Audi A1 e-tron: Extended Range Electric Vehicle for City Driving
03 March 2010
| The A1 e-tron. Click to enlarge. |
Audi’s A1 e-tron, introduced at the Geneva Motor Show (earlier post), is a range-extended electric city concept vehicle featuring a 75 kW (102 hp) peak power electric traction motor and a single-rotor single-rotor Wankel engine coupled with an electrical generator with a charge rating of up to 15 kW.
The four-passenger, two-door mega city vehicle (MCV) car can drive 50 kilometers (31 miles) emission-free in city traffic on its 12 kWh battery pack before the range extender kicks in. The Audi A1 e-tron was designed specifically for use in the metropolitan areas of Europe and North America and in the rapidly growing megacities of Asia and South America.
Motor and power electronics. The synchronous electric motor of the Audi A1 e-tron is mounted transversely at the front of the car. Its low mounting position has a positive effect on the vehicle’s center of gravity. Continuous output is rated at 45 kW (61 hp), with peak power of 75 kW (102 hp) available in short bursts. 150 N·m (111 lb-ft) of torque is continuously available, and peak torque is 240 N·m (177 lb-ft).
The electric motor sends its power to the front wheels via a single-speed transmission. The retractable selector lever on the console of the center tunnel used to choose between “Drive,” “Reverse,” and “Neutral” was taken from the first Audi e-tron.
The power electronics are mounted in the engine compartment above the electric motor. The most important components are the pulse-controlled inverter, which serves as the controller between the electric motor and the battery; the DC converter, which connects the high-voltage network with the 14 volt electrical system; a breaker unit to protect the high-voltage components; and the charging module.
Audi has developed a proprietary thermal management system to keep the battery, the electric motor, and the power electronics within their respective ideal temperature windows.
The socket for the standard charging plug is located behind the rings in the single-frame grille of the Audi A1 e-tron. A fully depleted battery can be recharged in approximately three hours from the 380 volt grid. A display immediately adjacent to the plug-in connection shows the current charge status and the charging time remaining.
The concept of the innovative Mega City Vehicle requires the electrification of key auxiliaries. The refrigerant compressor of the climate control system, for example, is electrically powered by a high-voltage electric motor that supplies only the amount of power needed at the time.
This increases system efficiency substantially compared to conventional concepts. Via a special circuit, the climate control loop also functions as a heat pump that regulates the temperature of the cabin and the battery.
The power steering of the Audi A1 e-tron is electro-mechanical. An electronic brake system makes it possible to tap into the recuperation potential of the electric motors. A hydraulic fixed-caliper brake is mounted on the front axle, with two novel electrically-actuated floating-caliper brakes mounted on the rear axle. These floating calipers are actuated not by any mechanical or hydraulic transfer elements, but rather by wire (“brake by wire”). This eliminates frictional losses due to residual slip when the brakes are not being applied. In addition, the servo unit received a new, demand-controlled electric vacuum pump.
Battery pack. The energy storage unit is arranged below the floor, assisting with the center of gravity and weight distribution of the vehicle. The battery pack is shaped like a T, with the short transverse beam filling the rear section of the center tunnel and the cross-beam filling that area in front of the rear axle where the fuel tank is otherwise located.
The 380 volt lithium-ion rechargeable battery has a nominal energy content of 12 kWh. It comprises 96 prismatic cells and weighs less than 150 kilograms (331 lbs).
| The range extender engine with its cooling fins displayed under a sheet of glass in the luggage compartment. Click to enlarge. |
Range extender. The A1 e-tron concept car has a Wankel engine as a range extender, but other compact concepts are also possible. The small single-rotor Wankel has a chamber volume of 254 cc and runs at a constant 5,000 rpm in its peak efficiency window. The electronics also consider navigation data such as the destination and route profile to automatically activate the range extender as needed.
The driver can also turn the range extender on and off as necessary with the push of a button The fuel tank holds 12 liters (3.17 US gallons).
Strengths of the Wankel engine include its nearly vibration-free and quiet operation, the small dimensions, and the extremely low weight. Together with the generator, which is powered by the Wankel engine and produces 15 kW of electric power, the complete assembly weighs only around 70 kilograms (154 lbs). This weight also includes the special power electronics, the intake, exhaust, and cooling unit, plus the insulation and the subframe.
