Supercharger question.

[snurdf]

Is it not necessary to have an intercooler with a supercharger set up? The majority that I have seen don't appear to use one but I have seen a couple with.
Is it that the supercharger doesn't generate loads of heat like the turbo does or am I barking up the wrong tree entirely?

[guy mayers]

Correct - the exhaust driven turbo generates the heat that an intercooler reduces. A supercharger just mechanically compresses the air using a belt drive from the crankshaft pulley. The aircharge will be heated by this process but not enough to cause detonation. The VX has a pop off valve on the inlet manifold to prevent backfires from stalling the rotors on the supercharger. Any slippage on the rotor would put them out of sync and wreck the supercharger.
Guy

[snurdf]

Cheers Guy, I suppose the ones I've seen with the intercooler set up must be big power high boost set ups.
Happy new year by the way. ;)

[strat6v]

The ones you have seen without intercoolers are running about 5psi or less boost. You can actually compress three psi inside your mouth!! For the increase in weight of all the parts its really not worth bothering at those pressures. Anymore than that sort of pressure, the intake charge will heat up and all your gains will be lost, you may even have less power than when you started.

If you fitted forced induction you would be penalised with insurance, higher capacity classes if you compete etc so you may as well have one boost more. With a mid engined car like the strat it would be best to use a dedicated chargecooler with a pre rad upfront and run a worthwhile amount of boost.

Choice of supercharger type needs thinking about also. For the best analysis, have a look on Whipple industries website for a comparison chart, they do blow their own trumpet a little but the chart is pretty accurate. Before you decide to do anything, go and buy 'forced induction performance tuning' by A. Graham Bell, agood book that will help explain things in more detail.

Oh, and the mini superchargers on ebay are the wrong size-unless you fitted two, but then things start getting heavy...........

[colin artus]

Have a look at the Rotrex unit; it combines the advantages of super and turbocharging.
Also bear in mind that above 7 psi in boost you will probably have to start reducing the static cr of the engine (ie new pistons) as well as intercooling and using detonation control (via knock sensor).

[strat6v]

There are quite a few centrifugal superchargers on the market, only thing is they have similar traits as a turbo. Don't do a great deal till the upper revs. Its all swings and roundabouts. The first questions should be what do i want the engine to be like and how much cash do i have available.

[vindi49]

Also see Practical Performance Mag (this months I think, has an orange TVR Cerbera on the cover) for a reasonable overview of the types of supercharger & how to sort a DIY version.
Russell

[Arthur]

Whole point of supercharging (increased air pressure at the inlet) is to get more mass of air into the cylinder.
When you compress air, you heat it. Mass per unit volume goes down. Cool it, and the volume reduces for the same mass. This way you can get more in.
If you don't use an intercooler, you'll still get benefit, but reaction with petrol vapour starts to take it's toll.
As someone above said, if it gets too warm, the petrol charge evaporates, rather than burning as finely atomised jet, and you end up in detonation country (goodbye squish). Plus the volume of evaporated petrol stops air getting into the cylinder - net result, much expense, no gain.
Also, exhaust temperatures rise - with a relationship of about 4 degrees exh temp for every degree rise in air inlet temp. So if your winter day temp rises from 10 degrees as was, to 60 degrees blown or charged, you can expect the final exh temp to rise by 240 degrees or so.
Plus you're burning more fuel, so the exh temps tend to be higher anyway.

So you plan to do several things - cool the compressed charge air, to get more in. Reduce the compression ratio (usually shorter rods, much expense) above 5 psi cooled, about 7psi uncooled. (cooling reduces the volume, reducing the volume shows as lower boost pressure). Change the exhaust and inlet timing to take advantage of the pressurised flow (less need for a rapid opening to start a ram effect). Use the excess air quantity (blower only, exh gas free, makes too much air; might as well use some to your benefit) to cool the cylinder in the overlap period.
Charger is a different compromise - all the air you use needs power to produce.

