One of the first questions that pops into my head when looking at a new concept car is how it applies to the manufacturer's future models. Subaru claims that the B5-TPH concept car, to be shown at next month's Tokyo Motor Show has no bearing on any future production car. I'll accept that Subaru are unlikely to make a spacious three-door coupe in the vein of the B5-TPH, but surely the technology under the skin of the concept must be something the Japanese company is working on.

TPH stands for Turbo Parallel Hybrid. As the name suggests, the TPH concept uses a hybrid powertrain to enhance the car's performance at the same time as allowing lower fuel consumption and emissions. Lexus is as such the only mainstream manufacturer to put this idea into production as yet, but the idea has surfaced on quite a number of concepts at the 2005 Frankfurt Motor Show and it must only be a matter of time before somebody else reveals another production version of the technology.

Subaru's implementation of the technology utilises a 2-litre petrol engine in parallel with a motor/generator fitted between it and the gearbox. The engine itself is a bit special. Being a Subaru, it naturally is in the horizontally opposed 'boxer' configuration, and is turbocharged to produce 256bhp. However, it runs on the 'Miller Cycle', in which the expansion or power stroke is longer than the compression stroke, resulting in much higher engine efficiency. This cycle is not in widespread use due to its limitations at low engine speeds. Subaru gets around this by using the electric motor's instantaneous torque (111lb.ft) to aid acceleration. The concept also makes us of a new type of manganese lithium ion battery, which charges to 95 per cent capacity in just five minutes and operates not just in city driving but also when cruising, the motor adding a further 13bhp to proceedings.

The shape of the B5-TPH is unusual, though it certainly takes cues from the current Impreza Sports Wagon and recent Saab concept cars. The side door cutout angles sharply into the sill and the tailgate itself takes up most of the rear of the car. We'll see what the interior is like for ourselves at the Tokyo Show in a couple of weeks.

Shane O' Donoghue - 30 Sep 2005