Four prototypes and one show car, each equipped with a different drive system, demonstrate how smart will be mastering the challenges of the future. With hybrid, natural gas or electric drives — the fortwo is an economical transformation artist.

It may be DaimlerChrysler’s smallest car, but the smart fortwo definitely measures up to its big brothers when it comes to vehicle engineering. For one thing, the little two seater’s extremely strong tridion occupant cell makes it one of the safest vehicles in its segment. And the smart fortwo is an agile car offering impressive fuel economy, regardless of whether it’s equipped with a gasoline or diesel engine. In fact, the smart fortwo cdi is the top-selling “three-liter” car, with 140,000 units sold to date. The three-cylinder gasoline version also boasts very economical fuel consumption, needing only 4.7 liters of premium gasoline to travel 100 kilometers.

Finally, thanks to the intelligent design concept behind the fortwo, it can be equipped with just about any type of drive system — which is why smart engineers in Böblingen, Germany, have already built five vehicle variants based on the two-seater: three different hybrid versions, one electric variant and a natural gas-powered fortwo. Although every member of this quintet relies on a different drive concept, they do have one thing in common: Occupants in each of these alternative compact cars needn’t make sacrifices in terms of comfort or driving pleasure.

In general, however, the current smart fortwo with the cdi diesel engine remains the model of choice. Klaus Badenhausen, head of Development at smart, explains the reason for this: “As long as customers are able to get diesel at any filling station, the smart fortwo cdi — which also has relatively low operating costs — will remain the most economical alternative.”

> Ready for the streets of London

The first customers to enjoy a smart equipped with an alternative drive system are in the UK. This past summer, the smart fortwo ev, which is equipped with an electric motor, celebrated its world premiere at the British International Motor Show.

And since November 2006, selected customers in the UK have been taking delivery of about 100 of the vehicles as part of a pilot project there. The electric cars will be used primarily in fleets that operate in the London metropolitan area. And because they produce no emissions, their owners are exempt from paying the London city toll.

smart showcar crosstown: Gasoline engine plus electric motor

The smart showcar crosstown demonstrates the impressive potential that the fortwo concept offers in terms of environmentally friendly drive systems.
The combination of a standard gasoline engine (45 kW/61 hp) and a 23-kW electric motor makes the crosstown even more economical than its production counterpart, despite the fact that it’s 85 kilograms heavier.

And the crosstown’s low fuel consumption of just under four liters of premium gasoline per 100 kilometers in the New European Driving Cycle (NEDC) is approximately 15 percent lower than that offered by the conventional fortwo.

Thanks to the vehicle’s automated six-speed manual transmission, which is also installed in smart production cars, drivers of the crosstown are assured a high level of comfort as well.

The recuperation of braking energy makes a major contribution to fuel conservation in hybrid vehicles. With hybrid drive systems, the electric motor serves as a generator during braking operations, converting the kinetic energy of the rolling vehicle into electrical energy. This electricity is then stored in the vehicle’s battery and later used for acceleration.

“The greater the braking power transferred to the generator by the axle drive, the higher the charge current for the battery,” Badenhausen explains. “The hydraulic brake unit kicks in, supplementing the braking force of the electric brake. But this only happens when the braking power, summoned when the driver depresses the brake pedal, actually exceeds that of the generator. And the battery must be fully charged for the hydraulic brake to assume sole control of the braking operation.”

The interplay between the electric motor and combustion engine makes it possible to operate each of the drive units at its respective optimal efficiency level. The electric motor can be used exclusively in neighborhoods where there are very low speed limits, for example. In such a situation, the battery must always be sufficiently charged, which is why the combustion engine channels some of its output into charging the battery whenever the engine operates at its minimum performance level. Operating hybrid vehicles in this mode generates additional fuel savings.

The smart crosstown also offers added comfort features: Its start/stop function turns off the combustion engine at red lights to conserve energy. When the light turns green, the electric motor assumes control, and the gasoline engine engages as needed.

The electric drive unit also bridges interruptions of traction force that last a few tenths of a second during shifting, which ensures that drivers enjoy smooth and continuous acceleration throughout all the gears. “The idling phases of the combustion engine alone offer a fuel savings potential of between five and six percent,” explains Badenhausen. “That’s why it makes good sense to shut off the combustion engine when the vehicle is stationary or only moving slowly.”

smart fortwo cdi hybrid: Lower diesel consumption

The crosstown certainly offers impressive fuel economy, but this performance is trumped by more than a liter by the smart fortwo cdi hybrid, which is equipped with a diesel engine and electric motor and uses only 2.9 liters of fuel per 100 kilometers.

The fortwo cdi hybrid operates in much the same way as the crosstown. Its even lower fuel consumption is due to the clever interplay between the standard 30-kW diesel engine and the vehicle’s 20-kW electric motor.
As is the case with the crosstown, this diesel hybrid also has an automatic start/stop device that provides for even greater driving comfort. As soon as the vehicle is to be put into motion again after coming to a halt, the electric motor can be engaged by simply depressing the gas pedal, which produces 20 kilowatts of power and rapid acceleration. If needed, the diesel engine can go into action to support the electric motor.