So, so far, we've added either a blower or a supercharger and intercooler. We've rebuilt the engine to reduce the CR. As a precaution, we'll add an uprated oil pump, and increase the underpiston oil jets (or add them if you don't have any) to stop the ally pistons melting. Well re-profile the valves, and fit new camshafts to re-time the valve gear. And this will be quite different from blown to supercharged.

Somewhere in there, we'll consult the publications, and talk to the experts, cos I hate the matching calculations and nomograms, and the actual doing of it is very much specific to engine.

Then apply the usual financial calculation. Price all the bits, carefully, twice, then multiply the result by three. You won't be far out.

Very mild charging may get rid of the need for CR changes and timing changes, but the results are likely to be very disappointing for cost involved, and are not a lot better than natural aspiration and a cam change. But if you have the will and the dosh, huge power is available.

Very broadly, blowers have lag. Smaller the blower, less the lag, but less the power available.
Superchargers have big power drain, belt drive problems, but no lag, and stupendous low-revs grunt.
At the end, there's not much difference in power available, or fuel consumption.
Blowers heads tend to have well modified inlet ports, not too much done to the exhausts. Problem is getting the stuff in, the exhausts don't matter so much, cos there's a blower turbine in the way anyway.
Supercharged heads tend to go the other way; lightly worked inlets, and full race exhausts. Getting it out is the main priority.
This is very general, you understand, see the pros for the full story for your own engine.
I believe Mr Carson once worked out that for reasonable supercharging of the 12V Alpha, the belt-driven charger would consume the same power as the 1000cc mini engine produced. Mind, it would go like stink............

Happy blowing
Arthur.

[strat6v]

Thanks for a top post Arthur, informative and educational :)

[CorseChris]

FWIW, I'm nearing the end of a supercharger installation on my 12V motor. Eaton M90, air/water charge cooler from a Jaguar XKR, first stab I'll use a 156 oil cooler rad/fan as a heat exchanger but expect this may be inadequate. Aiming for about 8psi but expect some drop-off at lower revs due to using the blower at the low end of it rev range.

Engine spec is more or less stock. 9.5:1 CR, port matching, stock valves etc, C&B 'mild' cams. Emerald K3 ECU, 350cc injectors.

For some real-world examples of supercharging the Alfa V6 check out Greg Gordons exploits on the Alfa 75. He advocates an M62 as the ideal size for the 3.0. I 'think' an M90 will work OK if not optimally, but as it only cost me about £100 I reckon it's worth a punt.

Do have to take issue with the popular notion that exhaust driven turbo charging is 'free' power though. I'm not an expert, but I'm happy to take the results of NACA (NASA) research on this topic.

Personally, I don't much care for the Rotrex style devices. They seem to have all the diasdvantages of a turbo charger and a blower. The only clear advantage they seem to have over other types is a quick and easy installation. Fine for an aircraft engine (which is what the Rotrex was designed for), not so great for a car.

Once I finally finish this project I'll post up the results, good or bad, as usual. Time will tell.

[Arthur]

Chris,

Best of luck! I'll be very interested to hear the results. I am currently sitting about 50 feet vertically above a single blower, which does around 15,000 RPM and supplies 2.5 Bar charge air to a 19,000 BHP diesel engine. I firmly believe that diesels are the true home of the blower.

As to free power, the theory is that as you supply more air, the actual compression ratio rises, despite the common low quoted value based on cylinder volume and stroke, etc. And this would be true, if there were no compromises needed, like using overlap air for cooling. But the main thing in high speed engines (mine does 105 revs flat out, with a bore of 600mm and a stroke of 3.2 metres) is that they don't scavenge well (scavenging is the purging of exhaust from the cylinder, and the recharge with fresh air), and this gets worse as the revs rise.
Any theoretical efficiency gains are pretty much wiped out by those two factors.
It's also the reason that there's not much difference between blowers and superchargers in practice, even though the efficiency theory says blowers work better.
The best you can say is that the exhaust gas is free - but the using of it costs money.

Correct me if I'm wrong, but don't most car diesel engines top out at around 5500 revs or so? Compared with anything up to 8 grand for petrol? And the bigger the engine, the slower it will go. Scavenging theory in practice!

