A commercial vehicle's design features are strongly influenced by its application. Depending on the operating conditions and job performed by the vehicle, a commercial vehicle has a partic- ular chassis, engine, powertrain, cab, suspension, and other spe- cific chassis equipment, as discussed in the following sections.
Chassis Frames
The frame of a truck or bus are the vehicle's backbone, sup porting the vehicle loads of freight, equipment, and passen- gers. All equipment, such as engines, suspension systems, and the cab, ultimately must mount or attach to the frame. A frame must be sized and built appropriately to be capable of supporting the loads applied to it while adapting to the forces that bend and twist the frame, such as when the wheels hit bumps or potholes or the load shifts. Two main frame designs are:
I. Ladder
2. Tubular-type frames
Ladder frames are the most common type of frame. They use two main large cross-sectional area side rails that are arranged longitudinally in the vehicle. Side rails are made from box or C-channel cold rolled alloyed steel. Smaller cross members join each of the side rails together using high-strength bolts or rivets to form a sub frame, providing good torsional strength to resist twisting and bending. The passenger compartment of the cab is attached to the frame after the frame is built. It does not require as much strength or rigidity since the frame pro- vides the primary structural strength. Many trucks, trailers, and buses are built on a ladder-type frame, such as the one shown in FIGURE 1. When used in semi-trailers, the floor mem- bers use dozens of small I-beams attached across two smaller main frame rails with each end attached to the trailer side plate. The king-pin area of the trailer frame usually has steel plate reinforcement.
Tubular frames do not use heavy side rails but, instead, as the name suggests, uses tubular steel to construct a ribbed structure of many lighter pieces of box sections. Attachment points for the engine and suspension system are present, but there is no separate passenger compartment cell like a ladder frame because the tubes integrate the compartment into a single frame structure. Structural integrity is provided by metal tubes, bulkheads, and box sections. Tubular frames are much more complex to build but are substantially lighter than ladder frames and provide much more room for passengers and freight (FIGURE 2). The passenger compartment area is safer since the frame cell is less likely to deform in an event, such as a vehicle roll-over.
Engines
Engines must have enough power to move heavy loads and accelerate up grades while providing good fuel economy. Diesel engines are primarily used for the following reasons:
1. Superior fuel economy compared to any other internal combustion engine
2. Higher torque output, especially at low speeds
3. Durability-lasting three or more times longer than spark-ignition engines
4. Fewer maintenance requirements due to the absence of a spark-ignition system
Battery-powered electric vehicles (BEVs) are rapidly becoming popular since they produce no noxious emissions at street levels. Very little maintenance is required by the electric motors too. Torque output is comparable or even greater than a diesel, but only at low speeds. Given the low cost of electricity to charge the batteries compared to the cost to purchase diesel fuel, BEVs are perceived to have low operating costs.
Torque is the most important factor in choosing an engine for commercial vehicles since it is torque that ultimately moves a load. Torque is the twisting force applied to the crank-shaft and then, finally, to the wheels. Torque is a function of cylinder pressure and it is the force that accelerates or moves a vehicle and its load.
Horsepower is a function of engine speed and torque. Horsepower describes how fast the engine can turn while producing torque. A high-horsepower, high-torque engine can produce lots of power at high engine speeds. In contrast, a high-torque, low-horsepower engine cannot turn as fast while it produces torque. The following is an equation for horsepower:
Torque rise is the difference between engine torque produced at rated speed (maximum engine rpm under load) and peak torque, or maximum torque an engine can produce. Torque rise is expressed as a percentage of torque at the rated speed.
To calculate torque rise, use the following equation:
For example, consider a vehicle with a peak torque rise of 1800 ft-lb (2440.5 N-m) and a torque at rated speed of 1200 ft-lb (1627 N-m). The percent torque rise is calculated as follows:
High-torque-rise engines are used for line-haul applications and require a steep increase in torque, such as when climbing hills or steep grades. Vocational vehicles and those used in urban environments, such as transit buses or pick-up and delivery vehicles, use low-torque-rise engines, which have less peak torque but more torque is available over a wider engine operating range. FIGURE 3 graphically illustrates the difference between low torque rise and high torque rise.
High-torque-rise engines allow the vehicle's driver to keep the vehicle in a higher gear range longer under load. This explains why high-torque-rise engines are only used in line- haul, on-highway trucks and buses. Torque is available in high- torque-rise engines only over a narrow speed range. At low speeds, little torque is available. However, its peak torque occurs at 10 to 15 mph (16.1 to 24.1 kph) or so below its cruising speed. This corresponds to about two-thirds of its maximum engine speed when the vehicle is correctly geared.
Low-torque-rise engines produce more consistent torque output over a wider engine rpm operating range. That makes them ideal for stop-start traffic and varying speed/load conditions. Torque output is not as high but more widely available.
Another difference between high-torque-rise and low-torque-rise engines is in their gearing. High-torque-rise vehicles generally use transmissions with fewer gears and wider ratio steps between gears. Low-torque-rise engines use transmissions with smaller ratio steps but need more gears.