Calculations on Power

I figured before I had this stuff in class, I would share it to get some feedback.

I'm working through the equations & problems to include in some physics lectures, & figured why not share some basic calculations to see if they were done correctly & what you think.


These are pretty straight forward, but the one that always confuses me is the fuel flow power equation up top- take a look & make sure my math is right. It's based on injector delivery & BTU's as the conversion from fuel power to HP, with about 30-33% efficiency (1/3 lost as heat, 1/3 lost as sound, 1/3 is actual power).

I used values from when I put my 05 S-197 on the dyno, 3rd gear (1.55:1), and grabbed the value of 221 lb.ft of torque. Multiply the torque by the rotational speed of the tire (91.32 mi/hr, & 742 revs of the tire= 1 mile)= 1129 RPM to get Horsepower to the rear wheels.

The next set of values I got are a little more hairy- we're working through calculations to determine the force of drag on the car at given speeds, and the horsepower requirement from the engine to overcome the force of drag.

This is a really roundabout way to get about the same values, but the idea is still the same- Take the speed you're traveling at (80 mi/hr), square it, & multiply that value by 0.0224 to get the total amount of force (drag) acting on the car at a given speed.

To calculate horsepower required to overcome that force of drag, calculate the speed (mi/hr) in feet/second, then divide by 550 (conversion) to determine the horsepower demand to overcome drag. This is just the demand to overcome drag, and excludes the force required to accelerate the weight of the car, or the other forces acting on the body as the car travels at a given speed.

Anyways, take a look if you have some time... let me know if I goofed up, and if this kind of stuff is interesting to y'all.

Thanks!

The answer for ‘power’ is ‘never enough’. Really, I hate math.

This is interesting. My PhD is in atomic physics so always nice to see some freshman physics being applied to things outside of our bubble. Your calculations look fine, although the non SI units are making me spazz but we need American units for this stuff.

I do agree with you on the injector flow rate conversion, its very widespread and I doubt linear at all, heat alone will change the delivery as well as the efficiency of the engine but I like it boiled down to some simple calcs etc.