Car Concept Design

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The OS Car will be a lightweight and aerodynamic two passenger car that will be fueled with ethanol biodiesel. The current plan is for the car to have a reverse trike configuration: two front wheels and one rear wheel which is the only driven wheel. The two main benefits of the reverse trike design are lower cost and weight due to one less wheel, and lower wind drag due to the highly tapered rear portion of the vehicle. The construction of the car will be a steel space frame covered with non-structural polymer or composite panels. The proposed powertrain is a hybrid hydraulic system which includes a single cylinder engine that runs a hydraulic pump which directly powers a hydraulic wheel motor used to drive the car and which compresses a gas in an accumulator to store energy. Gas in the accumulator will also be compressed via regenerative braking when the hydraulic motor is run in reverse to slow the car down. Cost control and quick development will be maintained by using donor vehicle parts and off-the-shelf parts. Components that cannot be easily manufactured such as brake calipers, steering knuckles, and rims will be sourced from a donor vehicle and components such as instruments, wiring, seats will be off-the-shelf components.

As the car design matures, some components from donor vehicles and off-the-shelf components can be replaced by scratch-built open source components. Also, forks can be made from the original car design for variants such a four or five passenger car, or perhaps a car with a hybrid electric powertrain.

Similar Vehicles

Some vehicles found so far that are similar to the Open Source Car concept:

Aptera 2e

Elio Motors Elio

Volkswagen XL1

Meyer Motors Duo

INGOCAR description Youtube video of news report on INGOCAR

Hydraulic Hybrid Powertrain

The OS Car powertrain will be a Hybrid Hydraulic System, making it a hybrid hydraulic vehicle (HHV). A hybrid hydraulic system was chosen over a hybrid electric system to avoid the weight of numerous lead-acid batteries or the cost of lithium-ion batteries. Since one of the objectives of the OS Car will be a long range, i.e, the capability to travel perhaps over a thousand miles without stopping to refuel or otherwise obtain more stored energy, the potential plug-in capability of a hybrid electric powertrain is not heavily valued. The basic hydraulic design for the OS Car is being modelled after the one described in Hybrid Hydraulic Drive Thesis. Here's a rough diagram of the hybrid hydraulic powertrain:


The current plan is to connect the hydraulic motor to the rear wheel with with a shaft drive which will be taken from a Honda Goldwing donor motorcycle. The hydraulic circuit will have an H-bridge design. The motor will be allowed to either drive the wheel using power produced directly from the hydraulic motor (blue line), drive the wheel using power produded directly from the accumulator (green line), or brake the wheel and regenerate pressure in the accumulator (red line) hydraulic drive.

One basic objective of the OS Car project is to exploit modularity in the powertrain design. That is, as many components as possible in the OS Car powertrain will be shared with power delivery systems on other GVCS machines. This plan is for the following powertrain components to be modularized:

  • single cylinder internal combustion engine running on alcohol. This engine will have an output from 10-20 HP and can be potentially be ganged in GVCS machines that require more power.
  • hydraulic motor
  • tubes and hoses for routing hydraulic fluid
  • accumulator

Using a fairly large accumulator would allow for good city fuel efficiency and good acceleration (highway safety).


  • The hybrid hydraulic powertrain will be designed so that the its single cylinder engine runs only at its most efficient operating condition. During most driving conditions the power required to propel the vehicle is on average less than the output of the engine at peak efficiency so the engine will only run as needed to maintain pressure in the accumulator. Regular starting and stopping of the engine will require it to have an electric starter and electronic fuel injection (EFI) so that it can be started quickly and reliably in all weather conditions. An additional benefit of an engine with EFI is higher fuel efficiency, perhaps 25% higher, over that of carbureted engines.
  • The smallest production gasoline engine with EFI found so far is the Subaru Robin "EX21 Fuel Injection" which has a maximum output of 7 HP. Its peak fuel efficiency is not provided online, so this value is assumed to occur at its peak torque operating condition, which, according to this plot, occurs at 2400 RPM where output power is only 4.8 HP. At an output power of 6 HP, engine speed is about 3100 RPM, where torque is only about 5% lower than peak torque, where efficiency is still probably pretty good at this speed, making the EX21 a good candidate for an OSCar with at cruise speed of 60 MPH. At max engine output at 4000 RPM, a top speed of only 63 MPH is predicted.
    • This engine comes with a pull start, so an electric start from a similar Subaru Robin model would have to be fitted to it.
    • It appears that an alternator is not an option on this engine, so one will have to be fitted to it.
    • The price of an EX21 with EFI could not be found online. A new EX21 without fuel injection was found priced at $381. A conservative estimate of the price of the EX21 with EFI is $600.
  • The next smallest gasoline production engine with EFI found so far is the Kohler ECH630 which has a maximum output of 19 HP. It peak fuel efficiency is also assumed to occur at its peak torque, which, according to this plot occurs at 2800 RPM where its output power is 16 HP, which is well over the 10 HP required for a 70 MPH cruise speed. At an output power of 10 HP, its speed is a bit over 1800 RPM where torque is about 7% lower than peak torque, where efficiency is also still probably pretty good, making the ECH360 a good, but oversized candidate for an OS Car with a cruise speed of 70 MPH. At max engine output at 3600 a top speed of about 88 MPH is predicted.
    • A price quote of $1607.46 plus $70 for freight was given for an ECH630 from a Kohler dealer on April 25, 2010.
  • Although, the EX21 and ECH360 are designed to run on gasoline, it's assumed that the primary fuel for the OS Car will be ethanol.
    • Technical requirements for adapting these engines to run on pure ethanol or at least E85 are unknown. At the very least calibration of the ignition and fuel injection systems would probably have to change.
    • One hope is that a variant of the ECH630 that's compatible with E85 fuel will eventually be |available.

