BRAIN BOX

It’s shorter than a Mini, carries two passengers, is fuel efficient and smart enough to be the car of the future, says David Fowler

The average vehicle in European city traffic contains just 1.2 passengers. Eight out of 10 passenger vehicles carry the driver alone, and the trend towards one-person households will make this worse. Congestion is increasing and parking spaces are at a premium. The majority of car journeys are short and for non-work related purposes. Cars spend 90% of the time parked or stuck in jams.

Conventional car design takes no account of these facts. The Smart was designed specifically to meet them. Manufacturers routinely claim to have re-invented the car concept: in the case of the Smart it may be true.

The Smart, designed in three years by Micro Compact Car (MCC), a joint venture between Daimler-Benz and SMH/Swatch, goes on sale in mainland Europe in spring 1998 and in the UK a year later. It defies convention in a number of ways: it carries only two passengers; is just 2.5m long – shorter than a Mini – and 1.45m wide. It weighs 680kg.

Through a combination of innovative features it will set new standards for fuel economy. Yet it sets out to be fun to drive, and to disprove the idea that small cars are less safe. Annual sales of 200,000 are predicted.

The maker argues that, as governments begin to embrace the idea of integrated transport planning, it could form a link between public and private transport. MCC predicts that special parking areas for Smart-size vehicles will be provided at airports, stations and shopping centres.

Smart is not what energy efficiency expert Amory Lovins describes as a hypercar, four times as efficient as a conventional car, though an even more radical hybrid version with individual electric hub motors for each wheel is under development.

But it does begin to demonstrate the compounding effect of savings as the car becomes lighter.

It will be launched with a three-cylinder 600cc turbocharged petrol engine giving fuel consumption of 4 litres/100km (70mpg); a diesel to follow is expected to hit the magic 3 litres/100km (95mpg) target. A non-turbocharged petrol engine is also planned.

The engine and automated gearbox are electronically controlled by a unified management system, with drive-by-wire technology.

Smart’s designers decided that there was no alternative to the piston engine at present. So at launch the Smart will be offered with the `Suprex’ three cylinder turbocharged petrol engine developed by MCC.

Boosting the power and torque through turbocharging made the use of a smaller and lighter power unit feasible. With sequential fuel injection and twin spark plugs, it develops 40kW (55bhp) at 5250rpm, and torque of 80Nm from 2,000 to 4,500rpm. It will accelerate from zero to 60km/h in only 6.5 seconds. For short bursts of acceleration, an `over-boost’ function can allow a temporary increase in torque of 10%.

Carbon dioxide emissions are expected to be less than 120g/km, so that the Smart will qualify for special tax treatment under Germany’s reduction of fuel consumption requirements.

The cylinder head and block are made from cast aluminium alloy, giving the engine a weight of just 59kg. Further weight reductions were achieved by a modular approach to the design of ancillary components.

The weight of the water-cooled turbocharger was halved, compared to a normal design, by integrating the turbine housing with the exhaust manifold. MCC claims it is the smallest but most efficient turbocharger in the automotive field.

Fuel economy is good because of the unit’s small capacity and low internal friction. Turbocharging also allows the engine to operate at part throttle more of the time, giving high part-load efficiency and increasing fuel economy.

Despite its light weight the engine is stiff, reducing noise and vibration. Power is transmitted to the wheels through a six-speed sequential automated gearbox with automatic clutch. To change gear the driver taps the gear lever forward to change up, or back to change down.

An electronic actuator then disengages the clutch and performs the gear change. Use of a sequential box with a cylindrical gear mechanism results in what is expected to be a simple, durable and lighter unit.

The engine and gearbox assembly is mounted transversely within the rear axle on three flexible mountings.

Development was carried out with partners Getrag for the gearbox and Fichtel & Sachs for the automated clutch.

Another innovation is the use of a specially developed integrated 32bit central control unit mounted on the air filter. This handles the engine management and gear change functions. As well as the usual engine management sensors, it receives signals from the electronic accelerator – there is no mechanical throttle linkage – and angular sensors for the crankshaft which match engine and gearbox speeds during gear changes.

MCC says integration of the electronics into one controller reduces the number of possible failure mechanisms, provides a high level of built-in diagnostics, and gives the option of having extra functions, such as drive slip control, added in the future.

Despite the Smart’s size, safety standards have not been compromised, says its maker. A stiff passenger safety cell mounted on a rigid double-skin `sandwich’ floor platform like that of the Mercedes A-Class, protects occupants against intrusion. The floor is 200mm higher than a conventional car, raising occupants above the bumper level of most cars. Major components are also located beneath the floor.

At the front, the bodyshell is strengthened by an additional cross member and high strength beams in the roof structure. Energy absorbing deformable `crash boxes’ absorb the energy of low speed crashes and prevent expensive damage to the floor structure or side members. The crash boxes bolt into place and are designed to be easily and cheaply replaced after a crash.

Because MCC has opted for a rigid passenger cell and has provided limited crumple zones, passengers are potentially subject to stronger deceleration in an accident than in a car with large crumple zones. As a result, seat belt tensioners are triggered sooner after an impact. But as a corollary, belt force limiters are needed to reduce injuries caused by the seat belt exerting too great a force on the passengers. These operate with full size airbags for driver and passenger.

With the Smart, MCC appears to have achieved its aim, in the words of Johann Tomforde, managing director for technology and design: `to prove to the European automobile industry and the discerning international customer that intelligent solutions which are, nonetheless, economical and ecological, can be offered even in the sub-compact class’.