Hybrid technology follow-up - some of you disagree with me on their underlying energy management voodoo. And that’s totally OK

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While I agree that a small diesel is best on the highway, I'd still say hybrids are better than standard petrol cars because they use Atkinson cycle engines. These engines are quite a lot more efficient than a standard petrol, but are low powered for their size, this doesn't matter as they have an electric motor to help out when you need to get up to speed quickly. - aussieguy69

I kind of agree with that - if you want fuel efficiency and not performance. Hybrids (and some other cars like the new Kia Seltos with 2.0-litre CVT powertrain) use the Atkinson cycle.

Atkinson is a thermodynamics hack - they leave the inlet valve open a bit longer and this ejects, by contra-flow, a little of the inlet air, from the cylinder. (It contra-flows back up the inlet port.) And that hacks the ratio of the compression ratio to the expansion ratio. 

Atkinson cycle engines have less compression relative to their expansion, and that derives greater thermal efficiency - at the expense of peak power performance. So you typically don’t see Atkinson cycle engines on exciting cars.

My point about hybrids is that the regenerative braking is how they do their energy management voodoo. This - and not the Atkinson cycle - is what sets hybrids apart. Capturing kinetic energy under brakes, converting it to electricity and using it to get going again. That is hybrid’s big trick.

On the highway, where regenerative braking opportunities are minimised, it would be better if you could simply wave Harry Potter’s wand and magic away the whole electric side of the hybrid system - because in those conditions it really is just excess baggage. 

If you want confirmation of this, look up the fuel figures and compare ordinary cars to hybrids. Hybrids are generally better on fuel around town than they are on the highway, even though urban driving is very energy inefficient driving (with lots of stop-start). The latest Corolla Hybrid, for example, uses 10 per cent more fuel out on the highway, compared with the ‘urban’ test.

It’s regenerative braking doing that, not the Atkinson cycle.

Aussieguy69 went on, representatively.

"Just one other thing, hybrids not only charge their batteries from regenerative braking, but they also charge from the engine if it's ever running but not under enough load to be most efficient.....then it turns the engine off and uses electricity until the battery is drained a bit, then re-starts the engine and repeats the process". - aussieguy69

Some hybrids will charge the battery with the internal combustion engine. Absolutely. Typically the ones with big batteries that rely on them heavily for conventional levels of motive power. This is also a nice idea, potentially, but unfortunately that’s not an example of energy management voodoo. It’s a compromise in hybrid vehicles where the electric side of the powertrain is designed to do a lot of the heavy lifting.

You really don’t get a thermodynamics or efficiency benefit from that - if you did, it would be a violation of the second law of thermodynamics. And that’s really just a happy fantasy.