New developments to help improve fuel economy.
Every motorist wants to save money on fuel, so it’s fortunate that plenty is being done to help us. Chris Randall looks at some of the latest developments.
During the 60 years that Car
Mechanics has been published, articles on saving money on fuel have been a regular feature. From handy driving tips to advice on maintaining your car so it runs as efficiently as possible, we’ve strived to help you spend less of your hard-earned cash at the pumps.
When it comes to car-makers, ever more stringent legislation has meant their focus on improving economy and emissions has never been greater. Right now they are having to get to grips with new testing regimes – the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) is far stricter than the previous New European Driving Cycle test – and, by 2021, an EU target demanding a corporate average of just 95g/km of CO2 will present further challenges.
The recent demonisation of diesel cars hasn’t helped, plus there’s the proliferation of electric and hybrid models on the market.
So where does that leave things for the internal combustion engine? Let’s take a look at what’s being done to make every gallon go further.
Weight is one of the arch-enemies of fuel economy. As cars get bigger and more buyers are tempted by an SUV, making vehicles lighter is crucial. One of the biggest advances is in the materials used for bodyshells, with many car-makers opting for a multi-material mix that ensures excellent strength and rigidity while being much lighter in weight.
Combining the likes of high-strength and ultra-high-strength steel ( both hot- and cold-formed), aluminium, magnesium and composites such as carbon-fibre reinforced plastic can yield significant savings.
For example, at the launch of the current Astra, Vauxhall claimed that it was up to 200kg lighter than its predecessor, with the body-in-white saving of around 20% (280kg instead of 357kg), while the current Audi Q7 SUV reputedly saved a whopping 325kg overall. The recently-launched Ford Focus’s 88kg reduction compared to the outgoing model was helped by lighter window regulators reinforced with glassfibre fabric.
Streamlining isn’t exactly a new area in car development, but it is worth noting how much work is being done on mainstream cars to reduce drag. Taking the latest Ford Focus as an example, the manufacturer’s data reckons that aerodynamic improvements (along with the weight reduction) contribute to a 10% improvement in fuel efficiency. This affordable family vehicle now has a coefficient of drag of 0.273 and benefits from features such as an Active Grille Shutter, which closes to reduce drag when less cooling is required, as well as ‘air curtains’ that guide airflow across the front wheels to reduce turbulence and enhancements to underbody airflow due to panelling over the central tunnel, fuel tank and rear axle.
Integrated Starter Generator (ISG)
Becoming common on a wide range of premium and bread-and-butter models, an ISG combines the function of the starter motor and alternator. It can be known by different names – Audi calls it a belt alternator starter – and some engines also retain a conventional starter motor for use during cold starts, but they all work in essentially the same way. Connected to the engine by a belt, it has a number of benefits, including a quicker-acting stop/start system and the ability to recover energy under braking which is then fed back to the car’s electrics. That ability to supply ‘free’ energy and provide electrical assistance to the powertrain contributes to fuel economy.
ISG is also a key component in mildhybrid systems. These differ from full hybrids by not actually driving the wheels directly, but can still produce useful savings. Audi and Mercedesbenz are keen employers of such systems, but they aren’t confined to premium brands. For example, Suzuki uses the same technology to save fuel with its Smart Hybrid Vehicle by Suzuki (SHVS) set-up. Using an ISG and a separate lithiumion battery to store recovered energy, the system can harvest power from regenerative braking (up to 12kw in the case of Audi’s system), power that can then be used to assist the engine during take-off from rest and during acceleration. More complex systems also incorporate ‘coasting’ functions, often in conjunction with 48-volt electrical systems, and when combined with smaller-capacity, more efficient engines, it’s easy to see how mildhybrids can boost economy.
Although pretty much limited to more expensive models at present, this technology will be filtering down to more affordable cars in the next couple of years. Able to provide four times the power of current 12-volt systems that are considered to be at the limit of their capabilities, it’s seen as a crucial development to support technology such as mild-hybrid systems and autonomous driving, as well as powering electric turbos or superchargers and climate control compressors. Combining 48-volt systems with starter-generators also means quicker engine restarting and allows higher levels of brake energy recuperation. And being capable of higher voltages with a lower current draw, there’s less need for heavy-duty electrical components, so wiring can be thinner and lighter.
