This is the first of two articles on future alternative energy sources for off-highway products, based on a presentation by AEM member company Cummins at the Association’s most recent Product Safety & Compliance Seminar.
As manufacturers look to develop off-road vehicles and equipment powered by alternative energy sources, it’s important to consider the various design, safety and servicing implications for end-users.
Machinery powered by alternative energies can share many similarities to that powered by traditional diesel powertrains. However, there will also be many differences. For the agriculture and construction industries to fully reap the benefits of alternative energies, a keen understanding of those differences is essential. Now is the time to develop that keen understanding.
“It is common to see on-highway technologies transfer to off-highway products with a few-year lag,”
said David Langenderfer, technical project leader at Cummins, who spoke to virtual attendees at AEM’s Product Safety & Compliance Seminar. In other words, that technology transfer could begin taking place sooner than later.
In off-highway applications, today’s most common powertrain continues to be diesel. Langenderfer said the future powertrain is likely to include a high-efficiency, spark-ignited engine with a choice of fuels: gasoline, natural gas or propane. From there, OEMs can step into a range-extending “hybrid” electric vehicle.
“The OEM still takes advantage of a conventional engine, but adds an electric machine to charge the batteries,” Langenderfer said. “The big benefit is that the combustion engine can continue driving the machinery if the battery ends up dying.”
Another future range-extending option could be a fuel cell that uses hydrogen to power a membrane exchange that creates voltage to charge batteries. Then, at the far end of the futuristic spectrum, is a path toward fully electric off-road vehicles and equipment. At that point, there will be much more for manufacturers and end-users to consider.
Drivers behind alternative energies
Conventional wisdom might suggest that “cost” is what is spurring interest in alternative energy sources. Langenderfer shared data from July 2019 that stamps a big question mark on that notion. For instance, the per-kWh price of diesel, gasoline and natural gas was relatively the same. However, the price of electricity was at least 37% higher, and the price of propane was roughly double.
Greenhouse gas (GHG) emission is where you start to see some meaningful advantages with alternative energy sources. Consider the following data measuring GHG emissions during normal machine operation:
- Combustion diesel engine: 296 g/kWh
- Combustion gasoline engine: 293 g/kWh
- Propane engine: 251 g/kWh
- Natural gas engine: 213 g/kWh
- Fully electric powertrain: 0 to 947 g/kWh (depending on generation method)
Fuel cost does come into play once you begin examining consumption. Generally speaking, alternative fuels are more efficient sources of energy. Take a look at the below data on thermal efficiencies:
- Diesel – 38%
- Port fuel-injected gasoline – 25%
- Direct-injected turbocharged gasoline – 35%
- High-efficiency gasoline – 40%
- High-efficiency natural gas – 40%
- Electric motor – 90%+
As you get to those higher efficiencies, fuel consumption goes down because the energy source directs more of the energy into the tractive effort of the machine. That reduces energy use — and that reduces an end-user’s operating cost.
Migrating to propane and natural gas
As Langenderfer pointed out, conventional powertrains are protected for multiple fuel types. Regardless if the machine is fueled by diesel, gasoline, propane or natural gas, there is commonality among the short block, engine mounts, and air and fluid connections.
“Conventional powertrains, independent of fuel type, also have the same FEAD (front-end accessory drive) components. Essentially, there is a plug-and-play piece with respect to conventional powertrains.”
That said, there are some variances when migrating to certain alternative fuels.
The fuel tank is one point of difference. Liquid fuels, including diesel and gasoline, utilize a similar tank made of either plastic or stainless steel. Natural gas and propane require a high-pressure tank.
The cylinder head will need to be constructed of aluminum for a spark-ignited alternative fuel. That is to help pull some of the heat out and eliminate knocking. With a diesel engine, on the other hand, a cast-iron cylinder head remains prevalent for long-term durability advantages.
Differences will also emerge when looking at the combustion piece of an alternatively fueled engine. For instance, diesel uses a high-pressure direct injector. Then, depending on the size of the engine, either glow plugs or a grid heater are utilized as a cold-starting aid. Conversely, a port fuel injector is used for natural gas and propane.
