The driveline might be one of the most overlooked series of components on fire apparatus, yet it’s one of the most important. Your vehicle has to get members to the scene efficiently, it must function as needed, and it must return everyone to the firehouse with a minimum of downtime. None of that is accomplished without a quality driveline.
In this article, we’ll examine heavier driveline components for ladder trucks, which are the kind of drivelines that are used for vehicles that have a gross vehicle weight rating (GVWR) of at least 54,000 lbs. (Although they can be used on lighter vehicles, these drivelines often are less cost-effective in those instances.)
Engines
When comparing a 12-liter engine with a 15-liter engine, bigger isn’t always better. Many look at horsepower and displacement as the deciding factors for engine selection, but torque is more critical for typical fire apparatus driving. Stop-sign-to-stop-sign performance is based on torque. Horsepower comes into play when pumping water and at highway cruising speeds on a grade.
For example, a Cummins 12-liter (X12) engine that’s rated at either 455 hp or 500 hp will produce 1,700 ft.-lbs. of torque. The Cummins 15-liter (X15) that has a 450-hp rating will produce 1,750 ft.-lbs. of torque, or about 9 percent more than the torque that’s generated by the 12-liter engine; 500-, 565- and 605-hp ratings will produce 1,850 ft.-lbs. of torque.
There are valid reasons in certain cases to use a 15-liter engine. If your response district is very hilly and steep or if you cover several hundred square miles and your response is more like over-the-road trucking, these engines could be justified. However, you still might not need a 15-liter engine. Properly setting up the overall driveline will accomplish the same things at a more reasonable cost.
The engine’s physical size dramatically affects other features and components of the apparatus. The 15-liter engine is about 1,000 lbs. heavier than the 12-liter is and requires much more air flow for proper cooling. Several manufacturers successfully packaged the 12-liter engine in their 94- and 95-inch-wide cabs, but the 15-liter engine pushes you into the wider 98- or 99-inch-wide cab. That doesn’t sound like much until you try to back that vehicle into an older firehouse, navigate narrow streets or look at the additional costs that are associated with the larger cab.
Depending on your cab configuration (raised roof, overall cab length, number of seats), the 15-liter engine could push you into a larger front axle, larger front tires or both. This affects the cramp angles for steering and increases turning radius, which reduces maneuverability. Once the axle load is calculated, you might have to reduce the number of seats in the apparatus or reduce cab length, if the calculated load goes beyond the axle manufacturer’s maximum weight rating. The engine tunnel is larger to accommodate the larger engine and the air flow that’s required, so the seating space for the driver and officer could be reduced even in a larger cab.
A relatively small 3-liter difference between the engines has an exponential effect on operating cost. The 15-liter engine burns fuel at a much faster rate than the 12-liter version does, just to name one operational cost increase.
Transmissions
A popular transmission for the larger fire apparatus drivelines is the Allison 4000 Emergency Vehicle Series (EVS). There are four models in this series, but the most common is the 4000 EVS. The 4500 EVS is used more commonly with the 15-liter engine, because it can handle higher horsepower input and can be set up with torque-limiting for higher torque. The other two models that are in the 4000 Series are the 4700 EVS and 4800 EVS units. They are used in aircraft rescue and firefighting (ARFF) apparatus.
Both the 4000 Series and 4500 Series transmissions are six-speed units. In vehicles that have a GVWR of 50,000 lbs. (or more than 1,250 gallons of combined water and foam), the apparatus is speed-limited to a maximum of 60 mph by NFPA 1901: Standard for Automotive Fire Apparatus. Because the 12- and 15-liter engines easily can reach top speed without using sixth gear, it’s common to lock out sixth gear completely. It also is common to program the transmission to use first, second, third and fourth gear when the “D” is selected and to enable fifth gear with the “Mode” button (economy mode). This eliminates some upshifting and downshifting, particularly in urban and suburban driving environments. Limiting the shifting reduces wear and tear on the transmission as well as some of the jerking during the shift cycles, which is more comfortable for personnel who are in the apparatus.
Allison currently ships the 4000 EVS and the 4500 EVS with the economy mode enabled. You might consider having this feature shut off. Doing so will change the shift points slightly, and the transmission will react better to real-world driving conditions. You can specify (or request at your preconstruction conference) these changes, which will improve the life and performance of your transmission for certain.
The 4000 EVS is a close-ratio transmission; the 4500 EVS is a wide-ratio transmission. The gear ratios between the two differ. In first, second and third gears, the difference between the ratios in the two transmissions is greater than it is between fifth and sixth gear. Fourth gear in both transmissions is a one-to-one ratio. Different gear ratios react differently with the engine during driving evolutions. The wide-ratio gearing can be a viable option to pair with the 12-liter engine, particularly with heavier apparatus.
During a recent project on a tower ladder quint that has a GVWR of 84,400 lbs., we paired an X12 500-hp engine to a 4500 EVS transmission and a 6.14 gear ratio in the rear axles. We were weight-limited on the front axle and couldn’t use a 15-liter engine, so the design required some creativity. Road test at final went very well: There was plenty of power (the vehicle accelerated well and didn’t lag on moderate inclines), and the pump test went flawlessly. This all was possible because of collaboration with the engineers, and it serves as a valuable lesson in selecting driveline components that work best for a department’s needs.
Something to keep in mind regarding engine offerings from Cummins and transmission offerings from Allison: The hardware doesn’t change across the different families in the product lines. The 4000 EVS transmission that’s used in a fire apparatus is the same as an Allison 4000 Series transmission that’s used in construction vehicles. The X12 and X15 engines that are used in fire apparatus are the same as the ones that are used in commercial trucks. What does change is the electronics and software, which affects characteristics and operation.
