- Jun 25, 2009
Can pulse and glide work to improve electric vehicle highway range?
Wayne Gerdes explains hypermiling before the Ford Fusion 1,000 mile challenge
Recently we got a question from a reader regarding the benefit of using the common hypermiling technique of "pulse and glide" with a battery electric vehicle. Given the relative scarcity of pure electric vehicles, P&G has mostly been used by hybrid drivers as one of the techniques to squeeze every last mile out of every drop of gasoline. There's a reason hypermilers use the technique: it is very effective. Since my own professional driver training has more to do with vehicle dynamics and getting through corners with the least possible loss of momentum I decided to consult with a true expert in the field of hyper-miling, Wayne Gerdes.
Wayne runs a web-site called CleanMPG.com that includes discussion boards where fuel sippers talk at length about getting more from less. At a recent media event we had a chance to talk with Wayne about what exactly is pulse and glide. At its core, it is precisely what the name implies, a pulse of acceleration to get the car up to some target speed, followed by a period of gliding, off the accelerator for as long as possible.
Find out more below the jump.
According to Wayne, there are two keys to getting the most out of pulse and glide: understanding the most efficient operating modes of the car you are driving, something that typically requires some practice, and testing. Hypermilers typically use a device like the Scangauge (right) to monitor the instantaneous fuel consumption of the vehicle. Getting the car up to target speed most efficiently will vary from car to car. Through a process of trial and error, drivers can find the most efficient rate of acceleration.
Once up to speed, the goal is to stay off the accelerator as much as possible, this is where gliding comes in. Maximizing the glide means cutting drag as much as possible to keep the car rolling. This is where Gerdes complained about regenerative braking. With all the hybrids built to date, automakers have tried to emulate the behavior of traditional vehicles. That means some amount of regen is programmed in to simulate the effect of engine braking when the accelerator is released. This, of course, means the vehicle will slow down when the pedal is released, not exactly perfect for gliding. Gerdes would prefer to see automakers provide an option for drivers to control the regen or at least turn it off except when the brake pedal is applied. Without regen, cars could glide significantly farther between pulses.
The other aspect of P&G that requires experimentation by the driver is how far to glide. At some point the vehicle speed drops to a point where another pulse is needed. How slow the driver can allow the car to go depends on road traffic and the acceleration characteristics of the car. Obviously, when there are other cars around, a glider can't drop the car's speed as much. An experienced hyper-miler can extract huge jumps in efficiency under the right conditions. Last December during the media launch of the Honda Insight, Gerdes managed 79 mpg (the EPA rating is 40 mpg city / 43 mpg highway). Stories of Prius drivers hitting nearly 100 mpg are common. More recently, Ford took a Fusion hybrid on a hypermiling road trip that managed to squeeze 1,445 miles out of a single tank.
Gerdes' limited experience with battery electric vehicles (BEVs) indicates that the improvements possible with P&G are likely much smaller than with hybrids or standard internal combustion vehicles, probably on the order of about 5-8 percent. The problem for P&G fans is that BEVs vehicles tend to be much more optimized for efficiency to start with, a benefit most of the time for something that leaves less room for improvement. In a hybrid, optimizing the use of the internal combustion engine can have a huge impact on the mileage numbers. Since a BEV has no engine, the driver only has the battery to work with. Clearly, driving style will influence BEV range, but it will likely only drop with aggressive driving rather than improve with careful driving.