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...as Michael says. CR is simply the total volume of the cylinder at the end of the suction stroke compared to the volume at the beginning of the power stroke. It's a measure of how much the charge air gets compressed prior to combustion (in the compression stroke). Forced induction (i.e. turbo or supercharging) doesn't change CR whatsoever.
In designing engines, of course you'd like to have your CR as high as possible, because as CR goes up, so does thermal efficiency and thus the power you get out of your engine for how much gas you dump in. The limitation is pre-ignition (a.k.a. knocking). Higher CR means more tendency toward knocking. When you're designing a forced induction engine, this becomes interesting because forced induction also increases tendency toward knocking...
Bottom line, your turbocharged or supercharged engine usually is designed to a lower CR than the same engine without the forced induction (see the classic 900). It's actually quite remarkable that Saab has been able to achieve CR on the order of 10.0 for their modern turbocharged engines. All this fancy engine management computer business is what allows it... basically via a more sophisticated knock sensor that can turn down your boost (i.e. less forced induction) if it detects pre-ignition. The other way of course is to use high octane gas which has a higher resistance to pre-ignition. Thus the reason your high CR turbo Saab wants high octane gas to perform to spec. (If you really want to see it taken to the extreme, check out the VW turbos, I believe with CR around 11! They require 93 octane). Also naturally aspirated engines with high CR (like a Ferrari) require the high octane fuel for the same reason.
Anyway, normal engines, even the forced induction ones, don't change CR on the fly. The new Saab one that J is talking about, does. It allows another dimension of adjustment with regard to taking an engine to the knock limit. Now you can not only control the amount of air and fuel going in, and the amount of pre-compression of that air (via turbo or supercharger), but you can also control the cylinder geometry to a limited extent. This means you can do a better job optimizing your thermal efficiency, which to the consumer ends up being power and fuel consumption.
None of this answers J's original question.. but yeah, the engine looks cool and I too would like to know if/when they're planning to introduce it to production models.
Cheers,
'Roo
'89 900S
'01 9-3SE
posted by 128.42.178...
, Sun, 8 Sep 2002 15:44:20
, Sun, 8 Sep 2002 18:48:54, Sun, 8 Sep 2002 15:14:37, Sun, 8 Sep 2002 15:37:53, Mon, 9 Sep 2002 18:05:24, Tue, 10 Sep 2002 04:35:33, Tue, 10 Sep 2002 19:38:55, Mon, 9 Sep 2002 08:57:48
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