What happened

Shares of Tortoise Acquisition (NYSE:SHLL) were trading higher on Thursday. Intense investor interest in electric vehicles has powered shares of Tortoise, a special purpose acquisition company (SPAC) that is preparing to merge with electric-truck drivetrain maker Hyliion, to major gains over the last few weeks.

As of 3:15 p.m. EDT, Tortoise's shares were up 9.6% from Wednesday's closing price.

So what

There was no big news driving Tortoise's shares higher on Thursday. What's happening is that investors, excited by the huge run-up in electric big-rig maker Nikola's (NASDAQ:NKLA) shares over the last month, are looking for the next Nikola -- and Hyliion is an intriguing candidate.

The two share some similarities. Both are looking to capitalize on the need for green alternatives to diesel-powered heavy trucks; both are early-stage companies; and Nikola went public (in early June) via a merger with a SPAC, just as Hyliion plans to do with Tortoise. (The deal is expected to close by the end of the third quarter.)

A Freightliner semi fitted with Hyliion's diesel-electric hybrid drivetrain.

Hyliion's hybrid and electric-truck drivetrains are compatible with all major truck manufacturers' products. The company will go public via a merger with Tortoise, in a deal that is expected to close by the end of September. Image source: Hyliion.

That said, the two companies are different in ways that should be important to auto investors eyeing this space.

Right now, Nikola has big plans to manufacture its own electric semis, a secret battery technology, and the company's founder making some noise on Twitter. But it doesn't have any revenue today; has just begun building its factory; and won't have any revenue until late next year (assuming all goes well).

Hyliion, on the other hand, is focused entirely on drivetrains, meaning motors and electricity sources that can be installed in semis made by established truck manufacturers. And it's not planning to build its own factory. Dana (NYSE:DAN), a long-established supplier of vehicle components, manufactures and ships Hyliion's products.

Now what

Hyliion is also different from Nikola in that it's already shipping one product -- a kit that converts a diesel semi to a hybrid -- and expects to be shipping a complete drivetrain next year.

Note that unlike Nikola's upcoming battery-electric trucks (and the Tesla Semi), Hyliion's electric drivetrains don't rely on heavy battery packs that take a long time to recharge. Instead, they'll be charged while under way by an onboard natural-gas-powered generator, or alternatively, by a hydrogen fuel cell.

Long story short: Nikola has very big ambitions. But Hyliion has a realistic product plan along with the partners -- and soon, the funds -- to pull it off, and that's why Tortoise's stock has been rising.