Why can't PHEVs be charged by the engine?

Question

I was reading a little bit about PHEVs here. There's also Rory's video here about why PHEVs are pointless.

The article says "The battery’s size means it can’t be recharged by the car as it drives along like a regular hybrid. Instead, just like a fully electric car, you have to plug in a PHEV in order to recharge the battery."

Why is this the case? What does the size of the battery have to do with the ability of a petrol/diesel engine to charge it? And are there engineering solutions in the near future that will allow us to have PHEVs that can be charged either by plugging in or by the petrol/diesel motor?

Answer 1

While searching using the terms "how do plug-in hybrids charge?" I discovered a number of inaccurate sources. As you suggest, the size of the battery has nothing to do with the ability of an ICE to charge it. The size of the battery would affect the rate of charge. PHEVs are engineered to use an engine mounted generator to charge the pack if the level is depleted beyond a certain amount.

A good example is the Chevrolet Volt:

Plug-in hybrids, like the Chevrolet Volt, operate in much the same way as a hybrid by providing an all-electric driving range using a battery pack. Once the battery has been depleted, the vehicle can slip back to being a regular fuel-fed hybrid and recharge its batteries using the gasoline-powered motor as a generator.

Answer 2

Are there engineering solutions in the near future that will allow us to have PHEVs that can be charged either by plugging in or by the petrol/diesel motor?

The future is now. :-) This functionality exists in the Toyota Prius Plug-in (at least in the 2019 model that I drive); so the claim in the article is just false.

In the Prius plug-in there's a "Charge Mode" button, which will charge the battery up to full capacity from the petrol motor while driving (but it runs more sluggishly, and uses more fuel, while doing so than the usual petrol-powered driving mode). It's not clear why you would want to do this, but the option is there.

Even if you don't use this "Charge Mode", if you drive a PHEV without plugging it in, it will still use the battery to "smooth out" the load on the engine – putting charge into it when demand is low, and taking it out again when demand is high. (It just won't charge it up all the way to the maximum.) In other words, it drives like a "normal", non-plug-in hybrid. But because the battery is much larger than a normal hybrid, you get an additional efficiency gain. So even if you never actually plug it in, a PHEV is much more fuel-efficient to drive than a conventional petrol or diesel car.