Forced induction? Compression?

[SaabJohan]

When the ignition is retarded and/or a richer fuel mixture is used this can supress knock, to some degree that is.

A pop off valves open when the boost pressure is high. A blow off valve open becasue of the pressure difference before and after the throttle, that means that it will open if the throttle is closed after boost preventing the turbocharger, or supercharger from surge. Note that blow off valves are only used when the turbo or supercharger is placed before the throttle.

An engine will deliver maximum power with an air fuel ratio, by weight, of about 12-13:1, this is rich, but to supress knock a even richer mixture must sometimes be used and this will cause power to drop. For maximum efficiency the engine should be runned lean, leaner than stoichiometric.

When the engine knocks, it is a detonation that happends after the spark has ignited the fuel (if it happens before it's a pre ignition). The more the ignition is advanced (up to the point of maximum torque setting) the power output will increase and so will the combustion pressure which increase the possbility of knock.

Positive displacement superchargers will give a constant boost pressure but not dynamic compressors like the centrifugal compressor, it will increase boost with rpm. However, using pop off valves are not a good way to limit boost (it can however be used as a safety device or in racing so a higher boost pressure than what is allowed isn't used, like in CART).

Using a high octane fuel is a good way to stop detonation, water injection can also be used but it has downsides.

[Sluttypatton]

Caught my mistake, 12:1 is richer not leaner...apparently I'm not on the ball today.

[Auto_newb]

Originally Posted by SaabJohan

When the ignition is retarded and/or a richer fuel mixture is used this can supress knock, to some degree that is.

A pop off valves open when the boost pressure is high. A blow off valve open becasue of the pressure difference before and after the throttle, that means that it will open if the throttle is closed after boost preventing the turbocharger, or supercharger from surge. Note that blow off valves are only used when the turbo or supercharger is placed before the throttle.

pop-off valves work only when the throttle is open, and blow off valves only work when throttle is closed?

An engine will deliver maximum power with an air fuel ratio, by weight, of about 12-13:1, this is rich, but to supress knock a even richer mixture must sometimes be used and this will cause power to drop. For maximum efficiency the engine should be runned lean, leaner than stoichiometric.

Leaner than 14.7:1?

When the engine knocks, it is a detonation that happends after the spark has ignited the fuel (if it happens before it's a pre ignition). The more the ignition is advanced (up to the point of maximum torque setting) the power output will increase and so will the combustion pressure which increase the possbility of knock.

Umm, doesn't the spark plug ignite the fuel? Wouldn't that burn pretty much all of it?
How is detonation so bad when it only happens during the power stroke?
Does the spark plug NOT burn all the fuel?

Positive displacement superchargers will give a constant boost pressure but not dynamic compressors like the centrifugal compressor, it will increase boost with rpm. However, using pop off valves are not a good way to limit boost (it can however be used as a safety device or in racing so a higher boost pressure than what is allowed isn't used, like in CART).

Using a high octane fuel is a good way to stop detonation, water injection can also be used but it has downsides.

Postive displacement: Constantly giving a set amount of PSI regardless of engine RPMS?

Why is it bad to use pop-off valves to control boost? Isn't it like a wastegate?

What's wrong with water injection? Doesn't it cool down the hotspots?

[Sluttypatton]

Pop off valves work when the boost pressure overcomes the spring load, opening the valve and venting boost to the atmosphere, this could be when the throttle is open or closed, just as long as the pressure is enough to overcome the spring. Blow off valves work by manifold vacuum; the blow off valve has a line connected to the manifold (usually), when the thottle plate is open the boost pressure is equal to the manifold pressure, however when the throttle plate closes the manifold is under vacuum and this vacuum is transferred through the line to the BOV where it overcomes the spring force that normally keeps the BOV closed and the BOV opens, venting trapped boost pressure to the atmosphere. This is the reason that many BOV's have filters on them, at idle the manifold is under vacuum so the BOV is open, however there is no boost being vented since usual turbocharged vehicles don't make boost at idle, so outside air is pulled through the BOV and into the engine.

The reason for spark advance and retard is because fuel is not instantly combusted the moment the plug fires, it is a controlled combustion. If for whatever reason the fuel ignites at a spot other than where the spark plug ignited it, there will be opposing flame fronts, which will cause pressure and temperature spikes when they meet. The pressure and temperature spikes reached by detonation are bad for the motor for three main reasons; they may scour away the oil film on the cylinder walls causing undue wear to occur, they may impose undue load on the piston which can cause extreme damage to it, and they can scour away the protective gas that normally helps protect the combustion chamber from the full temperatures reached by combustion.

Positive displacement superchargers move a fixed amount of air for each revolution. This means that the airflow increases linearly with engine RPM, resulting in a constant air pressure. A roots type supercharger is positive displacement. If a roots type supercharger is moving at x rpm is will deliver y amount of air, if it moves at 2x rpm it will deliver 2y air. Positive displacement superchargers give constant pressure since engine air consumption increases linearly with air flow through the supercharger.

