General Motors unveiled an experimental hybrid in 1968 that combined a heat‑driven power source with electric propulsion, predating the company’s later plug‑in models by decades.
Response to 1960s smog concerns
By the mid‑1960s, worsening urban smog prompted regulators to pressure manufacturers for cleaner solutions. GM allocated research funds to explore alternatives that could reduce tailpipe emissions without sacrificing drivability.
The effort focused on a compact European model that GM sold under several brand names. This small family car, known in Europe as the Kadett, was already familiar to American dealers and served as the chassis for the test vehicle.
Design of the hybrid test vehicle
Unlike conventional layouts, the prototype placed its heat‑driven power unit in the rear trunk, while a modest electric motor drove the rear axle. Energy storage cells sat forward of the driver, creating a front‑to‑rear balance.
The heat‑driven unit was a closed‑cycle device patented in the early 19th century. According to the College of Science and Mathematics at Fresno State, such a system converts temperature differences into mechanical motion by shuttling gas between hot and cold chambers.
Philips of the Netherlands supplied the unit, which delivered roughly eight horsepower. It ran on kerosene, a fuel that the device could burn while the electric motor powered the wheels.
Fourteen standard lead‑acid cells provided the primary energy, delivering a modest electric‑only range of fifteen to thirty miles. When the heat‑driven source engaged, the combined system could travel one‑hundred‑fifty to two‑hundred miles on a single 4.75‑gallon fuel load, achieving about forty‑two miles per gallon.
The design illustrated how a thermal engine could act as a range extender, a notion that later engineers would revisit when battery technology began to improve.
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Performance and testing
Weighing around 3,100 lb with occupants, the vehicle was heavier than its conventional counterpart. Acceleration was sluggish; a magazine test recorded a low‑speed sprint taking between ten and twenty seconds, though the take‑off felt smooth.
The test noted a break‑even speed near that low‑speed figure, where the electric storage could be replenished as fast as it was depleted. A later version swapped the original alternating‑current motor for a direct‑current one, shaving a few seconds off the sprint without extending range.
While the prototype never entered production, it demonstrated that a range‑extending heat source could coexist with electric drive, a concept that resurfaced in later GM projects.
Legacy and later experiments
In the early 1970s GM displayed another electric test mule based on the same chassis, this time without a range extender, to evaluate battery performance for a high‑speed record attempt. Around the same period, a fiberglass micro‑car equipped with a plug‑in hybrid system was built, but both remained laboratory curiosities.
When comparing the effort that year to the 2010 launch of a mass‑market plug‑in, the gap highlights how long it took GM to move from experimental to commercial hybrid technology. The early prototype showed technical feasibility, yet market‑ready solutions required decades of battery and control‑system advances.
Later, the company introduced the EV‑1 in the 1990s, a limited‑lease electric car, and eventually the Volt, which finally offered customers a usable electric‑only range paired with a gasoline generator.
Despite its ingenuity, the hybrid from that year never progressed beyond the prototype stage, and it remains a footnote in the company’s electrification timeline.
