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Does regenerative braking, using supercapacitors, really save fuel in the case of diesel city buses?

A 10-ton diesel bus, fully loaded, that slows down, on average, from 30 km/h to 0 km/h each minute and keeps like this for 18 hours per day loses a total of kinetic energy:

60min * 18h * 10000kg *((30km/h)^2)/2 = 104kWh

104kWh is enough for that bus to run for 1 hour at 30 km/h, fully loaded.

In other words, each 18 days such a bus, using an efficient regenerative braking system with, say, supercapacitors, can not save more than 1 day of continuous run for 18 hours, assuming all conversions are 100% efficient. If the overall conversion has an efficiency of only 50% (a more realistic case) the bus will have to run 36 days to save 1 day (18 hours).

For me, it seems that the regenerative braking is not worth the effort (supercapacitors plus an electric motor / generator plus other mechanical complications which increase the cost of the vehicle and the maintenance expenses).

Is my evaluation correct or I am missing something?

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    \$\begingroup\$ Regenerative braking on electric buses essentially just commutes the motor backwards to withdrawn energy, so it isn't much effort. I suspect that for an electric bus the capacitor is also unnecessary, since a several hour-class bus battery could likely absorb the breaking energy directly. \$\endgroup\$ – user1850479 Mar 2 '20 at 2:29
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    \$\begingroup\$ @Simplex11 Do pure diesel engine buses with regenerative braking actually exist? I agree that seems like a strange combination. \$\endgroup\$ – user1850479 Mar 2 '20 at 3:03
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    \$\begingroup\$ @Simplex11 If they're using a flywheel then what is the supercapacitor used for? \$\endgroup\$ – user1850479 Mar 2 '20 at 3:51
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    \$\begingroup\$ 1 HP = 0.7457 kW. If it's a diesel-only bus then how is the regenerated energy used? \$\endgroup\$ – Transistor Mar 2 '20 at 8:56
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    \$\begingroup\$ You're confusing primary energy (diesel) and secondary energy (electricity). And you're confusing driving a bus ar constant speed vs. Stopping regularly. The paper gives primary consumption including all the stops. Both together give about a factor 10 difference! \$\endgroup\$ – asdfex Mar 2 '20 at 9:44
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What you describe doesn't seem like it makes sense for a diesel bus, but it does for a diesel/electric hybrid. Replacing the mechanical transmission with an electric machine and power electronics offers quite a bit of advantage even without recovering braking energy. Your example shows braking energy recovery increasing the efficiency of the bus by close to three percent. Operating the engine in a more efficient manner also increases the overall efficiency. Mechanical transmissions for heavy vehicles are not inexpensive or maintenance free. Remember that railroad locomotives were long-ago converted to diesel-electric without the benefit of power electronics.

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  • \$\begingroup\$ Attention, I am talking about a diesel bus which only has supercapacitors that are only used for storing the kinetic energy of the bus that otherwise would have been wasted during braking. The bus I am referring to does not have batterie and the only electrical energy it can have onboard is from braking. Also, it is not a vehicle like a diesel-electric locomotive. \$\endgroup\$ – Simplex11 Mar 2 '20 at 7:10
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    \$\begingroup\$ Attention, I understand what you are saying, but what you are saying doesn't make much sense. Such a vehicle needs an electric generator to do the braking by converting the inertial energy to electrical energy. For that to work a power electronic system is required to control the motor and the charging. Once the energy is recovered, as electrical energy, the only way to use it is to propel the bus. Therefore it doesn't make much sense to say this is not a hybrid vehicle. \$\endgroup\$ – Charles Cowie Mar 2 '20 at 12:31
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    \$\begingroup\$ It is possible to use the motor only to assist the engine by combining the motor and engine effort through a complicated mechanical system. However that seems to be creating a more complicated system that will provide less benefit than can be provided by a less complicated system. It doesn't seem to make much sense, but there is always the possibility that someone has figured out a system like that makes sense for a bus duty cycle. If so, it must be more complicated than you describe. \$\endgroup\$ – Charles Cowie Mar 2 '20 at 12:42
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Does regenerative braking, using supercapacitors, really save fuel...

it seems that the regenerative braking is not worth the effort (supercapacitors plus an electric motor / generator plus other mechanical complications which increase the cost of the vehicle and the maintenance expenses).

You are forgetting the other reasons for using regenerative braking.

  1. Brake pad wear is reduced, which reduces maintenance costs. Buses typically do a lot of start-stop operation, so brake wear is much higher than in other vehicles. The brake pads themselves might be cheap, but the down-time isn't.

  2. Clutch wear is reduced. With a super-capacitor powering an electric motor the clutch doesn't have to slip when starting off. Even more maintenance savings!

  3. Pollution is reduced. Diesel engines tend to produce a lot of unhealthy particulates under heavy acceleration. By starting off under electric power, pollution can be reduced at bus stops where people are most at risk.

  4. Noise reduction. Diesel buses make a lot of noise when the engine is under heavy load. This is another health risk at bus stops and in city environments, as well as being very annoying!

Until diesel buses are outlawed (hopefully soon) regenerative braking is one way to make them a bit less obnoxious. Fuel usage is the least of their problems.

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  • \$\begingroup\$ My question is about the energy (diesel, fuel) saved by using a bus with regenerative braking. Such a vehicle could have the advantages you enumerated but this is a different discussion. \$\endgroup\$ – Simplex11 Mar 2 '20 at 7:17
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    \$\begingroup\$ It's not a different discussion. You suggested that the (small, but real) fuel savings were 'not worth the effort' because they would be swallowed up in 'mechanical complications' and maintenance expenses. My answer is pointing out that fuel savings are not the only benefit, and therefore the fact that they are small does not mean it's not worth the effort. The other benefits alone make it worth it, and the fuel savings are a bonus. \$\endgroup\$ – Bruce Abbott Mar 2 '20 at 7:37

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