Feature Machine building, frameworks & safety Building efficient, modern vehicles


The race is on to build efficient vehicles that produce low emissions, whilst retaining the performance, safety levels and refinement that we have all come to expect in modern cars. Mike Dickison, commercial director at the Faculty of Engineering and Computing, Coventry University, looks into the challenges


cars already? After all, we know what is needed to minimise emissions: a low carbon powertrain, minimum weight, low rolling resistance and ultra-efficient aerodynamics. The issue with achieving these goals is that, whilst they are all individually technically achievable, they can also adversely influence other important basic requirements.


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Low carbon powertrains Recent developments in electric vehi- cle drive motors have created highly efficient, compact and lightweight solutions that offer a huge improve- ment over traditional electric motors (which were originally designed for powering stationary equipment, so size and weight were not important). EV motors also take up less space than the typical internal combustion engine. While costs for these new solutions


are high, this will become less of an issue as production volumes increase. Batteries, however, remain a big


ow CO2 emissions means low fuel consumption, which is a major


purchasing factor for customers. So, why aren’t we all in electric


impact crush zones. This dictates that the battery pack needs to be positioned low down in the pas- senger compartment. The lower the battery the better, as this leads to a low centre of gravity, which aids the driving dynamics. However, it also raises the occupants, resulting in a greater frontal area. So, an EV powertrain adds weight


and is detrimental to aerodynamics – factors which are important in the quest for efficiency and low emissions. Adding an auxiliary power unit (APU) connected to a generator enables an extended driving range but also adds to cost, weight and complexity. APUs are generally gasoline or diesel powered, so


their CO2 emissions need to be consid- ered. Using a fuel cell as an APU is an area under development at Coventry University, using the purpose-designed Microcab as a test bed for durability and performance evaluation.


Weight issues The weight added by a low carbon pow- ertrain needs to be lost elsewhere, ide- ally by trimmed weight from the body


‘If reliable crash avoidance systems could be incorporated,


ultra-lightweight cars that do not require robust (and hence heavy) energy absorbing crash systems could be the way forward’


challenge for car makers. The cost and weight aspects associated with the most popular technology – lithium-ion – currently increases the overall cost over the vehicle life, potentially exceeding the savings made from the reduced fuel running costs. Manufacturers have introduced battery lease and rental schemes to reduce the financial risk for buyers but this cannot eliminate the fundamental cost issue. Whilst lithium- ion technology offers


a lighter solution than lower cost lead- acid units, it is still not unusual for the battery pack to weigh in excess of 150kg. Driving range is proportional to the battery capacity (and hence weight), so to achieve an acceptable range of, say, 100 miles, a high weight is inevitable. For safety, the battery needs to be a single unit and to be well clear of


Design Solutions JUNE 2013


structure, interior and other mechanical systems. Advanced materials, including carbon fibre and magnesium alloys can yield great weight savings – at substan- tial cost. Work, however, is continuing to meet this challenge. Coventry University is working with


seven other universities on a three year research project to investigate the use of recycled lightweight materials to create a fully recyclable ultra-light body structure for future cars produced in high volume. This project considers all aspects – materials, packaging, safety and aerodynamics. Coventry’s researchers are creating the multi- materials structure, with the other uni- versities contributing in terms of the development of lightweight metallic and polymer-based recycled materials, life cycle analysis and structural


Modern vehicles need to offer low emissions, a low carbon powertrain, and ultra-efficient aerodynamics


bonding and dis-


bonding (for ease of recycling). It isn’t difficult to slash weight by


simply deleting specification. In the 1960s, Bond Cars produced the eco- nomical 875 three-wheeler model. With this, a weight of under 400kg was achieved by using a composite mono- coque construction, plastic side and rear windows, three wheels instead of four, an all-alloy powertrain and the most basic of interior specification. But, the fragile construction and raw level of refinement would not be acceptable in today’s market. However, if reliable crash avoidance


systems could be incorporated, ultra- lightweight cars that do not require robust (and hence heavy) energy absorbing crash systems could be the way forward.


Tyres Tyre manufacturers continue to work on low rolling resistance tyres with promising results, but the benefit from a thinner tyre needs to be balanced against the reduction in cornering grip capacity. The lighter the car, the lower the forces that are generated, so lightweight cars can have thinner tyres, smaller brakes and low power whilst still providing an acceptable driving experience. Furthermore, recent research projects


have demonstrated that substantial gains can be achieved in terms of drag reduction, if a car’s exterior design is optimised for efficiency with aesthetics being a second priority. By ensuring close team-working between aerody- namicists and aesthetic designer further gains will be achieved. Developing affordable, low emis-


sion cars is beset by conflicting requirements. But, if reliable crash avoidance systems can be introduced, there is great scope for further sub- stantial improvements.


Coventry University www.coventry.ac.uk


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