Alternative FuelsThis is the second of two articles on the future of alternative energy sources for off-highway products. Read the first article here

The leveraging of alternative fuels requires several changes to vehicle architecture — some of which can impact the safe and efficient operation and servicing of a vehicle. As the construction and agriculture industries migrate toward alternative energy sources over the next decade, it’s a good time to familiarize yourself with these changes.

Fuel Tanks

CNG (compressed natural gas) is typically stored at roughly 3,600 psi. A well-constructed tank typically lasts 15 to 25 years. That said, regularly occurring tank inspections are vital. Unlike with liquid fuels such as gasoline or diesel, CNG leaks aren’t easily detected by puddles on the ground, etc.

LNG (liquefied natural gas) must be stored at -260° F. An LNG tank must include a methane detector to monitor for leaks.

Propane, or LPG (liquefied petroleum gas), is stored at 150 psi. Its energy density is 270x greater than its gaseous form. “When you get a propane leak, the fuel expands drastically once exposed to the ambient temperature,” said David Langenderfer, technical project leader at Cummins Inc., who spoke to virtual attendees at AEM's Product Safety & Compliance Seminar. “For safety, the tank must be 20x more puncture-resistant than a gasoline or diesel fuel tank. A propane tank also has a quick-release Type K15 dispenser connector.”

hydrogen fuel cell is extremely high-pressure — rated at 5,000 to 10,000 psi. A sensor is also necessary to detect for leaks.

Fuel Properties

Diesel remains the common energy source for the agriculture and construction industries. Its energy content is 130,500 btu per gallon. Natural gas’s energy content is a bit higher at 140,000 btu. Propane is a very fast-burning fuel with an energy content of around 90,000 btu per gallon.

Hydrogen is where things start to get interesting. Hydrogen can auto-ignite if slightly pressurized in a 7 to 50% concentration. Unfortunately, hydrogen fuel cell makers can’t incorporate an odorant to help detect a leak; doing so would damage the fuel cells. Hydrogen is also 57x lighter than gasoline vapor and 14x lighter than air, so it will instantly dissipate into the atmosphere.

“The big concern with hydrogen is one’s ability to see a flame,” Langenderfer pointed out. “Hydrogen burns very cleanly and is hard to see in daylight. If you had a leak and a fire breaks out on the vehicle, specific safety sensors and protocols are needed.” For instance, vent stacks should be incorporated into storage facilities, along with alarmed flame detectors in storage facilities and at fueling stations.

Langenderfer said current hydrogen fuel tanks include a plastic liner, carbon fiber-reinforced plastic layer to ensure pressure resistance, and a glass fiber-reinforced plastic layer to protect the outer surface. ISO/TS 15869 is an important standard to test for tank durability.

A Closer Eye on Electrification

Looking even further down the alternative energy spectrum, full electrification is on the horizon for off-road construction and agricultural machinery. Langenderfer said it’s a good idea to get ahead of the curve and begin learning about battery properties and key terminology.

First of all, battery systems are typically assembled with multiple modules; anywhere from two to eight depending on the energy needs of the vehicle.

BMS stands for battery management system. The BMS monitors and maintains battery charge.

Diodes refers to a two-terminal electrical connector allowing current to flow in just one direction.

HVIL stands for hazardous voltage interlock loop. It is a safety feature designed to protect the end-user. The HVIL is a low-voltage circuit that runs parallel to the high-voltage circuit, monitoring the connection of components and cabling to maintain safety for all of the high-voltage connections. “The HVIL connects to every device that has high voltage running to it,” Langenderfer pointed out. If the HVIL is disconnected, the entire battery system opens the contactors for safe service access.

MSD stands for manually operated master service disconnect switch. The MSD is simply a switch you pull to disable the high-voltage system.

Safety Considerations for Battery Systems

In addition to the features mentioned above, there are other safety elements built into a high-voltage battery system.

