2010 Prius Grill Blocking strategy

I'm actually surprised at that result since aerodynamics are only a portion of total losses - there's also rolling resistance (also substantially higher at speed) and other losses in the system (such as the inverters transferring current from MG2 to MG1 for heretical mode). And the Prius has a pretty low coefficient of drag, so I would expect those losses to be less significant until much higher speeds.

Any science showing where/when the aerodynamic losses become such a high % of total that they outweigh everything else?

I have been maintaining my tires (currently at 39/37), but it's worth a shot at going higher. I wasn't sure if it actually might be better to have lower psi (=more flex =warmer sidewalls) rather than higher to compensate for cold temps. But I'll try going higher first.

 

Actually I started with a drag formula Ken@Japan had reported from some Toyota sources:


The formula is:

drag (Newtons) = 190 + 0.42* (V*V)

0.42 = is the coefficient of drag, area, and air density on a standard day

V = velocity in meters/second
​

I don't have the reference handy but the EPA has gone with a second order, polynomial in their vehicle testing:

drag = A*(V*V) + B*V + C

A - derived from testing
B - derived from testing
C - derived from testing
​

When I was looking at it several years ago, the second order polynomial gave a drag curve for the NHW11 almost identical to the simpler, two element term. If you'll join the YahooGroup "Prius Technical Stuff" and search the message archives, I think you'll find the discussion. That is where I'd go back to find the source references.

Generally speaking, aerodynamic drag begins to predominate above ~45-55 mph. A second order effect, it gets really grim above 65 mph. In contrast, rolling drag remains nearly constant until tire resonance effects and/or poorly design transmission effects. But these usually don't show up except on race tracks at truly exciting speeds.

I have a copy of "Aerodynamics of Road Vehicles From Fluid Mechanics to Vehicle Engineering" Wolf-Heinrich Huncho, ISBN 0-408-01422-9, 1987, and it includes a section:

"Performance of cars and light vans", Hans-Joachim Emmelmann, pp. 83-105, Chapter 3 in which he states:

If you have an interest in modeling vehicle performance, I would recommend joining the YahooGroup, "Prius Technical Stuff" and check out both the message archives as well as file folders of images and user reports. It really is a treasure of engineering data. <wink>

Bob Wilson

 

"The motion of a vehicle is resisted by the following forces:
Aerodynamic drag W{d}
Rolling resistance W{r}
Climbing resistance (gravitational) W{c}
Acceleration resistance (inertial) W{a}

rolling resistance coefficient based upon the tire and at speeds under 100 km/h can be treated as a constant. It doubles going from ~100 km/h to over ~200 km/h (radial tires.)"

Good info - thanks! :thumb:

This helps explain why aerodynamics are the biggest loss at higher speeds (rolling resistance is effectively a constant, and climbing / acceleration resistance are also, assuming you're on a flat and traveling at a fixed speed). And in colder temps, air is more dense which means more resistance.

What about the supply-side? i.e.: what (where) are the losses in the engine/transmission in cold temps? And how do they compare (size-wise) to aerodynamic losses?

Based on what I know so far (which admittedly isn't very much), it seems like cold-temp losses in the Prius must be mainly on the supply side (since they happen at all speeds), with aerodynamics causing significant additional losses at highway speed.

Obviously can't do anything about aerodynamics except to drive slower in colder temps, but if there's something to be done on the engine side it might be worth exploring.

Pure speculation, but would simply having colder intake air be a factor? If so (and this gets beyond the scope of your average home mechanic), would using a heat-exchanger to pre-warm the air before entering the engine possibly make a difference?

 

The biggest losses are during the car warm-up. During this time the oil, both engine and transaxle, are cool and relatively viscous. It takes a little longer to reach operating temperature and then the excess engine heat, ~2/3d is waste and ~1/3d is converted to shaft, provides more than enough to keep everything at operating temperature ... after 30-45 minutes of highway power levels. The real problem is at lower speeds, 30-45 mph, typical of city driving, in dreadfully cold weather the cooling can trigger engine running just to maintain minimum, coolant operating temperatures, at least 60C.

You can slow down a bit, the drag force increases by the square of the velocity. So a little slower, say 65 instead of 70 or 70 instead of 75, can return summer like mileage. Also, look at warm-up including radiator inlet blocks and block heaters that also reduce drag.

That gets to be a little tricky. Our manifolds are heated and we push part of the air intake charge back into the manifold to go into the next cylinder. Other engines have a air inlet temperature effect but there are other issues including pre-ignition. But this is not an area I've done a whole lot of study.

Bob Wilson

 

I just got done with 100% lower block. I used 1/2" foam and I'm confident it'll hold. My install is slightly different than some here because instead of one continuous piece per row (with vertical slots for relief), I used 24 individual pieces. There are 6 slots per row, 4 rows. I found this much easier. With some skill you can sometimes make the vertical gaps disappear by butting up adjacent pieces. Each piece starts 12cm long, and then fine tune length to fit each slot. Used utility blade to cut.

