alt.energy.renewable

"David Williams" wrote in message
news:1205768059.916.1205722936@bayman.org...
>-> Steam locomotives used what I think was called a "cut-off valve" which
> -> allowed full pressure steam to enter the cylinder from TDC for a
> -> variable number of crankshaft degrees.

I don't know about locomotive engines in particular, but the theory is to use
a "poppet" valve that fully opens and then closes quickly, perhaps for a
variable amount of time depending on conditions. By being either fully open or
fully closed rather than somewhere in between, you reduce throttling losses.

A useful analogy to understand the difference between poppet and throttle
valves is to compare an analog power regulator to a switching power regulator.
The analog regulator would be like a conventional throttle valve, which
sacrifices efficiency for simplicity and may require massive heat sinks to
dissipate the waste power. The switching regulator would be more like the
poppet valve. The switching regulator is simply a high-speed switch that is
either on or off, but never in between, so it never acts like a resistor and
therefore dissipates very little heat, meaning that less energy is wasted. This
explains part of the reason why a switching power supply can be so much smaller
than an analog power supply.

Vaughn