Supercharged engine management theory

I'm trying to wrap my head around supercharging engine management concepts, but I'm missing something. I come from the turbo diesel world, where air:fuel ratio is not nearly as critical as in a gasoline engine, and where either a wastegate or variable geometry compressor vanes are used for boost control based on engine load....so bear with me.

I'm going to lay out some assumptions that I'll use in my discussion below. While not entirely accurate, it should be close enough to get the concept across.

First, we'll say that I drive where atmospheric pressure is 15 psi. (This allows me to say that 5 psi of boost would be a 30% increase in manifold pressure.)

Next, my engine is using 1 liter of air per revolution, normally aspirated.

My closed loop engine management strives for a 14:1 air:fuel ratio.

At 1 liter of air, 14:1 A:F results in 1 unit of fuel being burned.

I drive a 5-speed, and 2000 RPM = 60 MPH.

Finally, we'll say this is a "magic" supercharger that doesn't incur a fuel penalty or other losses just to turn it. (I know, but hey, it's theory, after all!)


Based on those assumptions, at 1000 RPM, 1000 liters of air are going through my normally aspirated engine, and I'm using 1000 units of fuel, per minute. At my normal cruising speed of 60 MPH/2000 RPM, I'm going through 2000 liters of air and 2000 units of fuel per minute.


Now I strap on a supercharger that makes 5 psi at 2000 RPM. Now I'm using 2600 liters of air at 2000 RPM (2000 liters + 30% more air because of the higher density)...which means now that I'm using 2600 units of fuel just to maintain a 14:1 closed loop A:F. This translates to 600 extra units of fuel consumed when supercharged vs. normally aspirated running the same speed as before.


So, am I missing something? Is there typically some sort of bypass or other method of not supercharging unless the engine is under load? Or does the supercharged engine just become that much thirstier under any given operating condition? Or do I just have to live with that 600 extra units of fuel at 60 MPH as the parasitic losses, and suck it up as the price I have to pay for being supercharged?

Thanks,
Keith

 

bypass valve on the supercharger I believe is the answer you are getting at.

 

Exactly.


The TRD blower for the 4.0 v6 hasn't affected my mpg at all..... Not unless my foot is at fault.

I can climb a long steep grade at 75 in 4th and still be in vacuum.

 

You would be putting the same volumes through the engine, my guess through reduced throttle. The same throttle position as before would give you more gas and air, and a higher RPM. RPM isn't tied to the throttle, gas/air are. Rpm is a result. Make sense?

 

Mrbear33....that's what I keep forgetting about...the throttle valve. So, that valve reduces air into the engine, bringing me back down the the perfect 2000 liters of air at 2000 RPM, and fuel matches. I've spent so long on fly-by-wire diesels, no throttle plate, no manifold vacuum, that I keep forgetting about that. In the diesel world, we used direct injection into the cylinders of just enough fuel to maintain RPM....and air flow was just to keep smoke down and burn all of the fuel in the cylinder. A smoking diesel is one where some combustion is taking place in the unpressurized exhaust pipe = wasted power.

2000GTacoma, how does the bypass work? Some or all intake air is diverted around the compressor, allowing it to spin at engine RPM but produce little or no boost?


Thanks
Keith

 

Q: What is this black plastic object on the side of my supercharger? What is a bypass valve?


A: That object is the bypass valve vacuum actuator. This unit has a vacuum port that connects to the supercharger inlet between the rotors and throttle body. This actuator opens and closes a bypass valve for the supercharger. This valve is open when throttle loads are low and closed when throttle loads are high. With the bypass valve open there is no pressure being created across the supercharger. This allows the supercharger to have an almost negligible parasitic loss in this condition. With the bypass valve closed, all airflow is routed through the supercharger and boost is created in the intake manifold. In some newer applications this actuator is also controlled by a solenoid.

 

http://www.eaton.com/EN/Eaton/Produ...motiveAftermarket/Superchargers/FAQ/index.htm


Sorry for double post... Paste option doesnt work right via phone. It always places the text in between middle of the other line


U could also call Eaton and ask them about the bypass valve operation for the M90 blower.

 

Do you guys know if the URD setup has a similar bypass or does it just run boost all the time?

 

I know they have a blow off valve, but I thought that was just to adjust the max boost. When the engine load is low does the BOV open all the way up then to allow for 0 boost basically?