Friday, September 14, 2012

Hypermiling in the Mitsubishi i-MiEV

As if electric vehicles (EVs) weren’t already energy efficient enough compared to their gas powered siblings, there are ways to extend even more range out of a charge.  Hypermiling is the act of using conscious actions to go farther by using less energy.  Several mechanisms exist for active hypermiling.  And there are a few other passive mechanisms that have been employed by others.  For the purposes of this discussion, “active” refers to actions that take place while driving and “passive” refers to actions that take place outside the actual act of driving but influence the driving range.

Active:  Hypermiling should be thought of as a challenge to see how far one can make a charge last.  But the activities described below should not be performed where the progress of other drivers is unduly impeded.  For example, don't pull out in front of oncoming traffic and accelerate at a pace so slow that it causes traffic to slow down behind you.  Either accelerate at a normal pace or wait for a larger gap.
·         Regenerative braking: This feature is not an option for non-electric vehicles.  Instead, non-electric vehicles with a manual transmission have the option of engine braking.  It is essentially the same action, but regenerative braking will not just slow the car down, it will return power to the batteries.  The biggest returns are gained from the highest speeds, and below 11MPH there is almost no recovered energy.  In regards to the Mitsubishi i-MiEV, there are three forward driving modes. 
o   Drive: Offers minimal regenerative braking.  In exchange, the slowing of the car when the accelerator is lifted is less abrupt.  This is the least used mode for the purposes of hypermiling.
o   Eco: The regenerative braking during Eco mode is between that of drive and Brake mode.  Eco mode also restricts the amount of power delivered to the motor which results in slower acceleration, but also less power consumed and thus longer range versus quick accelerations.  Eco mode is where almost all of the acceleration is performed and some braking is done.
o   Brake: The regenerative braking in Brake mode is the most aggressive of the three.  The response is not as harsh as someone stabbing the brakes.  It is closer to down-shifting one gear with engine braking on a manual transmission on a car with an internal combustion engine.  For the lone occupant in a vehicle it is not an issue.  If one or more passengers are along for the ride, it wouldn’t be advised.  The acceleration available in Brake mode is equal to that in Drive mode.  The majority of braking is done with Brake mode.  The balance is done in Eco to slow down more gradually and in Neutral to coast before coming to a stop.
·         Neutral/coasting:  Given that any of the three driving modes above will result in some regenerative braking (above 11 MPH), putting the transmission in neutral will allow the car to coast. It uses no battery power to propel the car forward, but also won’t receive any increase in battery charge.  Coasting in neutral would be most advantageous when going down a slight incline where vehicle speed can be maintained by gravity and the slowing caused by regenerative braking would not be desired.  Coasting can also contribute to range extension by putting the car in neutral around 11 MPH and coasting to a stop.  This technique takes time to develop since the operator needs to determine the distance from the eventual stop to begin coasting.
·         Shifting:  The truly "active" part of hypermiling is the act of shifting between the modes mentioned above.  Driving in only Eco mode would provide more range than driving only in Drive mode.  But to squeeze even more range out of the batteries, shifting between modes at the proper times will be crucial.  Shifting and anticipation are physical and mental counterparts to the range extending exercises.
·         Speed:  The slower a vehicle travels, the less energy is used to propel it.  Hypermilers rarely exceed the speed limit and several will travel about 5 MPH below the speed limit providing they are not holding up traffic behind them.
·         Acceleration:  Contrary to what the media might portray, electric vehicles are not slow by nature.  An electric motor has its full torque available at any rpm range and does not have to ramp up to achieve maximum torque.  That being said, if a driver is easy on the accelerator, longer overall range can be obtained by slower paced take-offs.  Using Eco mode to accelerate makes the task easier.  Driving efficiently could be visualized as driving with an open container of liquid.  Not as in 'don’t get caught with an open can of beer', but as in 'you have a bowl of hot soup in your lap'. If you accelerate too fast or brake too hard, the chances are some soup will spill.  The optimal rate to accelerate is close to that of a fully loaded semi truck or cement truck.  Likewise the optimal rate for braking is to use regenerative braking down to about 11 MPH, then apply the brakes to come to a complete stop.
·         Route:  Elements that can affect which route to choose are factors such as the number of traffic lights or stop signs, elevation changes, congestion, distance, average speeds, and elapsed time.  Here is a breakdown of how each influences the decision.  These are things that can apply to internal combustion engines (ICE) too.
o   Traffic lights: Let’s assume roads with two-way traffic are being used.  Turning right at intersections offers the best chance of uninterrupted driving.  Cars turning right on green have the right-of-way and in addition many traffic lights have a right turn arrow light while cross traffic is provided a left turn arrow.  If you catch this “bonus time” you can continue on your way.  If you are on a side street where the light is triggered by the presence of a vehicle, you may be able to proceed before getting a green light by waiting for a clearing in traffic.  The next best action at a traffic light is to go forward.  If equal time is given to each direction of traffic, a driver has a 50/50 chance of approaching a green light.
o   Stop signs:  Picking a route where cross traffic has to stop, but travel in your direction does not is the optimum condition.  In terms of EV’s, a 4-way stop is not much different from a stop where cross traffic does not have to stop.  An EV does not get penalized for “idling” the way an ICE does.  The positive side to stop signs are the opportunity for some regenerative braking.
o   Elevation changes:  A flat course is better for overall range than a hilly course, given both routes are an equal distance.  At some point there will be a trade-off where a shorter route over a hill is more economical than a longer route around the hill.  Absent any serious mathematical computations, simple trial and error may yield clear benefits of one method over the other.
o   Congestion:  The penalty for driving on congested streets is higher for ICE’s than for EV’s.  EV’s get a little pay back from stop and go traffic due to regenerative braking.  But having creep forward then stop through two or three traffic light cycles to get through an intersection is inefficient driving.
o   Distance:  The distance factor is a culmination of trying to find a route that offers the advantageous parts of hypermiling without having to go too far out of your way to achieve the results.  For example, it usually isn’t more efficient to drive three extra blocks to avoid a stop sign for the sake of being able to make continuous forward progress.
o   Average speeds:  Sometimes you can get away with driving 35 MPH in a 40 MPH zone.  By that I mean without holding up traffic behind you and becoming a menace to the roadway.  Hypermiling is about getting somewhere efficiently, but without being self-righteous and denying others their right to expect unimpeded progress.  Other times you could choose to use back roads or residential streets while traveling parallel to surface streets.  The lower your ‘traveling’ average speed (does not include acceleration or braking), the higher your overall range.
o   Elapsed time:  Elapsed time:  If a person isn’t using accessories such as heater, air conditioner, or radio, then the elapsed time is not too much of a factor.  However if using devices that will shorten their range, the route decision must also factor in total time the vehicle is on.  Just sitting at a stop light isn’t penalized if the A/C is off.  But if it is running, then you want to find a route that minimizes overall travel time.
·         Anticipation:  What does one do when the traffic light ahead has the ”DON’T WALK” light flashing for pedestrians?  It usually means the crossing light will become steady followed by the traffic light turning yellow.  Anticipation factors into hypermiling because the driver will have to determine if they can get to the intersection before the light turns yellow or if they should begin slowing down early.  If the chosen route has a sequence of traffic lights that are timed and require traveling at a certain speed or accelerating at a certain rate to not get stopped, then this consideration can trump the slow acceleration argument.  Another situation you may be faced with is when approaching a left turn.  If you spot a gap in traffic, it may require accelerating or decelerating to time the gap.  Sometimes a driver may have to carry more speed through a corner than is typical in order to turn earlier rather than come to a complete stop and wait for another opportunity to turn.
·         Accessories: Never place hypermiling above safety!  If the defroster is needed to be able to see out the front windshield, then use it.  But using the heater or air conditioning will reduce your range.  When looking at the estimated range on the i-MiEV, one can see the range decrease as the fan speed increases.  The lesson here is; if you use accessories that aren’t essential, your range goes down.  The Mitsubishi does have a seat heater for the driver.  They state that it is more energy efficient to use the seat heater to warm the driver than to use the vehicle heater to warm the entire cabin.

