Automatic Transmissions Gain in Favor
ACCELERATION and top speed are two of the most important performance requirements of a fire or crash vehicle. When the alarm sounds, getting to the scene fast, with all equipment necessary to combat the blaze, is of prime importance.
Suggested specifications by the National Board of Fire Underwriters (NBFU 19) for all motor fire apparatus state that from a standing start through the gears the vehicle shall attain a true speed of 35 mph within 30 seconds. And from a steady speed of 15 mph in direct drive, the vehicle shall accelerate to a speed of 35 mph within 30 seconds. Top speed shall be not less than 50 mph. The tests shall be with apparatus fully loaded—men, hose, equipment, water tank filled, etc. —and on paved roads, dry and in good condition.
To establish minimum and uniform performance standards for aircraft fire and rescue vehicles, the National Fire Protection Association has proposed vehicular performance recommendations. In this recommendation, equipment is classified in categories according to gross vehicle weight. Minimum performance requirements for acceleration, speed, gradability, etc., are shown. The following table from NFPA No. 414 lists the class by weight and gives the maximum time in seconds allowed to accelerate from 0 to 50 mph on dry pavement.
In a comparative acceleration test with a manually shifted unit the automatic was well in the lead. Figure 1 illustrates the acceleration performance of a vehicle in the Class 6 size. Although both units met the requirement of 50 mph in 50 seconds, the automatic not only exceeded this, but maintained faster acceleration throughout the entire range.
Translating this in terms of distance traveled, Figure 2 illustrates the travel in feet after 20, 30, 40, 50 and 80 seconds. At 80 seconds, both vehicles had accelerated to the same speed with the automatic running approximately 500 feet ahead of the stick unit.
This added acceleration and ability to maintain a higher travel speed was demonstrated recently in Minneapolis. The fire department was having difficulty with two ladder trucks, both equipped with stick transmissions. In traffic and on steep hills, the lead engine would outdistance the ladder units by several blocks. Since addition of the automatic transmission tractors, the ladder trucks keep right up with the lead engines on every run.
Cutting precious seconds off every fire run is not the only advantage offered by automatic transmissions. Motorists usually stop suddenly when they hear a siren. This means the driver of the fire apparatus with manual shift transmission frequently has to slam in the clutch and tramp on the brake. It necessitates repeated shifting through the entire gear range to regain top traveling speed. With the automatic, stepping on the brake is all that is necessary; the driver does not have to take his hands off the wheel or touch the gearshift. Relieved of the necessity of continuing shifting and clutching, cornering and driving in traffic is made easier. Full power shifting at all times enables the driver to focus his attention on the road.
Safety and control
To operate fully loaded on downhill grades is a requirement for fire and rescue vehicles. For satisfying this requirement, the automatic transmission incorporates a self-contained hydraulic retarder located between the torque converter and ahead of the range gearing. Several ranges of retardation are, therefore possible. The arrangement permits dropping the retarder and picking up the load in the same gear, a useful advantage when operating on rolling terrain. Using only one moving part, it is virtually troubie-free.
Mobility off highway
With the automatic transmission, fire and rescue vehicles have much greater mobility on as well as off pavements. In some areas, weather conditions can turn normally passable roads into a quagmire. In mountains and rolling terrain, grade and soil conditions present a real hazard. The automatic has the ability to shift from any speed to another in sequence, with out interruption of power transfer. The danger of hanging up due to mis-shifts or losing traction due to jerky clutches is eliminated. The torque converter provides the needed torque multiplication for smooth starts and fast acceleration in bad terrain and on steep grades. Figure 3 illustrates the full throttle performance of an Allison Automatic HT-70 transmission installed in a 30,000-pound pumper.
The case for the power shift transmission would be incomplete unless some mention is made of other tangible and intangible advantages gained by its use. It is at once apparent the automatic reduces length of driver training required to safely and efficiently operate fire apparatus.
It improves acceleration rate of the vehicle because the maximum efficient use of engine power with the transmission is specifically matched to maintain top performance. The shift points of the transmission are matched to power peak of engine with the same points on upshift or downshift.
It also reduces power train maintenance expense by eliminating the conventional friction clutch together with the accompanying adjustment and repair.
In addition it prevents engine lugging and stalling with resultant engine repair expense. The cushioning effect of the converter getting the load under way, smoother and faster at every change in gear ratio, reduces shock load and eliminates damage to driveline components.