The movement to commercializing alternatives to the internal combustion engine has focused on light-duty vehicles. This column has shared different research approaches such as batteries and fuel cells.

With the growth of hybrid electric vehicles, attention again is being paid to improving efficiency. In a previous TLT article (1), a real-time energy-management system was devised to optimize the use of the battery pack and the internal combustion engine. A reinforcement-learning energy management system was used to maximize efficiency. This system also was given the ability to learn through a reward-by-action factor. In a commuter study done in Southern California, this reinforcement model reduced fuel consumption by 12%.

In contrast, there has been limited development of a hybrid heavy-duty truck (see Figure 1). Dr. Leslie Bromberg, principal research engineer in the Plasma Science and Fusion Center at the Massachusetts Institute of Technology in Cambridge, Mass., says, “Efforts are underway to produce an all-electric heavy-duty truck. We believe this approach will be extremely challenging because the truck would require between 10 and 15 tons of batteries that would add excessive weight and be costly. Both size and weight would limit the payload the truck could carry and limit its driving range.”


Figure 1. A concept for a hybrid heavy-duty truck that contained a spark ignition engine and a battery was devised that has the potential for excellent performance while minimizing emissions. (Figure courtesy of the Massachusetts Institute of Technology.)

Consideration should be given to finding an alternative to trucks using an internal combustion engine, because such vehicles contribute two gigatons of carbon dioxide emissions globally of the seven gigaton emissions attributed to all vehicles. Bromberg believes the only logical alternative is to develop a concept for a hybrid heavy-duty truck that relies on both batteries and an internal combustion engine. 

A new approach to produce a hybrid heavy-duty truck concept has now been developed by Bromberg and his co-author and research scientist Dr. Daniel Cohn.

Spark ignition engine
Bromberg and Cohn outlined an approach for the development of a hybrid heavy-duty truck that uses a spark ignition (SI) engine in combination with a battery. They provided options that can be used to further enhance the performance of this heavy-duty truck. 

One interesting strategy used by the researchers was to not use a diesel engine but, rather, to propose that a flexible fuel gasoline-alcohol spark ignition engine be the preferred choice. Bromberg says, “We believe the SI engine has advantages over a diesel engine in running cleaner with lower levels of emissions, particularly when combined with a flexible fuel that can be a combination of gasoline and an alcohol (either methanol or ethanol).”

Diesel engines typically run at higher fuel efficiencies than gasoline engines. The researchers believe that this disadvantage can be overcome by optimizing the operating conditions of the hybrid heavy-duty truck.

Bromberg says, “In our approach, we are looking for the SI engine to operate in the sweet spot that is optimized by use of high-octane fuel.” Two strategies to optimize the SI engine include running at a compression ratio of 14 that is higher than the typical ratio of 10 and using a smaller gasoline engine. The latter case will reduce the amount of fuel required which will save weight.”

Bromberg says, “To achieve this objective, the SI engine will need to operate under higher temperature conditions than the diesel engine. This will lead to an exhaust emanating from the gasoline engine at a higher temperature (700-800 C) than for a conventional diesel engine (400-500 C).”

In Bromberg’s opinion, the higher operating temperature will represent a challenge to the lubricant industry to develop an engine oil that will be able to handle severer conditions than what are currently experienced. In their modeling, the researchers determined that the SI engine delivers the same power (380 kilowatts) as a diesel engine. 

The high compression ratio increases the likelihood of engine knocking. Bromberg says, “We proposed in our modeling to include two fuel tanks in the hybrid heavy-duty truck. One fuel tank would be gasoline while the second fuel is an alcohol that exhibits a high-octane value. The two fuels can be blended to the desired ratio to minimize knocking without adding too much weight.”

Pre-chamber jet ignition also is proposed by the researchers to maintain high efficiency. Bromberg says, “This approach can increase the combustion rate while minimizing engine variability from cycle to cycle.”

An intriguing option offered by the researchers is to use the flexible fuel option built into their modeling to propose alcohol enabled high performance exhaust recovery. Bromberg says, “Alcohols have the property when exposed to temperatures between 200-300 C and in the presence of a catalyst to decompose into hydrogen and carbon monoxide. The process is endothermic and can readily be conducted under the high temperature engine operating conditions. The hydrogen-rich gas can be reintroduced into the engine leading to potentially a 10%-15% increase in efficiency.”

The inefficiencies of the generator/motor can be compensated by the increased energy recovery using the endothermic reforming of the alcohols. 

In contrast, the engine gas recirculation (EGR) system present in a diesel engine will not be necessary with the SI engine and its absence also improves efficiency. 

From the battery perspective, Bromberg is not as concerned about the type as the size. He says, “We developed our approach with the feeling that a 200-kilowatt sized battery will be required.”

The researchers are conducting further modeling to optimize the engine size and the battery. Bromberg says, “We hope to propose this idea to the automotive industry as an approach for introducing electrification to heavy-duty trucks.”

Additional information can be found in a recent presentation (2) or by contacting Bromberg at brom@psfc.mit.edu. 

REFERENCES
1. Canter, N. (2016), “Improving the Efficiency of Plug In-Hybrid Electric Vehicles,” TLT, 72 (5), pp. 16-17.
2. Cohn, D. and Bromberg, L. (2019), “Flex Fuel Gasoline-Alcohol Engine for Near Zero Emissions Plug-In Hybrid Long-Haul Trucks,” Presented at the SAE World Congress & Exhibition in Detroit, Mich., on April 11, 2019.