Electric Vehicle

From RC-Autopilot Wiki

Jump to: navigation, search

Contents

New: AC drive

In Oct.2007 I'm working on an AC drive system to replace the DC version which is described further on. Please see the AC upgrade page for more details. Also new: Torque Motor

Opel Corsa Project

In April 2008 I sold my Microcar and all the related components, planning to open a REVA dealership [[1]] since I was convinced that the REVA [[2]] (apart from the lead battery) was really just the product I was trying to build. Unfortunately the Indian manufacturer wasnt convinced that it was the right time or person to bring REVA to Germany, so I had to give up that project also. Luckily I was able to sell my demo REVA to a German customer with only a small loss. Today (as of September 2008), there is still no German importer for REVA cars, although there would be quite a market.

Now, in September 2008, I'm planning to convert an Opel Corsa to electric, using standard 400V industrial components. See my Opel Corsa Project page.

Designing an Electric Vehicle (EV)

Inside the Microcar

This page documents my efforts in designing and building a light electric vehicle (EV), or electric car. I started thinking about this project in mid-2005, and finally started work in April 2006. I quickly had a drivable car in July 2006, and took it for some test drives. After being distracted by another project, the EV project sort of fell asleep. Now (Feb 2007) I've decided to publish more details and hope to generate interest and gain more momentum again.

Market Survey

There seem to be 4 classes of commercially available EV's today:

  • special EV's which look very unusual and are driven practically only by enthusiasts (Twike, City-El)
  • slow and/or heavy Neighborhood Electric Vehiles (NEV's) (45km/h) which use lead batteries (usually 10kWh weighing 250kg and more), have a range of 60-70km, and have 2 seats (Zenn, Reva/G-Wiz)
  • expensive EV's (70k+ EUR) which are similar to conventional cars, and just as heavy (or much heavier with batteries) Tesla Roadster, Lightning GT
  • show EV's (drag racing, etc.) which are individually built to demonstrate EV possibilities (but not for everyday use) Suckamps, Plasmaboy

I personally would never drive a car that is limited to 45 km/h. Even in a large city the traffic flow is often 60-70 km/h so it would be quite a disturbance. On a land road, it's like driving a tractor (and actually more dangerous, because people dont realize you are so slow until they are close up - a tractor is recognized from far away). And of course highways are out of the question. Worse, in some cities (like Mainz and Frankfurt in Germany) there is a river through the middle of town and many of the bridges across are highway only - so not being allowed there can mean a long detour. (In Mainz there are only 3 bridges, 2 of them are highway ones).

Project Goal

File:Motorträgereinheit.jpg
Motor carrier made from 4mm aluminium sheet
  • min. 110km/h (ideally 130 km/h) top speed, and allowed on highways
  • min. 100km range (at 60-80km/h speeds)
  • good acceleration, especially from a standstill (not to hinder traffic at stoplights)
  • 2+2 seats (rear seats for passengers <=140cm), so mothers can take their children shopping and to school etc.
  • Retail price incl. everything (batteries, charger, VAT) under 25k EUR (ideally unter 20k EUR)
  • Vehicle class L7E (400kg w/o batteries, 15kW, speed unlimited) (actually we're aiming for 400kg incl. batteries)

Examples to work with

  • Zenn, based on Microcar chassis, weighs 544kg incl. batteries (6x200Ah/12V lead gel, about 250kg), 40km/h max, range 50km
  • Twike, 246kg incl. batteries (2x38kg NiCd packs of 280 small cells), 3/5kw motor, 0-60km/h in 9 sec, 85km/h max)

So it seems quite feasable to build something like the Zenn, based on the same chassis, but with less weight and more capacity using Lithium batteries instead of lead, and higher speed using a faster motor. So thats exactly what we're working on.

Components

Microcar with original gearbox

The list of components I used so far. NOTE: This "kit" was used in the first version built in June 2006. Currently (Sept.2007) I intend to change to an AC motor, using the original Microcar differential+gearbox, and possibly also the variator (automatic transmission), I havent decided on that yet. Why do I want to change it? Because currently the car drives, and even reaches 100km/h as expected, but the initial acceleration is very slow (starting uphill from a standstill is very slow). Looking at the math, it's not suprising: With the low gear ratio of 1:2.2 and the max motor torque of 2x45Nm, the max acceleration is 198Nm at the wheels, or 750N (or 1.5 m/s² at 500kg). A normal car accelerates with 3 m/s², making our vehicle quite sluggish at the starting line. So if I replace the DC motors (max 2300rpm at 48V, meaning the gear ratio cannot be smaller than 1:2.2 if I want to go 100km/h) with an AC motor (max 10000rpm) so I can change the gear ratio to 1:8 (like the standard gearbox is) giving much better acceleration (400Nm at the wheels, 3.4 m/s² initially).

