Fuel-Cell Trucks and the Sleeper Enigma

Will the future of long-haul heavy-duty trucks be hydrogen or battery-electric? That question seems to preoccupy a lot of webinars and media stories. The arguments always seem to hover around two topics: refill times and payload weight capacity. While those two topics have some significant details to debate, I have yet to see an article that addresses a more fundamental issue — the sleeper.

Long-haul is a vague definition with a lot of nuances to it, but a conservative definition is that the truck regularly travels more than 500 miles per day and rarely returns to base. These vehicles tend to have sleepers. The EPA greenhouse gas regulations even define these tractors as primarily high-roof sleepers. Sleeper options from truck makers range from perhaps 36 inches to more than 70 inches in length.

In the alternative-fuel world, compressed natural gas vehicles have been making market share inroads for more than a decade. But when the North American Council for Freight Efficiency asked truck makers and fleets how many CNG tractors they have put on the road that had sleepers, the answer is somewhere near zero.

Why CNG Trucks Are Day Cabs

CNG vehicles overwhelmingly are day cabs. They universally store the pressurized fuel in cabinets behind the cab in the gap between the tractor and trailer. These fuel cabinets require approximately 24.7 inches of chassis length, according to one OEM’s body builder information. The fuel cabinets are frame mounted, not tied directly to the cab.

Comparing these to a comparable diesel is challenging, as vehicle specifications differ, with many option content choices available. A day cab with under-cab chassis-mounted fuel tanks can have a shorter wheelbase as a diesel-powered vehicle than one that is powered by CNG. A day cab also can be configured in such a way as to have the same wheelbase as the CNG truck. One example specification of a CNG day cab tractor I found indicated it had a 200-inch wheelbase. OEM information I found indicated 197 inches was as short as possible.

When migrating to sleeper cabs, the CNG cabinet will nearly always add length over a comparable diesel. The diesel fuel tanks on a sleeper truck are mounted on the chassis underneath the sleeper. The CNG truck, however, would have the fuel tanks mounted behind the sleeper, resulting in an additional nearly 25 inches of wheelbase.

That added length impacts front-axle loading, maneuverability, and likely the ride. Those factors may contribute to the explanation of why CNG sleepers are rare.

A number of fleets have opted to allow their drivers to stay overnight in hotels. This trend allows the fleet to use day cabs almost exclusively. The growth in regional-haul operations, where the driver returns to base each evening, also permits the use of day cabs. One fleet in NACFE’s Run on Less Regional was able to get a trailer to go more than 500 miles by swapping the tractors at the mid-point, allowing both drivers to get home each day. So there are operational duty cycles that can work for longer distances with day cabs.

However, the hydrogen or CNG long-haul sleeper tractor remains an enigma. Nikola’s fuel-cell vehicle plans indicate a sleeper version will be offered at some point in the future. Some aftermarket retrofits of CNG-powered vehicles have been done with sleepers. The majority of prototype fuel-cell trucks in use in California, though, seem tied to drayage operations where day cabs suffice.

A different take on this is that hydrogen may be the best zero-emission solution for autonomous long- haul trucking, where a day cab, or no cab at all, may be pulling the trailers.

If hydrogen is truly the solution for long-haul trucking, it would be in everyone’s best interest to start fielding prototypes with sleepers that actually permit long-haul driving. Selling hydrogen as a long-haul solution but only fielding regional and short-haul trucks is not helping the story.