Automotive Engineer

In a quest to reduce emissions and improve efficiency, many engineers spend their time trying to trim weight from a vehicle, reduce friction losses in the powertrain or improve air flow over the body. Each fractional improvement brings down the amount of carbon emitted from the tailpipe. But it can also boost performance and driveability too.

In 2005 Toyota and Subaru formed an alliance to produce new vehicles. A year later they began development of the Toyota GT 86 and its sister vehicle, the Subaru BRZ compact coupe.

Both vehicles have been designed to be driver-orientated, with emphasis placed on performance, using a naturally aspirated four-cylinder boxer gasoline engine with port- and direct-injection. Exterior and interior design was completed by Toyota, while the engineering of the powertrain, chassis and other areas was split between the two companies.

Tetsuya Tada, chief engineer of the GT 86, had a huge impact on the programme. His preference for a high-revving engine rather than using turbocharging technology shaped the vehicle. The 2-litre engine has a maximum speed of 7,400rpm, producing 147kW at 7,000rpm. It reaches maximum torque of 205Nm between 6,400 and 6,600rpm. The boxer engine was chosen to meet safety requirements.

“Crash regulations for vehicles and pedestrians meant that to develop a very low sports car we had two possibilities: a boxer or a rotary engine,” says Tada. The decision was taken to use a boxer engine because the technology was already used in Subaru’s Impreza and Legacy models.

The GT 86’s engine uses separate twin injectors for both direct- and port-injection, performing both high-pressure direct-injection into the cylinder and conventional intake port-injection, or direct cylinder injection only, depending on engine speed. The system improves the air/fuel mixture, boosting throttle response, power and torque over a wide range of engine speeds without sacrificing fuel efficiency.

The GT 86 uses 7.8 litres/100km of fuel when mated to the six-speed manual transmission, emitting 181g/km CO₂. But it still manages to reach 100km/h in 7.6 seconds and has a top speed of 226km/h. The vehicle’s Cd figure of 0.27 helps in this.

The 2-litre engine has 86mm bore and stroke and a compression ratio of 12.5:1. The crank pin has a diameter of 50mm to increase rigidity at high engine speeds. And the connecting rod’s shape has been optimised to reach those higher speeds.

But the GT 86 could also have benefited from Toyota’s hybrid technology, and there were strong calls from the company’s board to use an electrified powertrain. “At the beginning of the planning, board members asked us to use hybrid technology, but it’s impossible in a sports car because of the weight penalties. In the future, with the right battery and motor technology, maybe we can use hybrid systems,” says Tada.