Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Ardy123 on February 03, 2011, 06:44:35 PM
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If an engine has 2 gear settings for the super charger, one for high alts and one for lower alts, and the engine is left on the high alt one when not at high alts, would it blow the manifold gaskets?
I ask, because HiTech stated he could see adding control for the super charger gears in engines. If he were to add this, and one was in combat and the fight fell below the alt floor which the high gear is for, how long would it take for the engine to fail?
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That's a difficult question IMO. Some engines tolerate over boosting longer than others and it also depends on how close the FThs for different stages are to each other. If the first FTH is significantly lower than the second e.g. in a full throttle dive below second FTH with high gear on it would cause quickly a severe over boost situation where the engine would break down rather quickly.
What would be the mechanism then? I don't know but my guess would be that a blown gasket is the mildest result and I'd expect something a bit more radical when such a huge engine knocks severely. Maybe a destroyed piston or cylinder head i.e. what ever happens to be the weakest point, could also be a connecting rod or even crankshaft which could cause a bit more radical effect to the airframe, even an explosion.
Merlins also had an anti detonation screen to protect the engine detonating in direction of the supercharger but not all had this so I'd expect that overboost and too lean mixture could cause a detonation which could damage the supercharger. Not sure if this applies for this situation though...
-C+
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it wont happen every time but if it does, you would see a piston close to the size of your torso fly through the cowl, not changing the blower will also destroy the supercharger in some cases
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I would have to do research, but I am fairly sure nothing would happen.
There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.
2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.
HiTech
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I would have to do research, but I am fairly sure nothing would happen.
There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.
2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.
HiTech
Turbo superchargers had a waste gate.
Merlins had a setup that restricted the opening of the butterfly. The throttle lever might be full forward but the butterfly would be only partially open. If 15lb boost was the max boost, then that is all the engine got.
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Turbo superchargers had a waste gate.
Merlins had a setup that restricted the opening of the butterfly.
Any idea how this worked? But it seams to me that would tend to always over pressurize the inlet, and cause a large inefficiency in the charger do to the high out let pressure.
HiTech
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Any idea how this worked? But it seams to me that would tend to always over pressurize the inlet, and cause a large inefficiency in the charger do to the high out let pressure.
HiTech
from Allied Aircraft Piston Engines of WW2 by Graham White
To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.
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Interesting that they couldn't make the supercharger change gears using this mechanism.
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from Allied Aircraft Piston Engines of WW2 by Graham White
To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.
Interesting,sounds somewhat like a mass airflow sensor in modern fuel injected cars!I might be mistaken but isnt that how the idol works?
end hijack.....
:salute
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from Allied Aircraft Piston Engines of WW2 by Graham White
To eliminate this chore (pilot monitoring manifold pressure) R-R and the RAE developed an automatic boost control that allowed the pilot to "firewall" the throttle without concern for over boost.This task was accomplished by a servo system that read manifold pressure and opened the throttle butterflies far enough to give the maximum rated boost and no more. As the a/c climbed, the automatic boost control allowed the throttle (butterflies) to open until the critical altitude was reached, that is, the point at which the the throttle (butterflies) would be wide open.
Something I didn't know, but it still does not eliminate the possibility of a waist gate. Also was the butter fly before of after the super charger? If before it would make since to me for no need.
Ever seen the complete intake system diagram?
HiTech
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Something I didn't know, but it still does not eliminate the possibility of a waist gate. Also was the butter fly before of after the super charger? If before it would make since to me for no need.
Ever seen the complete intake system diagram?
HiTech
The book has a diagram. The butterflies (2 barrel carb) were before the supercharger thus restricting the amount of air being compressed.
The waste gate on the GE turbo supercharger was on the engine exhaust side, not the air intake side, of the turbocharger. The gate opened to restrict the compressor side pressure being produced.
SectorNine50, an aneroid switch was used to change between high blower and low blower. If the nominal change altitude was say 18,000ft, the change altitude would be say 16,000ft when going from high to low blower. If one was fighting around 18,000ft, one didn't want the blower to change every time one went below 18,000ft.
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If you restrict the air that goes IN the supercharger, it makes less boost, less heat, and takes less HP to turn.
A waste gate will not work on a crank driven supercharger, the waste gate vents exhaust gas past the turbine of a turbocharger into the exhaust pipe, to keep the turbine from spinning the compressor faster so that it does not make as much boost. There is no exhaust side turbine on a crank driven supercharger.
