The rapidly growing importance of energy efficiency in almost all engine markets has presented a variety of opportunities to develop technologies to recover waste energy from engines and vehicles—including waste heat recovery (WHR), reduced engine idling (start-stop technologies), and kinetic energy recovery from vehicle braking. Selecting the right technology is heavily influenced by the vehicle type, use, and duty cycles.
For vehicles with high mileage and large payloads, such as line-haul and heavy-duty refrigerated truck, WHR is the better choice for energy recovery. In applications where the engine load factor is lower, and the drive cycle consists of a large number stop and go events; hybrid powertrains are better suited. Some applications, such as large transit buses, are a bit ambiguous.
For engines with high load factors, WHR is a good match for fuel efficiency improvement. The benefit of WHR is highest at full load and diminishes at light load. It is important to factor in the additional weight of WHR systems when determining vehicle level fuel efficiency and freight efficiency impacts.
Two types of WHR of particular interest include turbocompounding and Rankine cycle. Both require the use of an additional expander in the exhaust stream to extract energy from the exhaust. For mechanical turbocompounding, the power turbine is connected to the crankshaft to supply the additional power. This is typically accomplished via a fluid coupling to allow for speed variation and to protect the power turbine from engine torsional vibration. There is also a gear set to match the power turbine speed to crankshaft speed. The engine fuel consumption improvement potential is 1.5 to 2.5% for typical line-haul applications. This estimate is in-line with other studies.
Another variation is electric turbocompounding where the power turbine drives an electrical generator. The electricity is used to power an electrical motor supplementing the engine output, to charge batteries in a hybrid system, or to power electrified accessories. Considering the combined impact of supplemental power for the engine and electrified accessories, a 3 to 4% improvement in fuel efficiency can be obtained for line-haul applications.
Electric turbocompounding has significant implications for the vehicle architecture if the full engine fuel efficiency impact is to be realized. This includes the electrification of vehicle accessories, the addition of an electric motor to supplement engine output, and an energy storage system to accumulate any energy from the power turbine that is not immediately used.
A Rankine cycle is another WHR option to extract useful work from the waste heat of the engine—most notably from the exhaust gas and exhaust gas recirculation (EGR) streams. The Rankine cycle describes a set of thermodynamic processes that extract waste heat from the engine and produce usable power—thereby reducing engine fuel consumption. Some systems can also seek to recover energy from the charge air cooler (CAC). The system includes a feed pump (FP) to drive the working fluid from the condenser to the three heat exchangers:
• Super heater-boiler—transfers waste heat energy from the engine EGR stream to the working fluid.
• Exhaust boiler—transfers waste heat energy from the exhaust gas stream to the working fluid. This is done downstream of the aftertreatment.
• Pre-CAC heat exchanger—transfers waste heat energy from the CAC to the working fluid.
A turbine expander takes energy from the working fluid to make mechanical power. The working fluid then passes through the condenser that rejects unused heat energy from the working fluid before starting a new cycle. The power generated by the turbine expander is coupled to the engine output shaft by a gear box. Alternative architectures can be used to make electricity, which in turn can power an electric motor supplementing the engine output, power electrified accessories, or charge a hybrid system battery.
The Cummins WHR system has been installed in a fleet of Class 8 trucks. Extensive effort results in a system with components that are packaged well with sufficient underhood air flow. Control systems provide robust management of the quality of the working fluid as it transitions through the components.
Extensive vehicle testing of the WHR system over a wide range of drive cycles (highway, regional haul, and inter-city) and ambient conditions indicates limited WHR from the CAC. Incremental fuel consumption reduction is achieved but may not support the added cost. The primary sources of economically viable energy recovery come from the EGR and exhaust streams.
Various levels of fuel consumption reduction are achieved as listed by drive cycle. The ranges represent data for a variety of ambient temperatures.
• Highway drive cycle (70% highway operation)—5.1 to 6.0%
• Regional haul (mix of highway and inter-city)—4.3 to 4.7%
• Inter-city (heavy transit bus and pickup/delivery)—2.5 to 3.7%
Vocational applications have a high proportion of stop-and-go operation and are ideal applications for hybrid powertrains, which recover waste energy from braking and idling and allow greater freedom for engine optimization. The U.S. GHG rule allows for the certification testing of hybrid engines and powertrains as a complete unit—the so-called "Power Pack Option."
A variety of powertrain hybridizations are available, ranging from basic start-stop to parallel full hybrid. Like all waste energy recovery technologies, fuel efficiency improvement potentials are heavily dependent on duty cycle as well as the size and type of component technologies and vehicle control strategy. As more hybrid technologies are deployed, the potential fuel efficiency improvement increases for amenable drive cycles.
The basic start-stop functionality for commercial vehicles shuts down the engine when the vehicle stops. The engine restarts when the clutch is engaged for manual transmissions or restarts when the brake is released for automated manual transmissions (AMT). Additional challenges exist with auxiliary power requirements for line-haul applications and service trucks (power take-off) where sufficient electrical storage devices are required while the engine is not running.
Hybrid powertrains provide specific opportunities for engine optimization that include:
• Reduced engine operating range with supplemental power from the motor—allowing the engine to operate in the region of highest efficiency
• Simplified aftertreatment due to reduced engine-out emissions through smoothing of transients and idle elimination
• Accessory electrification that allows parasitic loads to be reduced as the devices are operated on an as-needed basis
• Downsizing the engine may be possible in certain applications to meet average load compared to peak load—cost and weight reductions needed to partially offset the addition of hybrid components
• Downspeeding is facilitated as available hybrid boost can mitigate increased shifting frequency at highway cruising speed, improving driveability and durability
The challenges associated with waste energy recovery systems include:
• Increased vehicle cost and weight due to the additional components and complexity of the power management electronics for electrified vehicles
• Overall vehicle reliability due to the increased complexity
• Potential to not fully realize fuel efficiency improvements due to the high dependence on drive cycle, powertrain technology selection, and vehicle control strategy
This article is based on SAE International technical paper 2013-01-2421 by Donald Stanton, Cummins, to be presented during the 58th Annual L. Ray Buckendale lecture at the SAE 2013 Commercial Vehicle Engineering Congress.