Popping the gearbox into neutral and coasting – or freewheeling – was once frowned upon because you were deemed to not be in full control of the vehicle. However, in conjunction with sophisticated automatic and dual-clutch transmissions, it has become an essential part of saving fuel. Essentially, the electronics put the gearbox into coasting mode, decoupling it from the engine and disengaging clutches to reduce mechanical drag, in some cases switching off the engine at the same time.
During this operation, the battery (sometimes a separate lithium-ion unit) supplies power to electrical equipment and the engine is restarted almost instantaneously via an ISG. In Audi’s system, the mode can be activated at speeds between 34mph and 99mph.
It’s not just mechanical improvements that save fuel. Electronics are playing an ever greater role and nowhere is that more true than with systems that help you drive more efficiently.
Audi’s Predictive Efficiency Assistant claims to reduce fuel consumption on country roads by up to 10%. It works by pulling together a number of different technologies, including adaptive cruise control, satellite navigation and traffic sign recognition (plus coasting functions in some models), taking into account factors such as speed limits, the traffic ahead and route topography. Able to adjust speed to road conditions and alert the driver to slow down for corners, junctions and roundabouts, it promotes smoother, more efficient driving.
BMW’S ECOPRO system works in a similar fashion, using a gearshift indicator and the stop-start system, as well as coasting and a route aheadassistant, to achieve fuel savings of up to 20%.
You could find cars fitted with three-cylinder engines back in the 1980s, but they’ve made a strong comeback over the last few years. With the improvements that have taken place in turbocharging and electronic management systems, a wide range of car-makers see three-cylinder engines as an ideal way of offering decent performance with impressive economy.
In the Vauxhall Astra 1.0 ECOTEC, for example, you get 0-60mph in 10.5 seconds plus a claimed 64mpg. And if you think these engines are only suitable for urban runabouts, you couldn’t be more wrong. In fact, around a dozen manufacturers offer models with three-cylinder engines, in everything from the sporty MINI to SUVS and the usefully large Ford Mondeo.
Shutting down individual cylinders to save fuel isn’t a new idea, having been found on mainstream cars since the early 2000s. Often applied to larger, multi-cylinder units, it has become more widespread in recent years with Volkswagen the first to introduce Active Cylinder Technology on fourcylinder engines back in 2012. Taking VW’S current 1.5-litre TSI petrol unit as an example, the system operates between 1400-3200rpm under lowload conditions and at speeds of up to 81mph. It’s the two inner cylinders that are deactivated (the changeover almost imperceptible, with the driver notified by ‘2 cylinder mode’ appearing in the instrument display) as a pair of actuators disengage the cam lobes from the inlet and exhaust valves for those cylinders, so the valves remain closed and fuelling is cut. VW claims the 130PS version of this engine is 10% more fuel-efficient than a comparable petrol engine.
Ford has got in on the act: the company’s process of cylinder deactivation on three-cylinder units is a world-first, with its Ecoboost unit able to disengage or re-engage a cylinder in just 14 milliseconds – as they point out, that’s 20 times faster than the blink of an eye!
Spark Controlled Compression Ignition (SPCCI)
This latest technological advance is expected to appear in 2019. Developed by Mazda for its SKYACTIVE-X engine, it combines the spark ignition of a petrol engine with the compression ignition of a diesel, flipping seamlessly between the two. It works by switching from the stoichiometric 14.7:1 air/fuel ratio to a lean-burn mixture of 29.4:1. When the engine is cold or operating at high revs, conventional spark ignition is used; in lean-burn mode (around 80% of the time, according to Mazda), the spark plug ignites a small amount of fuel-rich mixture injected directly into the cylinder during the compression stroke. Compression ignition then takes over, resulting in a more complete and powerful burn of the mixture and therefore a stronger power stroke. It promises efficiency gains of 20-30% compared to Mazda’s current SKYACTIV-G petrol range.
48-volt systems are already fitted on high-level models. The technology should filter down to affordable vehicles in the not-too-distant future.
Smart Hybrid Vehicle by Suzuki (SHVS) is a mild hybrid that recovers energy from braking, storing it in a lithium-ion battery to assist the petrol engine later.
Mazda’s new SKYACTIVE-X engine will combine the spark ignition of a petrol with the compression ignition of a diesel.
Ford’s three-cylinder engine has the ability to shut down a cylinder when circumstances allow.
BMW’S ECOPRO electric system helps achieve fuel savings of up to 20%.