Mine, a MAN B+W marine engine, has a lot of stuff you'd kill for on a roadie, but it all weighs a ton (really, it's all steel castings) and it's all aimed at increasing the efficiency. But it works really, really well. Trouble is that around 690 tons in the front of a Mondeo would probably induce a little understeer. With a torque of around 2500 Tonne-Metres, it wouldn't matter you hit, it would just carry on. Oh yes, its also a two-stroke. We actually do the scavenging bit by having slots in the bottom of the liner, and supplying the air to those slots (funnily enough, it's called the scavenge space). When the piston uncovers the slots, the exh valve in the lid opens, and whoosh - gas out, clean pressurised air in. (Uniflow scavenging, for those who know).

So, then, lots of ideas for the next build!

All the best
Arthur.

[Elliott.L]

^^^Arthur, by the sound of it you must have the biggest Stratos in the World!!! :D

[strat6v]

He's just very good at fitting big things in small places ;) :D

[CorseChris]

All my plots and plans are based on a huge dose of ignorance topped off with no experience....but it keeps me happy.

Progress is painfully slow but I'm expecting a delivery of bolts so should be able to move it along another step soon.

Once it's all done, I'll be able to report back with before & after power figures, plus some figures on charge heating.

[chris.richard]

Does a wide bore/short stroke cylinder scavenge better than a narrow/long stroke one?

[Turbonutter]

All my plots and plans are based on a huge dose of ignorance topped off with no experience....but it keeps me happy.

Progress is painfully slow but I'm expecting a delivery of bolts so should be able to move it along another step soon.

Once it's all done, I'll be able to report back with before & after power figures, plus some figures on charge heating.


I'm with you on the suck it and see theory, it worked ok for me. Guess at a compression ratio, bolt a couple of turbos to the engine a quick trip to the rolling road and success it works. Shame about the fire 5 years down the road.......

[Arthur]

OK,

Long or short stroke.
The argument goes something like this. The longer you can expand the gas for, the better your fuel efficiency. The higher the compression ratio you can tolerate without blowing anything off, the higher your Diesel cycle peak temperatures will be, and the higher your fuel efficiency. Long, long strokes rule OK.
Course, you probably aren't worrying overmuch about fuel efficiency at this stage.
This means you are subverting the true aim of the blower in the interests of enormous power.
All stand and uncover.
So short stroke and high engine speed will work fine. Just not at the cutting edge of fuel eficiency. Or anywhere near it, truth be told. So fit a bigger tank, and as far as Global Warming goes, stick your fingers in your ears and go "Lah Lah Lah Lah". I mean, look at the early 80's Renault formula cars - 12,000 rpm, 1500cc, and 1500+ HP. Mind, you had to be Senna to stop the tyres chunking and the blower immolating itself, but hey, who cares.

best of luck!

[Shaun II]

Immolate - offer as a sacrifice by killing or by giving up to destruction; "The Aztecs immolated human victims"

I do enjoy Arthurs posts.

[Arthur]

Well, exactly!
Formula 1 all over again! Although the "sacrifice" lately seems to apply more to the drivers than the cars.
I recall Mark Blundell discoursing eloquently over Senna at that time - he reckoned that the cars had so much power that in qualifying trim, with about 4.5 to 5 bar boost, and the wings turned up, they would go flat round anything. He reckons it was entirely possible to kill a set of tyres in one corner. Apparently Senna only ever lifted at Stowe to stop the tyres chunking. The balance apparently was to find the maximum that would get the car round one fast lap without either chunking the tyres, or setting fire to the blower, and at this Senna was the master. Witness the number of times he ran into the pits with the blower on fire, but a time on the board.
Awesome.
Mind you, Mr Blundell also reckoned there was very little sense of acceleration in those cars - apparently, rather than the car proceed up the road, the horizon came hurtling backwards.
And they were put together much as proposed above - fit a huge blower, and see how it goes. Downside was a power characteristic that only worked on full boost, at narrow rev band. Off the boil, they were dead in the water. Too much boil, they were dead in the water. No engine management, carbs ony; must have been a nightmare.

Thanks for the vote of confidence, by the way.
All the best