One key to successfully increasing energy efficiency, reducing emissions and lowering energy costs trucks is a sound knowledge of drive and duty cycles. While the two terms are related, they have very different meanings. For those involved in designing or operating vocational vehicles it is important to know the difference, says Bob Johnson, director of fleet relations for NTEA (www.ntea.com), the Association for the Work Truck Industry.
“Simply put, a drive cycle defines how a vehicle is used,” he says. “A duty cycle defines how much a vehicle is used.”
NTEA represents companies that manufacture, distribute, install, sell and repair and purchase commercial trucks, truck bodies, truck equipment, trailers and accessories.
SPEED AND TIME
“Drive cycles are typically represented by a graph which plots vehicle speed against time,” Johnson explains. “Data for defining a drive cycle can be collected using various technologies, such as a dedicated onboard data logger, telematics or downloading data from the vehicle CAN (controller area network) bus.”
A CAN bus is network that connects individual systems and sensors as an alternative to conventional multi-wire looms.
Using the data represented in a drive cycle graph, critical vehicle operational statistics can be determined, he says. This includes such things as:
- Maximum speed.
- Average speed.
- Number and frequency of vehicle starts and stops.
- Idle time.
- Engine- off time.
- Total engine hours per cycle
“Other critical data, such as power export demands, can also be logged, but is typically represented as periods of engine idle on a basic drive cycle graph,” notes Johnson. “If your operations include periods of power export, collect additional data to provide operational information.”
This should include such things as:
- Power take-off (PTO) or other power export time.
- Dwell time during power export. This is when the engine is operating but no power is being exported.
- Engine rpm during the power export cycle.
- Power export load demand cycle (minimum, maximum, average, duration, etc.).
VEHICLE USAGE
NTEA’s Johnson says that by defining a vehicle’s duty cycle, information can be obtained on how much a vehicle is used, as opposed to how it is used. Typical duty cycle information includes data addressing:
- Hours/shifts per day.
- Days per week or other measurement cycle.
- Total miles per measurement cycle.
- Typical (average) load profile.
- Peak load profile.
NOT THE SAME
“When defining drive and duty cycles for your fleet, remember that not every vehicle will have the same profile,” warns Johnson, “and that the profiles for a given vehicle can vary over time.”
Take a delivery truck that is engaged in stocking shelves at local grocery and convenience stores, by way of example. A similar truck, owned by the same company and performing the same function for stores located in more rural areas, will have totally different drive and duty cycles.
“These variations can be critical when evaluating the effectiveness of various technologies and even in selecting chassis and equipment options when designing a vehicle,” he says.
Major drive and duty cycle variations over time are more common in cold climates than in warm regions, Johnson observes. “A municipal fleet in a Snow Belt area will have totally different drive and duty cycles during the warm weather construction season as opposed to the winter snow and ice control season.”
Utility and construction fleets in colder climates also have major variations in cycles, he adds, and in some cases, vehicles may be parked out of service for periods of time during bad weather.
“Drive cycles can differ in warmer climates as well, but typically, variations will be less radical,” he continues. “Regardless, when defining your drive and duty cycles, it’s important to define seasonal cycle variations or track a cycle for a period of at least a full year.”
DATA ANALYSIS
Understanding duty and drive cycles can also with help designing appropriate maintenance cycles for various classes of vehicles, project overall vehicle life, etc., Johnson of NTEA points out.
Most fleet managers design maintenance cycles for various classes of vehicles, plus project overall vehicle life, etc., Johnson of NTEA says. However, in many cases, it is a seat-of-the-pants analysis as opposed to having hard data.
By mastering drive and duty cycles, vehicles can be better designed with the most appropriate specs and options and vehicle maintenance programs can be optimized.
ADVANCED TECHNOLOGIES
Furthermore, knowing drive and duty cycles is also “especially critical” when implementing advanced technologies, of which there are many alternatives to choose from,” Johnson notes.
“Before making a selection, define your objectives and establish realistic performance measurement criteria,” he advises. “Then, select the technologies you are potentially going to implement.
“A careful analysis of your drive cycles will help you select the technologies that may be viable for your fleet. Then, you can analyze your duty cycles to help determine if your potential selections are viable based on financial performance and vehicle utilization.
“Most clean vehicle technologies achieve their optimum performance in specific drive cycles,” says Johnson. “By analyzing the drive cycles for specific groups of vehicles in a fleet, you can zero in on one or more technologies that will best suit that application.”
To illustrate this point, he cited several situations:
- The drive cycle for an application group includes very low mileage- per- analysis period, low average speed, starts and stops and a significant amount of incidental (traffic-related) idling time. In this case, the use of some type of hybrid or battery drive vehicle may be a viable alternative.
Technologies such as aerodynamics, low rolling resistance tires and vehicle speed-limiting systems will be of little or no value.
If the decision is made to stay with a conventional vehicle – as opposed to a hybrid or electric – idle management systems, accessory load electrification and powertrain optimization may provide a viable return.
- The drive cycle data shows that a vehicle operates primarily at high speeds (open highway) with a minimal number of stops and incurs a limited amount of idling time. Here, the adaptation of a hybrid powertrain or an advanced idle management system may not be practical. Instead, the use of an aero package, combined with low rolling resistance tires and a thorough chassis weight reduction plan would probably provide significant benefits.
- If a duty cycle analysis documents the fact that a truck being considered for conversion to CNG would need to refuel during a typical day of operation, the need for an onsite fast-fill (as opposed to time-fill) compressor station or access to another fast-fill facility would need to be factored in.
Well-designed and maintained trucks enhance productivity. Understanding the differences between drive cycle and duty cycle can help make certain that vehicles are optimized for their intended application.