As far as the original question, if you were to continue to over boost an engine, as you would if you left a two speed blower in high altitude gear at low altitude, you could easily destroy the engine. The excessive manifold pressure generated would make the engine detonate itself to pieces, probably knocking the rings off of the sides of the pistons, crushing the rod bearings, caving the pistons in, and seizing the pistons in the bores. Odds are, a Merlin rated at 80" of boost might make well over 100" of boost if the blower were left in high altitude gear. That kind of cylinder pressure could easily destroy the engine.
I thought I remembered reading that the two speed superchargers were commonly shifted by an aneroid bellows.
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CVH , Obviously if you over boost an engine serious things will happen very soon, but the question we are talking about does leaving a supercharger in high alt gear over boost the engine. With an automatic butterfly valve or other device are you sure it would over boost?
HiTech
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I would have to do research, but I am fairly sure nothing would happen.
There is a waist gate after the Super Charger that controls max manifold pressure. You would just be dumping more air out this gate. So you would not over boost the engine. Now I am not sure if anything would happen to the Super Charger, but off hand I can not think of any.
2nd think about it from a design perspective, would you really design something that would blow an engine just by a pilot forgetting to change gears? I would be fairly sure that it has happened many times.
HiTech
I would guess it creates an adverse effect. At higher alts the air is very thin. Now at low alts you are trying to turn a fan with much more resistance. It would be my guess that also accounts for the reasons you see flat spots in engine performance at a certain alt. where the trade off in engine power vs. boost could go either way.
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Don't they use pop-off valves on Roots type blowers?
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http://naca.central.cranfield.ac.uk/reports/1941/naca-tn-795.pdf
Just a link. Enjoy.
-C+
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Interesting link.
God but there were MEN in those days - imagine figuring all that out with the old slide rule folks....
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Generally nothing special will happen assuming that throttle valve is able to prevent the overpressure. The efficiency and power is just lower due to unnecessary workload for supercharger and higher temperature of the incoming air.
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CVH , Obviously if you over boost an engine serious things will happen very soon, but the question we are talking about does leaving a supercharger in high alt gear over boost the engine. With an automatic butterfly valve or other device are you sure it would over boost?
HiTech
If there is an automatic throttle valve that controls boost by referencing manifold pressure, provided it has the operating range to close the throttle enough, it will prevent excessive boost. In other words, if whatever automatic control can close the throttle ahead of the blower enough, yes, it can prevent excessive boost.
If there is no automatic throttle control to prevent excessive boost, it will over boost if you leave the throttle wide open. Example: Let's say that if you drive the blower at 1:1 (crankshaft speed) at or below 18K, and at War Emergency Power, it produces 80" of boost, which is all the engine will stand without detonating, provided it is cool enough. To produce the same 80" of boost at say, 27K critical altitude, let's say it requires you to drive the blower at 1.30:1 crankshaft speed (30% overdrive). So if you dive from 24K to 16K, leave the blower in overdrive, operating at WEP, then the blower is going to over boost the engine in just a few seconds, pretty much the instant it gets dense enough air, figuring 90% efficiency, you're going to get about 20-25% more boost, so now you have close to 100" of boost. On an engine already operating on the ragged edge, you have maybe a few seconds before it starts killing itself in a rather spectacular manner.
I'm not sure an automatic throttle control is present, I'd have to look at some diagrams to see if it is. Remember, these piston engine propeller driven fighter aircraft have a system that uses propeller pitch to control RPM, not throttle position. Throttle position is what governs boost on these systems. The throttle is controlled by the pilot, manually. So, assuming that the pilot left the throttle in the War Emergency Power position, unless the throttle itself, in the wide open position, presents enough restriction, if the blower drive system does not shift down to low speed, the boost will exceed the maximum rating.
To give you an example of how large a difference blower drive ratio makes, with a typical 8-71 blower on a 450 cubic inch engine (a properly sized blower for good performance and efficiency), drive ratio change from 1:1 to 1.2:1, or 20% over drive, a very common change, will take you from 8-10 pounds of boost (we don't use "inches of mercury or water to measure boost in racing) to 12-14 pounds of boost on a standard blower. On an engine that is operating as close to the edge as a World War II fighter engine, if it is designed to operate at 8-10 pounds of boost and you run 12-14 pounds of boost, the engine can be destroyed (catastrophic failure) in one ten second pass. Back when we were running the full 1/4 mile, or 1320 feet, and doing it in 4.4 seconds, we could make a small blower pulley change and predict almost exactly (within 20 feet) where the engine would reach an over-boost condition and blow up like a hand grenade.
