Heavy-Duty Air-Hybrid Engine Prototype Shows Up to 18% Improvement in Fuel Economy, Depending On Drive Cycle
Initial simulation studies of a prototype heavy-duty air-hybrid engine being developed by engineers at Volvo Powertrain, UCLA and Sturman Industries show a 4-18% improvement in fuel economy over a wide range of driving cycles. The amount of improvement depends on the cycle.
The addition of a low-temperature EGR circuit could boost that improvement by another three to four percentage points, according to the research team. The goal of the project, co-funded by the DOE, is to develop and demonstrate a heavy duty air-power-assist (APA) engine for a refuse vehicle with a 15% improvement in fuel efficiency and with emissions that meet the 2010 regulations. (Earlier post.)
Hyungsuk Kang from Volvo Powertrain presented the current results of this APA project at the DEER (Diesel Engine-Efficiency and Emissions Research) conference in Detroit.
The APA engine uses braking energy to work as a compressor, pumping compressed air into an on-board air tank (Air Compressor mode, or AC mode). During acceleration, the stored compressed air powers the engine—with or without burning diesel fuel—to get up to speed, or until the compressed air is depleted (Air Motor mode, or AM mode). Once the vehicle is moving along the engine converts back to a conventional diesel engine.
During engine acceleration, the high boost pressure also helps reduce the emissions of particulate matter. In addition, smaller quantities of fuel burned in each cylinder lead to lower peak temperatures, resulting in lower NOx formation. The APA technology also facilitates greater use of the engine brake, which in turn helps save fuel and reduces the frequency of brake service with its associated costs.
The APA engine is based on an 11-liter in-line six-cylinder diesel engine modified with a hydraulic valve actuator system from Sturman, an Amplifier Piston Common Rail System (APCRS) from Bosch, and a control system developed by UCLA. Compression ratio for the engine is 16:1. (Tsu-Chin Tsao, professor of mechanical and aerospace engineering at the UCLA Henry Samueli School of Engineering and Applied Science and his students are also working with Ford on a prototype light-duty air-hybrid system based on a 2.5-liter V-6.)
The variable valve actuator and the mode-switching controls are key enablers for the air-hybrid—in AC mode, the engine switches to a two-stroke mode, according to Kang. Other critical elements are the engine valve stem seals and external switching valves.
The air tank is designed to operate at 20 bar maximum pressure. Currently, in the experimental work, the system works at just 13 bar. The air tank also has a relief valve that can trigger at 15 bar.