PDA

View Full Version : air intake



BillSwagger
03-02-2010, 05:04 PM
http://www.raafwarbirds.org.au...01710-39%20abuse.pdf (http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/Allison%201710-39%20abuse.pdf)

I came across this read a while back and it lead me to thinking about air intake and how at higher speeds the faster moving air actually allows for higher boost pressures and engine outputs.

I know that supercharging is suppose to do this and works with the function of the blower, but does a blower exceed the efficiency of ram air?

It was explained to me that supercharging on aircraft is to help get better outputs at higher altitudes but doesn't necessarily mean improved performance at lower altitudes. Is this true?



Bill

BillSwagger
03-02-2010, 05:04 PM
http://www.raafwarbirds.org.au...01710-39%20abuse.pdf (http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/Allison%201710-39%20abuse.pdf)

I came across this read a while back and it lead me to thinking about air intake and how at higher speeds the faster moving air actually allows for higher boost pressures and engine outputs.

I know that supercharging is suppose to do this and works with the function of the blower, but does a blower exceed the efficiency of ram air?

It was explained to me that supercharging on aircraft is to help get better outputs at higher altitudes but doesn't necessarily mean improved performance at lower altitudes. Is this true?



Bill

M_Gunz
03-02-2010, 05:13 PM
True for sure. The zig-zag top speed by altitude graphs show this clearly.
Fixed-gear supercharger costs power to run and only benefits to full throttle altitude.
And it adds weight for every stage and doodah as well!
Best examples I know are to compare weight to low-alt performance of the light Russian
fighters to the high-alt German or American fighters.

Outlaw---
03-02-2010, 09:28 PM
There can be a huge performance increase at lower altitudes with turbocharged and supercharged engines. Superchargers provide instant power while turbochargers take a while to spool up. That's why dragsters use superchargers (instant power) and diesel trucks use turbochargers (increased efficiency at cruise).

--Outlaw.

Waldo.Pepper
03-02-2010, 09:50 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Outlaw---:
There can be a huge performance increase at lower altitudes with turbocharged and supercharged engines. Superchargers provide instant power while turbochargers take a while to spool up. That's why dragsters use superchargers (instant power) and diesel trucks use turbochargers (increased efficiency at cruise).
--Outlaw. </div></BLOCKQUOTE>

This may be true in the example you describe. However, if a pilot injudiciously (in for example the Cosair) engages the supercharger at lower altitude the engine may disintegrate and kill the pilot.

From Carrier Pilot by Norman Hanson.

"A youngster called Harris from 1830 Squadron was flying high -about 23,000 feet----Over Brunswick town at noon. He was tearing along at high speed with full supercharger going. (This was a small turbine of bewilderingly high revolutions positioned at the rear of the power unit, which at high altitude forced the petrol/air mixture into the engine under very high pressure.)

Quite suddenly the engine packed in through failure of the ignition system. (Both magnetos were pressurized for high-altitude flying, but at this stage the pressurization system wasn't yet perfected.) Harris glided down to around 16,000 feet, a height at which the magnetos decided to recommence operating and the engine cut in again.

The boy had already committed himself to a searing and explosive death. At this moment he had split seconds to live. When the engine cut out, he had obviously forgotten to withdraw the supercharger. Now, as the engine, from inertia, burst into life again at a high throttle setting, the thrust exerted on the bearings of the supercharger was too much. The rear of the power unit completely destroyed itself, with hardened steel tearing through fuselage, cockpit, wings-and Harris.

Down in Brunswick it was lunchtime and the main street was as busy as a bee. In the centre of the roadway stood a policeman controlling a school crossing. He looked upwards, perplexed by a strange whining. With commendable alacrity he leapt back a couple of paces as a Pratt and Whitney engine, weighing around two tons, hurtled down from a bright blue sky to bury itself in the tarmac in front of him. Nobody was hurt, but the Corsair was scattered far and wide and the young pilot had virtually disappeared."

na85
03-02-2010, 10:35 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Outlaw---:
There can be a huge performance increase at lower altitudes with turbocharged and supercharged engines. Superchargers provide instant power while turbochargers take a while to spool up. That's why dragsters use superchargers (instant power) and diesel trucks use turbochargers (increased efficiency at cruise).

--Outlaw. </div></BLOCKQUOTE>

A supercharger is also less thermodynamically efficient than a turbocharger.

