Chevy Bolt: A Tour Under the EV’s Hood

Some friends and I were talking about my Chevy Bolt and, of course, we had to lift the hood and look inside. Seriously, I had no idea what I was looking at. In a conventional car, the internal combustion engine dominates the space under the hood. It’s a part of our culture to know what the major components are. This doesn’t help you at all when looking under an EV’s hood.

Now I know what’s inside a Bolt. I produced a 4-minute Youtube video on it.

My Chevy Bolt prompts a lot more conversation than any other car I’ve owned. Unfortunately, the two most interesting components, the tiny drive unit and the half-ton battery, are both buried deep inside.

My newfound wisdom is mostly thanks to Prof. John Kelly’s WeberAuto Youtube Channel. Prof. Kelly is at Weber State University in Utah, and he’s produced a pile of instructional videos on automotive tech. I’ve been watching a series in which he disassembles a 2017 Chevy Bolt.

Terminology: Motor versus Engine

It is generally accepted that a gasoline motor is called an engine and an electric engine is called a motor. I use the word engine a lot because that’s what traditionally moves a car. So, when I ask: “Where is the engine?” the technical answer could be “There isn’t one!”

I think it’s more useful to say, “The Bolt has an electric motor.” Perhaps I should also reinforce the difference between a motor and an engine, but I don’t.

The original title of this post was “A Tour of an EV Engine Compartment.” I retitled it to minimize anguish over terminology. A lot of people will probably call the under-hood area the “engine compartment” regardless of the type of motor. I can’t predict if the EV under-hood area will ever be known as the “motor compartment” instead.

A Photo Tour

While I enjoy producing videos, I have limited patience for watching them. So here’s a textual tour. Since it’s easier to skip over text, I’ll include a few details omitted from the video. We start with the superficial features of the engine compartment:

Under the Chevy Bolt hood, pointing out fluid reservoirs (washer, brakes, and 3 for coolant), battery, HVAC, relays, etc.

There are three separate coolant loops, each with its own reservoir:

  • Cabin heating (upper left)
  • High voltage battery (lower left)
  • Engine and gray boxes (upper right)

Here are the four gray boxes that seem to fill the engine compartment:

The four electronics modules: the HPDM, SPIM, APM, and OBCM.
  • High Power Distribution Module (HPDM) – connects 350 VDC power to the motor and other auto components. The big connection in front is for high speed DC charging (50 KW). The orange cables generally carry 350 VDC.
  • Accessory Power Module (APM) – replaces the alternator in typical cars. The Bolt uses 12 VDC power to drive the typical equipment in a car, and to charge its 12 V battery. Black cables generally carry 12 VDC. The car needs a charged 12 V battery to start itself up; otherwise relays isolate the 350 V battery.
  • Single Power Inverter Module (SPIM) – converts battery DC power to three-phase AC to drive the motor. The motor can draw up to 400 amps. No one has said so, but I’m guessing the same circuits collect energy from regenerative braking to recharge the battery.
  • Onboard Charging Module (OBCM) – takes 120-240 VAC from the J1772 connection to charge the DC battery.

Here are structural mnemonics to remember which is which:

  • The top row handles DC only: high voltage on the left, low voltage on the right.
  • The bottom row handles AC-DC conversion: motor on the left, battery charging on the right.
  • The right column charges batteries: the 12 V on the top, the 350 V on the bottom.

Here is the engine’s drive unit and the four modules set up on a workbench.

Bolt engine components set up on a workbench with the drive unit

Photo courtesy of John Kelly, produced as part of his Chevy Bolt Deep Dive series on his WeberAuto Youtube channel. Used by permission.

The Bolt’s 1ET25 drive unit weighs about 170 pounds and generates about 201 horsepower. The drive shaft connects to the front axle.

The electric motor is on the driver side (right of photo). Its housing is obscured by components installed over it, plus the red jack.

The drive unit includes a 1-speed transmission, housed on the passenger side (left of photo). The transmission provides a 7-1 gear reduction. It contains about 13 moving parts, including 4 gears for the differential.


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