Weight Reduct ion-

is it a Valid Concept?

DR. HOWARD HORNFELD Consultex, SA.

Much has been made in recent years of--on standard (in preference to auto- the use of lightweight materials to reduce the fuel consumption of auto- mobiles. Dr. Howard Hornfeld, a plastics consultant with Consultex SA questions whether this is now a cost- effective philosophy.

This paper attempts to put into perspective some of the current ideas about the value of weight reduction and the use of lightweight materials (e.g. plastics) to achieve it. Since 1973 the world became markedly aware of the need to use more efficiently one of its most prized raw materials - energy. At that time oil was the symbol of that awareness, but in the decade since then, the precept of energy conser- vation has become more general than simple consideration of oil.

One of the most flagrant wasters of energy in 1973 was the automobile. Attacks upon an invention which, only the day before had been prized, became commonplace.

The initial approach to saving gasoline was simply to decrease the average mileage each existing car travelled, by car pooling and (in some parts of the world) car-less Sundays. This helped a little, but the incon- "venience was unacceptable to the mass of the car-owning population. There are no car-less Sundays now.

Improved mass-transit was con- sidered to be the most useful technique to decrease car usage and numerous metros, subways (underground rail- ways), U-Bahn and S-Bahn systems were built. These are widely used in Europe, but are not very successful in the US.

More efficient driving habits, in- cluding lower driving speeds, emphasis

matic) transmissions, reduced air con- ditioner use, etc. can also contribute to improved fuel economy.

But of course the ultimate burden of effective use of fuel in cars rests with the car makers; they must design a box to carry one or more persons comfortably, safely, quickly and cheaply from one place to another using a minimum amount of energy. It is necessary to determine where that energy is used, whether improve- ments in that area are practical and how significant they can be.

Driving habits The ineffective attempts at changing

driving habits by the introduction of car-less Sundays and car pools have already been acknowledged. While slower driving seems to have caught on in the US, it has not in Europe. The opposite result is found with the case for standard transmissions, but much of that is due to previous history.

In any case, very little has develop- ed in terms of changed driving habits to affect energy usage and, if anything, more mileage per vehicle per year is to

be expected with continuing growth in urban radius.

Fuel options Until a few years ago - and in most

countries still today - virtually the only energy source for cars was gaso- line. Several alternative fuels now exist, some of which are able to offer more energy per unit volume. LPG powers some 10% of the cars in Holland, gasohol is widespread in Germany, diesel fuel is about 10-15%

more energetic than petrol. Hydrogen has three times the energy of hydro- carbons. Ultimately the electric car, powered with lightweight, small power packs, charged from central electric plants will probably be the most efficient. It is clear that very important improvements in extracting the usable energy from any source will signific- antly affect the fuel economy of the car of the future.

Combustion engine characteristics While it is true that hot engines run

more efficiently than cold engines, it is clear that energy is wasted in heat- ing - and cooling - the engine. As a first step, more efficient use of that waste energy by recycling must be made. A recent study has shown that up to 78% of the energy provided by petrol is used for heating, cooling and running accessories, with only 22% used for actually pushing the car along. Clearly improved energy con- version could considerably improve fuel efficiency. There are many developments with alternative com- bustion cycles, with diesels probably the most widely used. But diesels operate under high compression and are therefore heavier than conventional engines. The Mercedes-Benz 200D weighs about 35kg more than its conventional counterpart.

Drive train The energy lost in transferring

combustion energy to kinetic energy through a heavy drive shaft and cumbersome drive train is already under attack, simply by shortening the drive train and removing the main drive shaft by the use of front wheel

728 MATERIALS & DESIGN, Vol. 4 APRIL/MAY 1983

drive. Although there is some trend to 4-wheel drive in certain markets (e.g. Switzerland) which decreases the efficiency, the major trend is towards front-wheel drive, with front and cross-mounted engines. These improvements are expected to add significantly to fuel efficiency.

Tyre performance Since tyres were first invented,

a number of technical developments have occurred: the introduction of synthetic rubbers, the use of numer- ous new belting materials (including high performance aramid fibres), the

changeover to radial design, tubeless systems, self-sealing puncture-proof tyres and attempts to injection mould complete tyres.