Chassis

Microcar MC2 Preference

For the chassis I have chosen a Microcar MC2 (or "Family Edition") which has the following advantages:

  • Available from the manufacturer as an empty chassis without motor/exhaust/tank/cooler/etc.
  • Weighs about 300kg as delivered
  • Is available with rear seats (2+2 seats)
  • Is available in version for a 15kW motor which goes 115km/h and is certified in England for that speed (which should make it easy to get our electric version registered anywhere in Europe)
  • Has had EuroNCAP crash testing done and is available with driver Airbag
  • It has all the nice stuff people expect in a comfortable car: electric windows, remote control lock, etc.

Microcar MC2 Family, built for 115km/h with a 15kw/35Nm gasoline engine

Battery Charger

Batteries

14 Li-Ion Battery Cells providing 10kWh of energy

(In this picture, the batteries are occupying the trunk, but this is only for easy testing - later they will be mounted completely underneath the car, or possibly even under the hood, using none of the interior space)

(Old) Motor

DC motors with belt drive
  • 2x 4,7kW (48V/110A) permanent magnet motors, can be run with 200A (2x 8,6kW) for 10mins [PERM PMG132]

The motors drive the axles over a toothed belt (HTD 8M) with a pulley ratio of 1:2.2 (22:48 teeth).

Assuming a max motor speed of 2300 RPM, and a vehicle speed of 115km/h at an axle speed of 1150RPM, this provides a theoretical top speed of 104,5 km/h. During max acceleration (450A) the 2 motors deliver about 150Nm combined torque to the axle, providing an acceleration of about 1m/s² (which would accelerate from 0 to 100km/h in 26 seconds).

(Old) Motor Controller

  • DC motor controller (48V/450A) [3]

DC-DC-Converter

DC-DC converter mounted on firewall

After searching unsuccessfully for an affordable DC-DC-Converter which was powerful enough to completely replace an auxilary battery, I finally found a Chinese company that built one for me: 40-60VDC input, 13.8V/50A output. Okay it's cheap and it has a loud fan that runs constantly, and I screwed it directly to the chassis so you can hear the vibration of the fan on the inside - but for testing it's fine, and later on I'll just put a temp.controlled fan inside and use rubber holders to stop the noise. The cost was <100 EUR so it's well worth it. Probably the production version will still need an aux battery, to keep lights+blinkers working in an emergency, even if the main battery is not working. Also to keep the clock running, and to provide comfortable start (relays etc.)

Heater (Standheizung)

I plan to use a diesel heater to heat the inside of the car during winter. A purely electric heater would use too much electric power and not provide the necessary comfort.

Advantages of EV's over ICE (internal combustion) cars

  • Regenerative Braking converts some of the kinetic energy back into useful power, reduces brake wear, rarely need to footbrake
  • Very quiet, practically silent except for wheel/wind noise
  • no gear shifting (not even reverse) necessary, wastes less energy, less moving parts (maintainance), less weight
  • Electric motors have a wide range of speeds at which they deliver full power, combustion motors only in a narrow range of RPMs
  • Can charge (refuel) at home, or anywhere else where there is an electric outlet, never need to visit a filling station again
  • Very low maintainance: No more oil changes, no clutch, transmission, rusting mufflers, or other service-intensive parts
  • Complete independance of oil prices, shortages, etc.
  • Very low cost per driven kilometer (10kWh/100km = 1,50 EUR/100km, comparable to 1.2 litres of gasoline (April 2007)
  • Lower tax and insurance expenses
  • Electric motors (especially brushless) have only 1 moving part, compared to hundreds that a combustion engine has - nothing to break!
  • Whenever you stop, the motor stops too! No more useless noise or burning fuel at stoplights etc.
The original variator, left the motor pulley, right driven pulley

Variator

The original Microcar uses a variator from http://www.cvtech-ibc.com

Originally I was considerung using a Variator to increase inital torque, and reduce the necessary motor RPM at higher speeds. But after looking at the efficiency of belt-driven variators (under 90%) due to the high friction of the belt (which is constantly squeezed into the pulley and yanked out of it's grip again), I abandoned that idea.

Ultracapacitors

Although not planned for this project, I want to mention them briefly. Ultracapacitors are just capacitors with very high capacity (several Farads). They could be useful in vehicle design because they can be charged and discharged very quickly and without wear, and so could i.e. almost completely store braking energy (like in stop-and-go traffic) and release it again for the next acceleration. Or they could be slowly charged from the battery pack while sitting at a traffic light, so the requested power surge when accelerating does not strain the batteries so much.

A useful size might be the energy amount released when slowing down the car from 100km/h to zero. If 90% of the kinetic energy is converted to electric, and 90% of the electric back into kinetic, then in a frictionless environment, almost 80% of the braking energy could be put back into accelerating again. (Unfortunately friction makes the balance much worse, but still worthwhile).

