Natural Gas: A Bridge from Diesel to Electrification

When the price of diesel soared to record-high prices in 2008, the development of natural gas engines accelerated for both on-highway medium and heavy-duty commercial vehicles. The development efforts were split between two focus areas. Some OEMs focused on the conversion of existing diesel engines to run on natural gas while others developed a special purpose engine optimized specifically for compressed natural gas (CNG), liquified natural gas (LNG), or renewable natural gas (RNG).

However, after the decline of diesel prices, much of the investment in natural gas engine development and infrastructure tapered off. It feels like a boom-bust situation for the industry. This is understandable as most OEMs desire diesel equivalent performance with a lower fuel cost, and natural gas has one-sixth the energy content of diesel. As the price differential diminishes, so does the motivation. However, the price of diesel has started to get back to a level where natural gas investment begins to make sense again.

The natural gas engine may also have renewed interest as an emission enabling alternative. Cummins Westport has already certified the ISL-G to meet current U.S. Environmental Protection Agency (EPA) and California Air Resource Board (CARB) emission standards. In addition, certain versions of this engine have also been certified as near zero emissions which meet future expected CARB ultra-low NOx requirements of 0.02 g/bhp-hr. As the list of cities, states, and countries requiring low emissions, zero emissions, or the ban of diesel engine grows, OEMs will search for alternative technology to meet market demands. Most vehicle OEMs have the Cummins Westport ISL-G and ISX12-G NG equipped engines as their engine offering in order to not be left out if natural gas becomes a key enabling technology in a highly restrictive emissions environment.

China may be another reason to expect a resurgence in natural gas engine technology. China is expected to become the world’s largest importer of natural gas as the government looks to diversify its fuel sources for emissions reasons. All heavy-duty commercial engine OEMs in China have natural gas engines in their portfolio to offer customers, and the penetration of the total volume of heavy-duty engines is higher than any other geographic area.

There are some technical challenges with natural gas equipped on-highway diesel engines. One of those is engine brake performance. In the conversion from diesel to natural gas, the engine’s compression ratio is significantly reduced, and the turbo is commonly downsized to accommodate a lower air-fuel ratio needed for stoichiometric combustion. The combination of these two changes can negatively impact engine brake performance by up to 30%. In turn, this will negatively impact the ability of the vehicle to maintain speed on a downhill grade without using the foundation brakes. An increased use of foundation brakes could lead to brake fade and increased brake wear rates. In some markets like Europe, China, and India, it could impact the ability of the vehicle to meet regulation E13C requirements for supplementary retarding and could require a reduction in Max Gross Vehicle weight certification.

There are solutions to this. Jacobs’ High Power Density (HPD) system, as well as a driveline retarder, will increase the total vehicle retarding power and reduce dependence on the foundation brakes. These systems will be able to restore or exceed diesel engine retarding performance, allowing the driver to increase safety and productivity.

The ability to have the natural gas engine perform at the same level as a diesel engine in positive and negative power is important for adoption of the technology. It becomes even more important as the natural gas engine faces a new competitor—electrification. More and more OEM development dollars are being invested in electrification technologies. While full electrification is still years away from meeting the current capability of the commercial vehicle market, it seems the natural gas combustion engine may play an important role as a bridge technology in the short-term future.