An Effective Braking System For Your Apparatus

An Effective Braking System For Your Apparatus

FEATURES

APPARATUS

Most firefighting apparatus gear ratios in the main automatic transmission than do regular trucks and trucktractors. The more powerful engine and high horsepower-toweight ratio found in fire apparatus provide excellent road travel performance using a less expensive four-speed automatic transmission. However, a fire truck with a four-speed automatic will not slow down as efficiently when the engine retarder is in the “On” position, as will a similar truck that is equipped with a fivespeed or higher transmission.

We’ve been taught never to force an upshift or a downshift in an automatic transmission. The driver is supposed to select a range and stay in it until he drops the speed to a crawl or comes to a stop.

Well, there’s exceptions to every rule. In this article, 1 will discuss the ways you can get the maximum use out of both the engine brake and the automatic transmission without tearing them up.

PROTECTING THE ENGINE

Engine retarders are inefficient machines that change the kinetic energy (weight X the speed squared, or wt2) of a vehicle into heat. The heat, then, must be properly transferred to the coolant, shipped to the radiator, and sent out into the cool airflow. Engine retarders become more efficient as the engine speed increases. Let’s see how we can use this fact to help slow down a vehicle with a fourspeed automatic transmission.

We’ve all heard stories of a driver who tore up an automatic transmission by upshifting and downshifting until he “blew” the engine, split the transmission case, or twisted off the drive shaft. Rough shifting by the driver is abuse in an automatic.

Most automatic transmissions are equipped with a self-protection device called the downshift inhibitor system. This system takes effect when a driver moves the transmission tower shift control from a high gear into a low gear at a vehicle road velocity that will cause overspeed and damage the powertrain.

The downshift inhibitor system tells the transmission to remain in the high gear until the vehicle speed slows enough to safely drop down to the next lower gear. This system will work in all gears, forward and reverse. While the damaging shifts are prevented, rough shifts still can put an undo strain on the transmission clutchpacks.

There is a stress reducing method of downshifting an automatic transmission. The method can be compared to the double-clutching shifts performed on multi-speed, range-shifting, manual truck transmission. Remember that all transmissions—automatic, powershift, and manual — should be shifted smoothly and quietly. This occurs only when the engine speed matches the vehicle road speed.

So, here’s a way to synchronize these speeds with an automatic transmission during downshifts. Then, once the engine is turning near the governed speed, the engine retarder performance will overcome the lack of automatic transmission gear ratios.

MARKING THE SHIFT POINTS

• First, check the truck’s maintenance manual for the engine’s rated maximum governed speed. This is measured in revolutions per minute (rpm). The actual engine governed rpm may have been readjusted too high. The first step is to manually move the shift control of the automatic transmission into the lowest range. This is usually first or first-second gear ratios.

Run the truck up to its maximum speed while at the engine’s rated maximum governed rpm. Record the speedometer reading. Continue doing this for each range of the transmission control.

• The next step is to apply a piece of colored tape just outside of the speedometer bezel to indicate the vehicle maximum road speed in each range. Be sure that the width of the tape and the contrasts of color are sufficient for the driver to read the numbers with a rapid look from the road to the speedometer. The driver must be able to tell at a glance if the speedometer needle is in line with each tape for the corresponding gear ranges.
• The final step of marking is to apply pieces of tape outside the engine tachometer bezel. Select a long straightaway where the truck can accelerate at wide open throttie up through the gear ratios automatically in the highest drive range.

Starting from a standing position, drive the truck down the straightaway at wide open throttle. Record the high engine rpm just before each upshift, and record the low engine rpm just after each upshift.

Apply pieces of tape to the tachometer as you did with the speedometer. The transmission shifts from one gear to the next usually will occur at about the same engine rpm. Therefore, you need the tape marks only for high and low rpm. The tachometer should have already been “red lined” at the engine’s rated maximum governed rpm. If it isn’t, you should add red tape to mark this rpm also.

DOWNSHIFTING SMOOTHLY

Earlier, we mentioned double clutching. Double clutching is accomplished when the driver pauses in the neutral position between gear ratios and briefly engages the clutch while the engine is at higher than idle speed, and then completes the shift to the intended gear ratio. Double clutching is necessary with a manual transmission in order to make the gears and shafts in the transmission speed up on downshifts and not drop too slow on upshifts.

In an automatic transmission, when the driver takes over, as well as during automatic shifts, the transmission shifts by engaging the next progressive gear ratio clutchpack, while at the same time—a split second later—releasing the original gear ratio clutchpack.

So, there is no time in neutral and no time when the automatic transmission is not in gear. Normally, the transmission’s torque converter slips during the shift and prevents harsh shift points.

For these reasons, smooth downshifting is an art to be cultivated with an automatic transmission. Here are some points on how to smoothly downshift an automatic transmission.

• When you are at a high cruising speed, you should make a nice, smooth downshift from the fourth gear ratio (“drive” range) into the third gear ratio (1-2-3 range). Make sure that the engine brake main switch is flipped to the “On” position and that the selector switch is in place for operating all engine cylinders. Leave the main engine brake switch in the “On” position all the time. Remember, it won’t retard unless the driver’s foot is off the accelerator.
• From the cruising speed, actuate the engine brake and slow down. Wait until the engine rpm drops below the low tape mark on the tachometer and the road speed also slows below the third gear ratio mark on the speedometer. (Caution: On these first few tries, it’s a good idea to let the needles drop below these marks by a considerable margin because you don’t want to put too much stress on the clutchpacks of the transmission.)

When both needles are below their marks, reapply the accelerator with only enough throttle to hold the truck’s road speed. Obtain a balance where the truck neither accelerates nor decelerates.

• The next step is to apply a little more accelerator pressure to increase the load on the engine and then rapidly downshift, moving the range shift control lever from fourth high to third high gear ratios. As the downshift occurs with the engine under power, it should increase its speed to match the road speed required of the engine by the new, lower gear ratio.

If you apply too little accelerator pressure, the truck will buck as the slow engine speed pulls down the speed of the truck. Too much accelerator pressure will cause the truck to jump forward as the engine throws the excess power into the rear axle. Both of these extreme downshifts have a negative effect on the transmission clutchpacks and on the truck’s entire powertrain.

• After you downshift into the lower gear ratio, you can actuate the truck’s engine brake. Because the engine turns over at a much higher rpm in third gear ratio than it does in fourth gear, the engine brake will effectively slow down the truck.

Be careful not to downshift while the engine brake is operating because this applies too heavy a shock load into the powertrain.

Also, that flexible driven plate between the rear crankshaft of the engine and the input shaft of the transmission won’t take many of these shock load errors without costly maintenance.

• Follow the same method when downshifting from third to second gear ratio.
• When you want to go from second to first gear ratio use the service brakes instead of downshifting. The speeds are low and the service brakes are cool at this point, so there is no need for the wear and tear of making a downshift.

SUMMARY

Downshifting:

• Helps the driver control the truck’s speed,
• Increases engine speed,
• Results in the torque converter being near lock-up speed.

By following the pointers on downshifting mentioned in this article, you will save the service brakes for their most important job—stopping the truck.