Tully__
03-03-2010, 04:27 AM
The amount of torque produced by an engine is determined by (among other things):
<UL TYPE=SQUARE><LI> The amount of fuel/air mix you can cram in to the engine
<LI> The pressure you can compress it too before the spark fires at the start of the power stroke[/list]

The maximum is limited by the pressure at which the fuel/air mix begins to detonate (ignite explosively due to pressure before the spark can start a controlled burn, causing rapid and severe damage to pistons and valves).

For a naturally aspirated engine (no turbo or supercharger), the amount of fuel and air you can get in is limited by (in laymans terms) how much the engine can suck in for a single intake stroke. To get the most out of it, you then have to have the highest compression ratio that doesn't cause the detonation mentioned above. Using higher octane rated fuel allows higher compression ratios before detonation occurs. The fuel itself doesn't produce any significant difference to lower octane fuels, it's the higher compression that yields the bulk of the power increase.

Now we come to the problem. As the aircraft climbs, the air pressure drops. In real terms, the engine doesn't suck fuel and air in, it is pushed in by the ambient air pressure when the engine reduces the internal pressure. Because the ambient air pressure is lower at altitude, less fuel and air is pushed in and less torque and power are produced.

To overcome the "sucking" limit of naturally aspirated engines, pumps are added in the intake path. While any pump will do, the types used in aircraft engines are superchargers and turbochargers.

When using these pumps, the engines are often designed with lower compression ratios as this can improve service life while the engine is not operating at full load. The pump is made powerful enough to make up the difference.

Because the air and fuel is now being mechanically forced in to the engine, the effects of reduced pressure at altitude is reduced. The pump is able to compensate for the lower air pressure.

With superchargers which are mechanically driven, the amount of additional air/fuel that can be pumped is limited by the speed at which the supercharger is driven. As the air pressure reduces, the amount of benefit at a given drive speed is reduced. To overcome this shortcoming, some aircraft are fitted with a two or three speed gearbox on the supercharger. This allows the pilot to select higher drive speeds at higher altitudes, however he must remember to lower the drive speed again as the aircraft descends or the combustion chamber pressures will reach the point at which detonation occurs and the engine will be seriously damaged. These are the engines which give the obvious steps in the performance at altitude charts, the steps occur at the points where the supercharger is switched to a higher speed.

Turbo chargers drive the compressor pump with a turbine in the exhaust system. This means that they give very little benefit at low throttle settings as there is not much exhaust pressure to drive the turbine. As aircraft operate mostly at constant rpm and throttle settings, this is not a big disadvantage, however as available air can dramatically affect the amount of exhaust pressure available a turbine designed not to blow up the engine at low altitude can rapidly lose performance as altitude increases.

To get around this, a common practice is to fit a turbocharger that is capable if producing far more pressure than required, but limiting the boost provided using a device known as a waste gate. This device is a valve in the exhaust manifold before the turbine. When boost pressure approaches dangerous levels, it triggers the waste gate to open and release exhaust before it reaches the turbine and limiting the boost provided.

In some aircraft the pump was set up in two stages one after the other. Each device would provide only part of the pressure boost. This allowed the installation of two compact devices instead of one large device and gave the system some benefits in terms of range of altitude at which the device was effective and reduced strain on the individual devices. These would be either a dual supercharger set up or a turbocharger pumping in to a supercharger.

The latter setup, sometimes referred to as a turbo-supercharger, it what was fitted to the P-47. The turbo stage in this aircraft was waste gated and at full power setting the waste gate would remain partially open until the aircraft reached an altitude at which the entire pumping capacity of the combination was no longer able to produce boost pressures dangerous to the engine. This is which the performance at altitude charts for the P-47 are a straight line up to quite a high altitude, the point at which the waste gate is fully closed.

Outlaw---
03-03-2010, 08:28 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Waldo.Pepper:
This may be true in the example you describe. However, if a pilot injudiciously (in for example the Cosair) engages the supercharger at lower altitude the engine may disintegrate and kill the pilot.
</div></BLOCKQUOTE>


If the waste gate can't respond in time it is easy to overboost any boosted engine. Even automatic systems could overboost an engine, especially the early ones. More than one Merlin had the blower case lifted because the pilot failed to reduce power when passing through the altitude at which the supercharger automatically switched to high gear.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tully__:
The maximum is limited by the pressure at which the fuel/air mix begins to detonate (ignite explosively due to pressure before the spark can start a controlled burn, causing rapid and severe damage to pistons and valves).
</div></BLOCKQUOTE>

It's worth noting that you can advance the spark a bit under boost to help prevent detonation and get just a tad more boost. The best way, of course, is to use an intercooler but, that's a lot of plumbing to deal with.

--Outlaw.