Fuel Smarts

Hybrid Electric-Drive Trailer Tandem Promises Quick Payback

May 2016, TruckingInfo.com - Department

by Tom Berg, Senior Contributing Editor - Also by this author

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Hyliion founder Tom Healy poses with the first prototype of the hybrid system. The company is currently driving and testing a newer and more improved, third prototype. Photos: Hyliion 
Hyliion founder Tom Healy poses with the first prototype of the hybrid system. The company is currently driving and testing a newer and more improved, third prototype. Photos: Hyliion

Hybrid trucks? Sure, though they’re somewhat complex and expensive, and interest has waned in the face of cheap fuel. Hybrid trailers? Comin’ up, and they’ll be comparatively simple and quicker to pay back their extra investment.

That’s the plan for a hybrid electric-drive trailer tandem being developed by a group of engineers at Hyliion Inc. in Pittsburgh, Pa. The device, which for now has no official name, is undergoing road testing and should be ready for the market in mid-2017, they say.

The system captures energy as a tractor-trailer runs downhill, and reapplies energy through a drive axle to help the truck run uphill. Operation is completely autonomous and, except for an on-off switch, the driver has nothing to do, explains Thomas Healy, the company’s founder and CEO. Electronic controls read road, terrain and speed conditions and try to keep a vehicle at a desired speed.

Healy has observed big rigs and wondered why their mass couldn’t be captured for reuse, like the hybrid electric car he now drives. Hyliion, the company’s name, means hybrid lithium ion, which is the type of battery that’s part of the system, says Robert Culbertson, the company’s marketing director. The firm started about a year ago to perfect the idea and market a product.

“We’ve been trying it out with different battery combinations and different motor combinations,” Culbertson says. “A lot of our supplies are going down in price, so it’s becoming cost-effective as well as environmentally beneficial.” The fuel savings is estimated at 21% for the electric-drive tandem alone. Trailer aerodynamics would save more.

The system would more than meet trailer targets in the proposed Phase 2 greenhouse-gas and fuel economy regulations, Healy says.  

When its design is finalized, the product will be a complete tandem with axles and a suspension that would replace a trailer’s existing tandem. Three people at Hyliion have changed out a tandem in less than 30 minutes. He envisions trailer builders eventually offering the electric-drive tandem on new trailers.

The suspension can be mechanical or air-spring, and a sliding tandem is the first type planned. A suspension supplier is being discussed.

Included will be a fairing mounted ahead of the tandem to smooth air flow and protect electronic, electrical and mechanical components.

Prototype uses belt and pulleys to transfer power between axle differential and motor-generator mounted above. Production version will put the motor inside the diff where it will be gear-driven.
Prototype uses belt and pulleys to transfer power between axle differential and motor-generator mounted above. Production version will put the motor inside the diff where it will be gear-driven.

How it works

A truck drive axle with a differential, now sourced from Dana, substitutes for one of the standard trailer axles. A 300-hp Remy motor-generator transmits energy to a battery pack that nestles between the rails of the slider box. When power is needed, electricity is sent to the motor, and it propels the trailer through the axle diff and shafts to the wheels.

The prototype uses a belt and pulleys to transfer power between the differential and the motor-generator that’s mounted above. Production models will mount the motor within the differential so the motor is gear-driven.   

“We’ve been working with Dana on determining differential gear ratios,” says Healy, a mechanical engineer. “We think there’d be one set for an operator who does pickup and delivery and another for highway use.”

Electronic controls use an algorithm to process data from wheel sensors and GPS monitoring, then determine what road speed should be maintained. Controls know when there’s wheel slip and alter the energy capture and power that’s applied. It works with the tandem’s anti-lock braking system to become electronic stability control, enhancing safety. Hyliion is working with Bendix on the ABS-related apparatus.

The hybrid-electric tandem weighs 400 to 500 pounds more than a standard tandem, but some of that would be offset by changing from dual wheels to wide-base singles, suggested a test driver whose testimonial is featured on the company’s website.

The craftsmen at Hyliion prepare a sliding suspension system for installation, a process which takes less than an hour.
The craftsmen at Hyliion prepare a sliding suspension system for installation, a process which takes less than an hour.

