Disclaimer: I own a Camry Hybrid but the most knowledgeable engineers (example: bwilson4web) appear to be in this forum. Also, my parents just bought a 2008 Prius and I'd like to help them out as well.

Question: It's starting to get cold here in New England and MPG is just starting it's cold-weather dip. In my Camry, I've noticed that I can't get my instantaneous MPG meter to read nearly as high on the highway in the winter as it does in the summer. It simply wants to stay low and is much more sensitive to throttle inputs. I thought that maybe the cold air was keeping some components cold during my entire trip (like transaxle) which was adding more resistance. I know that it is normal to have low MPG during cold startup, but I was expecting better MPG after cruising on the highway for several minutes.

I noticed that bwilson4web had done some experiments with blocking the grill during cold weather. Unfortunately, I couldn't conclude whether it would be worth it to try this myself. I know it would slightly reduce wind resistance, but that alone isn't enough to satisfy me. Also, I don't want to risk causing temperature issues with the lack of air flow in the engine bay. I would greatly appreciate some more info on this subject (and possible problems) if it is available. Thanks!

 

Hi Andrew,

Without doing a formal study, I've seen blocking the grill seems to improve heat retention. This should help on the MPG but I've not done a quantitative analysis. I have a Graham scanner for doing this but last winter I had other priorities to investigate.

The theory says it should help. The practice indicates it helps. But I can't give a quantitative set of numbers and say, "See!"

My subjective feeling is our cars are designed to work well in 100F weather, which means they have much larger air inlets than necessary when the weather drops to 50F or lower. I think if you can use an easily installed and removed block (aka., water noodle toy or water pipe insulation) you will be on safe ground. Should there be a problem, the car will no doubt report an over temperature and you can pull over and take off the blocks.

Sorry I don't have anything based upon experiments but with the cold weather coming on, I can start looking at a quantitative study. Right now, I'm looking at something else, the air mass flow sensor and mixture trim.

Bob Wilson

 

I greatly appreciate the info. The foam "noodle" idea sounds easy/safe so I'm going to try that one out.

Your experiments and postings are some of the most interesting and useful I've seen on the web and I thoroughly read all of your reports. I love all this technical stuff and hope to eventually use my software engineering experience to contribute to the community. Keep up the good work!

 

One area that needs investigation is the ODB interface of the Camry. If you have a laptop, you might look at getting one of the USB based, OBD interfaces and seeing if you can:

1. read all traffic - use this to figure out what is on the line
2. investigate 'probe' protocols - using a cheap scanner from Autozone or others, look at what they are doing when probing for error codes or information

This can lead to a way to capture engineering data from the Camry for similar studies.

The other approach, if you are interested, is to use multiple audio channels to read and record AC signals filtered to stay within a 1V peak-to-peak range. With a chopper, a system for flipping DC signals, it converts DC into an AC signal that can be recorded. This makes your laptop into a multi-channel, data recording system with as much depth as audio recording can hold. Look at Audacity to see what I use.

Bob Wilson

 

In the olden days, now gone by, the fan was often "hard-coupled" to the engine and the coolant thermostat used in cars had a substantial level of bypass even with the main "section" fully closed. Driving in and across MT, ND, etc, in the wintertime posed a challenge insofar as keeping the passenger cabin to a minimum comfort level.

A piece of cardboard to block some of the radiator airflow worked wonders.

The downside was that you had to be careful, watchful, to be sure the car didn't inadvertently overheat with virtually no airflow moving through the radiator while idling.

Your Prius may not have the same problem. The radiator fan is electrically operated and may be shut off completely in severe cold, and depending on the specific design there may not be ANY coolant flow to the radiator in those conditions. Coolant flow through the heater core might very well suffice.

There are lots of reasons for a hybrid designer to take extra caution to not allow the coolant to inadvertently get below optimum operating temperature.

So CAUTION is advised.

And another POINT.

Ford has been granted a US patent that might very well bear on this issue.

The patent involves using two techniques that are undoubtedly the result of a safely concern when operating a FWD hybrid in conditions that might lend themselves to encountering an icy roadbed, a slippery roadbed.

If the OAT declines below freezing, or even hovers nearby for an extended period, the level of regenerative braking used to recharge the hybrid battery is substantially reduced.

Additionally, if ABS activates as the brakes are applied, all year around, regenerative braking is INSTANTLY disabled entirely so as to allow ABS to be fully functional.

Given the level of patent cross-licensing going on between Ford and Toyota over hybrid technology it seems to me Toyota would be remiss in not making use of this same technique.

So yes, your Prius may not be very efficient, not nearly as efficient, during the winter months, certainly not if you live in the northern reaches of the US.

 

A picture is worth a thousand words.

It's possible the car could get too hot when the outside temp is over 70F or 20C. If so, remove the lower rad block, then the upper as the temp rises. The car warms up in 5 min or two blocks, whichever occurs first.

Coolant temp gets up to 95C with ambient at 20C. I believe the coolant boils around 120C or just above. The coolant temp slowly drops when the engine shuts off.

Coolant temp stays around 80C with ambient around 0C.

The blocking stops the car from cooling off due to air flow from motion or wind. The fans haven't run so far, even with ambient at 20C.

Material used is foam pipe insulation, available from your local home center. Cost was less than $5. Upper was 3/4" and lower was 1 1/2" as I recall.

 

RE: I just read your article regarding the sound analysis of MG1 and MG2. I'm going to try and implement that experiment on my car this weekend. My idea is to record the data to a laptop while using my digital voice recorder to constantly record what I'm doing (coasting, stopping, starting, etc). I'm guessing you did something similar to record your results.

I'll post my results when I'm finished. I can't wait to try this! Thanks again for your info and enthusiasm.


@wwest
Your post was very informative and got me thinking... After looking at my parent's new 2008 Prius, I noticed that Toyota made a great deal of effort to make every important part of the vehicle aerodynamic (especially the windshield wipers!). If modifying the grill could make a measurable/significant improvement in economy (without risking negative side effects), I think that they would have discovered that themselves. Despite that, I may try blocking just the lower grill to see what happens (like David Beale has shown in his attachment). If it doesn't help to much, at least it will help block some of the salt/sand that gets blasted through there in the winter.

@David Beale
That's a very slick modification you performed. It looks very solid!