In the present context, considerable efforts are in progress to decrease rolling resistance without loss wet- grip, by adopting new tread profiles and tyre cross-sections. Decreased rolling resistance plays a vital role in fuel efficiency; a recent publication states that a 10% decrease in rolling resistance provides a 3% increase in fuel efficiency. A 20% improvement, which would amount to a saving of about I0 pence (UK) per gallon at

present prices, is considered perfectly feasible.

Rolling resistance would also be affected by improvements on the other side of the interface - the road surface. One of the most important functions of a tyre's tread is the removal of water between the tyre and the road surface; improved road surface would also assist in that job. At the same time frictional energy loss between tyre and road could be reduced without loss of wet-grip. A recent study has shown that improved road surfaces could provide up to 2% improved fuel efficiency with no decrease in safety.

Alternative Fuels for IC Engines Although the Diesel cycle is in

theory less efficient than the Otto cycle, the ability of the diesel engine to work at a higher compression ratio and with a leaner air/fuel ratio gives it a better thermal ef- ficiency in practice. Moreover, the fact that it runs at "full throttle" all the time makes the fuel cons- umption benefits which it provides particularly evident under low duty, stop/go, urban operation. So there is clearly scope for energy conser- vation by increased use of diesel engines in transport.

However, the diesel also has a number of fundamental disadvant- ages, such as lower power/weight ratio, less smooth running, noise, smell and smoke. It also faces a major obstacle in the form of the USA regulations on exhaust parti- culate and nitrogen dioxide emis- sions. Moreover, scope for improving the thermal efficiency of the spark ignition engine, say up to 20%, is considerably greater than for the diesel. So, even if small, direct in- jection diesels can be developed, which should be more efficient than the indirect injection pre- chamber type, future prospects do appear to favour the spark ignition engine.

Liquid Petroleum Gas (LPG) is an excellent fuel for spark ignition engines, which has a high octance number and offers exceptionally smooth driveability performance. However, although an LPG car is cheaper to run than a conventional gasoline-engined car, it is more expen- sive to buy; therefore, like the

diesel engine, it will be most attract- ive to the high mileage user. Its market penetration may reach 5% within the next 10 years in Europe, although it is well above this level in the Netherlands and Italy.

Alcohol Fuels can be brewed from all manner of crops including pot- atoes, rye, cereals, sugar cane, wood, straw and almost anything capable of basically storing solar energy in a converted chemical form. Ethanol is lighter than water with a specific gravity, close to that of petrol, of 0.794. It differs from petrol in being fully miscible with water in all proportions. Methanol has similar characteristics, but is obtained from the destructive distillation of wood or fossil fuels such as coal; it can also be made by synthesis from carbon monoxide and hydrogen in a cata- lytic process.

As motor fuels, both types of alcohol suffer from inherent dis- advantages which make them un- attractive. These include an energy density around half that of petrol (for methanol) and two thirds (for ethanol). There are corrosive effects on the materials used in today's engines and fuel systems (see Vol. 3 No. 2), poor volatility (which causes starting difficulties in cold climates and the water affinity.

Many countries are considering the use of alcohol to ease the economic pressures caused by oil imports. This usage could take the form of either 100 per cent sub- stitution or the mixing of alcohol with petrol to stretch oil supplies.

Ford is actively engaged in develop- ing vehicles for both proposals.

The disadvantage of blended alcohol in design terms stems from the difference between its ratio of air required for chemically perfect combustion and that for petrol. In strategic and practical terms, Ford feels that there is a limit of 5% alcohol: bearing in mind that a 15% alcohol blend does not save 15% petrol, but only between 6.8 and 9.4% and that sudden upsurge in demand for fuel crops could have far-reaching effects on world food prices, alcohol strengths greater than 5% should be treated with caution.

Hydrogen is a very attractive fuel from the combustion point of view. It is difficult to handle and store as a gas, however, although there have been major advances in the development of metal hybrides (metal alloys which can store hydro- gen between their molecules and release the gas when heated). In the longer term, hydrogen holds high possibilities once the many problems of production, distribution and usage have been solved. It also forms the bases for several possible fluid synthetic fuels, including synthetic gasoline.

