Pulsing and gliding your way to better fuel economy

Jeff Ronning is a senior consultant with the Plug-in Hybrid Electric Vehicle (PHEV) Team at the Rocky Mountain Institute.

Ever since I bought a stick-shift Civic, I started some nerdy behavior. It's called pulse-and-glide (P&G) driving, and it gives me about 15 percent better gas mileage than the average for my car.

The premise behind P&G is simple: Normal, around-town driving rarely causes your engine to break a sweat. That's bad because the engine usually isn't operating very efficiently.

By changing your driving style, however, you can optimize your engine's efficiency and get much better gas mileage, without buying a whole new car.

Say you drive the family SUV out to the mall for a little holiday shopping. For argument's sake, let's assume your vehicle has a 4.7-liter DOHC 32-valve EFI V8 engine, capable of 260 horsepower (194 kilowatts).

Automobile engines generally get the best efficiency at around three-eighths of their peak power. That means your SUV is doing its dead-level best when you're asking (with the gas pedal) for 73 kilowatts.

How much power does it need to scoot down to the mall? Try seven kilowatts, to maintain the 35 mph cruise.

Today, engineers have to specify a powerful engine for hearty, 10-second acceleration. The result is that most cars rarely pass through their best efficiency.

Enter us P&G geeks. What we do here is to simply ask the SUV for about 73 kilowatts. Once we reach a desired speed, we give the car a break and drop it in neutral. Revs purr down to 800. Once we've slowed somewhat below target speed, we hit it again with the 73 kW.

Now you're asking, "How in the world does he know where 73 kilowatts is?"

Well, you don't have to hit 73 kilowatts like its some magic value. What you need to do is open the throttle without letting the engine rev high. This is why most P&G masters drive stick. With an auto transmission, when you ask for high throttle with your foot, the transmission controller thinks you want high throttle plus high revs. But high revs are the enemy of fuel economy.

For stick-shifters, here's the formula: You want medium engine speed during your pulses. Pick a gear that is going to keep you below 3500 RPM during the pulse.

Since you are your transmission's brain, it can't try to downshift when you put the pedal to the metal. That's right, go ahead and mash it ... this is momentary, right? (Unless you're driving a Lamborghini, you accelerated briskly, but did not get airborne.) After you slightly exceed your target speed, get off the gas, drop in neutral, and let it roll. Then repeat as desired.

The net result of P&G is that your engine gets to run near its best efficiency point much more often. Otherwise it's idling. If you have a car with real-time fuel economy display, you would see it alternate between moderately-worse-than-normal fuel economy and the maximum.

Interpretation: If you are pulsing about half of the time then your overall fuel economy will be about twice the number displayed during your pulse. This applies for the instantaneous display. If you also have a trip fuel economy display, then use that to see your overall effect.

There are a number of drawbacks to pulse-and-glide, which for many drivers may undermine the fuel savings.

First, in some states like Colorado, coasting in neutral (at least downhill) is illegal. Also, when in neutral you are at a safety disadvantage because of the delay of shifting back into gear. This is especially a concern if you tend to drive closely behind other vehicles (a bad idea for fuel economy, anyway).

Then there is increased transmission wear that results from frequent shifting, various emissions issues, and the perturbed driver behind you.

If those drawbacks keep you from pulsing and gliding down the road, here's the good news: hybrid cars achieve the same benefits of engine efficiency without all the baggage.

Their added electric propulsion systems help the engine by taking over during the inefficient engine points. They also supplement the engine with either positive or negative torque to reposition its operation to a more efficient point. Plus, they recover much of the kinetic energy that is normally lost in brake heating.

Driving a hybrid may not feel as good as walloping the EPA average fuel economy for your car, but it sure beats having to oscillate your speed and overwork your transmission.