wave tuning & turbo

[Horsewidower]

This may be of value to you, it eventually addresses turbos:

viewtopic.php?t=16763&postdays=0&postorder=asc&start=0

[ptuomov]

viewtopic.php?t=16763&postdays=0&postorder=asc&start=0

Interesting.

[ptuomov]

I think I’ve got an answer to my own question from Barnes’s book page 25 figure 1.17. It’s about being able to use the largest possible turbine while still efficiently spoiling it. There are three issues to consider: runner volume, runner length, and runner diameter. Any two will obviously determine the third. In any case, the small volume is desired to make the blowdown pulse hit the turbo hard. Then the diameter is as small as possible without causing excessive pumping losses. Length appears to be the least important, and thus usually short. This in a single cylinder engine. In a multi cylinder engine, I’m guessing that blowdown interference and the ability to get good merge angles increases the length.

[user-23911]

Everything works the same.......N/A or turbo.

The difference is the absolute pressure.

A small turbine causes a restriction at higher RPM and causes high EGT, just the same as a restrictive exhaust in a N/A application.
When you change to a bigger turbine, boost comes on later in the RPM band,causes less restriction and gives more boost if required.


At idle, whether turbo or N/A , it's the same. IM (average pressure) has vacuum, EM (average pressure) is at atmospheric, IM is at lower pressure than EM.
N/A full throttle, IM is close to atmospheric as is EM.
A large turbine.......at 1 bar boost. IM is at 1 bar boost, EM is close to that figure too, pressure differential across the engine is the same as with an N/A.

Increase the boost to 2 bar.
It's the same........if the average pressure in the exhaust rises more than the average pressure of the intake.......the turbine is too small.

[Circlotron]

Most street turbos have exhaust pressure greater than boost pressure. Only an extreme sized turbo on a real race engine will boost be significantly higher than exhaust back pressure.

If that is the case, how is it that all these guys on YouTube have managed to make gas turbines of of ordinary looking turbos. If the exhaust pressure on one of those was greater than the boost pressure then the exhaust would blow back out the compressor.

[user-23911]

Most street turbos have exhaust pressure greater than boost pressure. Only an extreme sized turbo on a real race engine will boost be significantly higher than exhaust back pressure.

Obviously an incorrect statement because you start off at low load, exhaust pressure is always greater than intake pressure.
I've yet to see a good engine that won't idle due to backwards flow.

But as you increase load, both intake and exhaust pressures rise. As I said above, if the exhaust pressure gets too high, it's due to the turbine being too small.

[Warpspeed]

Lots of reasons...

First a gas turbine does not have a restrictive engine between the compressor and turbine to limit flow.

Twenty or thirty years ago when many less wealthy people had two valve pushrod engines with fairly primitive sleeve bearing turbos, getting boost higher than turbine pressure was extremely difficult. It gets a whole lot easier with today’s four valve DOHC heads and ball bearing turbos with vastly better designed wheels and housings. Its now getting really easy getting boost up above turbine back pressure.

But its still true that if you want stump pulling torque way down low, with a fairly primitive design of engine that requires a small turbine, and its much more difficult to keep the back pressure down.

A whole lot depends on what you are trying to do, and what you ave available to do it with.

[user-23911]

A gas turbine isn't an engine.It's more like a flow bench.It's constant flow.

An engine isn't like a flow bench and isn't constant flow.

[ptuomov]

Everything works the same.......N/A or turbo.

The difference is the absolute pressure.

A small turbine causes a restriction at higher RPM and causes high EGT, just the same as a restrictive exhaust in a N/A application.
When you change to a bigger turbine, boost comes on later in the RPM band,causes less restriction and gives more boost if required.

At idle, whether turbo or N/A , it's the same. IM (average pressure) has vacuum, EM (average pressure) is at atmospheric, IM is at lower pressure than EM.
N/A full throttle, IM is close to atmospheric as is EM.
A large turbine.......at 1 bar boost. IM is at 1 bar boost, EM is close to that figure too, pressure differential across the engine is the same as with an N/A.

Increase the boost to 2 bar.
It's the same........if the average pressure in the exhaust rises more than the average pressure of the intake.......the turbine is too small.

Everything is most definitely the same as in a normally aspirated engine. Normally aspirated engine, for example, doesn't have the choice between a large volume exhaust manifold and a small turbo vs. a small volume exhaust manifold and a large turbo. Both spool up at the same rpm, but if done right (good pulse combination) the small volume exhaust manifold and a large turbo result in an engine that can run better at high rpms. Where's the normally aspirated engine analogy to that? In my opinion, turbo engine is very different on the exhaust side compared to a normally aspirated engine.