contactpatch

Why is a 'roots' positive displacement blower,
less efficient than a twin-screw, or centri, compressor?
Comparative P-V, or S-T graphs, welcome.

Damon

Here's a non-technical explanation..... it basically rips up the incoming air into little "bites" above the rotors and then reassembles them below the blower. Lots of ripping and tearing going on, air flowing one way and then back the other, netting out at more air being shoved into the engine, but not a "clean" shot from end to end compared to other methods.

flrtin1

"One significant difference between a centrifugal supercharger and a Roots blower is that the centrifugal is a true compressor rather than an air mover. This means that the air exiting the centrifugal is already under pressure. This is measured in the centrifugal’s rating of 70 percent adiabatic efficiency versus the Roots’ less efficient 50 to 60 percent rating. One advantage of the Roots blower over a centrifugal is the Roots is a positive-displacement blower, which means it can come up on boost almost instantaneously, while centrifugals require time to “spool up” to create boost. As you can see, there are a number of variables that make the decision a little tougher."

flrtin1

"The word "adiabatic" describes a process in which no heat is gained or lost - 100% adiabatic efficiency would be the perfect forced induction device, creating no heat gain whatsoever, probably impossible to achieve ever. And the closest anyone can come yet is around 80% efficiency.

The problem with heat is it defeats the original purpose - the hotter the air, the lower the density possible, and the extra power comes from dense air. Another problem from heat is ignition - the hotter the inlet air, the more tendency the engine will have towards detonation and pre-ignition (knock and ping), which damages the engine, besides diminishing performance. Drivers of blown vehicles tend to keep an eye on the temperature gauge.

The goal of efficient charging is to compress the air and to keep it cool, for maximum power. The cooler the intake charge, the denser the air and the more horsepower produced.

The greater the adiabatic efficiency with which a supercharger compresses air, the less the heat that gets added to the intake manifold. Efficiency is measured by the discharge air temperature at a given pressure. For 6 pounds of boost, a supercharger with intake air temperature of 185 degrees is more efficient than another at 190 degrees. Boost itself is only the measure of pressure the intake air is under, not an indication of the power produced as horsepower.

Which has greatest adiabatic efficiency
The Roots blower has the lowest adiabatic efficiency of all the forced induction designs (including the turbocharger, which has to start off with hot exhaust gases to deal with) - generally around 50 percent. The roots type is so inefficient because it doesn't compress the air directly, but delivers uncompressed air which wells into the intake manifold, becoming more compressed, but with additional heat gain from the turbulence and reverse flows of air mixing. Centrifugal superchargers can vary from 60% up to perhaps approaching 80% efficiency, as can turbochargers; both are more efficient at higher rpm, which is another way of calling them more inefficient at lower rpm. The twin screw supercharger normally delivers lower output temperatures, for adiabatic efficiencies of 70-80% across the whole rpm range."

contactpatch

Consider a , one cylinder, (2) poppet valves, air compressor.
.
Inlet valve (from the atmosphere), works as you would expect.
Outlet valve, could be timed in different ways.
(1) outlet valve, opens pretty close to BDC, or
(2) outlet valve opens when cylinder pressure is very close to
the pressure in the storage tank.
.........
I would think that case (2) would require less input work.
as case (1) would have full tank pressure during the
entire upstroke.
.........
Perhaps (1) vs (2), would be (1) external,, (2) internal compression or something like that.
............
OK, I'm trying to see a fundamental cycle difference
between a roots and the others.
.
Blower, compressor, 'just words'.

paul_huryk

English:

Roots blowers add a lot of heat into the air. They also may not properly distribute the fuel correctly to each cylinder - typically a carb is mounted above the unit.