grill blocking / aerodynamics

[gazz]

In the UK the weather is getting colder, on my Prius Gen II I blocked the grill. The thing is on the Gen III it does not look so right to do, the air looks as if it is funneled into the engine bay and the blocking will upset the aerodynamics.

Any thoughts on the blocking the grill on a Gen III aerodynamic.

Most of my driving is below 50mph.

Gary

[spiderman]

Originally Posted by gazz

In the UK the weather is getting colder, on my Prius Gen II I blocked the grill. The thing is on the Gen III it does not look so right to do, the air looks as if it is funneled into the engine bay and the blocking will upset the aerodynamics.

Any thoughts on the blocking the grill on a Gen III aerodynamic.

Most of my driving is below 50mph.

Gary

I actually found an increase of about .5 - 1 mpg increase myself. Get a baseline mpg that you can reproduce and try it yourself. A couple of threads around on suggested blocking.

[gazz]

Originally Posted by spiderman

I actually found an increase of about .5 - 1 mpg increase myself. Get a baseline mpg that you can reproduce and try it yourself. A couple of threads around on suggested blocking.

Thanks,

How cold is it where you are, the gen III in eco mode seems a lot better, does not loose as much heat. Will be interesting to see what it does in the depths of winter.

[spiderman]

Originally Posted by gazz

Thanks,

How cold is it where you are, the gen III in eco mode seems a lot better, does not loose as much heat winter.

Currently it is about 41F with lows in the lower 30s. My Prius is garaged without a EBH.

I have noticed my Prius seems to retain heat well (at least better than my 97 Camry).

[Rokeby]

The Prius, like any car, is designed to operate from the Arctic Circle to
the Equator. In very warm climes, you can't have too much ICE cooling
air.

Elsewhere, it is entirely possible to have too much. The problem is
once air is in the engine bay, it has to get out. That usually means
abruptly into the air stream under the car, or out through the front
wheel wells -- there are some exceptions in full bodied race cars;
louvered fender vents. The abruptness causes greater drag
somewhere down stream.

The aerodynamic MPG gain on grill blocks comes from having less
air in the engine bay to get rid of. This is of course is in addition to
helping keep additional heat in the engine bay for thermodynamic
efficiencies.

A word of caution. IIRC, the ICE runs most efficiently up around 180-
195 degF. With grill blocks in and air temps above 65 degF it is easy to
exceed these temps. If you're going to do full upper and lower grill
blocks, it is best to also have an ICE monitor, something like the
ScanGauge does fine.

The current info on the Gen III is the cooler for the inverter is high on
the radiator so fully blocking the upper grill may not be a good idea.
Fot the Gen II the cooler is low down on the radiator.

[gazz]

Originally Posted by Rokeby

The Prius, like any car, is designed to operate from the Arctic Circle to
the Equator. In very warm climes, you can't have too much ICE cooling
air.

Elsewhere, it is entirely possible to have too much. The problem is
once air is in the engine bay, it has to get out. That usually means
abruptly into the air stream under the car, or out through the front
wheel wells -- there are some exceptions in full bodied race cars;
louvered fender vents. The abruptness causes greater drag
somewhere down stream.

The MPG gain on grill blocks comes from having less air in the engine
bay to get rid of. This is of course is in addition to helping keep
additional heat in the engine bay for thermodynamic efficiencies.

A word of caution. IIRC, the ICE runs most efficiently up around 180-
195 degF. With grill blocks in and air temps above 65 degF it is easy to
exceed these temps. If you're going to do full upper and lower grill
blocks, it is best to also have an ICE monitor, something like the
ScanGauge does fine.

The current info on the Gen III is the cooler for the inverter is high on
the radiator so fully blocking the upper grill may not be a good idea.
Fot the Gen II the cooler is low down on the radiator.

Thanks, I might start by blocking a bit of the lower grill.

[Bica2go]

Here's another thread about grill blocking the Gen III. It has some useful guidelines about how much to block for given temperature ranges. I don't think you're likely to need a full block given the temps in the UK. You're a wee bit milder than us in New England (and our friend in Alaska!)

http://priuschat.com/forums/gen-iii-...-strategy.html

[Fred_H]

Just a reminder:

There are two different desired effects of grill blocking.
1. Increase combustion engine temperature
2. Decrease aerodynamic drag

Which lead to two interconnected things that one should keep in mind when grill blocking:
1. Try to reduce airflow mainly to the combustion engine radiator and not so much the Inverter radiator.
2. Try to minimise the total amount of air that goes through the grill, as opposed to routing it around the radiator after it passes through the grill.


Ideally, there would be three separate airtight plenums, one for each heat exchanger, each of which which would be separately regulated.

Practically, a more cost effective method is to block the air right at the grill, except for an area directly in front of the inverter radiator.

[Philosophe]

Originally Posted by Fred_H

Just a reminder:

There are two different desired effects of grill blocking.
1. Increase combustion engine temperature
2. Decrease aerodynamic drag

I doubt that reducing the air flow through the engine compartment will reduce whole car drag, and may in fact do the reverse…


With all the air tunnel time spent on the Prius, one could think that the air flow around the car was planned to efficiently blend in the air rushing out of the engine compartment. Hence, blocking the grille may disturbed that planned air flow and have a nagative impact, could it?


Anyway, this negative impact would probably be a lot smaller than the positive impact of having the engine warm faster (and stay warm) in the cold season.

Personally, I have fully blocked the lower grille and 50% (1 row only) of the upper grille. I want to keep some air flowing through the inverter radiator. Weather normals at this time of the year here are max 9°C and min 1°C.

[gazz]

Originally Posted by Philosophe

I doubt that reducing the air flow through the engine compartment will reduce whole car drag, and may in fact do the reverse…

This was the point of my post, could you be increasing drag by blocking the airflow at the grill.