# Supercharged engine

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#### nheistand

##### Member
I have an AMR300 small supercharger on my Predator 670cc V-Twin. See it running on YouTube:

Norm Heistand

#### blane.c

##### Well-Known Member
HBA Supporter
I believe some supercharged WW2 planes had to be careful on the ground with their rpms when the boost was set for high altitudes.
You have to reduce (occasionally and rarely increase) manifold pressure adjusted for density altitude when using Pratt R2,000's and R2,800's. This is done using a chart usually attached to the takeoff checklist. The "idea" behind turbochargers is that you get some free power from the exhaust gases to drive the turbo charger, this is often offset by the reduction in exhaust diameter (choke point) at the turbocharger. The main difference in performance between exhaust driven centrifugal turbochargers and gear driven lobed or screw superchargers is that centrifugal turbochargers are not really efficient at lower RPM's and do not add significant boost until the engine is in the upper power-band while geared superchargers are more effective through a broader power range. Centrifugal turbochargers can be set up to be efficient at cruise power and excess pressure (boost) can be relieved (automatically) at takeoff power settings. Geared superchargers can also be set up to relieve pressure automatically so for aircraft applications I see no clear advantage for either system. The P-47 Thunderbolt of world war two fame had the Pratt R2,800 with geared supercharger and multiple shift points to increase supercharger RPM for various altitudes, it also had exhaust driven turbochargers and could fly well into the 40,0000 foot levels.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
The Turbo needs a heat exchanger where a supercharger does not... ect.
Either a supercharger or a turbocharger may need (or at least could benefit from) a heat exchanger depending on the application because both increase the temperature of the intake charge. There are also some applications for each that do not strictly need one (e.g. many draw-through turbos get by fine without one).

#### rv6ejguy

##### Well-Known Member
Lots of misinformation here from the supercharger advocates.

For aircraft operating at very different altitudes, you need to increase the pressure ratio to maintain manifold pressure as you climb. If you bleed air off it to control boost, you generally heat the charge more on a centrifugal type- bad idea and a waste of input power. Transmissions/ clutches are complicated, heavy and maybe need quite a bit of maintenance. With a turbo, you close the wastegate as you climb, N1 increases- voila higher pressure ratio and infinitely variable, not coming in big steps.

As Nerobro stated, the need for intercooling is directly related to compressor efficiency and pressure ratio and modern centrifugal compressors are about TWICE as efficient as a straight lobed Roots supercharger. Apples to apples, turbos produce much higher hp than superchargers, not that that is important for aircraft usually.

Compressing air takes power, lots of it at the flow rates engines need so no belt on a blower = no hp increase. Not sure what the point of that was...

Modern turbos can generate torque peak anywhere you want, many cars have TP down in the 1400-1750 rpm range these days and can carry 80-90% of that torque right to redline- very broad powerbands indeed.

Look at the number of production supercharged GA aircraft compared to turbocharged. That pretty well sums it up. They work but just are not a good choice for aircraft applications.

#### Vigilant1

##### Well-Known Member
Supercharging is the only way to get desired power at the lowest cost, $300 for blower,$50 for pulleys and belt. I am thinking of building a 1600cc VW 4 cylinder for longblock $1400, supercharged$400, ignition $50, exhaust$50, carb $100, prop hub$150, and ready to go for \$1150. I challenge anyone to match that price for a zero time engine with reliable 70 hp.
If you want to build a supercharged 1600cc VW engine, go ahead. But in aircraft use it will give you no advantage over a naturally aspirated VW.

Power: Your 1600cc supercharged engine will produce less power than a 2180cc engine. You say you can make 70HP, NA 2180cc engines make more power. Unless you work some magic on the heads, the heat rejection is the limiting factor for cruise power.
Cost: Your installation will cost more--the VW parts cost about the same regardless of displacement (if they are comparable quality), and then you've got to buy the blower, etc.
Weight: All the VW engines, if configured the same (drive end, accessories, etc), weigh about the same regardless of displacement--within a few pounds. Your supercharger weighs something.
Reliability: You'll be running at higher effective compression ratios--that means a higher risk of detonation and other issues.

