Why do colder temperatures lower gas mileage under conditions of constant engine usage (e.g. warm engine, highway cruising)? I understand why gas mileage goes down for city driving -- the engine needs longer to warm up, and needs to stay on more to maintain engine temperature. But why am I seeing a drop of 5-10% in FE on the highway with a drop in temperature of ~20 °F? Its not just the change in gas, because my mileage goes back up to normal for the afternoon.

The way I see it, ICEs produce a vast excess of heat, hence the need for a cooling system and radiator. As the temperature drops, the thermostat should scale back coolant circulation to maintain engine temperature, and there shouldn't be a requirement to burn more fuel to maintain the temperature. I'm sure at some low temperature the engine can never get completely warmed up, but that shouldn't be happening at 37 °F.

Is there a benefit to putting a wind block (piece of cardboard) in front of the radiator during the winter to decrease cooling system efficiency and help keep the engine running at an optimal temperature? Can anyone out there with an OBD computer comment on whether they see a drop in engine temperatures on the highway on colder days?

What am I missing?

-Terence

 

The impact at steady state won't be as big as a cycle that involves lots of cold starting, stop & go, etc. But colder air is a little denser, and so a little harder to push through. Intake air, too, will be a little denser which means you burn a little more gasoline during each cycle. (Some vehicles are set up to mitigate this effect by heating the intake one way or another.) You also have to take into account the fact that bearings, transmission fluid, and other rotating assemblies may never get all that warm if the temperatures are cold enough. Colder lubricants mean higher viscosity which, in turn, means a little more drag. It all depends on just how cold it is.

 

On vehicles that are "stealth enabled" ( can run off battery only ) like the Prius and Hybrid Escape, a cardboard or foam radiator block helps A LOT in temperatures 50'F and below.

#1 You get the engine hotter, quicker
#2 You keep it warmer, longer, during all your EV segments.

There is a minimum temperature ( 140'F, 60'C ) in the Escape Hybrid that must be maintained to enable engine shut-down. I suspect the Prius has a similar setting.

In the city, I drive about a 1:1 ratio of gas powered miles, and battery powered miles. In temperaures below 50'F I have a real hard time keeping the engine warm enough to enable EV.

A cardboard block ( over the entire front ) works well. I'm considering making a more durable plastic model. The only harm is running the engine hard for extended periods ( over 65 MPH highway ) could cause an overheat on those exceptional winter days when weather exceeds 50'F. Make sure the spouse or other drivers are aware of the radiator "modification" so they can take it easy on the highway.

-John

 

I read some articles about the effects of wide open throttle (WOT) efficiency compared to the increase vacuum of partial throttle efficiency. Engineers agreed that the reason for better fuel efficiency in partial throttle was due to the warming of the outside air in the intake system. WOT was worst because of the faster rate colder air made it to the combustion chamber. Many intake designs draw air channeled directly from the front of the vehicle and not through the engine compartment. Perhapes if you could use the air which passes through the radiator during winter, FE would get better. Also, maintaining as low RPM you can would also give more time for outside air to warm in the intake sytem.

GaryG

 

A warmer air intake is definitely a plus. Liquid fuel does not burn. Only gasoline vapors burn. You are correct Gary, colder gas in colder air will take longer to burn, and/or not burn as completely. Looking at the FEH air intake system, it is not easily modifed.

Since the goal is to save gas ( and money ) a lot of $ would be needed for me to modify the air intake. Thus defeating the purpose.
Right now I'm debating the wisdom of building a plastic radiator block ($40) since I can make ( and already have ) a ( recycled ) cardboard one for free.
And how much petroleum went into the manufacture of the plastic???

 

John --

Can you tell me about your cardboard radiator block? I thought it was just going to be a matter of slipping the cardboard down between the grille & the front of the radiator, but that's not possible without tools, & I'd rather not take anything apart (I generally have 2 left thumbs when it comes to mechanical things). So the only thing I can see is covering up the front of the grille on the outside.

Am I on the right track? Thanks in advance!

P.S. - In the case of sustained highway driving, would turning up the heat in the cabin help draw excess heat off the engine? I used to do that in my old Toyota in the summer on the old stop-&-start IL tollway.

 

I've heard some good reports about using foam, water pipe insulation, cut into strips. It is weather proof, easy to form and low cost.

