14 | SIMPACK News | November 2009

CuStoMer APPlICAtIon | Nick Carpenter, Delta Motorsport

Delta Motorsport e-4 Coupe electric Car

eleCtrIC CArSEarly in the life of the automobile a battle raged between cars storing energy electrically and those carrying liquid fossil fuels. The first electric cars were built in the mid 19th century and enjoyed significant popularity because they were cleaner, quieter and much easier to use than the gasoline-fuelled vehicles that appeared towards the end of that century.In the following decades however, as the internal combustion engine was refined, roads were also improving and the network of filling stations grew. Thus, the greater range and shorter refuelling time afforded by these ever-developing fossil-fuelled cars gradually led to the demise of the electric car. Production of electric cars peaked early in the 20th century and the battle was essentially over by the late 1920s — or so we thought.

tHe CHAllenGe oF GoInG eleCtrICThe main problem with storing energy electrically has always been the low energy density (Wh per kg) of batteries relative to liquid fossil fuels. With a lead acid battery pack, the differential is around 300:1 — that is, for every 10 kg of gasoline carried on board, 3000 kg of lead acid batteries would be required to store the same quantity of energy. Regardless of how

Fig. 1: Motor installation in the front of the E-4 Coupe

DeltA MotorSPortDelta Motorsport (www.delta-motorsport.com) was formed in May 2005 by Simon Dowson and Nick Carpenter to provide design, engineering, build and project management services for the motorsport and niche vehicle sectors. Simon runs the production and operations side of the business while Nick looks after the design and technical aspects of all engineering projects. Between them they manage a team of experienced design and production engineers.

Since its inception, Delta has carried out a wide range of projects, including the design and build of 15 single-seaters and 2 two-seat race cars for the Grand Prix Masters series, engineering design for the Microcab hydrogen fuel cell-powered city car, a variety of motorsport aerodynamic development programs and a wide range of other motorsport, automotive and industrial consultancy projects.

tHe ProJeCtEarly analysis of the EV problem highlighted the disparity in energy density and thus the need to take absolute care with every last Wh of energy. At this point, the decision was made to start from a blank sheet of paper. A low drag shape was targeted, shrink-wrapped around a novel occupant layout that also kept the car’s frontal area as small as possible. Low weight — despite a 450 kg battery pack — was also going to be an important factor in achieving a range greater than usual for an EV. Detailed drive cycle analyses allowed Delta to understand the effect of each of these parameters so that balanced judgments could be made between aesthetics, performance, range, cost and many other factors.Once the fundamentals of the vehicle layout were understood, the team scoured the globe for batteries, motors, power electronics and the rest of the components necessary to build an EV. Some were available off the shelf but Delta decided

clean, quiet or environmentally-friendly battery-electric cars might seem to be, that sort of handicap was always going to limit electric vehicles (EVs) to niche applications.

There is hope for the electric car however. The latest generation of lithium batteries is over three times more energy

dense than lead acid batteries and a good electric powertrain is 3 times as efficient as an internal combustion engine powertrain (90 % rather than 30 %). So, measuring the energy available at the wheels, the ratio is now around 30:1. Although this sounds like much better news for the electric car, 300 kg lithium batteries will be required to replace every 10 kg of gasoline and this poses a huge challenge to anyone looking to build an EV.Engineering consultancy Delta Motorsport is based at the Silverstone race circuit, in the heart of the UK’s “motorsport valley”,

and early in 2007 they decided to take up this challenge. Since this time the engineers at Delta have been developing their understanding of the problems — and opportunities — that exist when designing a car that does not burn liquid fossil fuels.

Delta Motorsport is an engineering consultancy based in Silverstone in the united Kingdom. the company is run by technical director nick Carpenter and managing director Simon Dowson. Although their design skills and thought processes have been honed in the ultra-competitive world of motorsport, they thrive on the challenge of a wide range of racecar and non-racecar project work. one such project is the development of the e-4 Coupe, which is a very compact, light weight, low drag, 4-seat passenger car driven by electic motors and powered by lithium batteries.

“...300kg lithium batteries will be required to replace every 10kg of gasoline...”

SIMPACK News | November 2009 | 15

Nick Carpenter, Delta Motorsport | CuStoMer APPlICAtIon

Fig. 2: A dynamic event in SIMPACK

that others (such as the motor) would really benefit from a new approach, and this problem-solving philosophy has since given Delta the opportunity to protect (and license) the arising intellectual property.The biggest packaging headache in the car was undoubtedly finding somewhere to fit the substantial quantity of batteries, with their large mass and volume bringing a wide range of challenges. The motors, although narrow, were larger in diameter than many and thus also required careful consideration. It became immediately obvious that the “clean sheet” approach was really the only way of achieving an uncompromised EV layout. Dozens of options were explored in the coming months, and by late 2008 the fundamental package was in place — a motor for each wheel (direct-drive, but with the motors mounted inboard, see Fig. 1) and the batteries mounted in three trays underneath the car.

