Hybrid drama

8 min read

Toyota is throwing down the gauntlet to its automotive competitors with the launch in June of an SUV whose running figures rival those of cars it dwarfs. Dan Thisdell reports.

Some love them as the luxurious and practical approach to private transportation. Others loathe them as dangerous, polluting roadhogs. Nobody would call them economical. But from next month arguments over the merits or evils of the sports-utility vehicle, or SUV, will get more complicated as the latest new model hits the streets on 15 June.

Toyota calls its luxury new Lexus RX400h ‘a moving contradiction, an SUV with full-time four-wheel drive that performs like a big-engined brute, yet sips petrol like a family saloon’.

The advertisements promise luxury, fuel economy and low emissions, ‘a vehicle that offers you the best of both worlds, without asking you to sacrifice anything. A V6 engine delivers the power of a V8 while producing only a fraction of the emissions associated with a standard SUV.’

It sounds too good to be true. But the performance figures speak clearly: 34.9mpg and 192g/km CO2 emissions from a 3.3-litre V6. This compares to 23.2mpg and 288g/km CO2 from the 3-litre V6-powered RX300, which is otherwise nearly identical (a 3.2-litre V6 Vauxhall Frontera gets less than 20mpg and puts out CO2 at the rate of 344g/km).

The big difference between the two is the ‘h’ in RX400h, which stands for hybrid. By enhancing a petrol engine with an electric motor, Toyota has engineered a drive system that will lead the motor industry to offer more choice under the bonnet than it has in the 100 years since petrol internal combustion won out over steam and electric.

The Lexus is not, strictly speaking, breaking new ground. Toyota already did that with its Prius hatchback, which made its debut in Japan in 1997 and has proved popular in the USA and been voted 2005 Car of the Year by the European motoring press.

Featuring a less powerful version of the hybrid drive system, the Prius is impressively fuel efficient but many motor industry experts believe the market for environmentally friendly cars is limited; less than 10,000 were sold in Europe last year. If the big, powerful and luxurious Lexus proves popular with SUV buyers, though, Toyota’s rivals may soon have to do more than take note of this innovative technology.

According to Colin Hensley, research and development spokesman for Toyota Motor Engineering and Manufacturing Europe, ‘In future all Toyotas will be hybrids.’ He added that the Prius, the world’s first full hybrid and until the RX400h the only one in Europe, is a showcase for the technology that Toyota believes will define the automotive future, including hybrid drive, navigation, voice recognition and drive and brake-by-wire.

Prius leverages impressive results from its carefully balanced package: its fuel economy is better than 65mpg and CO2 emissions are just 104g/km on the standard European combined urbanmotorway drive cycle test, topping the league table on both counts.

For a car in the so-called D-segment — comparable in size to a Ford Mondeo (2-litre turbodiesel: 48.7mpg and 154g/km) or Vauxhall Vectra (1.9-litre turbodiesel: 49.5mpg and 154g/km) — those figures are remarkable. The only cars that come close to Prius are much smaller. Also remarkable is the overall energy efficiency of the Prius. The ‘well-to-tank’ fuel efficiency of petrol is 88 per cent, and a typical modern petrol car has a vehicle efficiency (‘tank-to-wheel’) of 16 per cent, translating into a mere 14 per cent ‘wellto- wheel’ efficiency.

For a turbodiesel the result is better, with fuel efficiency at 91 per cent and vehicle efficiency at 20 per cent, giving overall efficiency of 18 per cent. But the Prius, in its latest guise, manages 32 per cent overall by more than doubling, to 37 per cent, its petrol rivals’ tank-to-wheel efficiency.

According to Toyota, the relatively poor 50 per cent well-to-tank efficiency for deriving hydrogen from natural gas leaves even the fuel cell hybrid car lagging behind, at 29 per cent, in overall energy efficiency.

This high performance, of course, comes at a price. The Prius starts at £17,545, compared to £13,995 for Toyota’s D-segment Avensis. And while the car is apparently reliable, it is also very complicated and represents the extreme end of so-called full-hybrid design. The Prius features a stop-start system; the engine switches off when the vehicle is at rest and starts again when needed, but typically the car will set off under electric power alone.

