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Comment by maxerickson | Hacker News Reader

The low speed means they are using the power to accelerate against inertia, not to maintain speed against air resistance.

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0.7 m/s^2 is a typical acceleration for a city bus - most people won't fall over whilst standing at that acceleration.

A city bus perhaps holds 50 70kg passengers = 3.5 tons of cargo, and a lightweight bus design is perhaps 6.5 tons (typical bus=10 tons). Total = 10 tons.

Peak Power required to accelerate 0.7 m/s^2 up to 30 mph = 93 kilowatts.

Which is car territory. The cheapest tesla model 3 has a 208 kilowatt motor, so would be plenty enough power.

The bus is constantly starting and stopping. Regen doesn't recapture it all. Power isn't the issue, energy is.

Also, current hybrid busses with not-so-heavy batteries weigh about 15 tons without cargo. You are way off.

This math is for future busses designed for city use - so batteries will be much smaller and therefore lighter, and the bus construction itself will be much lighter because, as you point out, with a city bus pulling away ~5 million times in its lifespan, the cost of energy lost when stopping and pulling away far exceeds the cost of upgrading the frame to aluminium and other weight saving measures.

Buses have been designed for city use for ages. The double deckers are not designed for motorway use.

Regen can capture during stopping most of the energy consumed during starting, if well designed, while in a non-electric bus all energy is lost.

The mass of the bus does not matter, only the energy lost due to mechanical friction or electrical resistance, both of which increase much more slowly than the mass for bigger buses.

That’s a small bus. Buses here take about 100 people and are 20 tonnes fully laden.

okay - well the motor from the cheapest Tesla will still do the job just fine.

Inertia that can then be recovered, which you can't with air resistance.

There is plenty of air resistance in a low speed bus, which is shaped like a brick and has huge frontal area. That is still a lot of air to push.

Sounds like there is room for improvement here too ?

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