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Don't they use pop-off valves on Roots type blowers?
Yes, we use preset pop off valves with springs, the area is around 2 to 4 square inches, depending on the blower and the intake. That's an emergency valve, and is not intended be held in the unseated position for any real length of time, it's to prevent severe damage in case the engine were to backfire through the intake tract. The venting from the pop off valve opening releases fuel and air under pressure, in a relatively closed area, say, inside the engine cowling of an airplane, odds are leaking voltage from the ignition system would ignite the fuel and air, causing at least a small explosion.
On the Top Fuel car I crew part time, we also have a burst panel on the intake manifold, that, combined with the pop off valve, is intended to keep the blower from being blown off the intake in the event of an intake valve hanging open.
On turbochargers, if they are mounted in front of the throttle, we use a "sneeze valve", it's just there to keep the turbocharger bearings from being damaged if the throttle is slammed shut during high boost conditions.
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"and higher temperature of the incoming air."
And that is what I'd be worried about.
-C+
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"and higher temperature of the incoming air."
And that is what I'd be worried about.
-C+
Well, if you increase the boost, you increase the air temperature, unless you have an intercooler (found only on turbocharger equipped planes, best I can tell). Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds.
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Merlin engines from the 60 series on had intercoolers.
(http://www.enginehistory.org/TM/V6N1ICMerlin.jpg)
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Saw a blown Merlin off American Beauty at Reno a couple of years ago. A connecting rod snapped from being overboosted and it blew the whole bottom of the oil pan off. There was a hole big enough to stick your foot through. Obviously they're using ADI and running 120" or so, but that's the result.
[Edit]Also, good description of the P-51D automatic boost control in the POH in the AH Wiki. May be worth looking into a F4F manual or something older that had manual boost controls to see what the procedures were...
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All I remember is reading many comments by the Spit drivers talking about the 'thump' of the high blower kicking in as they hit a certain altitude. it never sounded like something they set on their own but that it was set for certain heights to kick in.
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Well initially they were manually controlled but it does not take much to impelent a device which monitors air density and boost pressure and if they both fall to certain level the second gear is clutched automatically. They even installed a cuckoo clock in FW190 and it did a fine job on engine management. :lol
"Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds."
AFAIK this is also what happens if you rotate the impeller too fast in too dense air and as the impeller starts to stall it causes the charged air to heat up which eventually leads to uncontrolled detonation.
-C+
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Saw a blown Merlin off American Beauty at Reno a couple of years ago. A connecting rod snapped from being overboosted and it blew the whole bottom of the oil pan off. There was a hole big enough to stick your foot through. Obviously they're using ADI and running 120" or so, but that's the result.
It also almost certainly had either aftermarket rods or Allison rods, as a Merlin never won Reno until around 1969 (not the exact year) or so when someone figured out how to put Allison rods in it. Several companies, such as Oliver, now make 4340 forged billet rods for the Merlin and the Allison. The oiling system on the Merlin is weak, and the big end of the stock rods will not stay round.
Of course, they're spinning around 4500 RPM as opposed to the original red line of around 3000. That 1500 RPM does not seem like a lot, until you consider that it is more than 40% above the original design rating, and until you hold a rod and piston in your hand, the stuff in them is massive. Boost does not break the rods, what it does to the rods is to force the big end out of shape, that pinches the bearing, which then sticks to the crank journal and spins, burning the rod until it becomes weak and breaks.
Tweedling methanol/water into the engine doesn't make any more power, in fact, they only put in what they have to in order to keep the combustion temperature down, any extra kills too much power.
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Well initially they were manually controlled but it does not take much to impelent a device which monitors air density and boost pressure and if they both fall to certain level the second gear is clutched automatically. They even installed a cuckoo clock in FW190 and it did a fine job on engine management. :lol
"Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds."
AFAIK this is also what happens if you rotate the impeller too fast in too dense air and as the impeller starts to stall it causes the charged air to heat up which eventually leads to uncontrolled detonation.
