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Saab's Variable Compression Engine
By Scott Memmer and Brent Romans
Last updated: 2001-01-25

With the recent introductions of the Honda Insight and Toyota Prius, Americans have had their awareness raised about hybrid vehicles and their ability to deliver excellent fuel economy and low emissions. But don't discount the internal combustion engine just yet.

Saab engineers have developed an engine technology that improves fuel efficiency by up to 30 percent, reduces tailpipe emissions, and doesn't negatively impact overall performance. Saab calls its new engine technology Saab Variable Compression (SVC). Interestingly, Saab engineers have been thinking about variable compression since 1981, and prototype engines with SVC technology have existed since 1990. At the 2000 Geneva Auto Show, Saab finally felt its SVC technology had advanced enough to show it to the world.

The SVC engine shown at Geneva was a supercharged, 1.6-liter, inline five-cylinder engine. With dual-overhead camshafts and four valves-per-cylinder, it looks pretty much like your average internal combustion engine. In fact, an objective in the development work on the SVC concept was to retain as many of the basic components of a conventional engine as possible. The crankshaft, connecting rods, pistons and valves are all of the same type as those of today's engines.

What distinguishes the SVC engine is the way it is split into upper and lower parts. Instead of having the cylinders located in the block like all modern vehicle engines, the SVC engine's cylinders are located in the upper section. This upper part is known as the monohead. You heard us right -- the cylinders are built into the head.

For those of you who may not know it, this is a very unconventional design. Traditional engine architecture places the cylinders below the head (or heads if the engine is of a V or flat configuration), between the valves and the block.

Everything else is pretty "normal" inside the Saab engine. The lower portion of the SVC engine -- the block -- contains the crankshaft, camshaft and pistons.

But here's the trick. Since the cylinders are built into the heads, the monohead can be adjusted (moved) relative to the block without affecting engine or valve timing. On the SVC engine, the monohead tilts at up to a 4-degree angle. Rubber bellows seal the monohead to the block.

By adjusting the "slope" of the upper part of the engine in relation to the lower part by the use of hydraulic actuators, the Saab engine can vary the volume of the combustion chambers. This, in turn, changes the compression ratio.

Compression ratio refers to the ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.

In all modern engines, the compression ratio is fixed and cannot be altered unless different engine components are installed. Saab's SVC technology allows a variable compression ratio from a low of 8:1 to a maximum of 14:1.

Although variable compression is what makes the SVC engine unique, Saab says the fuel efficiency of a conventional, normally aspirated engine would only improve by 4 percent if it were equipped solely with SVC. The potential of variable compression is most effective when combined with a smaller engine design (for better efficiency) and high supercharging pressure.

It's hard to argue with that given the impressive results. Saab says its 1.6-liter SVC concept engine generates 225 horsepower and 224 foot-pounds of torque. Maximum boost pressure from the supercharger is 40 psi.

These horsepower and torque figures are comparable to a normally aspirated engine of similar displacement (like a 3.0-liter V6 from a Nissan Maxima, for instance). Additionally, Saab says the SVC engine is more fuel efficient at light loads than a standard 1.6-liter normally aspirated engine.

Due to its variable compression ratio, the SVC engine can run at an optimum compression ratio of 14:1 at low engine loads to maximize fuel efficiency and lower harmful tailpipe emissions. Increased supercharger pressure at such a high compression ratio would cause detonation, so the compression ratio is lowered to 8:1 at high engine loads to enable maximum engine performance.

The SVC engine continuously varies the compression ratio according to driving conditions. This would not be possible, however, without an advanced engine management system. Saab engineers credit their Trionic engine management system with making the whole thing work. The current system, which Saab has been using since 1991 to manage their turbocharged engines, served as the basis for the specially designed SVC system.

The Saab Variable Compression technology illustrates yet another innovative success story in the search for more fuel-efficient cars. While manufacturers such as Honda and Toyota have gone the hybrid route, other automakers such as Saab have taken a more traditional route, exploring variations on the theme of the internal combustion engine.

The results so far look encouraging.