For example, orange cabling is used to signal a high-voltage line. Batteries feature either a cell vent or tearaway tab to allow for proper venting if a battery becomes overcharged. “A lot of battery architectures also feature current-interrupting devices and current-limiting fuses,” Langenderfer said.

Numerous safety standards are being developed for battery systems, including SAE J2910 which talks about essential PPE, the need for properly insulated service tools, and de-energization requirements after a disconnect is actuated.

The primary safety implications of electrification include overcharge, internal/external short circuit and thermal runaway. “The optimal most efficient operating temperature for the battery systems is 20 to 30° C,” Langenderfer said. “These systems can be air-cooled, but many are liquid-cooled with a chiller.”

Another safety consideration relates to connections. “When you’re measuring the terminals, you need to make sure you are connecting to the right part of the battery,” Langenderfer said. “There is a lot of energy there. If done incorrectly, you could end up melting the connector. This is why PPE like safety glasses and high-voltage protective gloves are so important.”

Something referred to as a rescue stick or maneuvering stick is good to have readily available. “A stick in accordance with ASTM F711 will help you move someone being shocked without getting shocked yourself,” Langenderfer explained.

Battery fires are also something to be prepared for. Langenderfer said it can be difficult to get adequate water to the location of the fire because a metal case surrounds the battery cells. But if you don’t get the area to cool down enough, the fire threat can continue for 20-plus hours. Furthermore, the effectiveness of a fire extinguisher is dependent on the chemistry of the battery.

“High ambient temperature is not your friend,” Langenderfer added. “It increases the aging of the battery and places a higher load on battery cooling. It’s important to understand what your high ambient temperature capability will be in order to properly size the battery thermal management system to provide the necessary cooling.”

Low ambient temperature is not a good friend, either. A battery becomes slower to react as the temperature gets lower. Thus, you may have to heat the battery up to that 20 to 30° C range in order for it to be efficient. “Manufacturers need to think about this when sizing their batteries,” Langenderfer said. “You still have to provide electric heat to the cab heater. But you also have to provide enough energy to heat the large battery pack.”

OEMs must also think about the other accessories that are part of the overall battery system. This includes thermal management pumps and fans, hydraulic pumps, air compressors, inverters and converters, etc. Where will these items go, how will they function, and what implications might that have on an end-user’s ability to maintain and service the vehicle?

Maintenance Considerations for Alternatively Fueled Vehicles

Langenderfer said that some propane and natural gas-fueled engines require unique spark plugs. That said, the lower carbon burn should result in longer engine life. Cold starting should also improve due to the fuel being gaseous, thus eliminating the need for a grid heater like you’d need in a diesel powertrain operating in very cold conditions.

With respect to hydrogen, it’s important to ensure that the balancer plant is durable. End-users will also have to stay on top of cooling system and fuel filter maintenance.

With respect to battery systems, significant preventive maintenance needs emerge around coolants, lubes and related filters, religious inspection of the HVIL and wiring, and the ongoing monitoring of battery health.

“Sometimes the battery system is located at a point on the vehicle where the vehicle is typically lifted in a shop,” Langenderfer said. “When designing-in battery systems, an OEM really needs to think about how the end-user is going to have to service the batteries.”

Similarly, fuel tank size is something to consider. Propane, natural gas and hydrogen tanks are typically larger than a gasoline or diesel tank. Plus, because those fuels are gaseous, regular inspections are critical to monitor for emerging defects and leaks. Thus, it’s important for an OEM to ensure that the fuel tank is easy to access for inspection and service.

Exhaust system routing also gains additional importance. “You don’t want anything to run right next to the fuel tanks, batteries or electric cables because you don’t want to damage or melt anything,” Langenderfer said.

As discussed in the first article, the benefits of fueling off-road machinery with alternative energy sources are clear. It is also clear that alternatively fueled vehicle architectures share many similarities with conventional powertrains. There are also many differences. Understanding those differences helps manufacturers design products that keep the construction and agriculture industries moving in a more energy-efficient, environmentally friendly direction — while still addressing the productivity, safety and serviceability needs of the end user.

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