Twincharging an engine that features a screw-type supercharger (ie it already has internal compression) in my mind has the following advantages and disadvantages:

Disadvantages:
It would still feature parasitic loss when the supercharger is "bypassed" and the turbo is powering away at higher rpm

Advantage:
The efficiency being better than a roots blower, would mean lower temps and more power

Can one of the gurus post their thoughts? PatPrimmer and Warpspeed had a lot to say on previous TwinCharge topics.

In short: what is the suitability of screw-type SC for twincharging project?||gatevalve

You've probably got it ... the screw type would give (theoretically) an efficiency loss during cruise, but a minor power improvement during the short period of acceleration until the turbo takes over.

The twin-charging application that I'm aware of (VW) only uses the supercharging for the first moment of acceleration in which the turbo is running "off design". As soon as the engine builds enough revs for the turbo to take over, it does. This leads me to suspect that there is little purpose in using a higher efficiency supercharger, because it is really only used for maybe a second at a time. Also, those engines are intercooled, so whatever minor extra heat comes from using a supercharger that is less efficient than it could be, gets taken away by the intercooler anyway.

At least in that application, the need to minimize parasitic losses during cruise and acceleration when the turbo is active takes priority.

'course, not every application is the same, I'm speaking only of that one, where VW's intention was to make a downsized and more-efficient gasoline engine; it's best not to throw away a few points of efficiency driving an inactive supercharger.

The twincharged VW engine has an electromagnetic clutch that completely disengages the supercharger when it's not needed, so the associated parasitic losses of the bypassed supercharger is not there.  In fact, I don't know of any contemporary OEM supercharged application that doesn't have a clutched disengagement during part load.

Care should be taken about discussing efficiencies.  While it is well known that screw compressors have internal compression, the general blanket conclusion is that this is more efficient than a positive displacement blower.  This is true at increasing pressure ratios, but in fact, at low PRs the difference approaches insignificance, and a Roots blower can get better efficiencies than a screw compressor because of less aggregate losses (the work of a compressor covers more than simply the process work).

Generally, if the PRs of the compressor stage is less than about 1.5, the differences in efficiencies are pretty small, and cost becomes the overriding factor.  However, the VW TSI engine has the blower delivering a PR of 2.5 barely above idle at 1250 RPM, and IMO a screw compressor would be better suited for this, but again, cost was the overriding factor.

In an SI engine, a large reduction in compressor work and charge temperatures can be achieved by injecting fuel at the compressor inlet, as Lotus has done with the Exige 265E and 270 Tri-fuel prototypes.

MORE NEWS