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Old 11-05-2018, 09:50 PM   #1 (permalink)
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City MPG vs Highway MPG- What Affects Each?

So I know that highway mpg is mostly affect by wind resistance but what are the main factors that change city mpg? Weight, Engine Efficiency??

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Old 11-06-2018, 01:41 AM   #2 (permalink)
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City MPG is most affected by weight and idle fuel consumption. All the stopping and then going takes a lot of energy, and then while stopped, the car is burning fuel needlessly. The test also factors in a cold start, which wastes a lot of fuel.
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Old 11-06-2018, 09:49 AM   #3 (permalink)
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The above was from this book:

Tires and Passenger Vehicle Fuel Economy



It would seem that aero not as big a contributor as some might think.
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Old 11-06-2018, 10:20 AM   #4 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
It would seem that aero not as big a contributor as some might think.
Actually, that infographic shows the vast importance of reducing load on the engine. Reducing that 11% aero loss to even 10% has a multiplying effect as we trace back the power usage to the energy source. 1% reduction in engine load will mean that the 69% 'engine loss' use gets reduced to 67%. So, for a 1% engine load reduction, you get a 'free' 2% reduction from 'engine loss' for a total of ~3% less fuel used.
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Old 11-06-2018, 02:04 PM   #5 (permalink)
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HWY mpg depiction

Quote:
Originally Posted by CapriRacer View Post


The above was from this book:

Tires and Passenger Vehicle Fuel Economy



It would seem that aero not as big a contributor as some might think.
I'm going to guess that the values shown,are for only for the EPA Mobile Sources dynamometer testing protocol.(which includes stops,idling,accelerations,decelerations).
*Real world,steady-state,highway cruising can demonstrate 36% thermal efficiency at the flywheel for an ICE.(higher for diesel).
*Powertrain losses are only 4% (under power) for a non-overdrive,manual transmission.
*Of the 'available' tractive power,on the highway,at 100 km/h,aerodynamic demands can constitute 80% of the overall road load.
*With rolling resistance absorbing the remaining 20%.
For decades I've considered the (b) table to be extremely intellectually dishonest.Almost conspiratorial in downplaying the significance to aerodynamic drag at highway velocity.
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Old 11-07-2018, 07:48 PM   #6 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
It would seem that aero not as big a contributor as some might think.
Actually of the 20% of the total energy that actually makes it to your drive axle, more than 50% is lost to aero drag.

But yes, if you could significantly improve ICE efficiency using coroplast and duct tape, I'd concentrate on that first

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