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FELA Efficiency in a City

Updated: Sep 18, 2022

People are happier when they are efficient in doing what they love.

The efficiency of our trips i.e. "travel time" is defined mainly by traffic along with city permeability, not the technical performance of our cars. The subject of this post is energy efficiency - reverse to energy use in Wh/km.

In a city where we go mostly alone, a sedan or SUV uses up to 1200 Wh/km. Being electrified, they need 3-4x less i.e. 250-300Wh/km. For the same trip, a velomobile (a faired trike) uses 2% of that.

Efficiency is ~50% of effective trip costs to you and to our environment that's proportional to a vehicle's size and weight, which in turn is proportional to the designed performance. Modern cars' maximum performance can only be exercised on runway-quality-empty roads i.e. almost never, and other than cultural conditioning, nothing can explain let alone justify our use of a car designed for 250km/h, 5-7 occupancy for personal mobility in a city of 60km/h speed limits, of poor permeability for that heavy and big vehicle.

Cars were conceived as carts where horses are replaced with engines. It took a decade or two to get people to want it. Marginal cars' cost to buyers' income ratio and cheap fuel made cars affordable and wildly adopted, and consequent car-centered city planning led to car dependency.

It's come to the promotion of cars as something more important than just a means to get from A to B. Cars have become a status symbol, dating site, means of crime, shelter, and wherever else that questions the sanity of the human race. Cars have become the second top expense for a household. The ridiculousness of the situation is shown by the phrase “I slept in my car” as a metaphor for the depth of personal social-economic fall. However, car-induced health and environmental costs are not that funny.

It’s not the design of the powertrain or its energy sources that define the lifecycle CO2 footprint of a car, but its size and weight matching its performance redundancy that results in waste of Earth resources.

To solve transport-induced problems we must start with our values and express them via the user requirements document (URD) for R&D or just a list of personal priorities for the vehicle to buy. Without clearity here, we get sold on what we don’t need but for a tenfold pay, e.g. 5-10 seater when we go solo 95% of the time, or "0 to 60" in 3 sec when doing so is not only wasteful but dangerous, or 500km range when our daily average is 50km. Whereas, in civilization of responsible individuals, we need personal vehicles (PV) that are as small as functionally possible.

The feasibility of PV that wraps its operator, weighs about 40kg, still, outperforms a 2-3x big and 10x heavy car (GVW) in a city, has been proved by velomobiles since their reserection fifty years ago. Minimization of PV makes human-machine essentially synergetic, which justifies the APV (active personal vehicle) or more specific HMSR (Human-Machine Synergy Rover) abbreviations.

The Wh/km is computed from FELA specs and standard schedule or speed profile (FTP-75 for city or NWFET for highway ~20km commute). To validate the computer model and to show that the size and weight of a vehicle are the main factors of the efficiency, Tesla and a heavier APV were modeled.

FTP-75 (Federal Test Procedure) is when a car goes 17770 m in 990m runs with 13sec intermittent stops. The first run is done at 91.2kph and the rest 17 complete test in 1874s that is at 50kph.

The first computing was for a relatively unfit guy (50kg weight, 1.6m height) who could jump up to 0.2m during 3sec - the energy applied in accelerations, and cruises were complemented under muscles’ 30W at 70rpm cadence. Total FELA efficiency in this test is 8.32Wh/km, where 1.19 Wh/km was the muscle's contribution.

When FTP-75 is computed for about the same size but a very fit guy in training mood (200W of muscle contribution), total FELA efficiency is 8.28Wh/km, where 5.49 Wh/km is of muscles.

When FTP-75 is done under “easy” 30W at 70rpm cadence by a 100kg 1.9m guy, FELA efficiency drops to 11.92Wh/km (43%), and “sweating” still increased by 25%. FELA makes it evidential cost of excessive body weight. To see a detriment from the extra curb weight of APV itself, FTP-75 was run for 290kg Raht Racer APV pedaled by the same guy. We get energy demand increased to 36.91Wh/km, only one-third of which is from muscles. So, APV weighing significantly more than 40kg is not “essentially synergetic” – human share decreases to 8% on cruise stretches. Absolute power demand jumps due to a proportional increase of all the resisting forces (rolling and transmission).

When an ordinary 77kg, 180cm, capable of 0.4m jumps in 10sec, takes 10km hwy in his 17km commute (HWFET schedule), under two “moods” 100W comfortable and 200W training, FELA would use 11.28Wh and 12.62Wh accordingly. In training mode, our guy spends 7.26Wh/km – 33 % more than “usual” commuting. Although protocols stipulate cruising speed on runs, higher acceleration or braking in aggressive mode increases average speed by only a few percent whereas your effort is doubled. It’s one thing when you read about danger and futile polluting but hyped agility of EV, and another when you feel exhausted by extra efforts for almost nothing in terms of trip time.

When our ordinary guy commutes similarly to FTP-75 protocol, FELA and he would spend 9.8Wh/km with a 50-50% split.

With FTP-75 (HWFET) and Tesla S input, our computer model yields 212Wh/km (216Wh/km). Along with validating our computer model, we see APV FELA is 212/9.8=22x efficient as Tesla.

Even though Tesla 3-4x efficient as an ICE car of comparable class, it’s almost at the same proportion overpriced, which indicates Tesla's dubious higher sustainability (margin in the auto industry is a few percent and production cost reflects embedded energy that is spent and eventually turned into pollution - one can't argue with thermodynamics).

The 20x efficiency of car-free personal mobility is only one of many benefits, so we’d better prepare ourselves mentally to live in car-free smart cities. Marketers are paid for promoting EVs, but the sooner reality replaces the jabber of marketers the less damage to the planet our children will have to control.

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