Squeeze more mileage from your hybrid car with this expert advice.
If you own a hybrid-electric vehicle (HEV or “hybrid”) or are considering buying one, you’ve probably wondered whether these cars really get the gas mileage their manufacturers claim. Well, that all depends on how you drive it. Hybrids are quite different from standard gasoline-fueled cars, and there’s a learning curve involved in optimizing their fuel economy.
My wife Rebekah and I have been driving HEVs for more than a decade. She purchased the original two-seater Honda Insight hybrid in 2001, and we immediately made a competition out of who could get the best mileage per trip. We developed several strategies for that vehicle, some of which did not translate to the 2006 Ford Escape hybrid that I purchased later. The two-seater Insight (made from 2000 to 2006) is considered a “mild hybrid”—one that never operates entirely in electric mode. For that reason, many of the strategies discussed here do not apply to that vehicle.
Most of the HEVs on the road are considered “full” hybrids—they can drive for several miles in electric vehicle (EV) mode alone. One of the keys to understanding how to optimize HEV performance is to learn how to make the best use of the battery that powers the electric motor.
All HEVs have both an internal combustion engine (ICE) and an electric motor, which gets energy from a battery pack. In many HEVs, such as the Toyota Prius, the ICE is an Atkinson engine which, while efficient at its optimal rpm, does not develop much torque at low rpm—so it’s rather inefficient during acceleration. The electric motor augments the ICE during acceleration—the faster you accelerate, the more battery power is used. Of course, most hybrids can also operate entirely in electric mode for periods and this uses a lot more battery energy.
The electric motor is used as a generator anytime you take your foot off the accelerator or apply the brakes. Additionally, once the car is up to speed and cruising along, the generator tops off the battery if it is low. This is known as a “cruising charge.” Regenerative braking switches the electric drive motor to generator mode, slowing the vehicle, recovering kinetic energy to convert it to electric energy, and storing it in the battery bank.
All DC motors can become generators if you turn the motor by force. The faster you turn it, the more electric power you can generate. In a hybrid, when you press your foot down on the brake pedal, the brakes are applied—but the motor is also used as a generator, which also slows down the vehicle. Some of the braking force is converted to electric energy rather than heating the brake pads and rotors. Besides helping recharge the battery, this also extends the life of the brake pads and rotors. The amount of regenerative brake energy depends on the battery pack’s state of charge. If the battery is low, the engine computer will configure regenerative braking to recover as much energy as it can. Many HEVs have two gauges—one that shows energy going into or out of the battery (charging or discharging the battery bank); another shows the state of charge. Along with the real-time mpg gauge, these are the two most important gauges to pay attention to for optimizing performance.
Driving a HEV like a normal car will yield fairly good gas mileage, but mastering regenerative braking means more energy stored in the battery and therefore less gasoline used. Driving more consciously is the key to wresting more mpg performance from regenerative braking. This is not always easy when you have passengers in the vehicle—especially children—but it becomes easier with practice. Most good driving guides stress paying attention to the road and planning as far ahead as possible, while also being aware of the vehicles behind you—and this certainly applies to driving a hybrid effectively.
To optimize the benefits of regenerative braking, plan all of your stops in advance and ride the brake gently for as long as possible until you come to a stop. Braking puts a lot more energy back into the battery than coasting with your foot off the accelerator. This is not always practical in traffic and doing this to extremes will certainly annoy the other drivers on the road. But used appropriately, it is an excellent HEV fuel-saving strategy.
To prevent wasteful idling, HEVs will usually shut down their ICEs at a stop, so plan your acceleration strategy according to how fast you expect to go when you take off. If you accelerate gently, the vehicle will remain in EV mode longer before it switches on the ICE. If you are stopped at the top of a hill, you can use gravity to assist your acceleration, reducing your need to depress the accelerator.
In city driving, accelerating very gently will keep you in EV mode longer if you remain below about 25 mph—any sudden pressure on the accelerator is likely to kick on the ICE. One way of looking at accelerating in a hybrid is to see how lightly you can hold your foot on the accelerator and still maintain the speed that you need. Most drivers tend to press on the accelerator much harder than is actually needed to maintain speed. Watching the real-time mpg display on the dashboard will help you get a feel for this (see “Scan Gauge” sidebar).
Let’s say you’re planning a round trip from your rural home to do some town shopping and your route in town consists mostly of low-speed driving (below 25 mph). If you can plan your driving so that you enter town with a fully charged battery bank, this will give your car the maximum EV range. For instance, if you have a long downhill run on the way into town, you can either drop the vehicle into low gear on that hill if it is quite steep or feather the brake pedal all the way down, or both. Using low gear turns the engine generator faster, extracting more energy from regenerative braking while also limiting the speed of the vehicle. Riding the brake gently without necessarily slowing the vehicle, especially at speeds above 45 miles an hour, will put a lot more charge in the battery than simply keeping your foot off the accelerator (coasting).
There are two thresholds to be aware of in most HEVs. One is the speed at which the ICE will always kick in from EV mode, even if you are going downhill or coasting. This usually occurs at about 40 mph. Similarly, you can drop your car into EV-only mode while coasting or slowing to below about 40 mph. The other threshold is the upper limit for acceleration in EV mode, which is about 25 mph. Being conscious of these thresholds will help you to maintain EV mode longer.
Here in rural Maine, most of our driving is on rolling, two-lane blacktop roads where the average speed is about 50 mph. My strategy is to allow the vehicle to slow on the uphill run, using the ICE minimally. At the hill’s peak, I feather the brake slightly, which causes the ICE to shut off and allows the vehicle to accelerate downhill using gravity. If the hills are gentle enough, by staying below 40 mph, I can stay in EV mode rather than having the ICE kick in for every uphill run. With no other vehicles in sight, I can allow the vehicle to slow below 25 mph at the peak of each hill, and then gain speed by the bottom of each hill, usually up to 45 mph. On steeper downslopes, I ride the brakes, staying below 40 mph and charging the battery. My Escape is nominally rated at 30 mpg on the highway and I have achieved up to 40 mpg using this technique. Of course, I pay attention to the road and resume a more normal driving strategy with vehicles behind me, or I pull over to let them pass.
Guy Marsden develops electronic prototypes of electronic products for individual inventors and small companies. He also manufactures differential controllers for PV-powered solar thermal systems. His website (www.arttec.net) showcases his sustainable efforts in considerable detail.
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Scan Gauge • www.scangauge.com