Sources: R.Q.E. Eden, Shell Inter- national Petroleum Company, Future Fuels for Motor Cars; Proc. Eng. Section of the British Ass. for the Advancement of Science, Annual Meeting 1980. ISBN 0 905332 14 8, International Journal of Vehicle Design, Special Publication SP1, 1982. ISBN 0 907776 00 0.

MATERIALS & DESIGN, Vol. 4 APRI l /MAY 1983 729

I{X)

80

40

ENERGY EFFICIENCY OF SYNTHETIC FUELS (ENERGY OUT/ENERGY IN)

UNIT COST OF FUEL ENERGY (1000 MILE SHIPMENT)

1 2 ~ 4, ,5

Fig. 1

:URRENT ELECTROLYSIS

I H~DROGEN FROM COAL GASOL,NE FROM O,t

Energy efficiency and unit cost of possible fuels.

Aerodynamics This is one of the most important

factors in improving fuel efficiency. Much has been done and is being done to understand and improve the aero- dynamic performance of automobiles. Aerodynamics can be influenced by both short-term and long-term deve- lopments - from the simple adding-on of a spoiler to the redesign of a car from basically shoebox to a projectile.

The aerodynamic coefficient c w of the car has become a fashionable talking point in the design factories of virtually all car makers. Most commonly heard is the desire to reduce this co- efficient from its present average of ca 0.4 (or a little less) to ca. 0.3. Less commonly heard, but probably more important,.is the effect of multiplying that coefficient by the effective frontal surface. This can result in a differing order of 'aerodynamic efficiency' when comparing different cars. For example, as seen in Table 2, the Volvo 760E has the same aerodynamic efficiency as the Citroen 2CV - despite a huge difference in their c w values.

The sleek-looking Lancia Gamma is as sleek as the Renault 5, despite its much greater frontal surface.

The effect of aerodynamic efficiency on fuel efficiency is difficult to cor- relate, as it is very dependent on aver- age driving speed. However, there has been a report on the effect of installing a rear spoiler on the VW Passat, increasing the weight of the car by 0.7kg. Its effect, however, is equivalent to having reduced the weight of the car by 23.5kg under 'standard German driving conditions'. This is a small blow-moulded rear spoiler; front spoil- ers are often said to be even more effective.

Much has been said about the lack of significant aerodynamic effect at low driving speeds, which is undoubt- edly true. What is less obvious is the increasing amount of highway driving and a certain polarisation which is developing. As more families live away from town centres, two relevant effects occur: (1) More highways are built into

town centres from surburban areas;

(2) Local shopping and entertain- ment areas arise.

Data from the USA show the in- crease in average metropolitan dia- meters from just under 1 mile in 1850 to 8 miles in 1910 and 25 miles by 1980. The population density in 1980 was about 3% of the 1850 urban density. One of the effects of these changes is the acquisition of two cars per family: one is a small, fuel- efficient town car which is driven at low speeds; the other, family (= drive to work) car, which will often be on the highway. This is admittedly a very American 1960s scenario, but it is certainly becoming reality in much of Western Europe. It is the heavier, family car which will be increasingly used mainly on the highways and which will benefit most from aero- dynamic improvements. At present, 55% of US mileage is highway driving and this ratio is rising.

Weight reduction In comparison to the factors dis-

cussed above, the Contribution made by weight reduction is minor indeed.

During the early years of the first oil crisis, auto makers felt the most efficient way to achieve higher fuel efficiency was to reduce the weight of the car without significantly chang- ing any of the comfort or design parameters. Frequently this has been

done by replacement of various heavy metal components by lightweight materials, most notably plastics. Soon the auto companies began to realise that, while indeed some weight reduct- ion was occurring, manufacturing costs were also decreasing. In many cases, replacement of the metal part by a plastic component entailed consider- ably less assembly and the installed

Table 1 Decreasing value of weight reduction to the automotive companies with time.