Fuel Efficiency Calculations

Here's the file that we're using to make rough fuel efficiency predictions: File:Fuel efficiency.xls

  • A rough calculation for fuel efficiency for a maximum cruise speed of 60 MPH is about 130 MPG. An engine designed to run at peak efficiency at 6 HP would be required for this max cruise speed.
  • A rough calculation for fuel efficiency for a maximum cruise speed of 70 MPH is about 100 MPG. An engine designed to run at peak efficiency at 10 HP would be required for this max cruise speed.


kind of like the Bike Revolution Mungo

The current plan is for the car to be a reverse trike with two wheels up front and one wheel in back. The benefits to a reverse trike are: less weight due to one less wheel, less variable cost due to one less wheel, and better aerodynamics due to the tapered rear end that can be achieved. Federal regulation requirements are much less onerous for a three-wheeled car than a conventional car which allows for a much lower cost to develop the vehicle. Many state regulations might also be avoided since many/most states treat a three-wheeled vehicle as a motorcycle (Need to research this.) Using a three-wheel design would allow for the back wheel to be directly driven which would obviate a differential, shafts, etc. See the Aptera for an example of a car with a shape that fully exploits the reverse trike configuration:

  • Frame

The frame should be a space frame, probably composed mostly of 4130 or cheaper steel tubes. Round tubing is probably better than square or rectangular tubing for most of the vehicle since race experience has shown that it is less likely to yield which makes vehicle ride more consistent over time.

  • Running Gear

The current plan is to use most of the running gear: tires, wheels, brake rotors, brake calipers, steering knuckles, steering rack from an NA Mazda Miata donor car on the OS Car.

    • Steering

We need to decide whether the car has front or rear wheel steering. I favor front wheel steering because it will make design of the car easier and probably cheaper because it will allow for more off-the-shelf parts to be used. I also think that front wheel steering will allow for better collision avoidance since, unlike rear wheel steering, it would not cause the rear of the car to initially move toward an object which a driver is trying to avoid. Handling at high speeds and extreme manoeuvres is probably also worse with front steering if the the info here applies to reverse trike vehicles.

We have so far decided to use front wheel steering rather than rear wheel steering to avoid possible controls issues with rear wheel steering and to avoid the task of designing a novel steering mechanism for the rear wheel. The current plan is to use the steering rack from the same Miata donor car that provides the wheels, brake rotors, brake calipers, steering knuckles, and other parts.

    • Suspension

Front: An unequal length double A-arm suspension will be used to achieve optimal control. The current plan is to use rotors, wheel hubs, and steering knuckles from a NA Mazda Miata for the front wheels. The Miata is a the best donor vehicle candidate that I found for front suspension parts because it's a rear wheel drive vehicle like the OS Car will be, the suspension has an unequal length double A-arm design which is optimal for performance, and because the hubs have a 4 x 100 mm stud pattern which allows for a wide array of choices for rims and tires.

Rear: The current plan is to use the swing arm, spring and shock absorber from a motorcycle. An automobile wheel is preferred over a motorcycle wheel but it's unknown at this point how practical it would be to mate one to the motorcycle parts. I'm currently leaning toward a Goldwing as a donor because an older donor can be found for a good price and because they have shaft drives which are quieter and require less maintenance than belt or chain drives. --Crank 17:04, 13 June 2011 (PDT)

    • Wheels

The current plans is to use 15 inch wheels with a 100x4 bolt pattern, probably from the NA Miata donor vehicle.

    • Tires

The tires will be obtained from the donor car or low rolling resistance tires will be purchased if funding exists.


  • The body will be composed mostly of of non-structural panels that will bolt to the frame via soft mounts.
  • The body will have a the desired low wind drag teardrop shape which is only possible with the reverse trike layout and exploited on several other ultra energy/fuel-efficient vehicles such as the Aptera, Urbee, and the NmG.
  • RepRap machines or modified larger versions could be used to produce the body panels for the OSCar, but it's unknown how resilient these panels would be to things like bumping and extreme heat and cold. If we find that RepRapped panels are not durable enough, then perhaps RepRap machines could be used to produce forms from which fiberglass or carbon fiber composite body panels would be produced.
  • The target drag coefficient of the OS Car is 0.15 and comparable to that of the Aptera and Urbee.
  • Plastic windows are preferred due to their lower weight over glass windows, but the windshield may have to be made of glass due to scratching problems with plastic.


It would be nice if, rather than the car be one chassis that accepts different components, it was a family of subassemblies that all came together to make a car. For example, four "suspension" subassemblies could be bolted onto a Power Cube to make a 4-wheeler, or the same four "suspension" subassemblies could be bolted onto a couple Power Cubes and a cabin to make a car, or even bolted onto a flatbed to make a light pickup truck.

An example of the varying sizes of vehicles that could be produced with the same basic components:

Utility ATV
Side by Side ATV

Utility ATV examples

* Honda Four Trax Recon independent double wishbone has 5.1" of travel; 22" tires; Length 75.0 inches; Width 40.7 inches; Height 42.1 inches; Wheelbase 44.5 inches; Seat Height 31.2 inches; Ground Clearance 6.0 inches; Turning Radius 8.5 feet; Curb Weight 432 pounds (Includes all standard equipment, required fluids and a full tank of fuel-ready to ride); starts at $3900
* The Four Trax Rancher AT is more expensive at $7000. It has 6.3" of travel; 24" tires; 46.1" width; 45.8" height; 49.4" wheelbase; 32.4" seat height; 9.1" ground clearance; 10.5' turning radius; 623lb curb weight;