Supercharging and VE

Hi,

I’ve been digging into Supercharging and all of the aspects surrounding it. I’ve watched the Webinar pertaining to Supercharging and it was of great assistance. However I’m still left with one thought that needs clarification.

Take for instance, we are tuning an engine with a Roots style positive displacement supercharger. I understand that this style moves a certain amount of air per revolution. Following this, obviously as engine speed increases so does airflow from both the Roots supercharger and the engine so effectively the supercharger is always flowing more air than the engine creating boost pressure.

However, the engine Volumetric Efficiency most of the time is non linear. This means at low rpm WOT, in theory there will be a higher boost pressure since the engine’s VE is low and then at higher rpm boost should decrease as VE increases. So in effect when boost decreases in this system wouldn’t total VE increase and vice-versa? And if so, if tuning on a Speed Density platform wouldn’t this mean at peak VE or lower boost pressure ignition timing should be retarded for safety and increased flame speed?

Thanks!

Not quite sure what your query is? You seem to be a bit confused between cylinder filling efficiency and boost pressure - 'boost' is best thought of as resistance to air flow into the cylinders, poorer airflow into the cylinders will result in higher boost pressure but approximately the same mass of air as the Roots supercharger is a positive displacement pump. less resistance less boost, and vice versa - like fitting it to a small or a large engine with same drive ratios.

You say a Roots type, so I assume it's a draw through type? With a unrestricted intake, it will be drawing a relatively fixed amount of air for each revolution and how much this is will depend on the size, or more accurately the displacement per revolution, and the rpm it is being turned at - and this will depend on the engine rpm AND the drive ratio between the engine and blower.

However there are worms in the apple. this can be summed up as charge density - how dense the air is in the manifold. There will be a restriction on the intake to the supercharger, a carbie (or two) or throttle body (or two) and depending how closed this is will reduce the air density (mass) entering the 'charger and so a lessed charge density may result, even after the compression from the 'charger.

I DO NOT like the use of 'volumetic efficiency' - it is a foul bastardisation that isn't correct terminology. Your confusion is part of that reason! I would suggest a better term is "MASS efficiency" - that is the important part and is a much more accurate description of what concerns us.

In 'theory' there should be the same proportion of air being draw in for each revolution of the engine, so what you're calling "VE" should be the same. There are issues, though, such as - supercharger efficiency isn't a straight line, by that I mean the supercharger will not move exactly the same amount of air at all rpm due to other inefficiencies, such as internal leakage past the rotors.

Another big complication is that the Roots type supercharger is notorious for it's poor adiabatic efficiency - it adds a lot of heat to the air, especially at higher boost pressures, which further increases the boost (pressure), but not density.

Normally, in the true sense of the term, you would run less timing at the highest volumetric efficiency as that is where best cylinder filling is achieved, with more before and after as there is less cylinder filling, but there is the problem of that heated air and so it may need to be adjusted for that.

NOTE, almost 5am here and got a cow of a headache, so will probably need to edit it with a clear head - and for any comments you may have.

My question should have been more simplified. I understand that boost is restriction. But my question is, wouldn't an engine with a positive displacement type supercharger, flow more air by volume when boost pressure is lower than when it is higher? In my head it would make sense that when the boost pressure goes down (less restriction) the engine will actually be more efficient than if the boost pressure would rise in regards to a positive displacement supercharger. You brought up a good point on using the term mass efficiency, as it takes into account adiabatic efficiency and other variables. I also understand that even if the engine were more volumetrically efficient, the mass efficiency can vary greatly hindering or helping performance.

Still my 2 cents worth - hopefully change isn't due...

No, with a positive displacement supercharger, it takes in a fixed volume (or mass for better understanding) of gas for each revolution, what happens after that isn't really relevant*. In simple terms, if it moves 200CFM and the NA engine's consumption is 100CFM, then the manifold pressure will be 2x pressure at the 'charger inlet so the same mass will pass through the engine, if the drive ratio is messed up, or someone makes a mistake and uses the blower on a larger engine that consumes 200cfm, then the blower will have as much passing through as the engine requires and the boost wil be 1x pressure at the charger inlet (same). If someone really screws up and puts the same blower, same ratio, etc on a big engine that consumes 400cfm, the blower will be still passing the same volume (mass) but rather than any boost, there will be MINUS 0.5x the inlet pressure - a partial vacuum.

I may have lost track a little there, but from that you should be able to see how the engine demand (how much air it rquires Vs how much the supercharger is allowing it to get) will affect boost. And also how, for the same engine, restricting the air entering the supercharger by partially closing the throttle, and so lowering the inlet pressure, further lower the boost/vacuum to the engine.

More to your point - many other types such as centrifugal (mechanical or exhaust powered), which use 'variable' displacement, are working directly against manifold pressure and so that will affect the volume (mass) of air passing through it. The greater the resistance (boost) the less the potential flow, and vice versa. With these, if you improve the engine breathing, so the true volumetric efficiency is improved, it is common to see 'boost' drop because of the lower resistnce to flow, but for more power to be realised as a greater mass of air is entering the cylinder. It is also not uncommon for a more efficient charge cooler to reduce boost, as the lower temperture of the charge lowers the pressure whileincreasing density.

Not sure if that helps, perhaps if you consider what happens if the outlets of each are completely blocked? The positive displacement wil continue, assuming no leakage, to continue pumping air until the pressure reaches a point where something breaks, whereas a centrifugal will simply reach a point where it will continue to spin but not increase boost - the airflow stalls.

*Well, not quite, simply compressing the charge by the blower will increase the temperature, and so further increase the pressure, but I'm leaving that out to keep it simple - hopefully.

I didn't quite put it properly before, but "volumetric efficiency" is simply the actual amount of gas in it vs it completely filled - it has NO allowance for density. As you picked up, a cylinder may have 100% VE (completely filled) but, if the charge density is only 1.5bar (say, a heated charge due to poor charge cooling in a forced induction engine), the overall Mass Efficiency is 150%, referred to ambient. But if a good charge cooler is used, that increases the density to 2.0bar, even if one reduced the VE to, say, 85% (the cylinder is only filled to 85% of what it could be) with a milder engine spec, the mass efficiency is going to be 170%!

It is the mass that allows power to be made, not the volume or comparative cylinder percentage filled in isolation.

This definitely cleared up a lot. Thank you for taking the time to explain everything.

I hope it helped.

NOTE, that is MY comment - others may disagree, take as much on as you can and make your own conclusions - in the end it will be you benefiting/paying for it.