The first step is to determine the weight of the truck and the number of axles required. Then, you need to define the performance criteria starting and road gradeability under maximum load, surface conditions (soft dirt, mud, sand), how fast you want the vehicle to go things of that nature, says Johnson. Then, it is pretty much a set process. You calculate the required horsepower and the required torque.
Once you have the engine selected based on the declared horsepower and torque, and determine the engine operating speed from the performance charts, you can select the final drive (rear-end gearing) to get the road speed required, says Johnson. Next, you can calculate transmission requirements.
A lot of people who have not been in this business a long time are really not aware of these procedures, says Johnson. Dealers will plug numbers into their computer programs and tell you this is what you need. Those programs do a very good job. The problem is they do not give you criteria to purchase from another manufacturer. It makes it hard to compare two or three brands. Its a matter of first determining what the truck is going to be, then defining the performance requirements, the engine, the rear end and, finally, the transmission.
For an on/off-road truck, these calculations need to be computed twice. You need to do these calculations for highway performance and off-road performance, says Johnson. Off-road gradeability might be 30%, but the maximum speed might only be 20 mph. On road, I might need 15% gradeability, but I need a road speed of 65 mph. In extreme situations, you may have to get a dual-range transmission.
Fortunately, most current engines have such wide torque bands, and with the gradeability achievable with many of the transmissions currently available, you can frequently get all you need without having to go to a dual-range transmission, he adds. That is good, because it is less maintenance and costs less [for a traditional transmission]. But you have to go through the process.
Depending upon the most important vehicle performance objectives, a powertrain can be selected that favors specific objectives, or that delivers a balanced all-around performance, says Dave Siler, marketing director, Detroit Diesel Corp. Specing for fuel economy should drive down the operating speed of the engine through thoughtful selection of drivetrain components and ratios. Specing for power and responsiveness should drive up the gradeability and startability of the engine/drivetrain combination.
Regardless of the specing objectives, the vehicles and drivers ability to do the job efficiently over the life cycle is the ultimate goal, he adds. Todays diesel engines produce more torque across a broader range of speeds than ever before. This simplifies the vehicle powertrain spec and allows the engine to do more of the work than before.
In todays world, there is a huge tendency to over-spec power, Johnson comments. Owners are going by past experience, and the technology has changed a lot. Engines have more horsepower and more torque. They have a larger torque band.
This tendency to go with what worked last time can mean you end up with a truck that is overpowered. It is going to cost you more money. It is probably going to add weight to the vehicle. It is probably going to burn more fuel, says Johnson.
The downsides to specing more torque and horsepower than required for a given application are a reduction in fuel economy, greater stress on the vehicle and its components and higher up-front costs, adds Siler. Its important to understand the vehicles expected performance, as well as the expected life-cycle costs, in order to determine the best possible specification for the job.
Its also important to keep performance expectations in check. Just because a certain engine can pull a particular grade at 65 mph doesnt mean it is the best choice for your application, says Johnson. Ask yourself if that criteria is really important. Can you live with going up that long grade at 55 mph and putting a smaller, lighter, more fuel-efficient engine in your truck?
On the other hand, you do need to leave a little margin. You never want to run an engine at maximum horsepower and maximum rpm, cautions Johnson. When you design your powertrain, you want to back the engine at least 5% off of its peak. You need a little reserve.