Reflections on the potential of human power for transportation

Tuesday, January 15, 2013

The Pedal-powered Porsche and the Ultra-Efficient Car

A lot of bicycle technology was incorporated into early automobiles; ball bearings, pneumatic tires, the differential and the list goes on. The efficiency demonstrated by modern human-powered vehicles should be an example for potential improvements in car design. While the two art-piece cars on view below are seen as jokes by the car community, they serve as a departure point to discuss a possible directions for aerodynamic improvements for that most popular commuter vehicle.
I don’t recall how I stumbled on the pedal-powered Porsche. After discovering it I asked the machinist at work, who is a Porsche devotee, about it and he just laughed. It seems it is a big joke among sports-car buffs. The people at Top Gear knew all about it. Driven by Richard Hammond, it holds the record for the slowest lap on their test track.  And there is a follow-on Enzo-Ferrari-like vehicle which appears to be slightly more aerodynamic due to the fact that the Enzo is based on a formula one (F1) car layout. More about the F1 layout later…
Now clearly, both these vehicles, created by Hanes Langder, should be categorized as art pieces. Weighing 100kg and having a huge aerodynamic footprint compared to any enclosed recumbent tricycle; the vehicles cannot be expected to be even as fast as an upright mountain bike. Nevertheless, the human-powered vehicle builder in me is bothered by the laughter associated with the vehicles. Pedal-powered vehicles can be viable transportation alternatives, especially with a small power assist, but the vehicles need to be ultra efficient for 1/3rd of a horsepower (250W) to be able to move the vehicle at anything greater than a walking speed. 
In contrast to the Ferdinand and the Fahrradi, RJK pedalectric velomobile is intended to be a viable commuter vehicle. I have no idea of the weight, but the cross-section of the vehicle appears to be smaller than Herr Langder’s creations, even if the open cockpit incurs more air drag. Of interest is the RJK has front-wheel drive and rear-wheel steering, which may result in a weight reduction from a shortened drivetrain. However, the vehicle’s speed in the video, with one peddler, does not seem to be very fast.
A minimal weight, minimum cross-section, maximum streamlined recumbent bicycle, such as the Varna Tempest, can probably cruise at about 50mph given a 250W input power.
Unfortunately, low visibility, difficult entrance and egress, limited steering etc. prevent such a vehicle from being a practical for commuting. The posts on this blog have spent probably an excessive amount of verbiage discussing how to optimize a vehicle that would use the bike lanes for commuting. See “Rx for a Healthy Commute”, below. But how could the lessons learned from human-powered vehicles make a more efficient car?



These lessons are minimize air drag, minimize weight and minimize rolling resistance. This discussion will primarily focus on efforts to reduce air drag. Weight reduction is assumed to come through use of lightweight materials, space-frame and/or monocoque construction techniques and optimization of load paths that route the payload weights as directly as possible to the wheels. Narrow profile, high-pressure tires would reduce rolling resistance.
Air drag is proportional to the cross-sectional area of the vehicle, the drag coefficient of the vehicle’s shape and the cube of the vehicle’s velocity. The goal in streamlining the vehicle is to reduce the form drag, the product of the area and the drag coefficient. If the form drag gets very low, then skin drag gets significant, and it is beneficial to reduce the total-wetted surface area of the vehicle.
The conventional-rectangular four-wheel layout for cars does not lend itself to being enclosed by classic streamlined shapes.  The difficulty lies with the wheels. Early cars left the wheels exposed and concentrated on enclosing the passenger and engine compartment, the fuselage. Since the top of the wheel is moving a twice the velocity as the vehicle, a rotating wheel has more drag than a static one. When the aerodynamics of the vehicle became more of a concern, the logical approach was to use a streamlined shape (teardrop) for the fuselage and enclose each wheel in a streamlined wheel pant as well. The structure that attached the wheels to the main fuselage also needed to be streamlined, often with wing-like sections. This approach minimizes the vehicle cross-section but results in added interference drag where the various streamlined elements came in contact with each other. The typical result is similar to the 1938 Hispano Suiza Xenia shown below.