Performance. The vehicle accelerates from 0 to 100 km/h (62 mph) in 10.2 seconds and has a top speed of more than 130 km/h (81 mph). The Audi A1 e-tron can also cover longer distances if the range extender charges the battery. The extra range, which is intended primarily for interurban driving, is 200 kilometers (124 miles).
According to the draft standard, the two different operating modes yield a fuel consumption of 1.9 L/100 km (123.80 US mpg), which corresponds to CO2 emissions of 45 g/km (72 g/mile). In electric mode, there are zero local CO2 emissions.
Audi’s use of a Wankel motor as a range extender is no less than brilliant. Wankel motors have higher power to weight ratios than regular diesels and gasoline engines. Moreover, they much cost less to produce and they are low in noise and vibration. In volume I believe Audi could produce their 15kW Wankel generator for less than 1500 USD. For comparison, GM’s 53kW Volt generator probably costs 6000 USD in volume.
However, Wankel motors are not very efficient and they are not very durable so they are not used for regular cars. Nevertheless, they could be a brilliant choice for PHEVs because efficiency and durability is not important for a range extender that will only be needed to make the 20% or so of the electricity that the vehicle will use during the year in a typical drive cycle. The remaining 80% of the miles driven is coming from the lithium battery that is at least 95% efficient. A potential serious problem with Wankel engines is that they are dirty. Has Audi solved that problem? Is it EURO 5 compliant and how much does it cost to solve that problem?
Still I would not be surprised to see a huge revival for the Wankel engine in the coming years. Apart from potential emission problems it is the ideal range extender engine.
Posted by: Account Deleted | 03 March 2010 at 12:51 PM
re: emissions of Wankel
As I recall, unburned hydrocarbons are the problem with Wankels (due to elongated combustion chamber), which can be easily cleaned up by an exhaust catalytic converter these days.
Doing the math: 124 miles from 3.17 gallons = 39.1 mpg in RE mode--not bad for occasional use.
Posted by: Nick Lyons | 03 March 2010 at 01:37 PM
I favor a free-piston engine for the range extender.
Posted by: ai_vin | 03 March 2010 at 02:16 PM
City PHEVs do not require very efficient genset as range extender (an insurance against running out of electrons) would very rarely be used. The very compact, cheaper, light weight (not so clean) Wankel is therefore a very good choice.
However, the genset in highway PHEVs will be used more extensively and should be the cleanest running light weight ICE or fuel cell whenever cost comes down.
Eventually, future improved batteries with 4x to 6x the energy density will negate the need for range extenders by 2020+.
BEVs equipped with 100+ Kwh quick charge batteries will meet most requirements without a range extender.
Posted by: HarveyD | 03 March 2010 at 02:25 PM
I hope constant-speed Wankel range extenders become a trend.
I rented one from Tx-Mo-Tx years ago and never forgot the tach and purr. Besides that, there's the light weight handling of a RX-8.
Posted by: kelly | 03 March 2010 at 02:29 PM
Contrast the displacement and complexity of this range extender wankel to that of the Chevy Volt..... uh.... sorry GM.
In related news, all chryslers still get horrible mileage and they always will.
Posted by: Ole Grampa | 03 March 2010 at 02:44 PM
Nick you are right this Wankel engine does appear to be rather efficient. Assuming it drives those 124 miles in 2 hours it will produce about 30 kWh of electricity or about 10 kWh per gallon of gasoline. Gasoline contains 36.6 kWh of energy so this is 27% efficient. This is high because it includes the efficiency losses in the generator. I don’t think a regular gasoline engine can do much better than that although a diesel generator can probably do 40%.
If an exhaust catalytic converter can make this engine EURO 5 compliant then that could add 500 to 800 USD to the cost of the car. That is OK.
Posted by: Account Deleted | 03 March 2010 at 02:59 PM
Neil Young's 1959 LincVolt uses a Wankel genset, I thought he had a good idea then and this seems like an even better idea now.
http://www.lincvolt.com/
Posted by: SJC | 03 March 2010 at 03:12 PM
It is very wise to use the ultra-compact and light-weight Wankel rotary engine as range extender engine. Peak efficiency of the Wankel is acceptable. The available internal space of a car is very important and is a strong selling point, and the more compact the drive train, more internal space is available with smaller external dimensions, thereby resulting in much lower curb weight and more energy efficiency.
For 60-80% of the time, the genset is deadweight that rob efficiency of the car, so the lightness of the wankel engine will more than make up for its slightly less thermal efficiency than a piston engine.
