Saab "light pressure turbo" technology?

[stillsearching]

I was curious who else is familiar with this, and whether tests show the theory to be valid and work or not. I'd think if it worked great everyone else would do it unless it was locked in patent/trademark type protection.

Basically the point of the Saab Low Pressure Turbo is a combination of turbo boost at low rpm and steep gearing. The turbo is sized to have full boost by 1500rpm or so if I remember right (or at least to be coming on significantly by then), there is zero turbo lag, it's basically undersized for the engine - top end HP improves almost not at all. A stock engine producing 125hp before, after the LPT might put out 140-150hp at max. What the turbo does is radically improve the torque curve especially at low rpm typically 30% or more. This makes the inline 4 have more like V6 types of torque so they do not have to rev up.

High compression ratios are maintained - the boost is light (no more than 5psi) so high compression for efficiency can still be used.

This is combined with steep gearing. The turbo almost acts like "displacement on demand" since whenever the engine gets loaded to where it would normally start to slow down in top gear, like due to aerodynamic load or going up a minor hill, the boost will come on. Thus you are allowed to use an engine that would normally be undersized for running at that rpm.


The problem is that cars that use this technology do not seem to show spectacular mileage. (they are not posting better mileage figures than other compacts, the LPT turbo numbers seem to be worse mileage than say other small 4cyl cars which may be comparable power levels even without boost) So i'm wondering if a 4cyl on boost at 1500rpm honestly has any advantage over a bigger engine at part throttle flowing about the same level of fuel...

I like the theory - to me it seems like a good strategy to try on any engine but maybe doesn't work in reality. Yet I was hoping someone here might know more..

[shovel]

Those supercharged Nissan Xterra/Frontiers from 10 years ago are somewhere around 5 PSI, so their ordinarily 180hp engine goes up to 210hp -

I don't know what their torque curve is like but I suspect their biggest advantage becomes obvious at higher elevations.

[thomason2wheels]

Interesting questions stillsearching, my belief is that hanging a turbo of any kind on an ICE runningat normal mixtures and temperatures, no matter whether the turbo is sized correctly or not will result in a net loss of economy compared to the same nonturbo engine. The one exception is the lean burn engine smokey yunick developed. Can you think of any other configuration besides, the one i mentioned? Talking strictly about gas as i have very little knowledge or experience with diesels.

[stillsearching]

Originally Posted by thomason2wheels

Interesting questions stillsearching, my belief is that hanging a turbo of any kind on an ICE runningat normal mixtures and temperatures, no matter whether the turbo is sized correctly or not will result in a net loss of economy compared to the same nonturbo engine. The one exception is the lean burn engine smokey yunick developed. Can you think of any other configuration besides, the one i mentioned? Talking strictly about gas as i have very little knowledge or experience with diesels.

I tend to agree, which is why I posted about a turbo bypass in the other thread. :^) That said the argument is exclusively that "the extra low rpm torque lets you hold steeper gears, shifting to a higher gear at lower speeds thus the system gives better mileage" than the displacement increase of 30% otherwise required to work the same. That's the ARGUMENT, I just don't know if it works as advertised, or how to discern which times it does vs it doesn't. The nature of it is that it's limited to lower pressure because compression ratios must remain as high as possible. 5psi ranges or so mostly help cylinder filling without necessarily causing any detonation risk. This is different than the technique of a very small 4cyl with high boost (14psi or more) which requires lower compression like 8:1, which damages efficiency when off boost. It's possible with the right design you could have both though if you added an octane booster (perhaps methanol) for "power mode" so you could keep high CR and the "light pressure" philosophy for all normal octane driving.

That said diesels almost universally 100% of the time increase economy with a turbocharger - more air = more power and more efficient burn, they dont require a fixed fuel-air ratio. The Saab LPT is for gas engines.


I hope to start experimenting on a vehicle in the future with a bunch of different strategies that i'm all posting about now as early research. I'm making a list of a bunch of different methods and hope to have a workhorse that lets me test most of them just on that one vehicle.