Slingshot Saab

ALASTAIR SLOANE looks at Saab's revolutionary new engine, which combines power and economy.

Saab called the new technology "Vepsilon", and said it was a fancypants engine with a variable compression system which boosted power and improved economy.

That was all the company was prepared to say early last month. Motoring writers would learn all about the new engine at the Geneva Motor Show, it said.

The Herald, meantime, guessed Vepsilon had to have five of something, because V is the Latin letter five and Epsilon is the fifth letter of the Greek alphabet.

Last week at Geneva, Saab unveiled Vepsilon. It's a supercharged five-cylinder petrol engine, a revolutionary 1.6-litre unit mixing slingshot power with slowcoach economy.

Only Saab doesn't call it Vepsilon any more - that was just a cryptic clue. Instead the five-pot prototype is called SVC, or Saab Variable Compression engine. Sad but true.

It's a Jeckyll and Hyde motor, one minute providing the power and economy of the average 1.6-litre unit, the next grabbing at the ground and delivering a 5.0-litre V8-like 168kW and 305Nm of torque.

The Swedish carmaker says there are three fundamentals to the SVC concept - reduced engine displacement, high supercharging pressure and a unique system for varying the compression ratio.

The SVC can develop 200Nm of torque and 112kW of power for every litre of engine displacement. In theory, therefore, a 2.0-litre SVC engine would produce 400Nm and 224kW of power.

These are muscular numbers. The 3.4-litre engine in the latest 911 Porsche produces 221Kw and 350Nm. Okay, the Porsche engine isn't boosted, but you get the picture.

The SVC is the brainchild of long-time Saab engineer Per Gillbrand, who sketched the concept more than 20 years ago. Saab says it is still at the prototype stage and more work is needed before an SVC engine could go into production.

The upper part of the SVC engine has a cylinder head with integrated cylinders. This is known as the monohead. The lower part consists of the engine block, crankshaft, connecting rods and pistons.

The compression ratio is varied by adjusting the slope of the upper part of the engine by a maximum 4 degrees in relation to the lower part. This alters the volume of the combustion chamber, which also changes the compression ratio.

The SVC engine can be varied to run at the optimum compression ratio of 14:1 at low load, or steady highway speed, to put the energy in the fuel to best possible use.

The ratio can then be reduced to 8:1 at high load, or under hard acceleration, to enable the supercharger to raise engine performance without the problem of engine "knocking."

The variable compression ratio also gives the engine great fuel flexibility, says Saab.

Since the compression ratio can be varied and adjusted to suit the properties of the fuel, the engine will always run at the compression ratio best suited to the fuel.

This means that exhaust emissions can be reduced proportionately to fuel consumption. The SVC engine meets all current and proposed European emission regulations.

Saab says the SVC concept allows the development of both small, fuel-efficient engines with good performance, and bigger engines delivering genuine sports car performance at reasonable fuel consumption.

Downsizing and forced induction have long been well-known concepts at Saab. It launched the turbo concept for passenger cars back in 1976 as one way of boosting engine performance without making the engine bigger, heavier and thirstier.

Conventional four-stroke engine development has now reached the stage where new parameters are needed to meet future demands for lower emissions and use of alternative fuels.

The various functions of the SVC engine are controlled by a special version of Saab's Trionic engine management computer.