Part of it is due to thermodynamics. A diesel engine works at a high temperature difference, with peak gas temperatures going somewhere in the >1000°C range.
High temperature differences lead to high efficiency >40%.
A human body can’t use high temperature processes, so has to stick to chemical workarounds which lead to additional losses.
A diesel car will typically run 20 km per liter. One liter of diesel is about 10 kWh. so if you put four people in the car, that’s 2.5 kWh per person. 2.5 kWh should allow you to walk for eight hours, or about 40 km. *quick edit: this is walking, cycling will get you further on 2.5 kWh!
Another thing is, diesel engines are water cooled, the heat is shed to the environment. This is about two thirds (varies 60-70%) of the energy produced, the > 40% efficiency you mention is only achieved by the highest performing engines around today.
Ok, lets assume an efficiency of 35%.
That means 3500Wh usable energy per liter of diesel.
A complete 500Wh charge of my ebike gets me ~70km at 20km/h without any pedaling (tried that out).
So the usable energy delivered by one liter of diesel would be sufficient to drive me and my bike a distance of 7*70km = 490km.
A person won’t be able to sustain 20km/h on a bike with the same low effort as walking at 5km/h, but even if that was the case, in 8h this would only get him 160km far.
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Edit:
Looked up some numbers for bicycling.
Driving 20km at ~20km/h burned energy is generally given in the range ~500kcal, meaning 580Wh.
That is better than I expected, as the 10kWh primary energy would get you ~350km far, so almost as good as the diesel engine.
(To be fair: this doesn’t yet take into account the CO2 footprint of food production, which is really abysmal. But that’s a different topic.)
Part of it is due to thermodynamics. A diesel engine works at a high temperature difference, with peak gas temperatures going somewhere in the >1000°C range.
High temperature differences lead to high efficiency >40%.
A human body can’t use high temperature processes, so has to stick to chemical workarounds which lead to additional losses.
A diesel car will typically run 20 km per liter. One liter of diesel is about 10 kWh. so if you put four people in the car, that’s 2.5 kWh per person. 2.5 kWh should allow you to walk for eight hours, or about 40 km. *quick edit: this is walking, cycling will get you further on 2.5 kWh!
Another thing is, diesel engines are water cooled, the heat is shed to the environment. This is about two thirds (varies 60-70%) of the energy produced, the > 40% efficiency you mention is only achieved by the highest performing engines around today.
Ok, lets assume an efficiency of 35%.
That means 3500Wh usable energy per liter of diesel.
A complete 500Wh charge of my ebike gets me ~70km at 20km/h without any pedaling (tried that out).
So the usable energy delivered by one liter of diesel would be sufficient to drive me and my bike a distance of 7*70km = 490km.
A person won’t be able to sustain 20km/h on a bike with the same low effort as walking at 5km/h, but even if that was the case, in 8h this would only get him 160km far.
============
Edit:
Looked up some numbers for bicycling.
Driving 20km at ~20km/h burned energy is generally given in the range ~500kcal, meaning 580Wh.
That is better than I expected, as the 10kWh primary energy would get you ~350km far, so almost as good as the diesel engine.
(To be fair: this doesn’t yet take into account the CO2 footprint of food production, which is really abysmal. But that’s a different topic.)