Drivetrain Drag on a Prius

It probably has more drag in the drivetrain, but the lower weight actually works against the Smart fortwo in a "glide" test. More mass = more inertia that the drag will be working against so the Prius will glide longer if the speed is sufficient to overcome differences in tire rolling resistance that would otherwise favor the smart. (Obviously it takes more energy to initially accelerate the greater mass, so there are many benefits of lower mass.)

Also, the drag coefficient of the smart is 0.29 (and I'm surprised it is that low with such a stubby shape) vs. 0.26 for the Prius, so the Prius will produce less drag. The cross section of the Prius is slightly larger but the difference only makes up about half the difference in drag coefficient from what I can tell. You might get a different or more pronounced result at a higher speed vs. a low speed in direct comparison. Rolling resistance will remain about the same while aero drag will increase roughly with the square of the velocity. Higher glide should favor the Prius, lower should favor the smart unless I'm missing something (as likely as not.)

 

There should be lower drag in the drivetrain of the Prius as well. Fewer gears to spin in the oil. You'd have to check the transmission design for the Smart to see if the gears are spinning in "N", some do, some don't.

 

It sounds like the transmission drag. what everone is suggesting, is probaly the main spender of energy. If I remember how a conventional tranny works, I believe the drive axle(s) is spinning a good number of the gears even in neutral. This, no matter how smooth, still spends energy that the CVT avoids. My 2 cents. probably only worth a penny at todays value of the dollar.

 

I'm sorry to read that you have been out-Smarted. <groan>

Everyone elses comments apply and these are just some ideas for you to consider:

• Prius transaxle - the only gears half-submerged are the differential gears. The rest are above the oil pool and lubricated from the small oil pump. There is very little loss internally.
• Smart transaxle break-in - some of the Prius transaxle oil tests suggests there is an initial load of early wear materials from the gears and leaching from the transaxle seals. You might want to do an early transmission oil replacement, say with the second engine oil change. This will get rid of the 'break-in' stuff and give the Smart a chance at optimum transaxle performance.
• Smart wheel alignment - we're finding some unexpected, strange wheel alignments in the Prius. It may make sense to have them tested early, before tire wear becomes an issue, and see if better alignment might help the rolling drag.
• All the other usual stuff - max pressure tires, avoid AC, higher energy fuel, slow-speed warm-up during first drive of day, selling Smart to get a Prius ...

That is my short list.

Bob Wilson

 

I believe that I read a post, here on PC, that said the Prius' rear alignment had a slight toe-in or camber, I don't remember which, for the VSC. Also, please, I'm not trying to be a SA, but I'm not sure what you mean when you reference a "higher energy fuel".

 

I suggest the OP should jack up the Smart, one axle at a time and spin the wheels by hand when in N, to see what drag can be noticed. Maybe the parking brake is dragging?

 

I found that some fuels in the Huntsville area had more energy per gram than others. As of last summer's testing, Shell, Exxon and Chevron 87 octane led the pack. The others were at least 5% less energy per gram including the premium. We have no ethanol diluted gas in the Huntsville area that I've been able to find. I have to drive to Birmingham or up to Nashville to find any.

This is difficult to measure and without a Graham or equivalent scanner, it would take several 'hill climb' tests to confirm using the built in displays.

Bob Wilson

 

Are you talking regular, premium, E10?

 

Consider state of tires

Tire breakin effect:

I have an Audi A4 with about 55000 miles on it.

I noticed the mileage getting steadily better over the first 10,000 miles, and assumed it was engine breakin and such.

At about 44,000 miles, I bought new tires of the same make and model as the original equipment.

Most of Northeast Albuquerque is built on a hill which puts a steady grade on many east-west streets that is quite near the amount required to sustain 40 to 50 mph in neutral when the wind is not blowing, at least for my Audi.

So I could say with no doubt at all that the drag was up appreciably with the replaced tires. I was slowing down on stretches which had had a slight speed up before. I even thought I could feel the increased drag.

With about 10000 miles on the tires, things have recovered.

The hill descent effect I'm describing here can't be due to odometer falsification because of tread depth, and I think the real component of reported mpg improvement due to that is very little, though not zero.

If that is part of your issue, your Smart will lose some of its disadvantage in a while.

My guess is that the tire thing and a disadvantage of the Smart in air drag are probably more important than any adverse difference in the drivetrain drag in the thread title.

While people commonly mention the drag coefficient, the aerodnamic slowing effect at a particular speed in comparing two cars must include three things: drag coefficient, frontal area, and mass. The product of the first two gives your drag, and dividing by the third is needed to compute equivalent decelerating effect. The Smart was already mentioned as having a somewhat inferior drag coefficient to the Prius, and though I don't have numbers, I'd wager it has lower mass per unit frontal area than the Prius.

 

What were you measuring to determining that some had more energy, HPR, MPG?

 

Is it a trick question? I am assuming your route is flat and you travel the same speed with Smart and Prius. Prius travels (coasts) further because it has higher potential energy stored due to heavier weight.

You can think backward if it is easier to see. How much energy will it take to accelerate Prius or Smart from 0-60 MPH? Prius will require more energy... therefore, Prius will go further as it has more energy to dissipate through mechanical, rolling and wind resistence.

The difference between Smart's and Prius' drivetrain drag is tiny compare to the 60% difference in weight.

 

I measured:

• engine shaft torque
• engine rpm
• mass-flow rate

Bob Wilson

 

I found a couple of car sites listing .29 for the Cd. I would have expected something like 0.31. Brochure doesn't say from what I could tell. Although stubby the shape is smooth, I'm still scratching my head over how it would be less than 0.30.

 

At 55 km/h, Prius has 500 Watts mechanical loss according to this graph at page 18. I am not aware of similar doc for Smart.

 

Please! I am not trying to be a WA. Is there any chance the difference could be a result of higher octane rather than a higher energy fuel? I thought most auto fuels had, basically, the same energy content, E10 aside. It is my understanding that the improved power and mileage that I'm getting on my Avalon, using premium, higher octane, over regular, is a result of it being able to adjust the ignition timing for firing at a a higher compression, thus getting more effiecency, more power and longer power stroke, out of the firing. but that the actual enengy content of premium is the same as regular. I'm just trying to learn. I appreciate the discussion.

 

Hi Dave,

Higher octane fuel burns more slowly than lower octane fuel. This allows the engine ECU to advance the spark timing which allows the engine to produce more power. The compression ratio remains unchanged since that ratio is a function of the distance that the pistons travel within the cylinders and the cylinder head design, neither of which are dynamically adjustable.

I recall when Bob was doing his gasoline brand testing. His statements are based upon testing various brands of 87 octane.