I wrote a few days ago on the joint efforts between GM and Daimler Chrysler Mercedes at trying to conquer the bear that is hybrid vehicle technology (click here for article). That effort is now being furthered by the addition of the Bavarian Behemoth, BMW. I prefaced the article with a reference to the Variable Compression Engine developed by Saab, which has apparently been shelved by GM.
In the event that some of you had forgotton about the operation of the SVC engine, like I had, I thought it might be beneficial to post the details here. The following is a reproduction of an article originally posted on the Channel 4 website.
If anyone from Channel 4 is reading, please don’t be upset. I’m placing it here for archival purposes, just in case your links rot. Just ask nicely and I’ll remove it if you’re offended.
Saab reinvents the engine
It’s not often that we see a major change in the architecture of an engine. Detail changes over the years, yes: four valves per cylinder, turbocharging and variable valve-timing, once exotic, have all become commonplace over the last quarter-century. But no-one has tried to vary the compression ratio, the amount by which the fuel-air mix is squeezed as the piston reaches the top of its compression stroke. Until now.
Saab, the company that made the turbo engine a mass-production reality, is behind a new idea that can cut fuel consumption by up to 30 per cent compared with existing engines of similar power. By allowing the compression ratio to vary between 8:1 and 14:1 – a normal modern engine is typically about 10:1 – Saab can run the forced induction that it always likes to use at much greater boost pressures, so forcing in more fuel-air mixture, while also letting the engine run much more efficiently when it’s not working hard and the turbo isn’t boosting.
The result is an engine that can deliver huge amounts of power and torque from a small cylinder capacity. From its 1598 cc, the prototype Saab Variable Compression (SVC) engine delivers 225 bhp and the same amount of torque (pulling power) in lb ft, while returning better fuel consumption than existing similarly-powerful engines. It shows that the more that an engine’s parameters can be altered to suit different types of driving, the more efficient it can be for more of the time. The lower friction levels of a small engine help, too.
How Does It Work?
The key is the monohead, an assembly consisting of the cylinder head and the cylinder liners in one rigid unit. This hinges within the engine block, pivoting along one side while hydraulically-powered rams force the other side of the monohead up or down as needed. This movement – a total hinging arc of 4 degrees – changes the distance between the top of the combustion chamber and the crankshaft’s centre line, so altering the chamber’s volume and therefore the compression ratio.
A rubber bellows-type gasket seals the gap between monohead and block, so crankcase fumes can’t get out and dirt can’t get in. And instead of Saab’s usual turbocharger, the SVC engine uses a supercharger which can deliver up to 1.8 bar of boost – that’s nearly twice as much as a regular Saab turbo manages – for a quicker response. This gets round the problem that a small engine doesn’t produce enough exhaust gas at low engine speeds to get a big, high-boost turbo spinning.
The low compression, and smaller cylinders’ better ability at shifting heat, are why the engine can run at such crazy boost without going bang. The supercharger’s drive clutch disengages at light loads, to save the energy that would otherwise be needed to turn it. Its throttle closes at the same time, and a by-pass throttle activates instead.
Making a small engine work hard is more efficient than making a big one work lazily, which is why just 1.6 litres of prototype SVC engine can propel a hefty Saab 9-5. As a 20-valve five-cylinder, with small pistons and valves, it can run more smoothly than a four-cylinder and the high boost puts back the torque that small-cylinder engines usually lack.
The monohead is tilted by five rams which resemble normal connecting rods. A shaft – which looks a little like a crankshaft except that all the cranks are on the same side – rotates under hydraulic pressure to move the rams. Changing from high compression to low is faster than the other way round, because cylinder pressures naturally force the monohead further away from the crankshaft.
When running gently, the combustion chamber is small, the compression ratio is high to burn the fuel as efficiently as possible, and the supercharger is disengaged
When you need more power, the supercharger clutch engages, the monohead hinges upwards, the compression ratio reduces and the SVC engine can cope with huge boost pressures. The compression ratio runs at whatever gives the best efficiency for the moment, across a range from 8:1 through to 14:1
The supercharger and its intercooler are encapsulated inside the air intake tract. This is to cut down the supercharger’s noise, which project leader Lars Bergsten describes as ‘tremendous’.
What’s it like to drive?
