Engine efficiency % improvent to mpg improvement - Fuel Economy, Hypermiling, EcoModding News and Forum

ngrimm · 03-03-2012, 02:46 PM

I can't seem to find what what I'm looking for. For instance if an engines total efficiency increased by 1%, is there a way to calculate a % mpg increase? Thank you.

mcrews · 03-03-2012, 04:31 PM

I would think that number is unique by engine, car and rear ratio.

a 3500lb car has a set cd and weight, but if the manufacure offers 3 or 4 engines, and auto or maunal and 2-3 different rear ratios, each model would respond slightly differently.
just my thoughts

redpoint5 · 03-03-2012, 05:04 PM

This question sparked an interesting realization on my part. If an engine were 30% efficient, and the efficiency increased by 1%, the overall mpg of the vehicle might see a 3% improvement. At minimum I would expect greater than a 1% increase in overall fuel economy. Of course, there are many variables at play here, and stating a 1% engine efficiency increase is way too simplistic.

ngrimm · 03-03-2012, 05:44 PM

So if everything else on the car remained the same and you reduce the fuel by 1% while maintaining the same engine power measured at the crankshaft. How would that weigh against the drivetrain, aero, etc % inefficiencies in the same car?

Ladogaboy · 03-03-2012, 06:28 PM

I'm not sure what you're trying to ask there. A 1% decrease in fuel usage would be a 1% decrease in fuel usage (i.e., 1 gallon per 50 miles would become .99 gallons per 50 miles).

The calculator provided in the tools section of this page might give you a lot of numbers to mess around with in order to calculate engine efficiency. Also, someone recently posted a link to the EPA's website on overall efficiencies in a car. That also might be worth your time to read.

http://ecomodder.com/forum/showthrea...epa-20754.html

drmiller100 · 03-03-2012, 10:29 PM

Quote:

Originally Posted by ngrimm

I can't seem to find what what I'm looking for. For instance if an engines total efficiency increased by 1%, is there a way to calculate a % mpg increase? Thank you.

ummmm, the mpg would go up 1 percent.

niky · 03-03-2012, 11:44 PM

You're looking at two totally different scales.

That 30% efficiency includes drvetrain losses, road friction losses, losses due to heat, losses due to accessories, etcetera.

The engine itself is nearly 100% efficient at converting fuel into power (except the fuel that is used to cool the engine and light the cats). But only 30% of that power goes towards pushing the car around. If you increase the efficiency of the engine itself by one percent and ignore all the loses caused by other stuff, then you're only going to gain a fraction of percent gain in efficiency overall.

ngrimm · 03-03-2012, 11:56 PM

Thank you for pointing me to the "tools", the aero, rolling resistance calculator helped me figure it out. I started with the sample car at .22% engine effic. and got 52.53 mpg @ 55 mph and changing the effic. to .23% gave 55.02 mpg or 2.4 mpg improvement or 4.56%. Then I added higher weight, frontal area, and CD to bring the car to 37.56 mpg at .22% engine effic. a 1% engine effic improvement gave 39.27mpg which is also a 4.5% mpg improvement.

Ken Fry · 03-04-2012, 02:28 AM

Quote:

Originally Posted by ngrimm

Thank you for pointing me to the "tools", the aero, rolling resistance calculator helped me figure it out.

.23/.22 = 1.045

.23 is 4.5% bigger than .22.

Suppose the base engine is 30% efficient.

.31/30 = 1.033

.31 is 3.3 % bigger than .30.

You don't need the any of the other data.

.02 is not one percent bigger than .01. It is twice as big.

redpoint5 · 03-04-2012, 02:46 AM

Quote:

Originally Posted by niky

That 30% efficiency includes drvetrain losses, road friction losses, losses due to heat, losses due to accessories, etcetera.

The engine itself is nearly 100% efficient at converting fuel into power (except the fuel that is used to cool the engine and light the cats). But only 30% of that power goes towards pushing the car around. If you increase the efficiency of the engine itself by one percent and ignore all the loses caused by other stuff, then you're only going to gain a fraction of percent gain in efficiency overall.

Not so. ICE engines are extremely inefficient at converting energy into work and the efficiency ratings do not include drivetrain losses or any other losses external to the engine itself.

As the math shows, a 1 point increase in efficiency on a 30% efficient engine is a 3% increase in overall efficiency.

Most Efficient Diesel Engine - 50% Efficient