What is more efficient in terms of greenhouse gases, buses with diesel engines or electric buses charged with Diesel Generators ?
In: Engineering
The exact answer will depend on the specific cases.
But in general a large scale generator in a power plant can be more efficient than having a small engine in a vehicle.
Generally speaking it’s the electric bus. Diesel buses need to keep the engine running even while stopped, wasting energy. Also under braking, electric/hybrid vehicles usually use the brakes to charge the battery. In classic diesel buses, the energy braking isn’t used for anything other than stopping the bus.
Diesel engines buses use engine waste heat for cabin heating, and thus they are much more efficent in cold climates.
BTW currently electric buses in central and nortern Europe run with diesel heaters on board, as batteries weren’t sufficent for both heating and motive power.
An electric powered, diesel generating hybrid vehicle is gonna be more efficient as you don’t need as large of a diesel engine as one used to move the bus. Plus, the diesel engine will be generating electricity 99% of the time it’s running, rather than idling.
Motors run constantly at or near their most efficient speed is one of the major ways that large generators are significantly more efficient than small engines. If you could only run the bus engine only at its most efficient speed you would be closer in efficiency to a large generator but the slow down and speed up of a small engine is really what hurts the efficiency.
People seem to be assuming you can just compare the overall efficiency of the diesel engine vs generator, as if the charging of batteries is 100% efficient! You can get very good rates if you keep within 20-80% charge capacity, but nowhere near 100% if you actually use the full capacity, and for the significant charge time there usually isn’t the option to fully optimise for the 20-80% cycle.
So it really is dependant on the exact route, number of miles/hours of service required before next chance to charge etc. But on average, over a year in all weathers, I would bet that an individual ICE diesel still wins. However, the benefit with batteries is that you can find cleaner sources of electricity, or at least have the generators outside of densely populated areas, reducing air pollution for most people. The nearest you’ll get to “clean” diesel is biofuel made from used cooking oils.
Buses for long trips in winter with modern diesel engines might be even to electricity in terms of consumption, which would make them better taking into account the production of the batteries. The more trips you make, the better the electric bus gets.
For buses driving around cities, the diesel engine can not compete with the electric one. Bus most buses are operated for a long time bevor getting exchanged, so the electrification will take time.
If you had fixed routes for electric buses with electric cables along the routes, you would get trains. Passengers trains are driving mostly electric for decades (at least in Europe) because they are so much more efficient and therefore cheaper. Diesel trains drive on remote routes with very few passangers because the initial cost for the electrification can only be split between a few passengers.
Electric buses because then you are decoupling them from dependency on fossil fuels.
Yes the grid that charges them is reliant on fossil fields but without that decoupling they would always need fossil fuels.
That and grids can turn renewable in a piecemeal fashion.
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