During acceleration, the motor can assist the engine. During deceleration and braking energy that would normally be lost as heat is recovered as electrical energy and used to recharge the battery. This regenerative braking is the biggest contributor to the car’s efficiency, accounting for about a third of the energy savings.

Toyota’s Hensley added that the extremely low-drag shape of the Prius is also very important here; when decelerating energy that isn’t lost to wind drag goes into the battery. Another quarter of the energy saving comes from the electrical powermanagement system.

When conditions allow the engine to run at high efficiency (such as when cruising), the battery can be charged, but when the engine would otherwise be operating at low efficiency (during sudden acceleration) it gets help from the electric motor. A system of planetary gears splits the engine power between the wheels and the generator, which in turn charges the battery and supplies the electric motor.

Critically, this series/parallel hybrid layout means the electric motor can power the wheels while the generator is charging the battery. Electronic control of the planetary gear power splitting device, generator and motor does away with a conventional transmission, creating in effect an electronically controlled continuously variable transmission. (In a parallel layout the electric motor doubles as the generator, so the battery cannot be charged while the wheels are being driven electrically; in a simpler series layout the engine drives only the generator, which supplies the battery and electric motor, which powers the wheels.)

Another benefit of the hybrid comes to the 1.5-litre engine. This is an Atkinson cycle engine, which uses variable valve timing to change the length of the compression and exhaust cycles independently. Thermal efficiency is higher than in a traditional Otto cycle, but torque is low, a factor made up for by the electric motor. In the RX400h the hybrid system is exploited to run the engine on an Otto cycle when lots of torque is needed.

‘That’s the joy of the hybrid — you can manage the system,’ said Hensley.

Fortunately, many of the benefits of hybrid drive can be had in much less complicated systems. Late last year, Citroën put on sale a ‘Stop & Start’ version of its C3 small car. The 1.4-litre petrol engine automatically switches off when the car comes to a rest and a combined starter and alternator restarts it within 400 milliseconds when the driver relaxes pressure on the brake pedal.

An automated manual transmission does away with the clutch pedal. According to Citroën technology spokesman Nicholas Lee, the very short restart time is achieved by electronic monitoring of the exact state of the engine’s combustion cycle at the moment it stopped.

Restarting consumes enough petrol to idle for three seconds, so there is a net fuel saving whenever the car is at rest for longer.

Lee said the system improves fuel economy by six per cent on the combined drive cycle test, but if the car is used heavily in urban stop-start conditions up to 15 per cent can be saved. A diesel version would be possible, but savings would be much less; diesels, with direct fuel injection, idle more efficiently than petrol engines, which when idling must pull air past a nearly closed throttle.

Such stop-start systems are called micro hybrids. Another example is undergoing trials by Ford and British automotive engineering consultancy Ricardo. Built into a Ford Transit van, the Hy Trans is a door-to-door delivery concept that uses a stop-start system and battery charging by regenerative braking.

The stop-start system runs on 42 volts to supply enough torque to start the diesel engine.

According to Ricardo senior technology manager Nick Owen, Hy Trans saves 3.7 per cent on fuel in the combined test cycle but more than 21 per cent in door-to-door operation because delivery vans otherwise spend so much time idling. Hy Trans highlights one important way that hybrid technology may cause automotive design to fragment into alternative systems.

A spokesman at Valeo Electrical Systems, which supplies stop-start systems for Hy Trans and the Citroën C3, 42V was specified in the Hy Trans project. But Valeo’s preference would be for a 24V system. That could be achieved with two 12V batteries in series for cranking and parallel for charging, or with ultracapacitors and one 12V battery to fluctuate voltage between 24V and 14V as needed.

The result would be better fuel economy due to ‘very selective’ regenerative braking — not possible with a 12V system.

Toyota uses a 500V system for the Prius, and the quest for greater fuel efficiency may bring to a close the days of the world-standard 12V system.

Another interesting approach to hybridisation also comes from Ricardo. With heavy turbocharging, 100HP has been achieved from a 1.2-litre diesel. An electric motor built into the flywheel kicks in as needed to make up for the turbo lag. Ricardo estimates that this i-MoGen concept of using turbocharging and hybridisation to decrease diesel or petrol engine capacity could be popular in Europe, where turbocharging is well accepted.