One great thing about the Saab concept -- to take nothing away from the hybrids -- is the performance factor. Granted, its fuel economy will probably never equal the Honda Insight's 70 miles per gallon. Still, for those who like their fuel economy served with a side order of hallelujah, this thing will most likely get up and go in a way the Insight never will.

We've driven the Insight. It's a wonderful idea whose time has come. We commend Honda. But the Insight's forte is not straight-line performance.

Saab hasn't announced official production plans for SVC technology. The engine is still in the concept stage. However, Saab does say that extensive testing has been performed, including 200,000-kilometer durability tests. Additionally, Saab wouldn't have gone to such marketing lengths at the Geneva Auto Show if it didn't think there was a future to variable compression.

Stay tuned, though, as Saab is now owned by GM. Never know what might happen with this technology if it gets perfected.





Keep in mind this engine is a prototype an was tuned for passenger cars in mind. this does NOT limit its performance engine applications however.

http://www.saab.com/home/GLOBAL/en/vepsilon/

 
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-Update-

Saab is now launching an entirely new engine concept named SVC, which stands for Saab Variable Compression. Owing to the SVC engine's unique design, it offers performance on a par with units twice its size but with the fuel consumption of a small engine. The SVC engine is a five-cylinder 1.6 litre unit producing 225 bhp, and it delivers no less than 305 Nm of torque.

What is unique about the SVC engine is that it features variable compression. In other words, the ratio between the piston's displacement volume and the volume of the combustion chamber is not constant, as it is in a conventional engine. Instead, the SVC registers current needs and decides how much the mixture of fuel and air is to be compressed in the cylinder prior to ignition. The upper part, the mono-head, can be inclined up to four degrees to achieve optimum compression, which means that the engine always works at its most efficient level.

It is this variable compression in combination with considerable overboost and a scaled-down cylinder displacement that makes the SVC design so strong and at the same time so fuel-efficient. Generous overboost means it is possible to supply more fuel to the engine as and when needed. This in turn promotes both greater torque and higher power output. A smaller cylinder displacement also means the engine is lighter and operates with lower friction, so it uses fuel more efficiently compared to a conventional engine. Fuel consumption can be reduced by up to 30 percent - while retaining existing performance levels.

25 years ago, Saab was the world's first manufacturer to develop a turbo engine for everyday use. Ever since, we have remained at the forefront of development in the field of engine efficiency. The new engine represents a major leap forward in petrol engine technology.

 
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Nice post Joe. Thanx for the info.

 
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yet another reason why i will always love saabs! nice post joe

 
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agreed. nice post! It was educational [IMG]i/expressions/face-icon-small-smile.gif[/IMG] I used to drive a volvo that was turbocharged and me and a friend ripped out a Saab APC ECU used to control the turbo on early turbocharged saab's and adapted it to my volvo. Saab has done wonderful things like this for a long time. It was good to read! Thanks!

 
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Saab

 
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Thanks Joe, you answered my request.

 
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how is that hard to argue!? it takes 40 psi to make 225hp and 224ft-lbs!? think abuot what hapens when you lower the compression on a honda b series motor and super or turbo charge it....if you were to run 40 psi you would have liek 700HP! (maybe not last very long), but to make 230hp you shure wouldnt need to run 40psi of boost! christ!

 
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But it does so relaibly...

 
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if you build a strong bottom end, you can easily run the needed 6-10psi of boost on a honda motor to make 250hp reliably....

 
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It's just different means to and end. Plus factor in that the Saab's ends include better emmisions and decent fuel economy.

 
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Dam, they actually found a replacement for displacement. Yeah, my jaw dropped also when they said its running 40psi. It’s is true you could get that hp with a 1.6L motor with quarter of the boost, but look at the torque that things putting out
It’s almost a one to one ration to hp. The thing that puts the argument to rest is the fact you’re getting about the same gas mileage as an NA engine if not less. Do that with normal turbo car.

The thing that troubles me is the rubber head gasket and the fact the head is moving.
The rubber head gasket is probably going to need to be change like a timeing belt.

Omega