Or the contemporary Enzo Ferrari …


This aerodynamic layout, enclosing with the fuselage and wheels separately originated with early motorized carriages. I like to think of it as the F1 layout, even though F1 cars cannot enclose their wheels.
The alternative approach is what most contemporary cars employ, enclosing the entire vehicle in one or more streamlined shapes. The upside is that interference drag can be reduced while the downside is the cross-sectional area of the vehicle is increased and the aero shapes are less conventional and less efficient. If the vehicle area must expand excessively to allow the use of a shape with a low drag coefficient, the resulting vehicle may not benefit from an overall reduction in form drag. The Schlorwagen, below, has a low drag coefficient of .15 and looks very streamlined from the side view.

 


 But in the front view, the cross-sectional area of the vehicle appears to be rather large.





The track (transverse wheel spacing) for the rectangular-wheel layout is determined by the width of the fuselage and the roll-over resistance of the vehicle when cornering. If the passenger compartment can be kept narrow, for example if the seating is reduced to two people in tandem, and the vehicle is allowed to lean like a motorcycle, that the cross-sectional area can be additionally minimized. See “The Drymer and Varna Lean Forward” below.
Going to a three-wheel layout can bury one of the wheels in the fuselage and thereby improve the drag coefficient, especially if the steered wheel is buried. The downside of this approach is that, for a given rollover resistance, a three-wheel vehicle must be approximately 50% wider track than a four-wheel vehicle. This adversely increases the cross-sectional area.
For simplicity, this discussion will concern itself with the more-conventional four-wheel rectangular layout. 
The design team at Edison2, who won the Progressive Automotive X-Prize for a four-wheel, four seat vehicle that bettered 100mpg, have been refining the F1 layout with the second iteration of their Very Light Car.




Interestingly, the top view of the Very-Light Car Mk2 looks a lot like a modern version of the Hispano Suiza above albeit with a much greater degree of fender seperation and more streamlining. Edison2 is probably at the leading edge for efficient vehicle design, both in aerodynamics and lightweight vehicle construction.



One means of reducing the air drag even more than the Very-Light Car is to narrow the passenger compartment by going to two-passenger tandem seating, similar to the Messerschmitt car from the 1950’s.

 It is of interest that the earlier Messerschmitts were three-wheeled with the single-driven wheel in the back. This is a later version which employed a larger engine. The rear seat of the Messerschmitt could hold a second person or haul cargo. An interesting design feature (shared by my friend Jerry Onifer) was that the center of gravity, c.g., of the vehicle was located in the rear-seat compartment. The idea was that the vehicle’s dynamic performance would remain independent of the load that was being carried in the rear seat.
So the vehicle I envision is similar to the So-Cal Bonneville streamliner in the photo at the beginning of the article. I would use narrower tires enclosed in wheel pants and the structure connecting the wheels to the fuselage would fit within horizontal wing shaped enclosures. Assume a vehicle height of 40” (that of a low sports car). If the c.g.is lower than 20”, for a one-gee rollover resistance the track could be 40”. This is less than ½ the track of a typical car. There could be some additional benefits to having such a narrow vehicle, since it takes up less width on the road.
Once you have a design for a vehicle that is aerodynamically efficient and light weight any source of motive power will benefit from the vehicle’s efficiency. I do not believe that electric propulsion is the magic bullet that solves fossil-fuel consumption problems. If the vehicle design allows for an extended driving range with batteries, the vehicle will get exceptional gas mileage with an internal-combustion engine as well.
The above being said, I envision the tandem ultra-efficient car being propelled by a hybrid power plant. The power source would be a small gas turbine driving an electric generator. The electricity from the generator would, in turn power an electric motor located in each of the four wheels. There would be onboard batteries to provide for increased power for acceleration and regenerative braking. There will be a weight penalty for using four motors instead of using one motor with four times the power. Having a motor in each wheel also violates the suspension principle of minimizing unsprung weight. On the other hand, having motors in each wheel that can be controlled independently eliminates three mechanical differentials as allows for unprecedented traction optimization under microprocessor control. Volvo used a turbo-generator and wheel motors in a concept car during the 1990s and Jaguar used twin turbo-generators more recently.

The X-Prize-winning Very Light Car weighed 830lb., was powered by a 30kW IC engine and had a drag coefficient of .16. Could a tandem two-seat version maintain the same drag coefficient, significantly reduce the cross-sectional area, weigh less than 200lb and be powered by a 7.5kW turbo generator? And would a pair of pedals get you to a gas station if you ran out of fuel?

Hephaestus  






2 comments:

  1. Speaking of aerodynamics why didn't You mention about dimpled car? It reduces consumption of fuel by 11%: http://youtu.be/eR5SlwNf4K0

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