Vacuum HP

[arcosine]

I was driving home last night and noted that the absolute manifold pressure was between 7 and 9 psia, so I wondered how much horsepower that was. I Calculated it to be 36 HP! That's more than is needed to propel the car on level ground.

saturn sc1:
14.2-8 = 6.2 psi differential pressure
1905 liter displacement
4 stroke engine
2042 RPM at 65 mph

Pv = pressure * volume/ time

Pv = (6.2 * 4.448/.0254^3) * (1905/1000/4) * (2042/60) * 1.34/1000 = 36 hp

units:
Pv = (psi* N/lbf * m^3/in^3) * (L/m^3/intake strokes per revolution) * rpm/(min/s) = N *m/s

N*m/s /1000*1.34 = hp

6.2 psid
1.905 liter displacment
2042.14153122327 rpm

power, KW 27.2
HP vacuum 36.5

[ecomodded]

I think your equation has an error.
Small to mid size cars use 25-35 hp to cruise at 50 mph.
Could it be your measurement was of the cars actual cruising hp with vacuum as a byproduct ?

[arcosine]

Show me the error.

[Daox]

According to the Aerodynamic & rolling resistance, power & MPG calculator here on the site, a 1998 SC1 uses just below 9 hp to maintain 50 mph.

Click here to see actual numbers

[RobertISaar]

where....... did this equation come from?

i'd like to think i'm pretty decent with math, but how you're calculating HP is entirely new to me. is it accounting for BSFC? volumetric efficiency? airmass?

it LOOKS like the equation only accounts for displacement, RPM and MAP, which will not produce accurate results.

[arcosine]

It came from first principals. power = force times velocity

Show me a better formula.

[pete c]

I'm no where near good enough in math to figure this one out other than to say that there is no way there is 36 hp worth of pumping loss.

I think that the actual formula would use vacuum and the cross section of the throttle opening, which is where the vacuum is being generated.

If I had to pull a number out of my ass, I would guess than pumping loss for a saturn being driven at 60 mph on level ground with a constant throttle opening, is somewhere around 5-10 hp. Likely closer to 5.

There are all sorts of smartypants on this board that know this kinda stuff backwards. i am sure one will chime in soon with a proper answer.

This topic does bring up the idea (again!) of using some sort of turbine as a throttle for constant power O/P like cruising on the highway.

[Patrick]

I'm not sure you can calculate HP that way, but if you can, I think you've got too many intake strokes per rev. For a 4 cylinder 4 stroke engine you should have 2 intake strokes per rev (intake only occurs every other rev on a 4 stroke engine: intake and compression on one rev, power and exhaust on the next rev).

[JRMichler]

I have an old SAE paper on engine friction laying around somewhere. My recollection is that throttling losses are the largest single source of efficiency loss / friction at part throttle. That's because the pressure drop is small at wide open throttle, and the flow rate is small at idle.

My thermodynamics (AKA thermogoddamics) is pretty rusty, but I think the correct equation has a natural log of the pressure ratio in it. Or something like that. And I'm not interested enough to look it up.

[arcosine]

Humm, intake, compression, power, exhaust, that's two revolutions, one intake stroke. Piston is working agaist the vacuum 1/4 of the time, doesn't matter how many cylinders.