Dakota Richline
“Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another.”
― Carl Sagan, Cosmos

Why Series Hybrids Can Be Elegant Electric Vehicles

Introduction

I’ve always been a fan of electric vehicles, but I’m not a fan of excessively heavy, expensive cars constrained by limited DC infrastructure. While many see hybrids as evidence of automakers dragging their feet on full electrification, they have potential as affordable carbon-neutral vehicles.

Definitions

For those unfamiliar with electric and hybrid vehicles, jargon used in this article is defined below.

EV/BEV
Battery electric vehicle. This is an electric car powered solely by batteries
ICE
Internal combustion engine. A gasoline/diesel powered engine like those found in a typical car.
Range extender
An ICE that generates electricity to power a vehicle
Traction motor
An electric motor that moves a BEV or hybrid.

Series and Parallel Hybrids

Most hybrid cars are parallel hybrids: the engine and motor are connected to the wheels through some sort of transmission. Battery sizes vary wildly: the 2021 Toyota Prius stores 8 times as much energy as the 2021 Honda Insight even though both work similarly. Parallel hybrids switch between running on gasoline, electricity, or both depending on a variety of factors.

In a series hybrid, the engine is not directly connected to the wheels; it generates electricity to power an electric traction motor. The BMW i3 Rex is a true series hybrid: a battery-powered electric car that happens to carry a backup motor for emergencies. Some series hybrids blur the line between hybrids and electric vehicles. For example, the Chevy Volt and Honda Accord use transmission clutches to seamlessly switch between series and parallel operation while driving, but only the Chevy is usable as a BEV. For the purposes of this article, series hybrid refers to electric vehicles with a range extender, such as the i3 Rex.

Disadvantages

A series hybrid has some downsides compared to parallel hybrids and BEVs. While many parallel hybrids use their engine for quicker acceleration, series hybrids are limited to the power of their electric motor. Parallel hybrids can also seamlessly switch to burning gas when the battery is low, without forcing the driver to slow down. When the battery of a series hybrid is depleted, the car’s average power is limited to the output power of the range extender: roughly 30kW for the i3, or a top speed of 90 km/h.

Maintenance and Reliability

A series hybrid is undoubtedly complex compared to a BEV, but it has significantly fewer points of failure than a parallel hybrid. Typical parallel hybrids are as complex as a normal ICE car, with an additional BEV powertrain. In comparison, the powertrain of a series hybrid is identical to that of a BEV, with an engine added to act as a range extender. Even if the engine completely failed, the car is still fully functional as a BEV. The combustion engine and battery are probably the weak link of the system; Elon Musk claims that his BEV powertrain can last 1.6 million km. Anecdotally, BMW motorcycle engines like the one used in the i3 Rex can last several hundred thousand kilometers before a rebuild (and likely much longer in a car that gently extracts 30kW of power from it on occasion). A series hybrid is as simple and reliable as a BEV with a high-end generator, compared to parallel hybrids that are even more complex than an ICE car with the same reliability issues.

BMW i3 engine and drivetrain [1]

BMW i3 engine and drivetrain [1]

BEV versus Hybrid Usage Models

Range is the largest issue with BEVs: who wants to pay a premium for a car that can’t travel great distances without recharging? Companies like Tesla are installing DC fast charging stations along major highways, but fast charging is stressful for the cars' batteries and the surrounding area’s power grid. A Tesla Supercharger can charge one of their vehicles in 15 minutes, but many competitors' cars and chargers are limited to a fraction of that speed.

The series hybrid is obviously ideal for someone that typically drives less than that car’s electric range in a day. If the driver averages 60 kilometers daily, why should their car carry enough batteries to drive 600km? Even when they do eventually use the full range on a long trip, repeated fast charging rapidly degrades that battery’s lifespan. If the driver owned a series hybrid, they would own a much smaller, cheaper battery that’s usable on a daily basis and have effectively infinite range when they need it.

Environmental Impact

Combustion engines are not ideal from an environmental standpoint, but there are many existing methods to negate their emissions. The engine in a series hybrid is likely to start less frequently than that of a parallel hybrid, and it always runs at its most efficient speed. In fact, a series hybrid owner could use E85 gasoline and their extended range would be 85% carbon-neutral! The constant-power nature of a series hybrid’s engine also means it can be easily replaced with a cleaner engine such as biodiesel, CNG, etc.

Combustion engines are made out of simple metals and are already easier to recycle than lithium batteries. Again: why should every EV owner buy a large number of rare earth metal-containing batteries for long drives when they could just burn a small amount of clean fuel on those rare occasions?

42.2kWh BMW i3 underfloor battery [2]

42.2kWh BMW i3 underfloor battery [2]

Fuel Cells

Hydrogen fuel cells appear to be a promising alternative to batteries and combustion engines, but their automotive use is extremely limited. A hydrogen vehicle can be refueled like a normal ICE car, but hydrogen fuel is not widespread; the only hydrogen fuelling stations are clustered around Los Angeles. Unlike fuel cell vehicles, the perfect infrastructure already exists for series hybrids: widespread BEV charging stations and gas stations everywhere, many of which currently sell clean fuels like E85 and 88 octane.


[1] “IAA 2013: BMW i3” by motorblog is licensed with CC BY 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/2.0/

[2] “File:Lithium-Ion Battery for BMW i3 - Battery Pack.JPG” by RudolfSimon is licensed with CC BY-SA 3.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/3.0

Last modified: 02/24/2021