At MIRA, the Motor Industry Research Association at Nuneaton, Warwickshire, an MG TF mid-engine roadster was boosted from 160PS (158HP) to 200PS and given all-wheel drive — and a 50/50 front-rear weight distribution — by adding electric power to the front wheels.

These are examples of ‘mild hybrids’, which fall between micro (stop-start) and full hybrids because the electric motor can assist the engine but not drive the car on its own.

This approach has been most thoroughly developed by Honda, which sells an Integrated Motor Assist (IMA) version of its Civic saloon as well as the IMAonly Insight coupé. The Civic IMA sells for about £1,000 more than its petrolonly counterpart, and improves fuel economy by about 20 per cent.

As Honda UK spokesperson Lawrence Pearce put it, the cost and benefit are on a par with diesel, and in the UK only about three percent of Civics sold are IMAs. Though Honda by its own admission doesn’t sell the IMA very hard, its market performance highlights the fact that in Europe consumers who want fuel economy have a huge choice in diesel cars.

According to Valeo, there is much interest from carmakers in micro and full hybrids, but very little in mild hybrids because the cost is too great compared to the benefits. Europe-wide, diesel sales account for nearly half the car market, so diesel is a big issue facing carmakers as they decide how much to invest in hybrid technology.

In the US, where diesel cars are rare, hybrids have been much more popular and Toyota has competition; according to automotive forecasting service JD Power-LMC, US hybrid sales were more than 80,000 last year and could top 200,000 this year, about half of which will be Prius.

But in Europe carmakers with a strong diesel range may not be quick to go hybrid. ‘Diesel’s got quite a life and it’s going to be hard to stop it,’ said Ernst & Young automotive analyst Steven Blackman. ‘I’d be slow to invest money in a factory to build hybrids.’

Richard Gane, a former automotive engineer now with PriceWaterhouse- Cooper’s automotive group, said that driving patterns are another reason Europeans aren’t as enthusiastic about hybrids. While Americans commute from suburbs into city centres, and benefit from the stop-start feature of a hybrid, most Europeans make that trip by public transport; much of their driving is motorway cruising, when a diesel’s efficiency is almost unbeatable.

Gane added that designs that stand the test of time are rarely as complex as today’s full hybrids: ‘They’re a lash-up, not an elegant engineering solution. The mechanical-electric hybrid is a dead end.’

This notion of hybrids being a ‘bridging technology’ is a recurring theme. DaimlerChrysler specifically regards hybrids as a bridge to hydrogen-powered fuel cell cars.

Gane is attracted to the simplified drive train of a fuel cell car, ‘half of which looks like a hybrid’, but in his view the big developments must come in fuel rather than how it’s used. Mixtures with hydrogen or biofuels could vastly improve the efficiency and emissions of reciprocating engines, he believes.

For Derek Charters, MIRA’s senior consultant for advanced powertrain projects, talk of hybrids as a bridging technology is a ‘bridge to nowhere’. ‘I love the dream, but let’s concentrate on the technology we’ve got. Hybrid technology is mature enough to deliver what we want to deliver,’ he said.

‘There will be a lot of different types of hybrids, with different energy converters,’ he said. Toyota’s Colin Hensley echoed that sentiment: ‘Our hybrid synergy drive is fuel neutral. You can go this way whatever fuel you use.’ Ricardo’s Nick Owen even dared to suggest that the battery-powered electric car may not be as dead as it seems. A leap forward (nanotechnology perhaps?) could make those zeroemission cars viable.

‘There will be a vast array of fuel and technology choices. The next 10 to 20 years are going to be exciting,’ he claimed. While an array of drive trains may soon confuse the simple choice between diesel and petrol, one theme may come to link them. Looking to the huge fuel savings attributed to regenerative braking,

Honda’s Pearce noted that any vehicle can boost fuel economy by 20 per cent almost by bolting something on. ‘In 10 years,’ he said, ‘I would think it would be deemed irresponsible to have a vehicle without some type of energy recovery system.’