high/low blower super charger gears

[Captain Virgil Hilts]

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.

[Strip]

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