7/4/12

Overview: BMW Active Hybrid 5

BMW’s Active Hybrid 5 starts off on completely the wrong foot. It’s not that there is anything questionable about the car’s design or engineering values; rather, that there is something profoundly mistaken about fitting a petrol engine to a vehicle supposedly intended to save energy and limit environmental damage.

BMW is not, of course, alone in building petrol-electric hybrids. But while Toyota (for example) may with some justification wheel out the excuse that its cars sell on price, BMW could unquestionably sell every diesel-electric hybrid it built, should it ever choose to build them. Besides, we are being over-indulgent even with Toyota, given that PSA’s fascinating and effective Hybrid 4 models use diesel engines.

The engine at which we have pointed a critical finger is the 2979cc in-line six, familiar from the 535i. It is directly fuelled and uses a twin-scroll turbocharger. Valve timing and lift are controlled using BMW’s Valvetronic system. Headline outputs are 306PS (at a rather busy 5800rpm) and 400Nm, which is delivered between 1200rpm and 5000rpm. The commendable torque curve quite makes up for the rather high power peak.

The engine is attached to a reworked version of ZF’s 8HP automatic transmission. This incorporates an electric motor that doubles as a generator, and a clutch for decoupling the petrol engine from the rest of the drive-train.

While ZF’s packaging of the hybrid drivetrain cannot be faulted — it all, bar the battery-pack, fits into and around a largely unaltered 8HP housing — it does mean that the motor/generator cannot be very powerful. There is simply not the space for a motor capable of delivering ‘normal’ road performance by itself. Its outputs of 55PS and 210Nm — a useful torque figure, as always with an electric motor — mean that the car’s maximum speed under electric power alone is limited: it is all out at 60km/h. This makes the Five very much a ‘mild’ hybrid. The maximum driving range on electric power is ‘up to four kilometres’ at an average speed of 35km/h, reflecting the modest size of the battery-pack. BMW refers to the hybrid Five’s economy and performance in terms of the 535i, which uses the same petrol engine; make the same comparisons with the 530d or 535d and the case for the hybrid looks much less convincing.

BMW AH 5 535i 530d 535d
Kerb mass 1850 1685 1695 1725
PS/rpm 340/— 306/5800 258/4000 313/4400
Nm/rpm 450/— 400/1200 560/1500 630/1500
0-100km/h (s) 5.9 5.9 6.0 5.5
Urban MPG
(l/100km)
49.5
(5.7)
27.1
(10.4)
44.1
(6.4)
43.4
(6.5)
Combined MPG
(l/100km)
44.1
(6.4)
37.1
(7.6)
53.3
(5.3)
52.3
(5.4)
CO2 g/km 149 177 139 142
Emissions EU5 EU5 EU5 (EU6) EU5
All cars fitted with ZF 8HP transmission, whether standard or optional.

The greatest (in fact only) advantage for the hybrid over the other cars here is in slow-moving traffic. At a standstill, all of these cars shut off their engines automatically. As speeds build, the hybrid relies increasingly on its inefficient petrol engine. When the driver releases the hybrid’s brake after a stop, the car will restart using either the electric motor alone or the electric motor and the petrol engine, depending on the high-voltage battery’s charge level and on how much power the driver wants.

There is a unique driving mode — Eco Pro mode — that is designed to achieve the best possible fuel-efficiency by making more frequent use of all-electric drive than the car’s conventional driving modes would. Like them, Eco Pro mode defines the behaviour of the powertrain, including the transmission.

Although performance under electric traction alone is rather limited, the motor’s substantial torque contributes usefully to acceleration when it is deployed in series with the petrol power-plant. You must bear in mind, though, that the combined output of the drive-train is not the sum of the outputs of the two power-units. The electric motor cannot draw sufficient current from the battery-pack to maintain its nominal maximum torque output for more than a few seconds.

The lithium-ion battery-pack, which was developed specifically for the Five, is mounted behind the rear seats, eating some of the car’s luggage space. An adequate boot volume — a little more than that of a 1-series — is maintained by the unfortunate expedient of not fitting a spare wheel. The battery cooling system is integrated into the air-conditioning cooling circuit. The battery comprises 96 cells; its capacity is variously quoted by BMW as 675Wh and 1350Wh. The high-voltage system is rated at 317V, and is linked to the 12V system by a voltage transformer. Interestingly (and sensibly), the air-conditioning compressor is powered by the high-voltage system.

The energy management functions of the power electronics take care of switching the motor/generator between its two modes of operation. The lithum-ion battery-pack is charged during the overrun, unless its state of charge is deemed critical, in which case it is boosted by the petrol engine. Under acceleration, the electric motor/generator works as a motor, increasing the drive-line’s torque output.

On the overrun, the petrol engine is shut off and decoupled from the rest of the drive-train. This is ‘coasting mode’. It happens at speeds of up to 100mph in Eco Pro driving mode, or 50mph in the other modes. Perforce there is no engine-braking. A start-stop system is fitted for urban driving. Additionally, the power electronics are linked up with the standard navigation system; this theoretically allows the drive-train to anticipate road conditions ahead, though — as BMW points out — the advantage ‘may vary according to the quality of the available navigation data’. Factors that might cause a change in the car’s normal operating strategy include the topography of the route and speed limits. For example, if the system knows that a downhill stretch is coming up soon, the car can invest all the high-voltage battery’s electrical energy in providing supplementary driving power, since the battery will be recharged during the forthcoming descent at no cost in terms of fuel consumption. On longer or steeper downhill sections, the electric motor’s generator function can recharge the high-voltage battery with no loss of speed. The operating strategy can also be managed to ensure that the lithium-ion battery is as fully charged as possible when nearing the end of the journey, increasing the electric driving range on the ‘last lap’.

