> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Michael Watterson
> Sent: 15 May 2011 10:08
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Water electrolysis -> H2 and O2
>=20
> more mpg (or kilometres per litre). No laws are broken. A turbo
charger
> is an example of "cheating". It doesn't give you anything for free. It
> can raise efficiency by reducing waste energy. You are not getting
> extra. just better less than 1 efficiency. Of course historically
turbo
> chargers may be optimised simply to get most power for same size
engine
> and this might even reduce efficiency, it lets you feed more fuel and
> air than otherwise possible.


It's a fallacy to think that a turbocharger makes an engine more
efficient - the energy consumed by the turbo (and it does use energy
over and above the wasted thermal energy in the exhaust due to increased
pumping losses) is not added to the energy output of the engine, but
instead allows you to burn yet more fuel.  In fact, spark ignition
turbocharged engines are typically less efficient for a given peak power
output than non-turbocharged.

The reason that turbo chargers are now more popular for economy reasons
is simply that they are paired with smaller displacement engines.
Off-boost, a smaller displacement engine has to work harder to produce
the same power as a larger one, i.e. the throttle needs to be opened
further.  This reduces pumping losses and increases the dynamic
compression ratio, improving thermal efficiency.  Since much of the time
driving will be done at lower RPM and off-boost, especially in urban or
under cruising conditions this gives a net improvement in economy.  With
the turbo spooled up and the engine producing higher power outputs,
efficiency will typically fall *below* that of an equivalent normally
aspirated engine (increased pumping losses, extra fuel needed for
cooling).  A turbo simply allows a small displacement engine to produce
sufficient maximum power without impacting on lower power operation.

Regards

Mike

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