  • E=0.5*C*U²
  • E=0.5*m*v²

550kg slowed from 100km/h to 0 releases 59Wh or 212000Ws (just like accelerating to that speed consumes this energy). Assuming deceleration in 15 seconds, would try to charge the batteries with over 14kW for 15 seconds. Assuming our 48V/200Ah battery pack, the charging current would be about 300A or 1,5C. We'd be lucky if 30% of this energy is actually stored in the batteries.

Storing 212000Ws at 48V in capacitors requires: 212000Ws = 0.5*C*48² = 1152C --> C=212000/1152 = 184F.

I just looked up a 16V/110F ultracap on Ebay, it's on sale for 300 EUR. We'd need to hook 3 in series to get 48V, but the capacitance in that series would sink to 1/3 = 36F, so we'd need 3 strings in parallel (9 caps total) to get our 110F back, and 2 more to get upto 182F, so we'd need total 15 ultracaps. 15*300 EUR = 4500 EUR, more than our 10kWh battery pack costs! So unless we find much cheaper ultracaps, this idea won't be economical.

Another example: 14F, 40V --> 80V/7F per string, 26 strings to get 180F, need 52 units. If the ebay deal goes through for the minumum bid (300 EUR for 120 units), we get our desired buffer for 150 EUR.

Of course, extra capacitors add extra weight to the car... another reason to just forget them :-)

Taxation and Insurance

Electric cars under 800kg max weight ("Zulässiges Gesamtgewicht") in Germany are tax free for the first 5 years, after that the tax is 22 EUR/year. For comparison, a 1200ccm Euro-4 "3-liter-car" (milage 3L/100km, lowest tax class for combustion cars) costs 82 EUR/year in taxes. Thats a saving of 410 EUR in 5 years, or 710 EUR in 10 years.

CO2 and Energy considerations

Closeup of the battery pack

Im Germany, electric grid power is generated by 10% renewable sources, 60% fossil fuels and 30% nuclear. The average CO2 production is 550g/kWh. Since the end customer can choose which company to get his electricity from, he can influence how much CO2 his household (and EV) uses, i.e, by selecting a purely nuclear or renewable energy source.

Driving an EV using 10kWh/100km or 0.1kWh/km thus produces 55g/km or less of CO2. In contrast, every combustion car (yes, even the hybrids!) produces between 100-500g/km.

Energy Comparison:

  • Audi A6 2.6 Avant Quattro, 1700kg curb weight, 121kW, uses 13l/100km super unleaded while traveling 180km/h (13.2l/100km daily average use in city/highway)
  • Energy stored in Super: 8,6 kWh/l --> 112kWh energy is burned every 100km (costing 13*1,30 EUR = 17 EUR)
  • efficiency combustion motor: max 35% --> energy actually used to move the car = 39kWh
  • this matches the theoretical calculation: 1,5m²*0,35cW*1,2kg/m³*0,5*(50m/s)³=40kW wind resistance at 180km/h - BUT 100km are covered in 0.56h at 180km/h so the real drain is 71kW! Due to wheel friction and larger front area?
  • with a 95% efficient electric motor, 41kWh battery power would be needed, costing 41*0,17 EUR = 7 EUR (if using a 90% efficient charger, 45.5kWh are actually drawn from the grid to charge the batteries)
  • driving electric (with no other modifications) saves 1 EUR/10km, or 100 EUR/1000km (monthly milage) or 20000 EUR/200000km (10 years car lifetime)
  • 40kWh Li-Ion batteries currently cost about 16000 EUR and weigh about 340kg. Obviously effort should be made to reduce vehicle weight as much as possible.
  • This is a rather extreme example (180km/h). The same scenario at 130km/h is much more civilized: Wind drag is down to 15kW (from 40), more than doubling the range from 100km to 250km. At 100km/h only 7kW, allowing over 550km range with the same 40kWh battery.

Links to other private projects and info sources

1 of 2 battery blocks, with cell balancers, temporarily mounted in trunk, but will easily fit underneath

EV component suppliers

DMC controller (450A version)

Commercial EV sources

  • http://www.phoenixmotorcars.com 35kWh/410kg Litium Titanate (Altairnano), 100kW/550Nm motor, 150mph, 160km range, 0-100km/h in 10 seconds, 1727kg curb, 484kg payload, Planning to start production in 2008, price for California fleet operators is 45.000 USD because of government incentives, real price estimated at more like 90.000 USD
  • http://www.teslamotors.com Planning to start production of a 100.000$ electric roadster in 2008

Video Marketing

Contact

Interested in helping, or more info? Contact me at info@leichtelektromobile.de

Personal tools