Prius fuel economy variability with the driver

<div class='quotetop'>QUOTE(tomdeimos @ Oct 19 2006, 05:52 AM) [snapback]334980[/snapback]</div>

Anyone care to translate the website into some form of understandable English for those of us who are not engineers?

 

<div class='quotetop'>QUOTE(jkash @ Oct 19 2006, 09:27 AM) [snapback]334986[/snapback]</div>

I think it says basically this: you drive more aggressively, you get lower gas mileage. DUH!

 

Basically, they said a couple of things (which we already know).

First, driving agressively leads to lower mileage (well duh, you're using more energy to accelerate and capturing less due to conversion losses and probably hard braking leading to wear on the physical brakes)

Second, the Prius was the one that performed the worst (relatively speaking) as the agressiveness increased. this means that the difference in MPG between non agressive and agressive driving was the worst.

third, the Jag they tested actually didn't better under agressive driving conditions (duh, it's a high performance sports car - the engine is designed to be run at higher RPM's).

So basically, they said that agressive driving is bad unless you have a high performance sports car. There's an optimal range for every engine to run in. When braking hard, hybrids regenerate less energy than they do when braking softly.

 

<div class='quotetop'>QUOTE(tomdeimos @ Oct 19 2006, 09:52 AM) [snapback]334980[/snapback]</div>

Well,

This is a somewhat obtuse comment by Argonne. The drivetrain efficiency goes down when you drive aggressively with the Prius, not because of the rapid accellerations, so much as the rapid decelleration. From scanning through the Argonne report, it seemed like the Prius was being put down because it performs poorly when people drive the way they like to drive, with an implication that rapid acceleration is the problem. The reality, as I see it, here is that all they proved is if you had a switch that would turn off regenration on the Prius, you get worse fuel economy. D'Oh, well of course!

Non-hybrid cars waste the same amount of energy when they brake from a given speed to zero, no matter what the decelleration rate. The brakes burn up the same energy of motion, no matter how long it takes the speed to change. And with the engine sized for acceptable accelleration, the engine is more efficient at higher output levels. So, driving aggressively in the XK-8 will be more efficient, because one accellerates at a more efficient engine output level, and the non-regenerating brakes waste the same energy whether one brakes slowly, or quickly.

The Prius regerates more energy if you decell at a optimum rate. Not too fast, not too slow. Decellerate hard, and the friction brakes kick in. It seems to me this is what Argobnne is calling "agressive" driving, and regeneration is part of the "drive train efficiency".

To me this report is phrased in a misleading manner. It makes it seem like a standard car is better for aggressive driving fuel economy. When in reality, the Prius and Insight still outperform the standard cars in fuel economy in the aggressive driving tests.

This report was apparently an attempt to jive fuel economies with real world car usage. To be true to that intent they should have added in a 10 minute "stuck in traffic" period to the test, as well as the agressive driving stages. Besides driving more aggressively than the EPA test, many drivers find themselves stuck in traffic allot more than in the EPA test.


Finally the point made "researchers found that the Prius had a much higher sensitivity to aggressive driving than any of the other vehicles tested. " is evidence that the Prius recovers more energy during braking than the other cars, and turning brake regeneration off (in effect) has the greatest impact. Which is evidence that the Prius has been optimized to the highest degree. It is totally misleading for Argonne to not mention anything about the requirement for a large engine in the other vehicles resulting in the trend found. Doing this evaluation in a purely experimental fashion, when the outcome is readily predictable by any well-educated engineer, is in effect a con-job.

 

<div class='quotetop'>QUOTE(tomdeimos @ Oct 19 2006, 11:47 AM) [snapback]335026[/snapback]</div>

Hi Tom,

I really do not buy that the Generator-Inverter-Motor path is that much less efficient than the Impeller-ATF Fluid-Turbine path in a mechanical automatic transmission at full power.

 

Every time you convert energy from one form to another, there are losses. So, you can either have the chemical energy converted to mechanical (as in an ICE), or chemical to mechanical to electrical to mechanical (as in the ICE-generator-Motor path. Since you're doing more conversions by going from the ICE through the inverter and motor, you loose more energy.

Of course, the worst path is to go chemical-mechanical-electrical-chemical-electrical-mechanical, as happens when the ICE is used to charge the HV battery.

 

<div class='quotetop'>QUOTE(eagle33199 @ Oct 19 2006, 12:20 PM) [snapback]335041[/snapback]</div>

Hi Eagle,

Your statment is incomplete. There are two forms of mechanical energy in a standard automatic transmission. There is shaft energy and there is fluid flow energy. And the input shaft energy is converted to the fluid flow energy, then back to output shaft energy. One way to know this is not 100 % efficient is that fluid coupled Automatic Transmissions have a fluid cooling loop. Just like the inverter in a Prius.

 

Short Summary

The Prius gets excellent mileage when driven gently and slightly less excellent mileage when driven aggressively.

The Jaguar gets crappy mileage no matter how you drive it.