Does the Prius do Snow?

Posted by John Dolan | Posted in Community, Hybrids, News | Posted on 16-01-2012

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A Gen II Prius Delivers the Mail in WisconsinNow that snow finally blankets the land that is a fair question.  So how does the Prius handle snow, really?  This time of year the winter ‘mannerisms’ of the Prius are regular discussion topics on hybrid owner chat boards.  By visiting sites like PriusChat.com you can read the accounts of new drivers and those with 8 to 10 years of winter driving experience in all kinds of weather with the Prius from the Rockies to the nether regions of darkest Canada.  The discussion topics on these chat boards range from reduced winter fuel efficiency to the various driver experiences of how well the Prius performs in wintery driving conditions.

The seasonal drop in fuel economy with the onset of winter is a little complex but easily understandable.  To start with, air is drier in the winter and cold air reduces the amount of energy that can be extracted from fuel.  In addition, winter refinery formulas also reduce fuel efficiency.  Any driver of a gas-only vehicle who is paying attention at all, has in all likelihood already noticed a drop in fuel economy.  Winter gasoline contains more butane, which according to Gasbuddy.com costs less, but also evaporates at a quicker rate and is more volatile.  Other factors, like the higher rolling resistance on your tires caused by snow-packed roadways and frigid wheel bearings contribute to decreased seasonal fuel economy, but in a less noticeable way.

So, all vehicles experience reduced efficiency during the winter.  The seasonal cycles with a hybrid such as the Prius are no different.  On the hybrid side of the equation, fuel economy takes a hit when the climate control system runs continuously to heat and defrost the cabin.  The heater-core in the Prius is supplied with hot anti-freeze which heats up the air passing through it.  Just like a normal car.  When keeping winter’s cold blasts at bay and maintaining the cabin of your hybrid all warm and toasty you are in effect, asking your gas engine to barely ever shut down.  On such days with the climate control running high, the dashboard indicator may show fuel economy in the 34 to 42 mpg range for the Gen. II Prius and 38 to 46 mpg for the Gen. III model.  Anecdotal reports say the 50-mpg average of summer may even fall to somewhere between 33 and 40 miles per gallon. That’s not really that much extra gasoline (up to 1 gallon extra every 100 miles), but Prius owners are known for taking their fuel economy seriously.

Further complicating the situation is the reality that battery capacity is reduced in cold weather, so the engine runs more often to top off the hybrid battery pack.  The performance of the nickel-metal-hydride high-voltage battery declines somewhat with the colder temperatures (just as it does for regular 12-Volt lead-acid starter batteries).  The less time the Prius runs on battery power, the more gas it burns, meaning fuel economy declines.

To say that Prius owners take their fuel economy seriously qualifies as a classic understatement.  Fuel efficiency awareness is heightened for the Prius driver dramatically by the Multi-Display Screen on the dashboard.  Every little nuance, information you were totally unaware of in your traditional vehicle, is displayed before you at all times.  If you had that information in front of you in any other car there is no way you could continue to believe that the effects of winter on fuel economy are minor.  Veteran Prius drivers do not let themselves get overly concerned with mpg calculations during the winter season without first considering the effects of cold weather on fuel efficiency.  Focus on the ‘big picture’ — the seasons change and soon enough the fuel economy cycle will improve with the warmer temperatures!  Remember, if the Prius suffers from reduced fuel economy during the winter months then other, gas-only vehicles are affected too, just to a greater degree!

Other than the palpable reduction of fuel economy that accompanies cold, wintry weather the other most common topic in Prius chat rooms seems to evolve around the winter driving experience itself.  Specifically, “How does a smaller car like a Prius handle in the snow?”   Comments are all over the board, ranging from those who claim the Prius is one of the best front-wheel drive small cars they have ever driven in snow to those who say it’s nearly useless in the stuff.  The majority of the folks commenting on traction issues (pro or con) in these forums own the Gen II Prius (2004-2009 model years).