Electric motors exhibit lower drive torque at high speeds, which is why the diesel engine remains in continuous operation at speeds above 70 km/h. If the driver wants to pass another car, for example, the combined output of the diesel engine and the electric motor, as a supplementary powerhouse, ensures enough power is immediately made available.

smart fortwo ev: Purely electric

Unlike the three hybrid variants, the smart fortwo ev (electric vehicle) is powered exclusively by an electric motor with an output of 30 kW (41 hp).

The advantage of this purely electric drive is that it results in operating costs per kilometer that are much lower than those for the smart fortwo equipped with a gasoline engine. The six euro cents per kilometer that the latter requires is certainly very economical, but the smart fortwo with electric drive costs just two cents per kilometer driven.

The electric compact car’s energy consumption is also reasonable: At 12 kilowatt-hours per 100 kilometers, it gives the vehicle a range of 110 kilometers, after which the battery has to be recharged. This is an acceptable figure, given the current state of battery technology. Nevertheless, it also clearly shows that electric cars are mainly suitable for short trips in urban settings. It takes only three-and-a-half hours to recharge the battery in the smart fortwo ev from 30 to 80 percent of its capacity. A complete recharge takes no more than eight hours.
Despite its range limitations, the emission-free electric smart offers superior driving performance. For example, its acceleration from zero to 60 km/h is about as fast as that of the gasoline-powered fortwo. And although the electric drive has to struggle a bit at higher speeds, it still reaches a maximum of 120 km/h.

Installation of the alternative drive system also presents no problem in the smart fortwo ev. Badenhausen describes how it’s done: “The electric motor is installed at the exact position in the rear of the vehicle where a combustion engine would otherwise be found. Also installed here is the vehicle’s transmission, which we only fitted with two gears because our drive concept doesn’t require anything more than one forward gear and a reverse gear.”

smart fortwo micro hybrid: Conserves fuel when idling

Consistently exploiting engine idling phases alone can actually reduce fuel consumption by as much as 13 percent. This has been demonstrated by the smart fortwo micro hybrid, which was built by smart engineers in cooperation with colleagues from Mercedes-Benz Development.

With this drive concept, a belt-driven starter generator serves as a starter motor and alternator. The key feature of the “micro hybrid” is a special automatic start/stop device that turns off the combustion engine — in this case a gasoline engine with an output of 45 kW (61 hp) — when the vehicle comes to a halt. The starter generator also shortens the time it takes the gasoline engine to reach the rpm level required for clutch engagement, by around 0.25 seconds. This may not sound like much, but it does noticeably reduce the relatively large amount of fuel that’s consumed when a vehicle is being driven away from a standing start.

The result is a decrease in fuel consumption of approximately five percent (New European Driving Cycle — NEDC), which means standard consumption falls from 4.7 liters to less than 4.5 liters per 100 kilometers. And the fuel savings are even greater when driving in congested urban areas, where the micro hybrid actually consumes 13 percent less gasoline.

smart fortwo cng: Natural gas for short trips

Unlike the monovalent drive systems in natural gas-powered vehicles from other manufacturers, the smart fortwo cng (compressed natural gas) offers a choice of gasoline or natural gas, because the vehicle’s combustion engine can run on either fuel. The car is therefore equipped with two fuel tanks — but that doesn’t pose any problems, thanks to the smart fortwo’s modular construction, which makes it possible to snugly fit the 12-liter gasoline tank and two 16-liter high-pressure natural gas tanks into the car’s underbody.

And drivers can even switch between the two types of fuel when the vehicle is in motion. “The transition between the two operating modes is automatic and extremely smooth,” reports Badenhausen, who bases this assessment on the many test drives that the car has been subjected to. “If the natural gas tank is nearing empty, the driver can simply continue driving to the next filling station by switching to the gasoline mode.”

With standard fuel consumption of around 3.2 kilograms of natural gas and 4.7 liters of gasoline per 100 kilometers, the smart fortwo cng has a range of 390 kilometers (138 kilometers with natural gas and 255 kilometers with gasoline). By comparison, the production model equipped with a gasoline engine has a 33-liter fuel tank and a range of just under 700 kilometers.
These numbers show that even a natural gas drive system can’t satisfy every need. But combining natural gas and gasoline operation offers big benefits for motorists who normally drive only short distances, benefits that are enhanced by the growing number of natural gas filling stations.

The five smart fortwos with alternative drive systems show that the brand can offer mass production solutions in a short period if demand warrants — and the 100 smart fortwo ev models to be used in a UK fleet test are a convincing demonstration of this ability.



The micro hybrid’s electronics use engine idling times to conserve fuel.


The cdi hybrid prototype has a display that shows the current drive mode.


Parade of vehicles: From the outside they look the same — only the rear lettering reveals the drive system that powers each of the five smarts.


The ev can easily be plugged in and recharged overnight.


The prototype’s test drives and test rig runs generate a large volume of data that is subsequently processed and analyzed on computers.


On the lift: Engineers at the smart Development Center can access various components best when working from below.


The cng can alternate between gasoline and natural gas operation.


The smart showcar crosstown has a transverse-mounted electric drive unit beneath its gasoline engine.


The smart showcar crosstown has a transverse-mounted electric drive unit beneath its gasoline engine.


Copyright © 2006, DaimlerChrysler AG