engine/turbo theory question

[Jorgen Karlsson]

> 	i was discussing turbocharges with a friend - well,
> 	explaning them, actually.  he raised a question that
> 	i never thought of:
>
> 	is there any overall power loss that comes from the
> 	pressure (backpressure?) required to turn the turbo?


The increased pressure in the exhaust manifold over the pressure on a n/a
car cost some power, a fraction of the power needed to drive the most
efficient centrifugal superchargers.

Super charger is driven by power that the engine has to _produce_ while the
turbo _prevents_ the engine from producing a few hp.

A properly setup turbo also produces power from just above idle, that is not
the case with a centrifugal supercharger. The lysholm screw superchargers
(not roots type) can produce boost at idle to maximum rpm but they need more
power to operate.

To get a turbo quick spooling the exhaust turbine has to be pretty small,
the exhaust runners also have to be small to keep maximum exhaust energy to
drive the turbo. The pulsing of the exhaust is important to keep, the
exhaust pulses should 'mix' as little as possible, this is also for
minimizing the spool up time. Remember that most turbos never reach
operating temperature in city traffic. A turbo charger is not at operating
temp until it glows bright red.

All of these design rules adds to backpressure and prevents the engine from
producing maximum power.

For racing maximum power is more important then drivability of idle, this
makes it possible to stretch the design rules a bit, the first measure
usually are to make the runners and the exhaust turbine larger. By
sacrifising the turbo spoolup when the car is not driven hard the exhaust
backpressure can be lowered a lot, the car is still responsive when driven
hard.


Jorgen Karlsson
Gothenburg, Sweden