Why do it? Maybe there are other good candidates out there to be supercharged--maybe a watercooled motorcycle engine with excess cooling capacity and the ability to run at high CR without any problem. Get a reduction drive, turn it at 6000 RPM, and try things out. But this VW engine is not a good candidate for what you are planning. If you like VWs (I do), just use more displacement if you need more power than a 1600cc can give. If you need more than 70-80HP, then look at a different engine.

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#### nerobro

##### Well-Known Member
Log Member
You have to reduce (occasionally and rarely increase) manifold pressure adjusted for density altitude when using Pratt R2,000's and R2,800's. *snip* The P-47 Thunderbolt of world war two fame had the Pratt R2,800 with geared supercharger and multiple shift points to increase supercharger RPM for various altitudes, it also had exhaust driven turbochargers and could fly well into the 40,0000 foot levels.
It goes a bit further than that. Centrifugal superchargers and radials happened sort of by accident. Getting good mixture delivery to the cylinders on a radial is not easy. Between long runner lengths, and gravity issues, bottom and top cylinders see very different mixtures. So, the logical solution was a mixing device. That mixing device was crank driven, and had this funny byproduct of producing some pressure.

That mixing device ended up being developed into centrifugal superchargers.

Now, eventually, engineers got interested in what they could do with the exhaust gasses coming off the engine. That lead to the turbo-superchargers we know and love. When applied to a radial, there was no reason to remove the mixing device....

Stacked superchargers and turbochargers are not unheard of. Most commonly you'll find hybrid turbo/supercharger setups on locomotives. They tend to be two strokes, and are dependent on a supercharger for scavenging. By using a sprag clutch, they can use the same turbocharger as a supercharger at low power settings. Also some engines just had the conventional supercharger behind a turbocharger setup.

Lots of misinformation here from the supercharger advocates.

*snip*

As Nerobro stated, the need for intercooling is directly related to compressor efficiency and pressure ratio and modern centrifugal compressors are about TWICE as efficient as a straight lobed Roots supercharger. Apples to apples, turbos produce much higher hp than superchargers, not that that is important for aircraft usually.

Compressing air takes power, lots of it at the flow rates engines need so no belt on a blower = no hp increase. Not sure what the point of that was...

*snip*

Look at the number of production supercharged GA aircraft compared to turbocharged. That pretty well sums it up. They work but just are not a good choice for aircraft applications.
I'm only an advocate in the "it's potentially easier to do" bracket. Turbochargers are essentially ideal for aircraft use. As long as money and injection/carburation isn't the question.

Screw (which look a lot like roots..) superchargers come pretty close to centrifugal designs. Enough so, that they can afford to use them on modern cars. Unless I had one handy... I wouldn't be looking in that direction though.

I mentioned running the engine without the supercharger active, to cover for hardware failures. If a turbocharger fails, you're looking at potentially a very large oil leak, reducing your flight time, and likely the need to rebuid the engine. Most superchargers are self contained, and a supercharger failure will just lead to a reduction in altitude (providing your were doing normalization) as opposed to total engine shutdown. It's potentially somewhat safer.

Have there been any supercharged O style engines? I can't think of any.

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
It goes a bit further than that. Centrifugal superchargers and radials happened sort of by accident. Getting good mixture delivery to the cylinders on a radial is not easy. Between long runner lengths, and gravity issues, bottom and top cylinders see very different mixtures. So, the logical solution was a mixing device. That mixing device was crank driven, and had this funny byproduct of producing some pressure.

That mixing device ended up being developed into centrifugal superchargers.

Now, eventually, engineers got interested in what they could do with the exhaust gasses coming off the engine. That lead to the turbo-superchargers we know and love. When applied to a radial, there was no reason to remove the mixing device....
Where might one read up on this story? It is at odds with the history I have read. Would like to hear more if you can point me to a source.

#### nerobro

##### Well-Known Member
Log Member
Where might one read up on this story? It is at odds with the history I have read. Would like to hear more if you can point me to a source.
I hope I can find it. I remember reading about it in a book when I was in college.... That's a long time ago now. But.. I'll try. And i'm open to being wrong.. it happens.