In one of the Prius techincal forums we are looking at putting a bias resistor in parallel with the thermister to bias it towards a warmer temperature. The non-linear thermister curve makes this fairly safe if done correctly. Too low of a resistor and the car with report it is too hot. Too high and it has no effect. But this approach needs to be designed, carefully.

The one risk is the ICE may accumulate more hours at a cooler than designed temperature . . . like it is being run in cold weather. Another effect, the ICE will not warm up so fast and cabin heat will suffer. One mitigation strategy is to use an electric space heater for vehicles with 1-1.5 kW inverters or 12V heaters. The Prius has some electric heaters built-in.

We are investigating this approach but initial results are promising.

Bob Wilson

 

In 2001 Prius there is space to put cardboard in front of the engine radiator. With added equipment I have watched the coolant temperature and it only rises to 'interesting levels' when the air is above 50 F and real hills are being climbed, when the ardiator is about 60% blocked. More generally in the winter I would expect no problems.

If your car does not allow easy access (like the newer Prius) there are other ways to reduce incoming air as Bob mentions. Don't get creative and block the inverter coolant heat exchanger though, because it like to be exposed to cold as much as possible. Some will remember the 'racing Prius' with that heat exchanger immersed in a bucket of ice water.

DAS

 

This really is not an issue of engine operating temp, as even when the engine is fully warmed up as a few other have said, but of increased drag:

For one, the air is denser when it is cold. This effect will be even more pronounced if it is humid. Since air resistance is the biggest factor responsible for fuel consumption, especially at highway speeds, even a small increase will make a big difference.

The other effect is a huge increase in rolling resistance. Rolling resistance is caused by rubber in the tire sidewalls and the ***** on the treads constantly flexing as they "flatten out" to form the contact patch on the road. Rubber that is cold requires a lot more energy to deform than rubber that is warm.

Another factor is that when a tire heats up, the air pressure increases slightly, reducing rolling resistance even further (the tire will hold a round shape more with a higher pressure). This is why it may be a good idea to run a few PSI higher ("cold" pressure, before the car has been driven) in the winter than in the summer, since the tires won't heat up as much.

 

Hi,
There are some differences in hybrid architectures, IMA vs. HSD, and even variation between model years that makes cold weather effects on the engine important. This is not a one is better than the other but rather they have some differences.

One of the key elements of Prius economy is the ability to auto-stop the ICE and sustain speed in just electric mode. In the case of the NHW11 01-03 Prius, auto-stop requires the ICE coolant temperature to be at least 70C. Below 70C, the ICE will not auto-stop, even at a light. Depending upon starting temperature, it can take over 5 minutes to reach 70C but the NHW20 can auto-stop within 80 seconds.

The USA version, NHW20 has a thermos of hot coolant but the Japanese NHW20 without the thermos, will go into auto-stop mode within ~80 sec., about 1/4th the time of an NHW11. So when these cars reach the first stop, the NHW20 is silent while the NHW11 continues to run. The data suggests the NHW20 has a lower ICE temperature threshold than the NHW11.

There is one other architectural difference, the transaxle drag. The internal drag of the Prius is a function of the transaxle oil. The NHW11 has a more viscous transaxle oil, Type T, and one of the changes to the NHW20 is use of Type WS, a less viscous transaxle oil. Early testing suggests the NHW11 can achieve a 6-10% fuel efficiency improvement by switching to Type WS. But both models depend upon mechanical and MG1/MG2 heating to bring the oil up to operating temperature. Testing last summer showed the NHW11 transaxle took 30 minutes to reach a stable operating temperature. Meanwhile, the ICE is generating mechanical power needed to warm-up the transaxle.

We have lay reports that an ICE block heater and a transaxle heater can significantly improve Prius efficiency in the winter. This is supported by observations that 'taking lunch' to re-heat the ICE and transaxle before the evening commute improves the total, daily commute efficiency. Except for the ICE block heater, these cold weather effects are likly to be Prius specific or have a greater impact on Prius performance than an IMA system.

These effects are common to all vehicles but depending upon the vehicle architecture, other effects may be in play. For example, I understand the 'lean burn' of the Insight requires warmer air than is regularly available in the winter. There are other reports that heated intake air helps some vehicles . . . but not everyone has seen that effect in the Prius.

Bob Wiilson