WEIGHT REDUCTION IS WORTH:

$2-4/lb 1974

$1/lb 1977

$0.50/lb 1978

$0.10/lb * 1982

* maximum

7 3 0 MATERIALS & DESIGN, VoL 4 APRIL /MAY 1983

Fig. 2 Automotive fuel tank moulded in HDPE (courtesy BASF); also showing filling gauge, electrical rheostat, float and suction filter, moulded in PBT and ocetal copolymer.

cost of the assembled items was therefore less. In the beginning this was an added -- almost unexpected - bonus. At that time, auto makers were prepared to pay more to reduce weight. Figures of about ~2/lb saved were considered acceptable. But times have changed: plastics are now increas- ingly used for cost reduction, not weight reduction. If the cost is not reduced, the weight reduction is al- most completely ignored (Table 1).

A classic example of this phen- omenon is the plastic petrol tank, which upon simple direct replacement of the steel tank immediately saves 4-5kg/car. The plastic item is indeed more efficient in carrying capacity and is at least as safe as the steel counter- part in a collision situation. But they were not cheaper at the time and for many years Volkswagen was virtually the only major user, and they used it on only one series of cars. Indeed, the price differential between the plastic and steel tanks was very marginal (in fact, at present they are essentially identical). It has recently been an- nounced that HDPE tanks will be installed on other major cars in the very near future, but note that it took ten years after commercialisation of the VW tank to achieve a 4-5kg saving with a well-tested direct substitution, mostly because there was no economic incentive.

On the other hand, plastic radiator

end tanks are now commonplace on European cars, from a position of nearly zero penetration five years ago. This represents a weight saving of perhaps one kilogramme, but a more significant economic saving in both manufacture and in installation costs. Similarly, valve covers in nylon are somewhat lighter than steel, or even aluminium, but they can be injection moulded more cheaply than covers fabricated in either metal.

The point is that weight reduction per se was considered important by auto makers only in the early attempts

to generate a fuel-efficient car. After several analyses it was found that the fuel economies attained with such weight reductions were marginal. The fuel saving produced by a one pound weight reduction was found to be nearly infinitessimal; depending on driving habits and patterns, the fuel savings per pound of weight reduction over a 100,000 mile lifetime is 0.9 - 1.1 US gallons, or about $1.50! In European terms, lkg weight reduct- ion corresponds to 7-9 litres in 160,O00km, on roughly SwFr 12 (equal to SwFr 1.50/yr/kg saving). In fuel terms 1 litre/yr/kg saved is a fair general rule of thumb. If every car in Europe weighed lkg less, the annual fuel savings would correspond to 2½ hours of Saudi crude pumping time; about 105 million litres or 650,000 barrels - per year! In the unlikely event of 100kg being saved, a fuel efficiency of 0.75 litres/100km would be attained.

To obtain weight savings by mat- erials substitution on conventional cars is becoming both more difficult and more dramatic. The easier sub- stitutions have already been done, such as plastic ashtrays. Exterior panel substitution, where initially small panels such as fuel filler flaps and sunroofs will be made in plastic, will be followed by major panels such as doors, fenders, bonnets and boot lids. These will be commercial on the GM P--Car due in 1984 (or there- abouts) and perhaps the Fiat VSS, Peugeot Vera and other commercial models expected by the end of this

Materials Engineering the New Vehicles

Even if weight reduction, as suggested by Dr. Hornfeld, ceases to provide a stimulus for innovative materials egineering, plenty of other stimuli remain.

The incentive for polymeric body panels will strengthen, spurred by no- damage criteria and immunity to rust. Aerodynamics favour the further adoption of profiled shapes, probably in plastics.

Alternative fuels are not without materials problems - methanol, for example, is a highly aggressive medium which poses problems for many common engineering plastics. What are its effects on steel?

The standard Brazilian production

engine for ethanol operation incorp- orates basic materials changes, which have been made to counter the increased corrosive properties of the fuel. These include tin plating of the inside of the fuel tank. With hydrogen fuels, what of hydrogen embrittlement? Plenty for the mat- erials technologist to be concerned with.

