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FELA Efficiency in a City

Updated: Feb 3

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, measured 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 trip effective costs to you and our environment. Those costs are proportional to a vehicle's size and weight and the designed performance. 

One can exercise modern cars' maximum performance only on runway-quality-empty roads, i.e., rarely. 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 a vehicle of conventional size and weight. 

Cars were conceived as carts where horses were 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 consequently, car-centered city planning led to car dependency. 

It's come to the marketing 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 or terrorism, 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 in the phrase "I slept in my car" as a metaphor for the depth of personal social-economic fall. Still, 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 — cars' size and weight matching their performance redundancy results in the waste of Earth resources and, less directly, in human capital degradation. 

To solve transport-induced problems, we must start with our values and express them via the user requirements document (URD) for R&D engineers or, simply put, list personal priorities for the vehicle's buyers. Without clarity here, we get sold on what we don't need for a tenfold cost, 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 resource-wasteful but dangerous, or 500km range when our daily average is 50km. In a 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 the velomobiles since their resurrection fifty years ago. Minimizing PV enables human-machine essential synergy, which justifies the APV (active personal vehicle) or, specifically, HMSR (Human-Machine Synergy Rover) abbreviations. FELA is a HMSR implementation. 

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, a 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 was done at 91.2kph and the other 17 completed tests in 1874s at 50kph. 

The first computing was for a petite guy (50kg weight, 1.6m height) who could jump up to 0.2m during 3sec - the energy needed for accelerations and cruises was complemented with muscles' work under 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 surprisingly increases to 8.28Wh/km, where 5.49 Wh/km is of muscles.

When FTP-75 is done under leisurely 30W at 70rpm cadence by a 100kg 1.9m guy, FELA efficiency drops to 11.92Wh/km (43%), and "sweating" still increases by 25%. FELA makes evident the cost of excessive body weight. The FTP-75 numerical test run for 290kg Raht Racer APV pedaled by the same guy shows a detriment from the extra curb weight of a vehicle: the energy demand is increased to 36.91 Wh/km, while only one-third of which is from muscles. So, APV weighing 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 guy, 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 muscular effort doubles. It is one thing when you read about EV's danger and futile polluting but hyped agility, and another when you feel exhausted by extra efforts for almost nothing regarding trip time.

When our ordinary guy commutes similarly to FTP-75 protocol, FELA would need 9.8Wh/km with a 50-50% machine-muscle 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 that FELA is 212/9.8=22x efficient as Tesla.

Even though Tesla is 3-4x as efficient as an ICE car of comparable class, it is almost at the same proportion overpriced, which indicates Tesla's dubious higher sustainability (the 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 intelligent 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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