·         Tire pressure: Conventional wisdom/Documented evidence states that if your tires are underinflated, the vehicle’s fuel/energy economy is reduced.  Some hypermilers look at the situation and apply extrapolated logic.  Overinflated tires should increase fuel/energy economy.  And probably they will.  But how much pressure to add above stated levels?  The drawbacks are the risks from overinflated tires.  What is the burst pressure at which the tire will blow itself off the rim?  Since tires heat up while traveling and that forces the tire pressure to increase, a static tire that is overinflated may not be a hazard.  In fact many people overinflate their tires while storing a vehicle to reduce the chances for getting flat spots on the tires.  But an already overinflated tire may reach burst pressure while traveling.  In addition to the safety risk is the financial risk.  Will an overinflated tire wear faster (at least locally if not uniformly) than a properly inflated tire.  The cost of early tire replacement could outweigh any financial gains from saving a few cents during recharging.  Lastly is ride comfort.  The higher the tire pressure, the stiffer the vehicle ride which typically transfers motion to the occupants.  With the caveats out of the way, the debate remains.  Should it be done?  How much could be done?  I am only informing others that it is being done.  I am not endorsing the practice.  But other anecdotal evidence is the fact that many SCCA racers routinely inflate the tires on their race cars to 40-45 psi.  And while they do worry about burst pressure and blowing out a tire, they are not as concerned about tire wear and a smooth ride.
·         Extra weight: How extreme can it get?  The i-MiEV does not have a spare tire; it has an emergency tire inflation kit.  Basically a can of Fix-A-Flat.  Part of the rationale could be the fact that the front and rear tires have different profiles.  The front is 145/65R15 and the rear is 175/60R15.  But unless it was a full size spare, it really only has to worry about fitting the same bolt pattern since it is for temporary use.  But in addition to saving cost for the manufacturer, it also saves weight; and that means requiring less effort to bring the car up to speed.  By the same token carrying around unneeded and unused items should be avoided.  I’m not advocating removing the interior door panels to shave off pounds or buying carbon fiber body panels to replace metal ones.  Just don’t carry around a 40 pound bag of dog food for three weeks.


  1. I disagree about avoiding Drive mode: it's the most useful for preventing unwanted regenerative braking without the danger of coasting in neutral.

    1. Thanks for taking the time to read my blog. I really want to inform the public even if most of the what I write about is my own opinion. In the spirit of informing the public, what is the danger of coasting in neutral, especially as it relates to an electric vehicle?

  2. @lee: The purpose of D mode is to provide ICE-like driving feel. It is not the best mode if you want to extend the range. B-mode is the best and if you don't like the aggresive regen, then do not release the foot from the pedal all the way. I wish there is easier switch between N and B. We have to juggle through D and Eco. From that reason I do not use neutral much.