I once expected an aftermarket industry to pop-up to hybridize old cars. Old rear-wheel drive manual cars sound easiest.

Generator attached to the flywheel instead of the starter motor, and then find some way to squeeze a drive motor by the rear diff (probably more complicated, but surely not impossible). I thought something might pop up once companies like Bosch started making generic e-axles for manufacturers to use.

With a small Li-ion battery (and all the usual inverters, etc.), it could work like a manually operated version of the dual-motor Honda hybrid system. Put it into neutral, and whatever energy is in the battery could drive the motor on the rear axle. The engine could automatically kick-in and drive the generator if there's not enough in the battery.

It would never be as economical as a purpose-built hybrid, but old manual cars are often reasonably economical on the open road, and terrible in city traffic. Targeting only the urban fuel consumption would go a long way towards keeping them useable, with the added bonus of making manual shifting optional in stop-go traffic. Small motors would be enough for city speeds, and with a small battery, it sounds cheap to me at least.

I doubt it could ever be a simple kit, but I thought people would have a go.

People probably are having a go. Here's one for pickups: https://edisonmotors.ca/trucks/pickup-conversion/

We had a guy give a talk at our local nerd meet-up about doing this to an old landrover with the hopes of commercialising it. Did a good job, but it was only for someone with money to burn.

Looks like he did commercialise it: https://www.jauntmotors.com/series

I recognise some of the photos from back then.

> Generator attached to the flywheel instead of the starter motor, and then find some way to squeeze a drive motor by the rear diff (probably more complicated, but surely not impossible).

Some vehicles have this as a “battery starter generator” where the “hybrid” functionality is entirely on the accessory belt.

https://www.bosch.com/stories/48v-hybrid-battery/

https://hackaday.com/2025/01/24/bosch-starter-motor-freed-fr...

I was thinking of a “feature” on manuals that ran electrical and accessory-belt stuff full blast during engine braking.

The best time to run your a/c or turn the seat warmers: when you’re trying to expel energy.

Would give you some extra braking without heating up the engine and take a small load off your brakes.

That or turn any vehicle into a mild plug-in hybrid for all electrical loads and flywheel the alternator until that time. Gasoline-powered generators are not very efficient.

Might get a few percent extra efficiency without having to touch anything mechanical.

Wouldnt the weight of the old chassis be a impedement to mods , older models tended to be more robustly buildt. Or what cut-off date for mods would be appropriate (last 10 years?) as beyond a certain cutoff would not be cost effective and most likely would be a hobby project done for the love of it.

Automotive engineer here. One consideration for the diff motor is that it would REALLY sag the diff. I think the only way to go would be a front and rear generator/motor combo in one housing to replace the transmission, powering the rear end with a driveshaft. A direct drive clutch between the front and rear motor/generator would economize driving and assure high power at high speeds.

My guess:

1) New cars are too cheap to make at scale.

2) Old cars are quite unsafe to drive, comparatively (airbags, backup cameras, lane departure alerts, blind angle alerts, etc)

In the US RWD manual cars have been exclusively sport/GT for decades. They aren't cheap and a hybrid swap and/or daily driving miles would destroy their value.

Stuff like this exists for bicycles. For mopeds and motorcycles there are electrification kits.

It should be possible to just replace the flywheel altogether with a generator/motor that both brakes and powers during each cycle, cancelling vibrations. Then more generation during deceleration, and more power during acceleration.