I'm not doing the upper until I figure out an esthetically pleasing way to do it.

It'll be around 4F (-15C) here in Winnipeg next week so I'm hoping for results. I'm avg 5.8L/100km for past 250km (40mpg). Tires 39/37.

 

Looks good! Keep an eye on the FE and report back.

 

If you put flames on the paper on your plate it increases the HP 10 fold

The pipe insulation is very easy to use

 

I have blocked my upper with duck/duct tape stuck to the surrounding bodywork - apart from the lower section which is just clear of the bottom edge. This allows air pressure to force cold air to the Inverter Radiator through the gap as that section of tape is forced back, acting as a kind of crude flap. Not the most pleasing way to do it but effective.

Just hope the adhesive does not cause damage to the paint when the tape is removed in warmer weather.

 

I had to mess with these photos to make the blocking clearer, but you can see what I have in place right now - about 85% lower and roughly 90% upper (small gaps at each end). I had tried 100% upper and lower, but I think I got overheating on the highway.

The idea of having a "flap" which will open up more at higher speeds is a good one, I think, although it would probably require some fine-tuning to make it work right.

Ideal would be automatically adjusted louvers tied in with the outside temp gauge and coolant sensors.

 

Very helpful data. Regarding #2, I was expecting the rolling resistance to decrease with lower temperatures. As the rubber stiffens with deceasing temperature, I thought this would decrease the rolling resistance too. Of course, this is for tires with a constant pressure. Keeping them at a constant pressure regardless of the temperature is a different story... Moreover, the decreasing fluidity of all the lubricants of the drivetrain will more than likely overcome the little loss in the rolling resistance of the tires. But for what concerns the tires alone, I would go for lower resistance with lower temperature. Do you have any numbers on this?

 

One of the values that can be displayed with the SGII is the intake air temperature. I have watched it and was a bit surprised at how warm it is considering the winter weather. It was warmer than the outside air temp even at highway speeds so it is being heated somewhat. I'm sorry I didn't document anything so this is just an observational report.

 

Yeah, I think the other major factor that was recently uncovered was that rolling resistance degrades significantly with colder temps. So at highway speeds where aerodynamics and rolling resistance are most important, cold air makes BOTH worse...

I'm thinking the grill block doesn't do all that much except improve warm-up time - which is a big factor for people doing short trips, especially if they like warming up the cabin, but is only a tiny fraction of the gas used for longer highway trips.

 

I took my '07 in to the dealer last week to have it looked at before I trade it in for a '10 which I get next week. The manager had the mechanics remove all my grill blocking, telling me it was going to fry the transmission.
I explained to the manager when I got the car back that I had had tons of trouble with my need to either have a hybrid car, or use cabin heat. I gave him the link to a post I put here in the past with evidence that the car has been bad in the winter.

Here's the link to my post:
http://priuschat.com/forums/gen-ii-...ng-off-defrost-turns-heater-2.html#post778268

I'm hoping the heat recovery approach in the '10 has solved the problem, but I'm warning everyone that not all dealerships are friendly to grill-blocking.

 

So when the engine in your new Prius gets too hot and a engine bearing goes Kaflooey how long after seeing your 'Grill Block' will it take the Toyota dealer to tell you that you're Factory Warranty has also gone Kaflooey ? I'm betting seconds !

OK Call me crazy, but I'll take the original automotive designers and engineers at Toyota who actually knew what they were doing, and wager that they don't require any Home Grown Back-Yard gimcracks tacked on to their industry leading design work by people who know far less than they do about the cars requirements and protection.

But don't let me stop you, I'm looking forward to the posts when all this causes another anti-Toyota Flatulance Epidemic the way the floor mats did.

 

The '10 is great for heat. I've got lots of experience with all 3 generations here in Minnesota.

As for their concern, simply point out how temperature still doesn't exceed the norm. Those of us with a ScanGuage can clearly see that. All the blocking does is prevent heat loss below that.

Then, ask why he's concerned about 195°F being too hot. It's not and we know it. My dealer does too and has no issue with mine.
.

 

if you insist...

YOU ARE CRAZY!!!

now that you feel better about yourself, let me ask you...in your "craziness"

what makes you think that your Toyota is optimized for ALL weather conditions? from 30 below to 110 above?

or is it that its set up to accept a wide range of temperature and driving conditions??

hmmm... that would mean that maybe, quite possibly, could be, that in specific situations, other things can be done to improve gas mileage.

ya know, like taking a slower, less traveled route with less traffic control?? or maybe even BLOCKING THE GRILL when its cold out

 

i have been taking my car to my dealer for 5 years and EVERY car has had grill blocking and they have not touched or mentioned it to me

 

As a full year has gone by, can you confirm that the numbers used in the table are still valid; do you feel any adjustments should be made?

Thanks again for your work; as I don't have a ScanGauge, I rely on your feedback and others' to adjust my grille blocking.

 

Currently the temps here are the mid 50s to lower 60s. I have not seen(SG) the coolant temp rise above 197F and is often at 195F. I still have the lower grill blocked 100%.