Fuel Economy and RPMs and VEs

[Fuelie Fan]

How you would figure it out if you had all the information (equations simplified):

Your first “known” is your desired speed, V_car
From this, you can compute losses due to aero and rolling losses in the tire. You can also make a first estimate at drivetrain loss (this is an iterative problem)

Road Load Power = P_tire+P_drivetrain+P_aero

At constant speed, the engine produces just enough power to match all the losses. If it were producing more, you would accelerate. If it were producing less, you would decelerate.

P_engine = RLP

So, you know how much power you need to produce. Now what you need (and probably don’t have) is a complete BSFC map for your engine. This gives you the engine’s efficiency in fuel consumtion per horsepower (lbs_fuel/[hp*hr]) versus torque and engine speed. Since horsepower = T*N_engine/5252, you can draw a constant power equal to RLP, and use it to find the highest efficiency point ALONG that line. Follow the intersection point down to the RPM axis to give you the engine rpm.



KEY POINT
You will NOT be at the engine’s peak VE/BSFC. If this were the best for economy, GM would gear its V8s to cruise 70 mph at 3000 rpm.

Now that you have the desired engine speed and desired vehicle speed, gearing can be calculated:

N_engine = V_car*gearing factor (NOT just rear end gearing, also has tire size)

Using the engine speed and gearing, you can now check you estimated drivetrain losses using engine, transmission, and rear end data, and re-calculate using more precise info for road load power.

General guidelines: every single component of the road load power is speed-sensitive. In other words, the best way to save fuel is to slow down! Assuming you don’t want to slow down, you can minimize the drivetrain losses by at least slowing IT down with tall gearing.

You engine’s BSFC map will have it’s highest values at high torques, in other words closer to WOT. To have a high torque without having high power, you need low RPMs, so here is another reason that tall gears help increase efficiency. More throttle means less pumping losses, greater efficiency.

As pointed out, an engine’s efficiency will fall off if the rpms get TOO low, but to say precisely where would require the BSFC map

[BluEyes]

So, you know how much power you need to produce. Now what you need (and probably don’t have) is a complete BSFC map for your engine. This gives you the engine’s efficiency in fuel consumtion per horsepower (lbs_fuel/[hp*hr]) versus torque and engine speed. Since horsepower = T*N_engine/5252, you can draw a constant power equal to RLP, and use it to find the highest efficiency point ALONG that line. Follow the intersection point down to the RPM axis to give you the engine rpm.

That BSFC map is the key.

Definately hard to get ones hands on as well. If you've got Engine Analyzer or something like that you can build one up with a bit of work though. Find the RPM's that your engine will be turning with the gearing options available and then drop the carb/TB CFM rating untill you get down to cruise HP at the right RPM's. The gear ratio that gives the lowest BSFC at cruise RPM and cruise HP will the the best for economy. I guess I repeated what you said but maybe a bit easier to understand. Let me tell you, it's amazing what little CFM numbers you've gotta punch in - around 20 or so. And the BSFC in a throttled situation is very different from WOT BSFC (much higher). For some real fun you could change the ambient conditions in Engine Analyzer and see the effect of weather/elevation...

I've used the BSFC output from Engine Analyzer to calculate the theoretical MPG that some of my cars should be getting and it actually agrees pretty well with real world numbers from road trips and such so I'll say the technique is definately worth a shot.