VeHICle DYnAMICSHaving settled on this radical layout, Delta approached INTEC Dynamics to carry out a range of analyses to support the program. Time was very tight because of the quantity of work required over the course of 2009, so a work plan was discussed that would — very quickly — evaluate design decisions and highlight any concerns with the car’s ride or handling.The first step was to define the targets for the car since the ride and handling dynamics should be driven by its target market — comfortable and “safe” for a luxury car, stiffer and more challenging for a sports car. Although Delta’s target was to be at the sporty end of the range, care was taken to ensure the characteristics were suitable for the road rather than the race track!With a target identified, analysis of some of that car’s basic characteristics was undertaken to put it in a zone where the later

practical ride & handling work could be limited to fine tuning rather than having to find solutions to unpleasant surprises.The position of the batteries below the floor and within the wheelbase challenged the team’s beliefs of what a road car could be. For example, the centre-of-gravity height is lower than that of a modern Formula 1 car, and

the central location of the batteries leads to a low yaw inertia and thus a low dynamic

index. The dynamic index (DI) of a car is a good subjective indicator of a car’s perceived agility. The DI positions the centre of rotation relative to the rear axle and as such describes the magnitude and direction of yaw rate induced slip angle of the rear tyres. A lower dynamic index generally indicates a more agile, lively feel but if it’s too low the car may become difficult to drive. The effects of the DI on the rear tyre slip angles were confirmed using SIMPACK (see Fig. 2), and the low DI means that it suits the stated objective of producing a car with a sporty feel. This information also helped the team to set specific targets for suspension tuning to ensure sufficient disturbance rejection, resulting in a car which is exciting but safe to drive near the limit.

SuSPenSIon DeSIGnThe car’s suspension and steering is — in most respects — fairly conventional, with double wishbones all round and with the steering rack located behind the front wheel centreline. Based on the initial geometry layout, the suspension geometry was tuned and compliances adjusted in an iterative process. The main focus was on achieving stable toe- and camber-change characteristics through a variety of ride and roll attitudes, as well as giving the car sufficient Ackermann. The compact nature of the car meant that one of the challenges was to achieve these characteristics with relatively short wishbones and — because of the

“Having settled on this radical layout, Delta approached

INTEC Dynamics to carry out a range of analyses to support the program”

Fig. 3: An example of some of the intermediate results that are tracked when iterating on the suspension design

Fig. 4: Typical results for a full vehicle analysis; in this case a constant radius cornering event

16 | SIMPACK News | November 2009

CuStoMer APPlICAtIon | Nick Carpenter, Delta Motorsport

four wheel drive capability — relatively little flexibility in the location of the steering rack. Unusually, the team at Delta also specified that the front subframe, wishbones, dampers and uprights should be used at the rear as well as the front. With all four wheels being driven, some of the targets (such as the scrub radius) could be carried across relatively easily. There were, however, many other elements within the brief. One of these was to maintain stable roll centres at the front and rear with an inclined axis between them (lower at the front) to build driver confidence on turning into corners.As a result it was decided not to simply translate the suspension and subframe from front to rear, but to turn it through 180-degrees at the same time — thus the right front suspension is also used for the left rear. Small adjustments were necessary to the geometry (such as those required to achieve the inclined roll axis), but these were all achieved within the design of the chassis, leaving the suspension components common from front to rear.Once the geometry was settled, the next task was to tune the compliance characteristics of the suspension bushes (see Fig. 3). Delta did not have the time or money to invest in the design and manufacture of a full set of bespoke bushes, so a proprietary top-hat section bush (available in a range of hardnesses) was chosen as the start point for the analysis. The target for this phase of work was to achieve consistent toe change characteristics during a range of braking and

cornering manoeuvres, whilst at the same time achieving a sufficiently high camber stiffness (for predictable handling) and a sufficiently low recession rate (for good impact performance). A compromise was found that achieved all the desired attributes whilst still delivering a fairly stiff suspension that will contribute to the sporty feel that is desired for the car. This was achieved with

a very limited range of suspension bushes and enabled the necessary compliances without introducing a large number of bespoke parts.

Full VeHICle AnAlYSISThe final step was to carry out analyses of several full vehicle manoeuvres to check the effects of the many design decisions on the overall handling of the car. For example, the linear understeer gradient was determined by simulating a constant radius cornering test during which the car speed is slowly increased (see Fig. 4). The linear understeer

gradient is defined as the gradient of the steering wheel angle versus lateral acceleration plot, in the linear (low lateral acceleration) region. The constant radius

test also gives a first indication of the limit behaviour of the car. In addition, transient simulations were performed to study other factors such as the ride frequencies and roll stiffnesses (see Fig. 2) and their effects on the dynamic response of the car. This combination of results allowed the team to decide on the most appropriate spring rates and anti-roll bar rates. An interesting outcome is that the desired roll stiffness distribution can be achieved with only one anti-roll bar (at the front), largely because of the low centre of gravity.

Over a very short period of time, the various steps within the multi-body ana-lysis supported many design decisions, and this enabled the program to advance quickly enough to fit within the tight time-frame. A wide variety of targets had been set at the start of the program for the various handling and ride characteristics that were felt to be desirable for this compact, sporty car, and having achieved these, the team feels confident that the car will be very enjoyable to drive.

X-PrIZeDelta expects the first car to be running before the end of 2009, and this will allow a short period of testing and development of the car’s major systems before it is shipped to North America in April 2010 to compete in the Progressive Automotive X-Prize (www.progressiveautoxprize.org). The goal of the Progressive Automotive X-Prize is to inspire a new generation of viable, super fuel-efficient vehicles that offer consumers more choices. Ten million dollars will be awarded to the teams that win a stage competition for clean, production-capable vehicles that exceed 100 MPG (miles per gallon). The Progressive Automotive X-Prize will place a major focus on affordability, safety, and the environment. It is about developing real, production-capable cars that consumers want to buy, not about science projects or concept cars.In parallel with Delta’s involvement in the X-Prize, a further five vehicles will be built to take part in a 12-month UK demonstrator project for ultra low carbon vehicles. Delta is part of a consortium of niche companies being brought together under the umbrella of EEMS (Energy Efficient Motor Sport) in the Accelerate program (www.eemsaccelerate.com/index.html).

AnD FInAllY…The project has been a great example of how SIMPACK’s versatility means that it is not only the tool of choice for many mainstream OEMs, but it can also be applied very quickly and efficiently to low-volume production cars and unique and exciting individual projects.

Fig. 7: Placement of batteries