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Car efficiency

Pressures from regulators and environmentally conscious customers are creating cars that are more fuel efficient and easier to drive. We’re almost at the point where cars can drive themselves. Welcome to the future.

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Changing times

Remember cars that needed servicing every 1500km? Or motors that needed re-conditioning after 160,000km? Or body panels that seriously rusted out at just a few years old? When second-hand cars cost more than new? This was the local car market in 1975.

There were some cool cars from that period such as the Range Rover, the Mercedes 280E and the Triumph 2500S. But today’s cars are faster, safer, more economical and more reliable. They’re also longer-lasting and cheaper. Some of the advances over the last few decades have come from customers’ increasing environmental awareness, some are due to rising energy prices and others are the result of public-safety campaigns.

Most of all, car making has been revolutionised by vastly improved manufacturing technologies and economies of scale. Now it’s cheaper to build cars in huge highly automated factories and ship the car to various markets – rather than ship crated components to be (inefficiently) assembled closer to home.

Most cars manufactured during the 1970s had few electronic components – maybe just a car radio. The engines used an ignition system developed during the 1920s and fuel was delivered to the engine by carburettors. But the 1970s also saw the development of the low-cost micro-processor, which made cheap electronic fuel-injection possible. This has substantially improved both exhaust emissions and fuel economy. Electronic engine controls have also allowed engines to develop more power for their size.

That’s where cars have come from. But where are they going? Energy costs continue to rise and with increasing environmental awareness the pressure is on to further improve car efficiency.

Key advances

Here’s our take on 10 key technological and human factors that have made a difference.

1. Vehicle weight

The 1970s fuel crises prompted calls for car manufacturers to reduce fuel consumption. One of their responses was to make cars lighter. This saves on fuel – especially during stop-start around-town driving. But lighter cars constructed using the technology of the time resulted in poorer crash resistance: lightweight cars of the 1980s were particularly bad at protecting their occupants. Pressure to improve crash resistance led to heavier cars – a factor that makes fuel consumption worse.

The results of this can be seen in some simple comparisons. For instance, a late 1970s Honda Accord weighed 945kg; the current Accord Euro tips the scales at 1555kg. Similarly, a late 1980s Mazda 323 was 936kg and today’s equivalent (the Mazda 3) is 1322kg.

Advanced design techniques and high-strength steels keep body weight in check while maintaining strength. Aluminium and composite (mostly carbon-fibre) materials are also being increasingly used for body structures. These materials are more expensive than steel (composites considerably more so), which is why they’re being used in higher-priced vehicles. A current automotive engineering challenge is to reduce the cost of lightweight materials.

2. Wind resistance

Some 1920s and ’30s cars with upright radiators and separate headlights had less wind resistance when they were going backwards rather than forwards! More wind resistance requires more fuel. The drag caused by wind resistance increases dramatically as the vehicle’s speed increases – double the car’s speed and fuel consumption increases eight-fold. That means wind resistance is less important for city driving but much more important on the open road.

Modern car body shapes are developed in wind tunnels to make the car “more slippery” to the air passing over it.

3. Rolling resistance

Try pushing a car and its rolling resistance quickly becomes apparent. Most of this comes from the tyres. Push a car with a flat tyre and the rolling resistance from under-inflated tyres becomes obvious. The rolling resistance of tyres affects fuel consumption at any speed.

Some vehicles now have tyre-inflation warning systems that alert the driver if a tyre is under inflated. There is also an increasing number of fuel-efficient tyres. These tyres look like standard tyres, but they often differ in their materials, design, tread pattern, and performance. You can identify them through EECA’s ENERGYWISE approval mark. (See our tyre tests for information on more fuel-efficient tyres.)

4. Lighter engines

Modern design and manufacturing technologies have allowed designers to make lighter engine components. Aluminium is increasingly being used for major engine components such as engine blocks and cylinder heads, which traditionally were iron. Inlet manifolds are now commonly plastic instead of cast aluminium. All this has combined to reduce engine weight.

5. Engine “boosting”

Forced induction – sometimes referred to as “boosting” – was once the realm of aftermarket car-tuners but has now gone mainstream. Adding a turbocharger or supercharger to an engine can substantially increase engine power without adding much weight. It also increases engine efficiency (power output for fuel burned) and brings much better cruising fuel economy.

A good example of this trend is the Ford Focus. In Europe the Focus can be bought with a standard or a small turbocharged engine. The standard 1.6 litre engine develops 92kW engine power and has a combined (city + open road) fuel consumption of 6.4 litres/100km. The turbocharged 1.0 litre engine also develops 92kW but has a much lower combined fuel consumption: 5.0 litres/100km. These two engines give the Focus near-identical top speed and acceleration times. The 1.0 EcoBoost engine has won the International Engine of the Year award for the third consecutive time and is available here in some Ford Fiesta models.

6. Hybrid drive

This combines petrol and battery-electric power in one package. When a car slows down energy has to be lost. This lost energy is usually converted into heat in the brakes and released into the environment. With a hybrid car that energy is captured by an electric generator and stored in a battery. The stored energy can then be used to supplement the petrol (sometimes diesel) motor. The supplementary electric power means the petrol (or diesel) engine can be tuned for optimum fuel economy rather than power.

This energy capture and reuse means hybrid vehicles save most fuel when they’re used for stop-start city driving. The popularity of hybrids such as the Toyota Prius for city taxis is no accident.