With respect to the aftertreatment system, diesel engines are well-known for components including a DOC (diesel oxidation catalyst), DPF (diesel particulate filter), SCR (selective catalytic reductor) and AMOX (ammonia oxidation catalyst). With a spark-ignited engine, a smaller TWC (three-way catalyst) is utilized.
“As we get further and further down the emissions threshold, you’ll start seeing these catalysts move closer and closer to the engine to help mitigate the amount of fuel required to heat them up,”
From an exhaust system perspective, the peak temperature for diesel exhaust is around 760° C, whereas a spark-ignited engine runs much hotter at 850 to 950° C.
“As we design vehicles with these spark-ignited engines, we’ll have to think about that higher temperature in relationship to where the components are near the engine,” Langenderfer pointed out.
Fuel leaks present some unique challenges with certain alternative fuels. With a liquid fuel such as diesel or gasoline, leaks are easy to spot through regularly occurring visual inspections. But with gaseous fuels such as propane and natural gas, there is no visual cue that a leak has emerged. Thus, a leak warning system is a necessary addition to the vehicle or equipment. Evaporative system requirements must also be met.
In terms of overall vehicle architecture, Langenderfer said there are few differences between diesel and gasoline powertrains. With natural gas, however, it’s likely that two or three fuel tanks will be needed to provide the necessary volume of energy to achieve the same production output (i.e. mileage).
“Because natural gas is such a good fuel, one benefit is that you can likely downsize the engine,”
For instance, the typical diesel engine is a 6.7L. Natural gas could require a 2.8L engine.
“This could provide some additional space under the hood in the engine compartment — without sacrificing power and torque,”
Langenderfer pointed out.
Hybrids, hydrogen, and full electrification
A range-extending electric/diesel vehicle is also relatively comparable to a conventional powertrain architecture — with one additional benefit.
“You are able to go down to a smaller engine because you’re splitting the power between the battery and the engine gen set,”
In this architecture, the vehicle has a 150kW generator, 74 kWh battery, and a 180 to 200 kW electric machine in the back.
“Depending on the performance requirements of the vehicle, this architecture could also have a gearbox,”
Langenderfer pointed out.
Additional componentry includes a three-phase inverter to convert the AC power from the generator so it can charge the DC battery. A second three-phase inverter is also needed, this time between the battery and electric machine. A DC-DC converter is also required since there is no longer an alternator to charge the 12V or 24V battery.
“An OEM must also add an on-board AC converter/charger so the end-user can charge the battery overnight,” Langenderfer added. “There are also a couple of different connectors such as a Level 2 AC SAE J1772 connector which is 240V AC.”
One thing OEMs must put some thought to with a hybrid architecture is vehicle accessories.
“With a diesel/electric architecture, the engine may not always be running as you operate the vehicle. Therefore, you’ll need a way to electrify accessories such as the power steering pump, AC compressor and cab heater.”
With a range-extending battery/gasoline vehicle, not much changes other than going from diesel to gasoline engine, or even a natural gas engine. This reiterates the point made earlier that OEMs want to have some commonality between powertrain configurations. This gives end-users the ability to choose the best fuel type for a given job site.
Looking further out on the horizon (2030-2035), a range-extending hydrogen fuel tank vehicle enters the discussion.
“As hydrogen fuel costs come down and the cost of a fuel cell comes down, OEMs could look at replacing a conventional internal combustion engine with a fuel cell. In this case, all electric accessories will remain similar — but an OEM will need a high-pressure fuel tank for the hydrogen fuel.”
When off-highway machinery evolves to fully electric vehicle architecture, Langenderfer said a much larger, 150 kWh battery will be required since no engine or hydrogen fuel cell will be helping power the vehicle.
“OEMs will also need to think about the liquid cooling for the battery, inverters, convertors and electric machine itself. Manufacturers will need to think about where they want to package all of those heat exchangers.”
Langenderfer pointed out.
Manufacturers will also encounter a Level 3 DC charger for faster charging, because end users will require a quicker turnaround in order to make fully electric machinery practical for everyday operation.
Speaking of everyday practicality, OEMs looking to leverage alternative fuel sources must consider the implications with respect to productivity, safety, and the ability to service and maintain vehicles and equipment.