Pop off valves are a poor method of boost control because they tend to be inaccurate, noisey (some people may like that), and environmentally unfriendly on draw through systems. They do however have some merit as safety devices, just in case the wastegate fails.

I have never used water injection, so SaabJohan would be better at explaining his beef with it.

[SaabJohan]

The largest problem with the pop off valves is that that they blow off air that could be used, and the compressor will need to pump that air anyway.
With a wastegate the turbine power is controlled, therefore the compressor will not do unneccesary work.

Stoichiometric air fuel ratio is for gasoline about 14.7:1.

The spark plug will give a spark, then with a short delay the flame front will start to travel, first with the laminar flame speed (usually 0.3-0.5 m/s for normal fuel). The combustion products will have a different density and will therefore push the flame outwards at a much higher speed, say 20 to 50 m/s or even higher at high engine speeds. All this will take some time, usually we want the combustion to be done some after TDC, perhaps 15 degrees or so, lets say that we ignited the fuel at 20 degrees before TDC the combustion had a duration of 35 crankshaft degrees, this can be recalculated to time if the engine speed is known.

When the engine knocks this is usually a detonation around TDC, after the fuel is ignited. The preoblem is usually largest on mid engine speed.

For info about water injection look in the thread about water injection.

[Auto_newb]

Oh so there IS a wastegate for superchargers... Didn't know that. But if pop off valves blow off air that could be used, what's so different about what a wastegate does?

BTW, I still don't know what a BOV does, maybe on turbos, not sure on superchargers

Originally Posted by Sluttypatton

The pressure and temperature spikes reached by detonation are bad for the motor for three main reasons; they may scour away the oil film on the cylinder walls causing undue wear to occur, they may impose undue load on the piston which can cause extreme damage to it, and they can scour away the protective gas that normally helps protect the combustion chamber from the full temperatures reached by combustion.

How does it scour away the oil films?

Does detonation give the crankshaft a sort of "shock"?

What is the "protective gasses"?

[2turboimports]

no wastegate for superchargers...at least not in the same respect as a turbocharger. The turbo wastegate regulates how much Psi is run by diverting exhaust gases to the exhaust thereby limiting the amount of air going back to the turbocharger.

the supercharger 'wastegate' is like a valve on the intake side of things that will regulate boost if it goes over a certain amount by bleeding it out of the intake tract.

[Auto_newb]

so a wastegate on a supercharger bleeds pressure out of the intake, and the turbo wastegates bleed out of the exhaust?

[2turboimports]

as far as i know, yes

[Sluttypatton]

I'm not actually 100% sure on how exactly detonation removes the oil film, as I have never seen it happen or any research on the subject, this is just what I have learned from personal research. I would assume, however, that it has to do with the increased temperatures, pressures, and vibratory load imposed when the pressure front meets the cylinder walls.

Detonation certainly does give the crankshaft a shock, however, this is not the most concerning thing. The real issue is the shock it imposes on the piston, this can be severely damaging, especially to cast pistons.

The protective gasses are stagnant combustion products and unburnt air/fuel that remain relatively motionless lining the combustion chamber, I believe this is due to the surface friction between the gasses and the combustion chamber (but I may be wrong). This motionless gas acts quite effectively in protecting the combustion chamber from the full heat produced by combustion because it isolates the combusting gasses from the combustion chamber. As soon as detonation occurs it quickly removes this barrier gas and exposes the combustion chamber to the full temperatures attained by combustion, piston temperatures rise quickly, followed by everything else. This rise in temperature then promotes preignition.

[-= c a m a r o =-]

Originally Posted by matt11583

turbos compress air to get more air into the same size area. the more air, the more fuel. lower compress normally does mean higher top end.turbos have more power because they have a bigger top end. they dont have ass much as n/as at low end power. off the line a n/a will beat it, then later the turbo will catch up when the turbo starts spooling.

superchargers have bovs to make sure that it doesnt put too much air into teh engine. if there is more air than it can put in, the bov will open to let the extra air out.
pistons and headgaskets are the best way to lower compression.

its the wastegate that controls the boost the turbo makes, excess boost is let out thru the wastegate. the blow off valve lets excess air created by the spinning turbine escape when you let off the gas, thats what makes that psssssst sound.
*superchargers dont need blow off valves because they're belt driven. and dont be so sure that a n/a engine will beat a turbo off the line, that depends on the setup and if the turbo lags.

technically u dont need to lower compression, thats just for added safety, if u run higher octane, or lower boost u can avoid lowering compression. higher boost needs lower compression to keep the engine from detonating. dish pistons can lower compression, dome pistons can raise compression.

[SaabJohan]

Superchargers will need a blow off valve if they have the throttle after the compressor.

As mor most turbo engines on the market they give more more power at mid engine speed for a given peak output compared to a NA engine so often the stock turbocharged engines are very fast. As for racing cars these can often have boost before they even start to accelerate. If turbochargers was allowed in the faster dragracing classes they would likely beat the compressor engines, due to their higher power output.