Tuesday, 6 May 2008

A blast from the past #3

Two speed supercharging.

There is, as the popular saying would have it, nothing new under the sun. So when I read a press release from Antonov Automotive Technologies proclaiming 2-speed superchargers the way forward I recalled that the idea has been tried on piston-engined aircraft. In aviation the idea was to maintain performance at altitude so as the aeroplane climbed the pilot would select a gear to speed up the supercharger. It was a crude but reasonably effective way to compensate for the thinning of the air.

A supercharger is an air pump and while different types have different characteristics but, broadly speaking, the faster it runs the more air it moves. Antonov’s idea is to run the supercharger relatively faster at low engine rpm to boost low-speed torque without blowing the engine up at high speed. They don’t say this, of course, but that is what it amounts to. The behaviour of the supercharger should be fully automated and imperceptible when driving. The goal is to allow vehicle manufacturers to use smaller engines that are more economical.

I’m not sure how plausible their claims are. Superchargers are not the most efficient means of increasing performance. I’m not convinced by anyone who claims that superchargers don’t have the heat issues of turbochargers. Anyone who has used a bicycle pump knows that compressing air makes it hot, the air isn’t particularly bothered how it gets compressed. Two-speed supercharging is an interesting idea but I wonder when engineering fashion will swing back towards big, engines in low states of tune combined with high gearing – the sort of thing seen with the BMW 525e and Renault 5TS.


Si Hollingworth said...

Ah, Lambert Lambert Lambert... you've got the right idea with two-speed superchargers but your historical background has them the wrong way round. The Merlin X had a two-speed supercharger, but it worked at lower boost at low altitude (where the air was denser anyway, and high boost would push the engine into destructive detonation), going into high-boost at higher altitudes (where the air was thinner and the boost the engine "saw" was less). It was automatically selected, though, not pilot-controlled.

Having said that, some writers have made a total hash of describing it, confusing "two-speed" supercharging with "two-stage" supercharging, a very different thing. I need to read more about this, as the way I've seen it described so far looks like Antonov have got two superchargers which they switch to according to demand and revs; the trouble is, they seem to use a centrifugal supercharger at low revs and a positive-displacement blower at high revs, which is totally the wrong way round! Either it's badly described (which is what I'm putting my 10p on) or they're on a hiding to nothing!

Oh, and while I'm kicking a man while he's down, although it's true that air doesn't care HOW it's compressed, all other things being equal, with turbos and superchargers "all other things" are not equal. Using an exhaust-driven pump to compress air is bound to get it much, much hotter than would be the case if the pump didn't have exhaust gas going through it. I've seen turbos glowing red, orange and in one case, yellow, but I have NEVER seen a supercharger any other colour than bare alloy. An extreme case, but I think it proves the point. Turbos have always had issues with underbonnet heat, and although superchargers add some heat to the intake charge, it's nowhere near as bad as a turbo'd engine has to deal with.


Sorry about that, John, but I had to stick my oar in. Good blog, BTW... :-)

Fourwheelsteer said...

There is no positive displacement blower, just a centrifugal supercharger. The bulk of the unit - having seen one sectioned at the SMMT test day - is an epicyclic gearset. At low RPM the planet carrier is locked by spring pressure and the supercharger runs at roughly 1.3 times engine rpm. Attached to the input shaft are a series of bob-weights, as the revs rise they overcome the spring pressure and lock the input and output side of the epicyclic together so the supercharger runs at engine speed.