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Shifting
Gears the Right Way
This topic
is mostly concerned with shifting gears and the associated clutch
actions after the rig is in motion. To first get the rig started
moving, refer to our clutching topic
which explains in great detail, what you can do to improve the
initial start out.
Torque
and Power
To understand
shifting, you have to understand the difference between torque
and power. Torque is the twisting force that the engine exerts
on the drive-line to make the wheels turn. Torque is the holding
power of the engine. As you start up a grade, the truck speed
slows a little, engine power decreases, but engine torque increases
and drags the truck on up the grade
Power on
the other hand, is the ability of the engine to accelerate the
truck. Engine power is increased by raising the engine RPMs. All
engines have more torque at lower RPMs, and less torque at higher
RPMs. All engines have less power at lower RPMs, and more power
at higher RPMs.
If you don't
want to accelerate the truck, then keep the current shift gear
and the torque will increase as the engine RPM decreases and the
holding power of the engine will be maximized without a gear shift.
Torque
Limit Considerations
Ok, so now
we all know that holding a gear will get the most torque out of
the engine. As the grade gets steeper, the engine RPM will decrease,
and the engine torque will continue to build. At some point, the
engine torque output matches the load torque requirements and
the truck holds its speed, or the RPMs keep falling and a downshift
is required. The number of cylinders in an engine will determine
the interval between firing pulses. The pressure developed in
the engine cylinders is seldom exactly synchronized with the throw
vector angle of the crankshaft, which results in a fractional
acceleration of the crankshaft above mean crankshaft speed at
each firing pulse. These torsional forces due to the power stroke
are amplified at slower engine RPMs, when the interval between
firing strokes is greatest. These torsional forces can damage
the driven components of the drivetrain at high torque, low RPMs.
If you don't
downshift, when the RPM is low, the engine can handle the torsional
forces but the dirvetrain can't. When you downshift, the engine
gains more RPM, and the whole process repeats as the engine either
holds its RPM, or continues to drop RPM while torque increases
once again. Remember, when you downshift, the truck and its load
is moving slower, so less work is being done by the engine per
unit of time. The engine torque is multiplied a little more by
the transmission downshift, so all these factors work the engine
less.
Engine Performance
Let us look
at engine power and torque curves for a moment. The Caterpillar
3406E engine is spec'd at 1350 lb/ft @ 1200 RPM, and rated at
355 hp at 1800 RPM. This means that for the fastest acceleration
of this rig, you would want to keep the RPM near 1800 RPM. To
be precise, you would want to go past 1800 RPM by one-half of
the next gear RPM drop. So if the next gear drops 150 RPM, then
you would shift at 1875 RPM.
For holding
grades, you would let the engine fall down to 1200 RPM before
downshifting. This would allow the engine to achieve maximum torque,
which may be enough to hold the grade in that gear. If the tach
continues to fall below 1200 RPM, then drop that required gear.
Fuel effeciency
is usually achieved at the engine's sweet spot. The sweet spot
is usually midway between the rated torque RPM and the rated power
RPM. For this rig, best fuel effeciency on flat land would be
achieved at 1500 RPM.
Double
Clutching
Double clutching
is the art of using the engine RPMs to control the speed of the
transmission input shaft between gears to match gear speed. When
it is time to upshift, the transmission input shaft is turning
the same speed as the engine. To engage the next upshift gear,
the engine RPMs must be lower and the transmission input shaft
must also be turning slower RPMs to match the gears inside the
transmission.
An upshift
double clutch consists of the following steps:
- Let up
on the throttle, and at the same time push the clutch part way
to the floor, enough to release the clutch (do not push the
clutch all the way to the floor if you have a transmission brake
on your transmission)
- Pull the
gear shifter into neutral position and let the clutch back out
for an instant as the engine RPMs fall (this slows down the
transmission input shaft)
- Push the
clutch back to the release position and pull the gear shifter
into the next gear (this engages the next gear), then release
the clutch.
A downshift
double clutch consists of the following steps:
- Push the
clutch part way to the floor, enough to release the clutch (do
not push the clutch all the way to the floor if you have a transmission
brake on your transmission), then pull the gear shift into neutral
position.
- Let out
the clutch and match the engine RPMs to slightly higher RPM
than required for the downshift gear match.
- Push the
clutch part way to the floor again, and push the gear shift
into the downshift gear, then release the clutch.
In both cases,
the clutch uses the engine RPMs to get the transmission input
shaft to the proper RPM for the next shift point, so that the
gears easily mesh.
Jake Brake and Clutching
There is
a lot of arguing amongst drivers about whether or not to use the
jake brake during shifting. During
the shifting double clutch, the conditions are correct for jake
brake activation. If your gear shifts are close together, then
the jake brake will fight you on the engine RPM match while shifting.
However,
if the gear shift result in large RPM drops, or if the shift is
on a steep grade which results in rapid truck speed loss, then
use of the jake brake may very well help you to grab that next
gear quicker. In this situation, the truck speed is falling rapidly,
and the engine RPMs are falling normally, so during the double
clutch, the engine RPMs are yanked lower by the jake brake. This
might be helpful to get that next gear in place during a slow
and steep grade climb.
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