Saab’s engineers warned us that this was a prototype car, in case cynical journalists jumped to the wrong conclusions. ‘It’s a bit noisy,’ they said. They needn’t have worried, because the SVC engine works brilliantly.
We can’t report on the mpg claims, but performance is very promising. Saab turbos usually have a soft, slightly elastic throttle response (although they’re much better than they used to be) but the SVC’s energy is near-instant. There’s a tiny delay when you press the accelerator as the supercharger’s clutch engages with a squeak, then pow! Off you hurtle, reeling in wonderment that 1.6 litres can propel a bulky 9-5 automatic with such verve. There’s torque from way down in the rev range right up to the rev-counter’s warning zone, accompanied by a five-cylinder hum that’s more Volvo than Saab.
The prototype’s trip computer has been rejigged to show boost pressure and compression ratio, and it’s fascinating to watch what happens when. That forward leap sees the first measurement rocket and the second plummet, but both are changing all the time as you speed up, slow down or just cruise. Under gentle acceleration the supercharger clutch engages more gradually, for a smooth drive.
Here, then, is an engine that’s both fun and frugal. Yes, it is a little grumbly at the moment, especially when cold, but concentrated acoustic development should have that and the supercharger squeal licked. Having a top half of an engine able to move relative to a bottom half was never going to be a recipe for top refinement, but it’s fascinating to watch with the bonnet open as the throttle is blipped hard, and see the engine bend before your very eyes.
The SVC project began in 1990, and at one stage Saab even had a 1.4-litre, six-cylinder unit on the go. If Saab decides to go ahead with SVC, it could have an engine in production within four years which would cost about the same as the current asymmetric turbo V6.
But the possibilities go further. Saab is already talking about using alternative fuels that the engine could detect and adapt to, from methanol (very high-octane and able to exploit a high compression ratio) to diesel, perhaps with spark-assisted ignition. However, what’s next from Saab is Combustion Control.
Saab Combustion Control, or SCC, is a tricky concept to get your head around. It’s driven by the need to comply with both US emissions rules, which demand the near-total cleaning-up of exhaust gases that only a three-way catalyst can give, and European rules which are less stringent about nitrogen oxides (as long as most are dealt with) but are going to get tighter on greenhouse gases (carbon dioxide, emissions of which are in exact proportion to fuel consumption).
Sounds boring, but it has to be done unless car makers have two different engine ranges for the different markets. Future lean-burn Euro engines will produce too much NOx, but absorption catalysts will absorb 85 per cent of it, which is enough for us. That’s no good for the US, though, especially Los Angeles with its photochemical smogs.
Saab’s SCC uses exhaust gases to fill the cylinder mainly with an inert mixture under gentle driving, so that the small amount of fuel-air mix that’s left is in the chemically-correct, or stoichiometric, ratio that a three-way cat needs to work properly. What’s in the cylinder is not ‘lean’ in the usual sense, therefore, although the economy benefits are the same.
How SCC Works
When running gently, the SCC engine adjusts its valve timing so that the exhaust valve, as well as the inlet valve, is open on the induction stroke. This sucks exhaust back into the engine to give a very diluted mixture that’s still able to burn if you give it enough time. Fuel is then injected directly into the cylinder through a combined spark-plug injector. To make the mixture burn, the ignition timing is advanced a long way and the spark is made to jump a huge 4 mm gap (a normal plug has a gap of about 0.7 mm).
When you need more power, the valve timing changes to a more normal setting so the engine can take in clean air (helped by a boosting turbo, of course – this is a Saab, remember) and the ignition timing retards. The cylinder pressures are greater now, so the spark can’t jump that big gap. Instead, the spark passes directly to the top of the piston, which has a small peak ready to receive it. When it’s time for the spark, the piston is far enough up the cylinder for the gap to be exactly right.
As well as these two extreme modes, the engine can operate at stages in between them. When you move the accelerator pedal, you’re adjusting the engine’s output at first purely by altering the valve timing, bringing a throttle into play only when the engine’s working harder.
As well as being cleaner, the SCC engine is more efficient under light load because it doesn’t suffer from the pumping losses found in a normal engine whose throttle is nearly closed. Imagine trying to breathe through a straw: that’s something like what’s happening when you’re trickling along with a normal engine, and it’s not very efficient.
Saab, which has patented both SVC and SCC, is now trying to get other car makers interested in the ideas. Other brands within the General Motors empire could be the first to benefit.