The hybrid Five’s four-zone climate control is unique to this model. It can be activated before a journey begins to cool the interior — heating relies on the engine’s coolant being warm. The system is designed to keep the interior climate constant during breaks in the journey and when the car is in coasting mode with the combustion engine switched off.

As always with hybrids, the instrument cluster and central display show hybrid-specific information of sometimes questionable usefulness, especially for a driver in the process of driving. The displays are: energy flow and recuperation, electric boost (shown in the rev counter), operating status, battery charge level and fuel consumption history. The battery charge level warrants a dial in the main instrument cluster, but doesn’t get one; the rest is a pointless distraction.

Perhaps all of this data processing would be useful if the driver could collect it in a manageable form at the end of a journey and use it to analyse his or her driving?

The Active Hybrid 5’s chassis and structure are shared with other Fünfer saloons. Notable features include aluminium construction of the bonnet, front side panels, doors and various chassis components. BMW claims that the use of aluminium doors alone results in a weight saving of around 23kg compared with conventional steel doors. BMW actually has an Aluminium Competence Centre based at its Dingolfing plant, where the Five is built. Multi-phase steels and hot-stamped ultra-high-tensile steel are used in the structure to improve its strength-to-mass characteristics. In the hot-stamping process, galvanised sheet steel is cold-formed, heated to a temperature of over 900°C, then immediately water-cooled to a temperature of around 70°C and hardened. This technique gives the components between three and four times the strength of conventional sheet steel.

Double wishbones locate the front wheels, while multiple independent links look after the rears. The power steering is electronic, using the Bosch speed-sensitive Servotronic system. Dynamic Damper Control is available as an option. The Dynamic Stability Control system, which stabilises the vehicle by applying braking pressure and reducing engine power, also incorporates functions such as Dynamic Traction Control, the anti-lock braking system, Cornering Brake Control, Dynamic Brake Control, Brake Assist, Fading Compensation, a Dry Braking function and a start-off assistant.

Boot space is adequate, at 375l — 145l down on a regular 5-series boot. There is no spare wheel.

The ‘Driving Experience’ control switch offers Eco Pro, Sport+, Sport and Comfort settings. If the optional Dynamic Damper Control is specified, it also offers Comfort+. These different modes adjust the accelerator response characteristics, engine response, the power steering characteristics, the DSC settings, the shift characteristics of the automatic transmission, and — if the car is equipped with Adaptive Drive — the damping characteristics as well.

Options include active seat ventilation, active seats, Comfort Access, sports automatic gearbox with shift paddles on the steering wheel, Soft Close Automatic function for the doors and boot lid, hands-free boot lid opening function, electrically operated glass roof, and high-quality audio and rear-seat entertainment systems. Bi-xenon headlights, with an adaptive function and cornering lights, are also optional extras.

The optional Active Protection function, controlled by the central safety electronics, is designed to reduce the risk of injury in a collision. If an accident risk is detected, the system closes the side windows and (if one is fitted) the sunroof. At the same time, the front seat-belts are pre-tensioned; if Comfort seats are fitted, their backrests are adjusted to an upright position.

A fair range of BMW’s Connected Drive features are fitted as standard or as options: Head-up display, park distance control, rear-view camera, surround view, adaptive headlights, high-beam assistant, speed limit information, BMW parking assistant, lane change warning system, lane departure warning system, BMW Night Vision with pedestrian detection, internet usage, extended integration of smartphones and music players, real-time traffic information and apps for receiving web radio and using Facebook and Twitter.

Production

The BMW Active Hybrid 5 is produced at the Company’s Dingolfing plant in Bavaria, alongside the other 5-series models, the Sixes and Sevens, including the Active Hybrid 7. The three model lines share a common architecture.

Dingolfing is the BMW Group’s largest production plant worldwide.

BMW Active Hybrid 5
Cylinders 6I
Block/head Al/Al
Valves 4
VVT IE
Aspiration Tts
Bore/stroke 84.0/89.6
Swept volume 2979cc
Compression
ratio
10.2:1
Engine PS/rpm 306/5800
Engine Nm/rpm 400/1200
Electric motor type Synchronous
Motor PS 55
Motor Nm 210
Combined PS 340
Combined Nm 450
Battery type Li-ion
Battery capacity 1.350kWh
Maximum speed 155
0-100km/h 5.9
Urban MPG
(l/100km)
49.5
(5.7)
Combined MPG *
(l/100km)
44.1
(6.4)
CO2 g/km * 149
Emissions EU5
Transmission
— I
— II
— III
— IV
— V
— VI
— VII
— VIII
— Final drive
A8
4.714
3.143
2.106
1.667
1.285
1.000
0.839
0.667
2.929
Driven wheels Rear
Fuel tank 67l
Kerb mass 1850
PS/t 183
Nm/t 243
Length 4899
Width 1860
Height 1464
Wheelbase 2968
Track
— front
— rear

1600
1627
Cd x A 0.28 x 2.35
Tyres (standard) 225/55R17
* With standard wheels and tyres.
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