Active tilting set to roll out

The tandem two seater urban car that aims to ease congestion and have low emissions for the inner city. Justin Cunningham investigates

Mention a three-wheeled vehicle to most people and they will affectionately remember the Reliant Robin, probably with Del Boy and Rodney at the wheel. But while the idea of going around on three wheels may draw a wry smile and soon be dismissed, the concept is beginning to be seriously re-examined.

A three-year European Commission funded research project has the objective to research and evaluate a small clean vehicle for urban use. Five academic institutions carried out the design work with BMW being technical coordinators.

Delegates from Eureka magazines the recent Design Day in Nottingham will have seen result of the project, a prototype of the 'Clever' vehicle. The rather ungainly acronym stands for Compact Low Emission VEhicle for uRban transport. But the vehicle itself is rather elegant and sleek in appearance.

"The majority of vehicles on the road only have a one or two occupants, so there is clearly a need for a small, low emission vehicle," says Dr. Benjamin Drew, a research officer from the Centre for Power Transmission and Motion Control at the University of Bath. "The idea was to combine elements of both a car and motorbike, taking the advantages of both and negating the disadvantages. We needed to produce something that had the comfort and safety of a car, but that also had a minimal road footprint and low emissions."

The vehicle uses a number of technology innovations including the use of lightweight materials and an internal combustion engine that runs off natural gas. As many filling stations do not sell compressed natural gas, exchangeable 6l fuel tanks have been developed.

Additionally the chassis and full body tilt up to 45° during corning. This was the focus of the work carried out by Bath University; the tilt actuation system and the active control system.

"The reason for the tilting chassis is because we wanted to have the vehicle track no more than 1m to bring advantages to traffic congestion as well as parking," says Drew. "But that creates a rather unstable vehicle in corners. To solve the problem we decided to tilt the entire chassis to balance the lateral forces with gravity."

The usual approach to improving stability is to lower the centre of gravity and increase the track. But neither of these were an option. So the tilt system devised divided the vehicle in to two modules. The front unit consists of the front wheel and passenger compartment, and the rear unit contains the engine and transmission with the rear wheels. Fixing an actuation mechanism between the two modules allows the vehicle to ease in to corners, and eliminate handling stability issues.

The tilting mechanism uses an electronic control unit and an hydraulic actuation system. Two hydraulic cylinders power linear actuators at the interface between the two modules, set up in a 'V'.

Although a tilt-by-wire set up, there is a physical connection between the front wheel and the steering wheel meaning the vehicle would avoid issues over a steer-by-wire system.

"There are two methods of tilting a vehicle," says Drew. "Direct Tilt Control where an actuation system physically rotates one part of the vehicle against another. Or Steer Tilt Control where the vehicle is effectively free leaning.

"After some initial analysis we felt, that the most feasible method to employ within the project time-frame was Direct Tilt Control. Through simulation of the control algorithm, we developed the software necessary for the electronic control unit."

The team also ran in-depth simulation of the hydraulic system, and by combining the two models, they tuned various parameters to ensure reliable operation of the tilting system. The next stage was to build the mechanism and test it for real on the prototype.

To initiate and control the tilt angle two main methods of operation were evaluated. The first saw the tilt actuation system controlled directly by buttons on a controller and so it could be actuated while stationary. And secondly automatic, where the control unit took measurements of the driver and the vehicle, and actuated the tilting system when driving.

"We went through a process of testing firstly the steady-state handling," says Drew. "That's the ability to maintain balance while following circular paths of differing radii at various speeds. We then tested transient handling, left and right corners following each other, at various speeds. A typical transient manoeuvre is a figure of eight, or a lane change."

"While steady-state performance was good, transient performance required some improvements. Particularly with heavier drivers, the car took a long time recover from tilt compared to the input from the driver on the steering wheel. The solution was to increase both the hydraulic system pressure and the response in the control unit to make the system react quicker to transition."

BMW is currently considering the commercialisation of many 'alternative' vehicles, as are most other automotive OEMs, with several three wheeled tilting vehicles earmarked for 2010. The significant driver is that European legislation will impose limits on the average emission from any manufacturer's fleet of vehicles. Including a vehicle like Clever, which produces only 60g/km of carbon dioxide, will significantly reduce the overall average. As a comparison, the Toyota Prius produces 104g/km of CO2.

The University of Bath is continuing research into tilting vehicles to increase the sophistication of the tilting system to enable it to perform well with slippery or loose surfaces and in emergency situations.

Pointers

CLEVER is comprised of a lightweight aluminium frame clothed in plastic body panels. The frame was specifically designed to perform well in accidents, with crumple zones and collapsible components to ensure that the occupants of CLEVER are safe.

In addition to the frame, Clever includes seatbelts for both occupants and an airbag for the driver. Impact absorbing materials were also used in the cabin to reduce serious injury to the occupants.

Three prototypes were constructed specifically for crash testing purposes. The results of frontal impact, side impact and a compatibility crash test - Clever crashing into another car - ensured Clever met a 3-star score in the EuroNCap tests.

Specification at a glance
Configuration: 3-wheeled tilting vehicle made from aluminium space frame with plastic body
L: 3m
W: 1m
H: 1.35m
Seats: Tandem seat constellation
Engine: Single-cylinder 230cc natural gas engine
Weight: 395kg
Max speed: 100kph
Acceleration (0-60 kph): 7s
Range: 200 km

Author
Justin Cunningham

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