The dynamics of a turbo system

[Rollins]

Hey guys, as some may know I am looking at getting a 951. Because of this I must ask this question, probably a silly one, but how exactly does a turbo system work? I see you guys talking about them and I am totally turbo illiterate. Someone please, clue me in.

[1.2gees]

Overly simplified:

Exaust gasses exiting from the cylinder are routed through the exaust turbine of the turbo. This is connected to the propeller on the intake side of the turbo via a shaft. The faster the exaust side spins, the faster the intake side spins. The spinning propeller on the intake side of the turbo pressurizes the air going into the engine.

Engine is saved from overboost (distruction) by some means to let either exaust gas out before the turbo, or route a portion of the intake charge somewhere besides the engine.

Also, many systems have a way for the intake charge to be routed before the turbo, to keep boost up, during shifting.

Turbo lag: The time it takes for the turbo to start producing boost, after throttle is applied. The size of the turbo, and the mass of it's rotating parts, as well as flow characteristics, etc. all have a role.
Ahmet

[Blackfoot]

turbocharger is an air pump that is driven by exhaust gas. it pressurizes the intake to force more air and fuel to go into the cylinder, and that means more horsepower.

there are two sides of the turbo; the turbine side, which is spun by those exhaust gases as they leave the engine, and the compressor side, which is driven by the turbine side and compresses the intake air.

almost "free" hp.

[drew1]

Rollins,

A turbo charger is bolted to the exhaust manifold outlet. The impeller at the exhaust, the hot side, is turned by exhaust leaving the cylinders and passing though this impeller housing down the exhaust pipe.

A shaft connects the exhaust impeller to the intake or cold impellar and turns it. This intake impeller, in the intake housing gets air from breather, air filter, and pumps it at a higher pressure up to the throttle body or intake. The idea is that there is about twice as many air molecules in a pressure of two atmosheres of pressure about 15PSI guage as regular atmosheric pressure. A CIS injection, 930 & 931, have the fuel distributor which feeds the injectors between the breather and turbo intake impellar. Many, most 930 & 951, have an intercooler that reduces the pressurized air temp; therefore closer to twice as many air molecules at twice atomsheric pressure. The intercooler is between the outlet side of the intake impeller and the throttle body.

A turbo would just keep building more and more pressue, when you have it floored, if it weren't for the waste gate. On a simple system, 931, pressue from the duct between the cold side outlet and throttle body is fed to the waste gate. When this pressure overcomes the spring/diaphram of the waste gate , the wastegate opens and lets exhaust out to the exhaust pipe, bypassing the turbo hot impeller. This regulates the maximumum pressure of the system.

When more air is let into the intake system, more gas has to be let in too. The CIS dopes this through the warm up regulator lowering control pressue while fuel system pressure is the same and DME does it by increasing pulse of the injectors. There are rising rate fuel regulators that work much the same as CIS principle, more fuel with more pressure at injectors.

When you let off, Your intake duct still is pressurized. To help the turbo spool up quicker, the is sometimes a valve activated by differntial pressue between the intake manifold behind the throttle and the duct. This valve lets off the pressure & the turbo spools up quicker, on gear shifts, etc. This valve is actually in the cold housing of the 931 turbo.

There are boost tricks:
Wastegate shims & heavy springs
relief valves that let intake pressure build up before letting it signal the wastegate. (Lindsey)
Electric solenoids that don't open the signal line for a certain time or certain pressure. (HKS, GReddy)
signal bleed mechanisms using air regulators or valves. (Performance Products)

Hope I'm halfway clear.

drew1

[Blackfoot]

Originally posted by 1.2gees:
Engine is saved from overboost (distruction) by some means to let either exaust gas out before the turbo, or route a portion of the intake charge somewhere besides the engine.

a.k.a. wastegate.

[wolfrpi]

BTW Blackfoot - I checked how quick boost came on in my 951 and I'm at full boost (15 psi) by 3000 rpm in 4th gear. Ya gotta upgrade!

Rollins - you're gonna have a ton of fun - just don't "spin" it...

Erick