CID ...........static vs. dynamic

[JHardwick]

Okay, this is a dumb, good for nothin question, but it's been in my mind for the last couple of days and I've not had time to look for any formulas. It came to mind watching the steam amble out of a gas powered Dodge, while seeing the plume of dust behind me kicking up the road grit.



Here is the scenario:



2-360 CID engines ... ... 1 is a naturally aspirated gasser, the other a turbo charged diesel.



If you turn each of the cranks 1 revolution, each engine will displace 360 cubic inches of volume and air ... ... ... ..... this is not really static, but in terms of operation it kinda fits.



Now, in operation, the gasser operates at a vacuum in the intake, and the diesel operates at a positive pressure in the intake. They are both still displacing 360 cubic inches of theoretical volume, but the actual air volume has to change, no?



I think this could get very tricky throwing combustion into the mix, not to mention a gasser continues to build vacuum, while a diesel continues to build boost.





But, I'm curious as to what the true volume of each of the engines are.

 

[RTillery]

The gasser with the butterfly in the throttle body closed is displacing 360 cubes of an air pressurized at less than the atmosphere.



The diesel has no butterfly, is displacing the same area, but it is positive pressured air made more dense by the turbo as the piston makes its stroke.



I remember my old shop teacher explaining "effective compression ratio". How does a piston sweeping the same volume, create a varying amount of power. The amount of air/fuel mixture in a gas engine varies with the butterfly. As it is opened more air/fuel is allowed into the cylinder, therefore more mixture is compressed, therefore more power is made. This corresponds to a low effective compression ratio at closed throttle/butterfly, to a high compression ratio at open throttle/butterfly.



In a diesel, we get the same result, but go about it a little differently.

Instead of a butterfly to control the amount of air, we let all in that will go. When we call for more power, more fuel is added, which spins the turbo faster, which caused more air to go in, which raises the effective compression ratio, and causes smoke to come off the rear tires.

 

[sag2]

The fact is, both DISPLACE 360 ci no matter what. But when running, both engines MOVE different amounts of air. As the RPM's increase, the non turbo engine moves less air because of losses from restrictions in the intake system.

The turbo engine can move more than 360 ci of air per revolution because of pressure in the intake. In theory, the engine does not suck the air in, atmospheric pressure pushes the air in. With the turbo, the atmospheric pressure is higher, so it pushes more air in, and overcomes the intake losses.

 

[wsteffes]

The amount of air an engine uses also depends on the volumetric efficiency of the engine.

Just some quick numbers here,

359 cu in -- at 75% efficiency -- at 3000 rpm



a vacuum would be less than 233 depending on how much vacuum.

0 boost would be 233 cfm

10 boost would be 369 cfm

20 boost would be 496 cfm

30 boost would be 619 cfm --Hx35 best efficiency is around 600

40 boost would be 736 cfm

80 boost would be 1178 cfm--Also the minimum flow for bottom turbo with twins



75% I used is just a number I picked, I'm sure the actual number is less for a stock cummins 5. 9

Just some rough numbers to use when picking turbos and air filters.