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Comment by master_crab | Hacker News Reader

It's alluded to in the article, but the energy extraction from combustion in power plants is also more efficient than the combustion engine in your car (almost double in the case of natural gas using combined cycle).

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Diesel electric systems are highly efficient ~85%. This is practical now in power plants, trains and shipping.

The same could be achieved on a small scale if someone would simply make it

i am no mechanical engineer, but i believe there is a practical limit to size of a turbine. I dont think you can have combined cycle engine in a car

Wasn't that sorta the premise of the Chevy Volt? Though I think most HEV still include a direct drive from the ICE.

Personally I'd be interested in a pure electric drive HEV with a high efficiency diesel generator. Give me just enough battery for around town (100 miles maybe?) and just enough generator to extend my trip to ~300 miles. The generator doesn't need to be large (and expensive) enough to be capable to drive from directly at highway speeds, but just enough to extend the range when needed.

Heck make the power units "modular" in 3 ft x 4 ft boxes and let me mix and match power sources. I'll start out with a 1 battery and 1 diesel setup, but could upgrade to 2 battery power packs later if I drive around town a lot. (^_^)

I was joking but actually modular "power packs" would be cool.. You could replace one at a time vs a full $20k battery. Or take one out and plug it into your home. Maybe even rent a extra diesel pack for that coast-to-coast trip. Hmmm, wish I had the energy to start random companies. ;)

> The generator doesn't need to be large (and expensive) enough to be capable to drive from directly at highway speeds, but just enough to extend the range when needed.

So you would need to park and let the generator slowly charge your car? Why not just charge the car at a charger at that point?

Make the generator large enough so that it can generate enough power to directly run the electric motor for highway driving.

Yah I figure use a super charger. Maybe refill gas too. If you could save $20k I'd say it'd be worth the hassle on the occasional road trip.

The main reason would be cost and weight. Full EV card are still too expensive for many. I'm guessing there's a sweet spot that'd be much cheaper than either but would provide more range when needed. But as the other commenter points out the ICE + 20 mile pack might win out on efficiency of scale.

But why include the gas engine at all if it doesn't extend your highway range? Keep your small <100 mile battery pack, skip the extra cost and weight of an engine.

A car only needs 10kW or so on average to do highway speeds, but you need peak power around 30kW at minimum (or about 70 to satisfy most people's expectations).

You only need the generator to be able to extend the driving time to around 8 hours to cover 99% of use cases, so if your battery can do 2 hours then the generator only needs to produce 3/4 of the energy in the minimal case.

What you are proposing would require the generator to be 3-20x as large as the alternative.

Sorry, maybe I was not clear. Assuming you are correct, I am saying the generator should produce 10kW of power, and OP is suggesting that the generator would generator < 10kW. I'm saying the generator should keep up with the energy demand of cruising on the highway.

You can get a range multiplier even below that.

A car that uses 10kW average and has a 50km range is usable with a 7.5kW generator and stopping to charge every 200km. Knowing you have the ability to stop and charge or limp at 3/4ths speed to the next charger would allow more confidence pushing the boundaries.

A small battery means a 15 minute break to charge every 2 hours is viable and not a deal breaker for someone who only needs range infrequently.

I’ve seen talk before about EVs with gas range-extenders as a possible design. But in a rough sense, we’re at that point with all the new PHEVs on the market. Usually they have 20-45 miles of all-electric range, and then have normal regeneration beyond that. I understand that the power/weight/efficiency trade offs of a PHEV are not the same as having a range extender, but I wonder whether it doesn’t matter, that the economies of scale of current PHEVs and iterative improvements will provide most of the benefit at lower cost.

A nice thing about current PHEVs is that they have smaller batteries than full EVs, which could matter in a battery-constrained market.

The BMW i3 comes with a petrol range extender. Not sure how much that adds in practice.

The problem with the generator is it's more cost, weight, and maintenance. But it still makes sense in many scenarios, which is why many cars do something like this, like the BMW i3.

I wonder how reliable the portable diesel generators are. The huge ones in datacenters are known to have trouble starting up sometimes, so they're tested frequently.

Much more reliable than gasoline and the added benefit of always operating in the peak performance range as a generator.

Diesels require higher compression than gasoline and must be built stronger as a result. Which explains the higher cost of diesel engines in general.

Diesel engines, like all internal combustion engines are limited by the Carnot Theorem. The maximum theoretical efficiency for diesel is in the low 80% range.

No actual diesel engines can muster up much higher than 60%.

> Diesel electric systems are highly efficient ~85%.

That's 85% of the crankshaft energy moving the wheels. Once you multiply in your ICE efficiency which will be around 30% for anything that fits in a car or truck you're looking at 25%

Burn the same deisel in a big turbine and chemical->wheel efficiency going via power lines and a battery will be in the 30-40% range, but with the advantage of being able to mix in solar.

Moving a giant metal and plastic box for each individual human is still a gross waste of resources though and we should subsidize mass transit and active transport infrastructure rather than spending vast amounts of common money on roads and parking lots.

This number is implausibly high. Combined cycle natural gas is only 65-70% with a large amount of engineering put toward capturing waste heat

Yeah, it is.

But heat recovery is also paired with diesel, depending on the usage, with nearly complete conversion efficiency.

All the calcs for electric are bullshit. Every engineer that isn't a bleeding heart acknowledges this. They have either fudged the production cost or disregarded the recycling cost or ignored the mining expense, drive like a puritan with no climate control... on and on.

Well then how about an electric car driving full out in freezing temps? Or with the A/C max?

I can take nearly 100% of the waste heat from a diesel genset working at maximum efficiency with 42% conversion

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