05/28/2003

Otto, Rankine, and Boost

Back to the otto cycle.  A classic otto cycle engine has an intake,
compression, expansion and exhaust stroke and is known as a four stroke cycle
engine.  

When you add an external compressor to the engine, much of the work of
induction is removed from the intake stroke and transferred to the compressor.

In fact, when the average intake pressure exceeds atmospheric pressure, none
of the work of transport is done by the intake stroke. What we have now is a
compound cycle - part otto -part other.

Consider now the Rankine Cycle.  Working fluid is compressed to a constant
pressure and heat added at a constant temperature continuously.  The working
fluid is then fed to an "expansion" device where it is extracted into rotating
work, the working fluid condensed and returned to the compressor.  This is
closed cycle.  Open cycle rankine engines vent the used working fluid to the
atmosphere.  

A simple piston engine, where working fluid is introduced at tdc, the pressure
unchanged and being greater than the pressure under the cylinder, drives the
piston to bottom dead center, whereby the working fluid is exhausted to either
a condenser or the atmosphere.

This device is a zero expansion rankine cycle piston engine.  

Now consider a super atmospheric engine.  A compressor raises the pressure of
the working fluid above atmospheric to a constant temperature and pressure
continuously.  The working fluid is then admitted to a cylinder, and the
pressure difference causes the piston to move downward to bottom dead center,
where the intake closes.  The charge is then compressed by the compression
stroke.

We now have two engines and two cycles working on the same reciprocating
assembly.  

The first cycle or engine is a rankine cycle open engine, where the
atmospheric pressure air is raised in pressure and heat by the compressor and
the working fluid drives the piston downward.  This is superimposed on an otto
cycle engine where the former intake stroke has been replaced by the rankine
cycle power stroke.  Clear as mud right?

This is the basis of all compound engines and is usually referred to as a
direct compound engine.  Others, such as a turbo compound, place a turbine
geared to the rotating assembly in the exhaust stream.

Now let us consider Brake Mean Effective Pressure.  This is the pressure if
applied uniformly over the power stroke of a four stroke cycle engine would
realize a quantity of Brake Horse Power.  

This is the indicated mean effective pressure ( the average pressure actually
applied to the piston ) over the same period minus the Friction mean effective
pressure ( the pressure that must be overcome to make the bmep ).

We can consider the portion of bmep contributed by the zero expansion rankine
cycle engine as either reducing the FMEP or as a direct portion of bmep.
Since all friction is accounted by the FMEP, cylinder pressure is directly
proportional to bmep instead of adding to Imep.

As long as the work needed to compress the charge and "expand" it in the
cylinder is less than the work required to use the same amount of air by a NA
otto cycle engine, there will be a net increase in efficiency.

The work of compressing this charge is taken from the previous cycle as FMEP.

Thus if you follow Schietzer and later's concepts, a super atmospheric engine
is not a pure otto cycle engine.

Applying the rational of Rankine cycle to the inlet shows a direct
correlation.  If you change from a zero expansion cycle to a partial expansion
cycle, you will see an increase in power recovered from the intake charge.

What happens here is that the inlet charge is "cut off" early and allowed to
expand in the cylinder.  This is similar to the Lysholm effect - where the
intake valve is closed early under boost.  Within reason, the earlier the
charge is cut off, the greater the expansion, and the higher the power from
the working fluid - providing the working fluids pressure is increased
proportional to the earlier occurrence of cut off.

This is the principal underlaying the 2 cycle inlet supercharging valve from
schietzer.

The higher the inlet pressure, the more energy is recovered during the "inlet"
stroke.  With a constant output power and rpm, the higher the pressure - the
greater the work developed is split between the inlet and the expansion
stroke.