-C+
Yeah, like I said, I'm pretty sure the blower drive speed was controlled by a aneroid bellows that shifted at a preset altitude. An automatic throttle control to use the throttle to control boost could be made much the same way a "throttle stop" is used in drag racing today (I hate them), the driver, or in this case, the pilot, would open the throttle all the way, but an automatic device would be able to close the throttle at a pre-determined boost level, with a telescoping/sliding segment in the throttle cable, or a second set of throttle plates.
Yes, compressor stall will heat the air, but it also causes pressure surges, and then you have serious inefficiency that kills performance, not to mention you can damage the supercharger.
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FYI ...
The water/alcohol injection on a Merlin (or any engine) does not appreciably affect combustion
temperatures. It does however affect the intake air temperatures and helps prevent detonation
or pre-ignition. Detonation can cause cylinder pressures to spike well over normal limits and shocks
the piston, rod, ring package, and crankshaft. In fact spark knock (as its sometimes called) is the
audible sound of two wave fronts colliding. In a over boosted condition this would be the most likely
cause of damage to an engine.
Strip
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All I remember is reading many comments by the Spit drivers talking about the 'thump' of the high blower kicking in as they hit a certain altitude. it never sounded like something they set on their own but that it was set for certain heights to kick in.
I’m reminded of Bobby Oxspring's experience flying Spitfire IXs with 72 squadron in North Africa during early 1943:
"The second stage supercharger of the Merlin 61 had an automatic barometric control gauged to cut in at 19,000 feet. Since barometric instruments are notoriously imprecise, the effect on a squadron climbing for altitude at high power settings meant that twelve superchargers cut in at slightly different times. The effect of the formation with these widely fluctuating power settings was like shuffling a pack of cards and trying to keep the same suit together. To overcome this discrepancy, we decided to climb well above the automatic setting utilizing the manual override. On a radio order from the leader we flipped the 'auto' switch and cut the superchargers in together. We were then able to contend with the foe at high altitudes, but it was prudent to keep the knowledge of our capability from him as long as possible. Bearing in mind the efficiency of the enemy’s 'Y' service, we devised a code for 2nd supercharger engagement. The formation leader's instruction for simultaneous action by other pilots was 'Up your pipe'; Up your' being the warning, and 'Pipe'’ being the executive command to cut in. The stratagem had the desired effect and 'Pipe' sent the squadron soaring aloft like a pack of homesick angles."
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If an engine has 2 gear settings for the super charger, one for high alts and one for lower alts, and the engine is left on the high alt one when not at high alts, would it blow the manifold gaskets?
For Spitfire IX see:
(http://www.spitfireperformance.com/jl165rrspeed.jpg)
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FYI ...
The water/alcohol injection on a Merlin (or any engine) does not appreciably affect combustion
temperatures. It does however affect the intake air temperatures and helps prevent detonation
or pre-ignition. Detonation can cause cylinder pressures to spike well over normal limits and shocks
the piston, rod, ring package, and crankshaft. In fact spark knock (as its sometimes called) is the
audible sound of two wave fronts colliding. In a over boosted condition this would be the most likely
cause of damage to an engine.
Strip
Actually, alcohol, be it ethanol or methanol, has about 1/2 the BTU content of gasoline, and the BTU content for water in this case is nil, since water adds no energy to the combustion process, and occupies space normally filled with something that actually burns. When I was racing my turbocharged Buick V6, the MWA injection would lower combustion temperatures about 100 degrees, as noted by the accompanying drop in exhaust gas temperature. Before I quit fooling with it, we switched to pure methanol injection, which did not drop temperatures as much, nor did it cost as much power.
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Out of curiosity how were you measuring combustion temperature?
Strip
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Out of curiosity how were you measuring combustion temperature?
Strip
We were measuring coolant temperature rise and EGT. It's really simple, if your EGT goes down with no other change than injecting methanol and water, you certainly reduced combustion temperature, as they are closely related. It's why the EGT on a properly tuned engine on methanol will be about 250 degrees cooler than the same engine properly tuned on gasoline. You'll also note that on methanol, the problem is not keeping heat out of the engine, but rather getting heat in the engine. Not only that, a close examination of components will tell you about where your combustion temperature is.
If you add methanol and water, which reduce temperature, to the combustion process, the temperature will go down. Unless of course you do something else to bring the temperature back up. Which defeats the purpose.
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Actually I think (at least in the Corsair) that you certainly could over-boost the engine. Hence the caution in the operator's manual to reduce manifold pressure before switching to a higher blower as you increased altitude. I'd have to go look, but I'm 99% positive there was also an automatic blower setting to preclude damage if you forgot to shift to a lower blower setting in a dive. I could be thinking of the auto mixture setting though.....
