Wednesday, 2 January 2013
Saab tilting engine
Saab's talent for innovation was immense. It was always inventing gadgets, like the thermo-accumulator that warmed engines before starting up, and drive-by-wire. When I was researching my 1997 book, it let me in on secret research it was doing on the variable-compression engine. In the event its announcement was delayed until 2000 but it showed the heights to which Saab ingenuity was rising. I had to excise this reference before publication:
Yet the Holy Grail of engine design, a variable compression ratio engine was an elusive goal. The single cylinder research engine Sir Harry Ricardo devised in 1920 to calibrate fuels, had an adjustment to bring on piston knocking and determine octane ratings. The world's first variable compression engine, his E35 had a threaded cylinder barrel which wound in or out of the cylinder block, and a telescopic drive to the overhead valvegear. Generations of engineers tried to reproduce it as a workable multi-cylinder with a low compression for low-speed running and a high compression for lean-burn economical high-speed cruising.
The goals seemed incompatible and the practicalities insurmountable until Per Gillbrand and his engineers at Södertälje revealed the fruits of seven years work on a new generation of engines with pivoting cylinder heads. These tilted to adjust the compression ratio inside the combustion chambers automatically and constantly, according to speed and load. The result brought an improvement of 30 per cent in fuel consumption, and an increase in efficiency, which allowed a 1.4 litre engine to do the work of a 1.8 or even a 2.0 litre.
The production obstacles were formidable, and Gillbrand and his team had to evolve an engine in which the head was integral with the cylinder block, to make it work. New techniques of engine construction were developed, enabling valve seats to be machined before the cylinder liners were in place. The hinged block, which tilted through some seven degrees to vary the pistons' penetration of the combustion chambers, was moved by a hydraulic ram actuated from the electronic engine management system. Precision was everything, and to gain the benefits of the most profound breakthrough in engine technology since the advent of the turbocharger, Saab introduced its first ever engine-driven supercharger.
PICS Saab 92 and J21 aircraft. I have the artwork for the tilting engine somewhere in the archives; the engine is a 1.9TiD.