Dreamlinerwannabe
But, I don't understand why do we have to have a "rev-up, throttle back" principle.
Why do we have to do this? Why sometimes we have to select RPM first, then MP?
And how do we know if that RPM setting is optimal?
Also, it's saying that propeller setting controls RPM, throttle control MP.
The reason for the change in speed before increase in manifold pressure is to ensure the engine doesn't detonate with the increase in BMEP. This is often refered to as "OverBoosting" the engine - but it can also occur in normally aspirated engines as well.
You can easily achieve detonation in a motor car with a manual transmission - with a slow speed in a high gear push the throttle all the way open and you will hear - and sometimes feel - the engine "Ping". This is detonation and it is very destructive to aircraft engines.
Below is an excerpt from the Pratt & Whitney Engine Operation Manual 01 - 100 that was used in training during WWII.
Page 27 - 29
DETONATION
Normal combustion is rapid, but it is by no
means an instantaneous explosion. The charge
burns evenly and smoothly, the flame front advancing
at a measurable rate-about 35 feet per
second when combustion begins, increasing to
roughly 150 feet per second, and finally slowing
down as the process nears completion.
If sufficiently heated and compressed, any
combustible mixture of gasoline vapor and air
will catch fire. Accordingly, if the temperature
and pressure of the unburned portion of the fuel-air
charge reach critical values, combustion
will begin spontaneously and simultaneously
throughout the unburned charge. The result is
a sudden and violent explosion known as detonation.
Detonation occurs so quickly that even high
speed cameras, which slow down normal combustion
to a snail's pace, fail to retard its progress
sufficiently for exact analysis. It is accompanied
by an abrupt pressure rise and violent
pressure fluctuations of extreme rapidity. The
engine is unable to turn into useful work energy
so explosively released. The recurring shock
pressures are carried to piston, cylinder, and
hold-down studs, and the fatigue stresses set up
in the materials quickly lead to the failure of
these parts.
Detonation also causes a rapid rise in cylinder
temperatures, and thereby aggravates the very
conditions which produced it. These high temperatures
can rapidly destroy the piston, cylinder
head, exhaust valve and guide, and damage
other parts by burning and erosion.
Similar in its results to detonation, and frequently
accompanied by it, is pre-ignition. The
latter is caused by uncontrolled ignition of the
charge ahead of the normal flame front, because
of contact with some "hot spot" in the combustion
chamber, such as an incandescent spark
plug. As a result the timing is too far advanced;
the engine loses power and overheats ; local temperatures
at the hot spot rise rapidly; and the
engine may be damaged, if it is not quickly
stopped.
Detonation-free operation is altogether normal
and entirely possible over the full range
of rated engine performance, even under the
most adverse conditions. Nevertheless, detonation
is the most likely as well as the most destructive of
the possible consequences of improper engine operation.
CONDITIONS LEADING TO DETONATION
Among the conditions which may lead to detonation
the most important are :
1. Excessive manifold pressure. As manifold
pressure is increased, so is the pressure of
the charge entering the cylinders. The latter
is multiplied many times during compression
and combustion, and, if the initial pres-
sure of the charge is too great, a critical
value may be reached which will result in
detonation. Excessive manifold pressure may
be caused by too wide a throttle opening or,
on some engines, by the use of too great a
degree of supercharging.
2. Excessive carburetor air temperature (c.a.t).
As the temperature of the charge air at the
carburetor is increased, so is the temperature
of the fuel-air mixture entering the cylinders.
The latter is further raised during compression
and combustion, and, ' if the initial
temperature of the charge is too high, a critical
value may be reached which will also
result in detonation. Hot "free air" entering
the induction system, inadequate inter-cooling
in the case of multiple stage superchargers
or too much carburetor pre-heat, may
cause excessive c.a.t. High impeller speeds,
the consequence of high engine rpm or of
improper operation in the "high" impeller
gear ratio, will cause a sharp heat rise -
through the supercharger, and, as a result,
the charge will not be sufficiently cool when
delivered to the cylinders.
3. Excessive cylinder head temperatures. The
temperature and, indirectly, the pressure of
the unburned portion of the charge may be
raised to critical values as a result of excessive
cylinder head temperatures alone.
4. Improper grade of fuel. If the fuel used has
an anti-knock rating (i.e., resistance to detonation)
lower than that called for by the rating
of the engine, detonation will follow any
attempt to operate in the high power range.
5. Malfunctioning of the ignition system:
Whenever the engine is operated in the high
power range, detonation is likely to occur if
the timing of the spark is too far advanced.
It may also occur during high power operation
in a cylinder where only one of the two
plugs is functioning.
6. Lean Mixtures. The tendency to detonate
varies with the fuel-air ratio, and mixtures
at or near best power are the ones most
likely to detonate. Combustion chamber
temperatures may be lowered most effectively,
and detonation thereby most readily
inhibited by enriching the mixture beyond
the best power setting.
Detonation imposes one of the most important
limitations on engine performance, and the operator
must at all times so control conditions as to
avoid any which might lead to detonation and
the consequent damage to his powerplant.
Page 101
TRANSITION FROM TAKE-OFF
TO CLIMB
As soon as the field and surrounding obstacles
are cleared, reduce power at least to
Normal Rated. With constant speed (variable
pitch) propellers the reduction should be accomplished
in steps as follows:
1. Retard throttle to reduce manifold pressure
to about 2 in. Hg below that for Normal
Rated power (with fixed part throttle,
manifold pressure will rise as rpm is reduced).
2. Retard the rpm control to Normal Rated
rpm.
If a further reduction in power is desired,
proceed as follows:
1. Lower manifold pressure by 2 in. Hg.
2. Lower engine speed by 200 rpm.
Continue in successive alternate steps until
the desired engine speed is reached, finally adjusting
the throttle to the desired manifold pressure.
This is not to be construed to mean that the throttle
should never be advanced with a low rpm. The engine -
is not affected by the position of the throttle.
It is affected by the manifold pressure resulting from the
throttle position.
Bold and Italics for emphasis are mine.
Hope this helps