-dtrip
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You will find that the reason the water/meth injection allows you too run higher boost
is the intake air temp. The injection brings the intake charge temperature down and
limits the susceptibility to detonation. The slightly lower combustion temp is a by product
and not the actual cause/effect.
Strip
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In the hot rod diesels they use water/meth to lower egt's so that they can continue to add fuel. The reason they use water/meth and not straight water is that it has better heat transfer, just like the coolant in your radiator.
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You will find that the reason the water/meth injection allows you too run higher boost
is the intake air temp. The injection brings the intake charge temperature down and
limits the susceptibility to detonation. The slightly lower combustion temp is a by product
and not the actual cause/effect.
Strip
No, that is not what we found. Lowering the intake charge temperature actually increases the density of the intake charge, thereby driving the cylinder pressure up, which defeats any possible detonation reduction effect of temperature reduction. Methanol has a much higher octane rating than gasoline, and water simply does not burn in the combustion process, it turns to steam. Methanol, because it contains about 1/2 the BTU's as gasoline, burns around 250 degrees cooler than gasoline, reducing combustion temperatures, the water, turning to high pressure steam, creates something of a buffer, preventing two flame fronts from colliding, and further cools the combustion process.
We started running 120 octane race fuel, which stopped the detonation, and then started injecting pure methanol for the cooling effect when we ran the car as a non intercooled 1984 model. We eventually switched to propane, as it cooled it even further. But the intake charge cooling only made up for the fact that there was no intercooler present, it did not help detonation.
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Cap
Lowering the intake charge temperature actually increases the density of the intake charge, thereby driving the cylinder pressure up.
Are you speaking pre or post ignition. If Pre, can you explain why a density increase would also be a pressure increase when simply dealing with at compression ration? Because I seem to be missing something.
HiTech
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Cap
Are you speaking pre or post ignition. If Pre, can you explain why a density increase would also be a pressure increase when simply dealing with at compression ration? Because I seem to be missing something.
HiTech
If you drop the temperature of the intake charge, you increase the density, more air and more fuel occupy the same space. If you put a denser charge into a cylinder, the cylinder pressure will be higher, even before ignition. Simply put, if you increase the air density by 10% by cooling it say, 20 degrees (an example, not an exact figure), you will have more cylinder pressure on the compression stroke as well as on the power stroke. You've put more air/fuel in the same space as before, and you'll squeeze it into the same smaller space as before (static compression ratio). When you compress that air/fuel charge, you're going to heat it back up, and in fact it's going to get to about the same temperature as it would if the intake charge were not cooled by 20 degrees, simply because you are compressing it, and it is in direct contact with hot engine components.
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CVH, are you saying the the point of adding water is to cool the mixture before the intake?
I thought that the point of adding water droplets to the mixture was that during compression, when the temperature rise, it is delayed the water phase transition into gas. This lowers the
over all rise in temperature and delays detonation, while at the same time increase the pressure because liquid water has turned into gas.
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CVH, are you saying the the point of adding water is to cool the mixture before the intake?
I thought that the point of adding water droplets to the mixture was that during compression, when the temperature rise, it is delayed the water phase transition into gas. This lowers the
over all rise in temperature and delays detonation, while at the same time increase the pressure because liquid water has turned into gas.
No, I'm not saying that at all, that is what Strip is saying.
Once again, introducing water into the combustion process lowers the temperature, because water absorbs heat and turns to steam in the process. The resulting steam slows the speed of the flame front, and if two flame fronts form, tends to insulate them from each other.
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If you drop the temperature of the intake charge, you increase the density, more air and more fuel occupy the same space. If you put a denser charge into a cylinder, the cylinder pressure will be higher, even before ignition. Simply put, if you increase the air density by 10% by cooling it say, 20 degrees (an example, not an exact figure), you will have more cylinder pressure on the compression stroke as well as on the power stroke. You've put more air/fuel in the same space as before, and you'll squeeze it into the same smaller space as before (static compression ratio). When you compress that air/fuel charge, you're going to heat it back up, and in fact it's going to get to about the same temperature as it would if the intake charge were not cooled by 20 degrees, simply because you are compressing it, and it is in direct contact with hot engine components.