Finally that old chestnut, the Second Law of Thermodynamics, points to the need for higher temp- erature engine capability. How long will it be before the work of aerospace companies such as Rolls-Royce, on structural high temperature materials, including ceramics, can be applied to the reciprocating automobile engine?

MATERIALS & DESIGN, VoI. 4 APRIL/MAY 1983 731

Table 2 Indicators of aerodynamic efficiency.

AERODYNAMIC EFFICIENCY

e f t . frontal c w c . . . . f surface = f - "-"

Citroen GSA X -3 1.81 Talbot Matra Murena 1.77 Porsche 924 Turbo 1.76 Mazda RX-7 1.66 Renault Fuego 1.83 Fiat Panda 45 1.72 VW Scirocco 1.71 VW Polo 1.74 Ford Escort 1.86 BL Mini Metro 1.71 Mitsubishi Colt 1.60 BMW Series 5 1.95 Porsche 928S 1.95 Lancia Gamma 2.11 Renault R5L 1.73 Audi 2000 2.03 Citroen 2CV 1.65 Volvo 760E 2.16 Chevrolet Citation 1.99 Peugeot 604 2.05 BMW Series 7 2.19

0.32 0.58 0.33 0.58 0.34 0.60 0.34 0.60 0 . 3 5 0.64 0.39 0.67 0.39 0.67 0.39 0.68 0.375 0.70 0.41 0.70 0.46 0.74 0.39 0.76 0.39 0.76 0.37 0.78 0.45 0.78 0.41 0.84 0.51 0.84 0.39 0.84 0.42 0.84 0.44 0.90 0.46 1~1

decade. These would represent major weight savings (see, for example, the BL ECV3, reported in this journal in Vol III No. 6) but the driving force for these substitutions is economic, not environmental. Even these considerable weight savings projects are not being implemented for weight savings pur- poses, but for money savings. It is expected that these materials will permit cheaper and more flexible assembly and repair, while providing safe, comfortable and attractive trans-

portation. The weight reduction is an added benefit, but has almost no bearing on the motivation behind substitution.

But there is yet another hidden aspect to this weight reduction/fuel economy picture. As fuel efficiency increases, further increments become less meaningful. In improving mileage from 15 to 20 miles per gallon, a considerable benefit is enjoyed; a similar improvement from 35 to 40 miles per gallon is far less interesting,

Table 3 Decreasing economies yielded by increasing fuel efficiency.

ECONOMIES YIELDED BY INCREASING FUEL EFFICIENCY

Mileage Rate

15 mpg 20mpg

35mpg 40mpg

Gads. Used

1000 750

Savings 250

428 375

Savings 53

Gallons

Gallons

Cost

$2000 ~1500

500

$ 857 S 750

106

as seen in Table 3. The data represent a car driven 15,000 miles per year, paying US 2/gal. of gasoline. For the first 5 mpg improvement 250 gallons are saved and the cat owner spends $500 less for gasoline. =For a similar improvement of 5 mpg from 35-40 mpg, the savings are only 53.5 gallons and $107. Obviously further improve- ments are even less interesting, for strictly weight saving-fuel efficiency effects. The additional research and development efforts for decreasing fuel consumption become cost/per- formance questions, to determine where the effort will be most re- warding: in further weight reduction, or perhaps in the other areas dis- cussed above?

Fig. 3 Gearshift housing and gate components moulded in 30% glass fibre reinforced Nylon 6 (courtesy ICI).

Fuel economy is obviously still a necessary goal and efforts by all parts of the auto industry are re- quired to achieve such efficiency. It is useful, however, to note the relative benefit to be gained from improve- ments in the various factors:

10% weight reduction yields 1.5- 2% fuel economy; 10% aerodynamic improvement yields 5% fuel economy; 10% rolling resistance reduction yields 2-3% fuel economy; 20% aerodynamic improvement is roughly equivalent to a 67% weight reduction. The true reason for using alternative

materials - especially plastics - is not weight savings, but the more important cost savings, as well as the functional improvements available through the much greater design flexibility of polymeric materials.

Over a 15,000 mile (e.g. about one year's distance in the US. Assume US $2/gallon.

7 3 2 MATERIALS & DESIGN, Vol. 4 APRIL/MAY 1983