What is it?
Volkswagen’s Golf 1.5 TSI MHEVs (mild hybrid electric vehicles) are engineering prototypes equipped with the firm's new 48V electrical systems, which are likely to become commonplace in the next couple of years.
The 48V systems are simple, mainly because, unlike a full hybrid or plug-in hybrid, they operate at low voltage, making them relatively cheap and easy for manufacturers to install.
They consist of a belt-integrated starter generator (BISG) and 48V lithium ion battery that's about the size of a conventional 12V battery. The 12V battery is retained to drive the conventional electrics and is charged from the 48V system. The BISG can boost acceleration, recover energy and store it in the lithium ion battery when the car slows or the brakes are applied. It makes stop-start much faster and smoother because of the extra power compared to a 12V starter system.
We briefly tried two versions, the Golf MHEV and Golf MHEV Plus, at Volkswagen's Ehra-Lessien proving grounds. The first is a straightforward front-wheel-drive Golf MHEV powered by a 148bhp 1.5-litre TSI turbocharged petrol engine, equipped with an 8bhp BISG and a prototype of the all-new DQ381 seven-speed DSG automatic transmission. The MHEV Plus has an additional 34bhp electric motor driving its front axle.
The MHEV Plus we tried is one of two configurations – the second motor can also be used to drive the rear axle, effectively giving the car part-time all-wheel drive. In that guise, torque to the rear wheels is less than that of an engine-powered all-wheel drive system but still provides useful traction off-road, and engineers say it can be used to improve cornering balance on the road by neutralising understeer.
Despite the extra input of the electric motor-generators, the total engine's power is likely to stay roughly the same, with the torque of the electric motor-generators being used to reduce emissions and improve economy (by around 7mpg) and driveability rather than increase performance significantly.
What's it like?
The regular MHEV feels fully integrated to drive and pretty much production-ready. Its power delivery is smooth and linear and stop-start events are as smooth as promised. The main difference between this and a conventional Golf is that the gearbox can decouple from the engine, allowing it to shut down so the car can coast to save fuel. That’s where the smooth restarts are of most benefit. When the car is slowing with the engine running, the BISG acts as a generator to charge the 48V battery.
The MHEV Plus is more noticeably a prototype, mainly because adding an additional motor creates a far more complex system. Bringing the second motor into and out of play and smoothly blending the torque between the two motors and the engine is extremely complex, so at this stage, slight steps are noticeable. Production versions will have fully refined software, making transitions seamless.
The advantage of the second motor is that when the Golf is coasting and the engine shut down, the second motor can perform regenerative braking to recover energy. The standard MHEV can only recover energy when the engine is running, so engineers think that the MHEV Plus will give greater economy.
We weren’t able to drive the all-wheel-drive version but did observe an engineer extracting a Tiguan from deep, wet sand using it. In front-wheel-drive form, the SUV was stranded, but with the rear drive switched on, it managed to drive out of trouble with reasonable ease on winter tyres. In a similar demonstration, the Tiguan was driven up a steep slippery slope that it couldn’t manage in front-wheel-drive mode.
Should I buy one?
The likelihood is that in a few years time, you may not have much choice. Electrification of cars to recover and reuse kinetic energy through regenerative braking is the only way manufacturers will meet future CO2 and emissions regulations.
However, the good news is that MHEVs are even better to drive than standard cars. The electric boost neutralises turbo lag, sharpening response to the accelerator; the systems don’t add as much cost as a high-voltage PHEV or full hybrid system; and the fuel consumption benefits should be significant, as long as cars don’t become further bloated with even more heavy features.
Given the option, at the right price, an MHEV will be an all-round better choice than a standard combustion-engine car.
Volkswagen Golf 1.5 TSI MHEV prototype
Where Lower Saxony, Germany On sale 2019 (est) Price TBA Engine 4cyls, 1498cc, turbocharged petrol plus electric motor Power 148bhp Torque na Gearbox 7-spd automatic Kerbweight na Top speed na 0-62mph TBA Fuel economy 48.5mpg (estimated) CO2 rating na