high/low blower super charger gears

[Mike Williams]

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:

[Captain Virgil Hilts]

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.

[Strip]

Out of curiosity how were you measuring combustion temperature?

Strip

[Captain Virgil Hilts]

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.

[dtrip61]

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

[Strip]

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

[SIK1]

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.

[Captain Virgil Hilts]

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.

[hitech]

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

[Captain Virgil Hilts]

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.

[bozon]

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.

[Captain Virgil Hilts]

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.

[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.

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

[Captain Virgil Hilts]

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?

[hitech]

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