Posted by: Roger Pham | 03 March 2010 at 03:22 PM
I hope a Wankel gets to show its stuff in a genset.
I owned an RX7 and it was a sweet setup.
A Wankel is inefficient and dirty but effiency is not very important in a genset and the exhaust is easy to post-clean.
A Wankel makes an excellent genset for a concept car for the reasons already covered plus, because:
1. It brings excitement (A Cruz engine? How dull)
2. Development and manufaturing costs are not applicable - it's a concept car.
2. Same for sales price.
3. MPG is determined by the copy writer.
Posted by: ToppaTom | 03 March 2010 at 06:48 PM
"the complete assembly weighs only around 70 kilograms (154 lbs)"
Such a compact, lightweight genset could be optional and standardized.
Buy a one-moving-part BEV cheap. Rent/install the genset if driving cross-country. Clock genset operation hours like tractor motors. Easy access/repair/rebuild (swap in a new Wankel), swap in future better batteries, etc..
Body wise, swap in future better lighter panels, customize/alter appearance, ..
In other words, negate the annual 'new' model planned obsolescence GM perfected.
One could actually have inexpensive, flexible, minimal oil usage/pollution spewing personal transportation.
Posted by: kelly | 04 March 2010 at 07:44 AM
This is an interesting engineering solution to a (probably temporary), problem. The EREVs approach is clearly superior to the pure BEV, with today’s battery technology. But the true EREV, like the Volt suffer, from a relatively marginal, but real price and cost problem.
Audi engineers answer this by building a ‘not-quite EREV’. It substitutes a tiny, lighter, gen-set incapable of maintaining charge, in a tiny, lighter, ‘A-segment’ car. So it is not a 'Charge Sustaining' mode of operation.
What it is instead is a is a 'Slower Charge Depleting’ mode of operation. This allows a smaller cheaper battery. But that battery will suffer a beating due to the occasional deep discharge. OTOH, the EU does not have an assinine CARB bureaucracy ruling, that requires a PHEV battery to be warranteed longer than a BEV battery. And longer than any drivetrain component in a conventional drivetrain.
After the plug-in mileage is drawn down, the gen-set is engaged much like a CS Volt's engine is. But it is insufficient to replenish or maintain battery charge. It merely slows that depletion, until the car must grind to a stop in 150Km. Hopefully that range is sufficient for most days of use.
Significantly, it does allow the optimization that comes from electric miles first, that substancially augments fossil mileage, in an EREV architecture.
It has a smaller battery than a BEV or an EREV, making it cheaper, and lighter. And it has an alternative to the BEVs needing a tow truck, when it runs out of juice. It has a built-in re-charger, that can operate by the side of the road to rebuild charge, without need of a tow truck. This provides and partially relieves ‘Range Anxiety”.
Posted by: Stan Peterson | 04 March 2010 at 11:03 AM
"The four-passenger, two-door mega city vehicle (MCV) car can drive 50 kilometers (31 miles) emission-free in city traffic on its 12 kWh battery pack before the range extender kicks in."
"..a single-rotor single-rotor Wankel engine coupled with an electrical generator with a charge rating of up to 15 kW."
If 50% SOC is used, like the Volt, then there's ~5 miles/kwh * 15kW => 75 miles per hour of electricity generated.
Perhaps the small 3 gallon gas tank is intended to promote plug-in electricity use. Another article stated the genset can also be manually switched on when desired.
As electric traction motors are mass produced, one can assume the motors will be much more powerful and much less expensive than the 4, 5, 6, 7.. speed transmissions presently required by ICE powertrains.
Posted by: kelly | 04 March 2010 at 01:06 PM
Audi is on the right track in terms of genset size. Smaller is better. The next step is to reduce the size of the car yet more. A city commuter does not need to carry a family. Seven-eighths of the time it only carries the driver.
As regards the comment that all Chrysler cars still get poor gas mileage comment, my 1998 5-speed Neon with 200,000 miles has averaged 38 mpg in mixed driving over the 85,000 miles I've had it. My 2000 Plymouth minivan with 4-cylinder averaged 28 mpg in mixed driving. I don't use hypermiling techniques.
Posted by: fred schumacher | 05 March 2010 at 06:27 AM
TT,
I had an RX7 once and thought it was great. The one thing I noticed was the smell of aldehyde on cold start due to incomplete combustion. The smell of paint thinner made me wonder, it is similar to what they get burning alcohol fuel in Brazil.
Posted by: SJC | 06 March 2010 at 02:08 PM