help, but a 4 pot or 6 probably not worth it.


James

There is a short chapter on Tractor-pulling in Hugh MacInnes' Turbochargers.
Tandem turbocharging is used to get high boost (90+ psi).

Basically it is the turbo equivalent of a two stage supercharger, compressing in two stages has big advantages especially if you can cool the charge between stages.

Is this not "sequential" turbocharging? "Compound" usualy refers to a turbo that recovers mechanical energy which is directly added to the engines output (rather than just used to compress air so even more fuel can be burnt).

I can't see how this would work without some kind of valve to direct the exhaust gas flow between the small and large turbos?

Hmm, I was thinking more of a twin turbo thing where you'd have a smaller turbo that spools up quickly, and a larger turbo for the real power. The smaller one is kind of used to prevent turbo lag (well, reduce it) in massive chargers

the two are different from my reading....

compound turbocharging is different to a turbo-compound engine though the two sound very similar, turbo-compound engines are like you say driven from the crank

I found a video

The more widely used term you should be searching for is "sequential turbos"

this is where the first turbo is used for low down engine speeds, and the (first and) second for higher end.

S

http://en.wikipedia.org/wiki/Twin-turbo

I dont think I am,

on a sequential system, I dont believe that the smaller turbo feeds the larger turbo, they are just ran at different times, one then the other, small for low rpm and then big for high rpm - the RX7 had a sequential twin turbo

subaru legacy twin turbo is also sequential, i could be wrong but i always thought that one fed the other.

from wikipedia

quote:

---

Sequential turbos refer to a set-up in which the motor utilizes one turbocharger for lower engine speeds, and a second or both turbochargers at higher engine speeds. Typically, larger high-flow turbochargers are not as efficient at low RPMs, resulting in lower intake manifold pressures under these conditions. On the other hand, smaller turbos spool up quickly at low RPMs but cannot supply enough air at higher RPMs. During low to mid engine speeds, when available spent exhaust energy is minimal, only one relatively small turbocharger (called the primary turbocharger) is active. During this period, all of the engine's exhaust energy is directed to the primary turbocharger only, providing the small turbo's benefits of a lower boost threshold, minimal turbo lag, and increased power output at low engine speeds. As RPM increases, the secondary turbocharger is partially activated in order to pre-spool prior to its full utilization. Once a preset engine speed or boost pressure is attained, valves controlling compressor and turbine flow through the secondary turbocharger are opened completely. (The primary turbocharger is deactivated at this point in some applications.) In this way a full twin-turbocharger setup provides the benefits associated with a large turbo, including maximum power output, without the disadvantage of increased turbo lag.

---

Yes, it's a different thing to what you se on cars to get around the lag from a big turbo by using a small one along side it to boost from low revs. With compound turbos one blows into the next, all the gases flow through all the turbos one after the other.
As mentioned it's used in tractor pulling where they're looking for power levels on a big scale, an engine that left the factory with 100-140bhp will see boost levels like 200psi & make over 1000bhp. You dont get anything for nothing though & engine blow-ups happen in a literal sense, like the block will split between the crank webs & base of the bores!
Don't know if there's much advantage over conventional turbo set-ups in it for making the levels of boost you'd want on an engine that you want to be reliable.

Some diesel racecars tried it.

Heat and weight are your biggest enemies. Then there's the space needed.

Turbo into a super is common on heavy equipment. But they usually divert the turbo straight into the engine at a set boost pressure.

Where's BT? His big ships have some beautiful turbo/super systems.

I went past a place in Totton the other day, where they rebuild this ship stuff. They had a turbo sitting outside, must have been 4' across! And that's a little one!

Cheers,
Nev.

We used to install Volvo KAD44 engines into our motor yachts which were Supercharged and Turbocharged The Roots type superchargers were geared up and crankshaft driven through electric clutches which disengaged at 1900 rpm. There was a flap valve which allowed which ever induction path ran at the higher pressure to feed the air to the intercooler.

This set-up worked really well at delivering seamless boost throughout the rev range. I have never seen this used in a car, Why not?

quote:

---

Originally posted by plentywahalla
We used to install Volvo KAD44 engines into our motor yachts which were Supercharged and Turbocharged The Roots type superchargers were geared up and crankshaft driven through electric clutches which disengaged at 1900 rpm. There was a flap valve which allowed which ever induction path ran at the higher pressure to feed the air to the intercooler.

This set-up worked really well at delivering seamless boost throughout the rev range. I have never seen this used in a car, Why not?

---