An APU, too

In addition, energy captured in the battery pack can be tapped to run air conditioning and heating in a sleeper cab, Healy says. A reefer unit might also be powered from the batteries. For sleeper-cab HVAC, voltage would be stepped down from 400 to 110 by an inverter that’s part of the electronic controls. Depending on ambient temperatures, there would be 20 to 30 hours’ worth of power available.

Because the system doubles as an auxiliary power unit, federal law exempts 400 pounds from the 80,000-pound gross weight limit, so “the truck wouldn’t lose any payload,” Healy says. The APU function would take total fuel savings to about 31%.

Projected price for a complete tandem is $29,500, but the projected fuel and dollar savings would supply a payback in as little as six months, according to the company. A lease-purchase plan would cost $500 per month for five years, and monthly fuel savings would more than cover that.  

“We’ve got this 80,000-pound vehicle moving around, so why can’t we get some energy out of it?” Culbertson says in recalling the idea that set development in motion. “The parts are pretty well tested, the motors, and it all works.” 


  1. 1. Dennis O Taylor [ June 04, 2016 @ 09:31AM ]

    The Hylion (concept) reminds me somewhat of the "Power Dolly" concept which combined an engine and automatic transmission on a single-axle dolly. The idea then was to provide an extra push for under-powered tractors on mountainous routes. I believe the maintenance issues outweighed the performance advantage, so the idea was short-lived. In the current case, I can see some advantage for stop/go duty cycles. For long-haul, the amount of energy that must be stored (on a descent) can be fairly large (think of a 5% grade that runs a few miles). The question is whether that much battery capacity can be provided. If that is possible, then you can reduce the displacement/power of the tractor engine and save fuel on the "flat" portion of the route, yet still climb grades at a sufficient speed. So, basically, the Hylion unit is replacing the function of the engine brake on the descent, but also returning a reasonable fraction of the braking energy on the ascent.

  2. 2. Steve [ June 04, 2016 @ 11:40AM ]

    Too much drop & hook going on, I'd rather replace twin screws with a single screw and this concept as a tag axle. Loaded trailers are already too heavy with a 40'bridge

  3. 3. AJ Emanuele [ June 06, 2016 @ 07:14PM ]

    Dennis, you are exactly right. The engine brake on descent is replaced by the Hyliion product that creates electricity to be used to power an ascent, the next acceleration, a refer, or other equipment while idling.

    The performance and maintenance of Hyllion has been tested on the road and this time is much different than the Power Dollys you mentioned. Similar in concept but different in execution. It truly is a set it and forget it solution for over 30% fuel savings.

    Long-haul is a great fit but stop/go duty cycles are also seeing incredible results. Brake blocks with wear and tear are great examples of wasted frictional energy. Hyliion turns that destructive force into electricity that saves you money. Really amazing stuff.

    Stay tuned! I appreciate your engagement.

    Feel free to use the Hyliion contact form to reach out and talk further.

  4. 4. Ivan Pearson [ August 28, 2016 @ 11:10AM ]

    BadAss!!! I'm getting one cash no soon they come out!!!:-) my first trailer purchase that would save me money on my first year as an owner operator

  5. 5. Mark MacDonald [ March 03, 2017 @ 02:02PM ]

    Have you considered a single axle power dolly design for doubles?

  6. 6. Bob James [ August 19, 2017 @ 02:51AM ]

    It appears this whole tandem system is being drive by a fan belt, when most are driven by a very big engine and propellor shaft, with several thousand Newton Metres torque, what torque will this FAN BELT be able to transmit and how long will it last while doing this work?
    This may seem like a moot point, but so is the fitting of this 300hp electric motor inside the differential housing, what is going to be removed to make enough space for the electric motor to fit inside? I ma only and experienced Electrical and Automotive Engineer so I may be missing some very salient points of the design, or maybe it is just a big Emperors New Clothes trick.Bob

  7. 7. Patrick Emerick [ November 06, 2017 @ 02:16PM ]

    This is pretty cool! I'd agree that drop and hook could be a problem for ROI, but I could see an owner-op or small fleet really benefiting from something like this.

    Bob - As an experience automotive engineer you should see the electric motor and realize it's definitely not 300hp. You probably only need 15-40hp to reduce fuel consumption by the stated 10-20% based on typical road load conditions here in the US. The correct v-groove belt could definitely function and last in that power range.


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