I'm still missing something Cap I agree more air molecules & hence more gas & power, but if you start with with 40 " and have a 2 to 1 compression ratio, don't you end up with 80 " regardless of density?
Or are you saying the increased destiny causes more of a temperature rise during compression, and hence more pressure?
HiTech
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I'm still missing something Cap I agree more air molecules & hence more gas & power, but if you start with with 40 " and have a 2 to 1 compression ratio, don't you end up with 80 " regardless of density?
Or are you saying the increased destiny causes more of a temperature rise during compression, and hence more pressure?
HiTech
With a 2:1 compression ratio, you'd start with 80 cubic inches, and end up with 40 cubic inches. Compression ratio does not work like boost does.
Now suppose that with an intake temperature of 200 degrees, you have a density of 50%, so the volume of the intake is filled by 1 million molecules. Suppose that if you lower that intake temperature to 125 degrees, you have an density of 75%, so the same intake volume now contains 1.5 million molecules. Now, pass those into a cylinder. Now, the volume of the cylinder will be the same, and so will the volume of the combustion chamber that the contents will be compressed into. Now, which create more pressure compressed into the same volume, 1 million molecules, or 1.5 million molecules?
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Now, which create more pressure compressed into the same volume, 1 million molecules, or 1.5 million molecules?
Trying hard to understand Cap, but something still isn't jiving in my head.
This question can be answered either way depending on the temp. In fact to start with before compression we had the very case you described, volume the same, different temp, more molecules/density but the same pressure.
First Boyle Law, pressure volume ratio is constant, I.E. double the volume 1/2 the pressure if temp remains the same.
http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH5104
So it appears to me that any different in pressure based on starting density, has to be do to a difference in the temperature rise based on the starting density.
But if memory servers the change in heat is also based only compression ratio , correct?
HiTech
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Yes, mechanical compression ratio is a determining factor in both heat and pressure. That's why engines with superchargers are almost always built with a lower mechanical compression ratio.
The more you compress a gas, the closer it becomes to liquid form. You cannot compress a liquid. A denser gas will approach a liquid state faster during compression and yield a higher pressure.
Keep in mind, while MWA, or any variation thereof, will lower the temperature of the intake charge, making it more dense at the same pressure, the temperature of the cylinder, the piston, and the combustion chamber will not be lowered appreciably, when the cooler, denser intake charge reaches the cylinder, it will rapidly absorb that heat, and reach a temperature nearly the same as the intake charge would have been had it not been cooled by the MWA.
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Captain,
Your logic that compressing a gas brings it closer to liquid form are severely flawed when applied to this conversation. The gases found inside the combustion chamber are well above there critical temperature. Critical temperature is the point where a gas, not matter what pressure, cannot form a liquid. Nitrogen and Oxygen in this circumstance will closely follow Boyle's First Law and are infinitely compressible.
The compression ratio found in a blower engine is, as you've said, often quite lower than a natural aspirated engine. This is done for a couple of reasons but engine reliability is the main goal. The pistons are often thermally stressed due to heat and the lack of a cold incoming air charge to cool them. Lowering the compression takes heat away from the piston, head and cylinder assembly. Plus an open combustion chamber is conducive to good flame travel and resisting preignition.
During any conversation about boosted engines the subject of intake air temperature is bound to come up. There is a very good reason for this, the intake air temp closely correlates to when an engine will detonate. Too much boost coupled with a high intake temperature will cause the mixture to preignite or detonate. In this application where durability is a prime goal detonation will determine the danger point.
Detonation occurs when the incoming mixture is compressed, and heated, to a point where it spontaneously combusts. Lowering the intake temperature will allow the mixture to compress more before this comes into play. About this time octane rating also comes into play, the higher the rating the less apt a mixture will detonate. Ceteris Paribus the higher the octane rating the more boost pressure and ignition timing an engine can run. Obviously many things can affect the exact point of detonation but this is the most common one. This is why many engines had uprated horsepower when running high octane fuels, regardless of water injection.
When water (or alcohol) injection is on the mixture is sprayed within intake tract cooling the incoming compressed (and heated) air. Furthermore, depending on the exact engine configuration the air will have only a couple hundredths of a second to absorb any surrounding heat. Within that time frame it will have been subjected to actual compression and heating for even less. Not unlike waving your hand quickly through a fire, the air does not have time to gain heat.
Engines are able to run more boost while running water injection because of colder charge temperature and consequently better detonation resistance in high boost engine operation.
Strip