7. More-efficient automatic transmissions

Automatic transmissions are popular because they require less driver skill and effort. But traditional automatic cars use more fuel than the same car with a manual transmission. The fuel economy of automatic cars has been improved by increasing the number of gears and incorporating a “lock-up” feature which eliminates slippage by locking the engine and transmission together for open-road cruising.

Many modern cars have dispensed with the traditional automatic transmission and instead use a continuously variable transmission (CVT). This is a belt and expanding pulley system which varies the transmission ratio continuously (rather than having the distinct steps of a manual or a traditional automatic).

Double-clutch automatic gearboxes (DSG and other acronyms used by manufacturers) are another recent development. They’re found mainly in European cars – notably vehicles produced by the Volkswagen group (such as Volkswagen, Audi, Skoda, and Seat). The system combines manual gearbox efficiency with the operation of an automatic.

8. Diesel engines

These were once largely restricted to commercial vehicles. Now – thanks to turbochargers, electronic fuel-management systems and exhaust filters for capturing diesel soot – diesel engines have undergone a transformation in power output, economy and emissions.

Diesel-powered cars are popular in Europe. But that’s not the case here, where they’re subject to a road-user charging system (RUC) that’s different from the excise tax applied to petrol vehicles (which is paid at the pump). This different charging system has tended to make small efficient diesel vehicles less attractive to run than their equivalent petrol versions. That’s unfortunate because modern small diesels drive really well, produce less atmospheric CO₂ than petrol engines, have a great range between refills, and are ideally suited to vehicles that travel big distances.

9. Electric vehicles and alternative fuels

Electric cars have been around since the early days of motoring. They’ve always lacked range rather than performance, and to some extent they still do. The key factor is their battery energy density (how much electricity they store for their weight). Early cars had very heavy lead-acid batteries; next came a move to a nickel-metal hydride; and now the cars use much lighter and more energy dense lithium-based batteries.

The development of lithium batteries continues with the car maker Tesla – in conjunction with Panasonic – recently announcing plans to build a multi-billion-dollar factory to make lithium batteries for the automotive market and possibly other uses.

The 1980s was the golden age for CNG and LPG in this country. Since then alternative fuels have become niche products. While the cheap-shale-gas revolution in the US has sparked new interest in CNG-powered vehicles, it seems unlikely that interest will resurface here.

10. Driver behaviour

How and where you drive can make a big difference in fuel economy:

  • in any driving situation, repeated severe braking and acceleration uses more fuel than smoother driving (see “Hybrid drive”)
  • highway driving means faster speeds, which uses more fuel (see “Wind resistance”)
  • stop-start driving in the city uses more fuel (See “Hybrid drive”)
  • opening windows at highway speeds means extra drag and will increase fuel use
  • running the air-conditioning with the windows open means the air con needs to work harder and uses more fuel
  • careful route planning to avoid traffic congestion and hills will minimise fuel use - a longer route may use less fuel.

Buying a fuel-efficient car

Here are some tips for selecting the most economical car.

  • Decide on the type of vehicle you really need for your lifestyle.
  • Once you’ve decided on the type, look for the most modern (safest) and most fuel-efficient option available.
  • SUVs tend to be heavier and thirstier than cars. Only buy one if you truly need the space or the off-road capability.
  • Check the vehicle’s fuel-efficiency label.
  • Turbos increase power and efficiency. A modern turbo-engined car can produce very good economy – as long as you’re gentle with your right foot.
  • Modern small diesels are super-efficient – but our system of road-user charges makes them no cheaper to run than petrol versions of the same car.

Ditch the driver

The technology already exists (in experimental form) to create driverless cars – particularly for city driving.

Imagine you have an early flight to catch. You use your smartphone to order and pay for a driverless car to take you from home to the airport. A short time later the small, electric, pod-like driverless car arrives and takes you the quickest way to the airport – guided by GPS and using built-in sensors to avoid obstacles (including other vehicles).

Other days you use the same service to commute to and from work, saving parking hassles and costs or waiting at cold draughty bus stops.

This could become a reality over the next decade or so. Driverless cars will be a disruptive technology. They will reduce the need for families to have more than one car; they will make city taxis obsolete; and they could reduce the need for peak-capacity transport infrastructure and some transport services.

Driverless cars could also be applied to motorway driving but this could be more technically difficult because the higher speeds involved mean a greater danger to passengers if something goes wrong.

In 2013 we drove the latest Honda Accord V6, which was fitted with several of the latest systems designed to reduce driver workload and so avoid accidents. Features such as radar cruise control, lane assist and proximity sensors all assist the driver – and would also be essential components of a truly driverless system.

What form such a system could take – or even whether it’s feasible – is uncertain. One vision has cars “linked-up” electronically to form a motorway train. There are two advantages to such a system: the “train” would take up less motorway area because the gap between cars would be smaller; and nose-to-tail crashes would be much less likely because as soon as the lead car brakes, the others brake.

Fuel-economy labels

Fuel-economy labels can be found on new and used cars from dealers. They’re also making their way on to online vehicle-listing sites such as Trade Me.

The fuel-use figures are useful for comparing car against car. They’re compiled from standard lab tests according to a fixed formula, so don’t expect to get these exact figures in actual driving. You might get close but you’re unlikely to match them.

Is this a problem? We don’t think so: the figures give a valid basis for comparing vehicles when buying a car.

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