Even a small hybrid sedan has the potential to do well on snow and ice.  Hybrids, like the Prius, tend to be front-wheel drive (FWD) vehicles.  While not as sure-footed in snow as all-wheel drive, front-wheel drive cars have the advantage over their rear-wheel drive counterparts in that the heaviest part of the car — in this case the Hybrid Drive System, is on top of the drive wheels.  This helps to push the drive wheels down through the snow and gives them better grip.  Because electric motors are more efficient at distributing torque than internal combustion engines the hybrid has an additional advantage in the snow.  It is precisely due to this huge amount of torque from the electric motor that traction control technology is applied to hybrids.  The purpose of the system is to prevent wheel slip and loss of traction. Because electric motors provide maximum torque from 0 rpm, on slippery roads the wheels spin easily–whereupon the traction control promptly brakes the spinning wheel. The result, owners say, is halting vehicle acceleration with an accompanying beeping sound from the skid alert as the wheels lose traction, are slowed down and stabilized; then the cycle repeats itself as the car slowly digs itself out of a traction loss.
Spinning tires are never a good thing.  Loss of grip may cause safety issues and will definitely increase tire wear.  Traction control helps prevent all that.  Brute force is not the most effective method of getting through snow anyway.  A slow and powerful dig is what traction control is designed to provide.  Like many other aspects of driving the Prius — the vehicle prompts some re-learning.  Putting the pedal to the floor in a Prius is the last thing you want to do, especially when there is a loss of traction due to a slippery surface.
Beginning with the Gen III Prius (2010) the Traction Control system (TRAC) was re-designed, along with 90 per cent of the hybrid drive components in the Prius.  In previous models, mostly from 2004 to 2009 (Gen II), the electric motor design had more torque (295 ft. lbs.) and as a result, the TRAC system was more aggressive, cutting power (throttle) at the mere hint of wheel spin.  TRAC in the Gen III Prius functions more like any other Toyota (not the Gen II Prius) in that it does not cut out all torque (throttle) in a slip condition.  Instead, TRAC in the current Prius will try to provide adequate torque (by utilizing a more powerful 80 hp electric motor with 153 ft. lbs. of torque) to maximize friction between the wheel and the road.  Without going into too much detail, Toyota engineers simply moved traction control responsibility from the hybrid system control unit to the skid control ECU (electronic control unit).  As a result, the skid control ECU instantly determines the state of the vehicle when wheel slippage is detected and operates the brake actuator to apply braking to the slipping wheel.  At the same time, the skid control ECU initiates cooperative control with power management (the hybrid drive system) in order to adjust motive force.  In this way the TRAC system in the Gen III Prius can constantly maintain stable vehicle traction and minimize wheel slippage.
A hybrid like the Prius (with a ground clearance of 5.25 inches) handles snowy conditions just fine.  The misconception is that more clearance is needed.  Most urban and suburban roads are plowed before accumulation of that depth occurs.  In fact, you can always drive through piles deeper than that anyway!
If you have traction issues more demanding than normal winter driving, just upgrade to a more aggressive snow and ice tire. The factory mounted all-season tires on the Prius are a good choice for most drivers.  More aggressive snow and ice tires do involve an efficiency penalty (and they are usually noisy, too) but their better road gripping ability is an acceptable trade off for some hybrid owners.  The traction on any vehicle could be improved with a more aggressive tire.  To that end, many Prius owners with unique challenges during their winter driving cycles have upgraded to heavier snow and ice tires such as the Bridgestone Blizzaks or the Nokian Hakkapeliitta RS which are both great winter tires.  But for the rest of us, the factory tires and the TRAC system on the Gen III Prius work just fine for basic winter conditions.  The Prius can handle the winter weather.  Cold weather and slippery driving conditions present challenges for all vehicles, but the hybrid system in the Prius is well-equipped to deal with them!
Sources:  Priuschat.com, Toyota Prius Owner’s Manual, Hybrid-cars.org, Greencarreports.com, Hybridcars.com/forums, MilwaukeeHybridGroup.com, Drive$mart America

Gen III Prius Demonstrate TRAC On A Icy Driveway:

Demonstration of Gen III Prius Vehicle Stability Control:

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