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Comment by londons_explore | Hacker News Reader

Busses currently have big engines and fuel tanks (or motors and batteries).

That intuitively makes sense - obviously a big vehicle needs more energy than a small one like a car.

However, typical city bus routes spend most of their time under 30 mph, cutting aerodynamic drag by a whopping 90% compared to 70 mph highway cruising that a car does.

With more work on rolling resistance (buying super-good bearings and fancy tyre designs), regen round trip efficiency, and energy use of the passenger cabin (heat pump heating, double glazing), I could see busses needing similar size batteries to electric cars and still being able to do a full days city work.

In turn, that makes the energy source fairly irrelevant from both an economic and a social perspective.

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As an example, most of Dublin Bus’s electric fleet have 450kWh batteries for a range of 320km. So, bigger than a car, but not as much as you’d expect given that the buses are 20 tonne fully laden and take a hundred passengers.

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

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 ?

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.

I'm not sure how important double glazing is when you're opening the inner air volume to ambient air every few minutes, but maybe?

It's possible something could be done about that with air curtains, PVC strip curtains, etc.

Climate control energy varies widely depending on geography - and appropriate door energy saving approaches will probably depend on where the bus operates, and possibly even the season (ie. winter doors swapped for summer doors), or extra batteries added in summer/winter to account for the extra energy use.

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