Economy cars often have miles-per-gallon displays that reveal an important fact. A car that at steady 75-mph interstate speeds delivers 45 mpg goes much less far on a gallon of gas in rural or around-town driving. In my case, the usual number is 26 mpg. The usual reason given for this is that stop-and-go driving requires repeatedly accelerating the car then dumping its kinetic energy into its brakes at the next stop sign. This is part of the reason, and is why automakers now work very hard to make cars lighter because it takes less energy to accelerate a 3,200-pound car than it does a 4,500-pound car.

But there is more to it. In a conversation I had some years ago with Ducati CEO Claudio Domenicali, he noted that if you make a plot of four-stroke spark-ignition-engine-specific fuel consumption (fuel burned per horsepower, per hour) at all rpm and throttle openings, you will find that the highest fuel consumption is roughly 2-1/2 times greater than the least. That's a mouthful. Two and one-half times.

Here is why: We know that friction loss increases as rpm rises; inertia and gas loads on bearings rise. But what is less well-known is that so-called pumping loss increases at small throttle angles. In between those extremes—high rpm and low throttle angle—is a region of least fuel consumption. Engine rpm is low to moderate, cutting friction, and the throttle is open enough to reduce pumping loss. That gives the economy car its 45 mpg during interstate cruise and is the other part of why it gets an average of 26 mpg on slower secondary roads and around town (and, of course, the combustion engine gets zero mpg at stoplights and while patiently waiting in frustrating traffic jams).

Engineers looked at those facts and saw a possibility: What if we don’t run the combustion engine at all during slow running, stop and go, and in traffic jams, common situations in which its specific fuel consumption is high? What if, instead, we power the car during those times with an electric motor driven by a battery that is charged by the combustion engine? An electric motor has no pumping loss and it doesn’t idle; it just stops when not needed, using zero energy.

And so, the hybrid automobile was born. It uses its combustion engine—which gives excellent range, served by the nation’s 160,000 gas stations—during conditions in which it gives its best fuel economy. But in conditions that make combustion engines much less economical, we’ll switch to the electric motor.

Ducati Multistrada on black background
A hybrid Ducati Multistrada? Where would you place an electric traction motor, its power supply, and a battery capable of driving it a useful distance?Courtesy of Ducati

You hop into your hybrid, switch on, and as you depress the “gas” pedal, the combustion engine does not start. Instead, the electric motor propels the car across the parking lot and into traffic. Once conditions are more favorable for combustion-engine fuel economy, your gasoline engine starts and seamlessly takes over the propulsion task. And it also puts back into the electric motor’s battery the energy just used. If traffic slows or there is a red light, the combustion engine shuts down and the electric motor, because it is more efficient in slow going or stop and go, takes over.

An electric motor has no pumping loss and it doesn’t idle; it just stops when not needed, using zero energy.

Another benefit is possible: regenerative braking. Because electric motors can also be made to act as generators, casual braking can be performed by operating the electric traction motor as a generator, taking kinetic energy out of the car’s motion—that is, slowing it—converting that into electric current, and returning it to the battery. That process cannot be 100 percent efficient, otherwise everything we use could run on perpetual motion, so only about two-thirds of the car’s kinetic energy can be returned to the battery in this way. But that’s a lot more than zero, which is what happens when you have only conventional friction brakes at each wheel.

All of the above increase the around-town fuel economy of hybrids significantly. But, of course, fuel economy is not the only issue in the great task of planet-saving. There are also the added cost and resources required to give the car a second propulsion system, with the rare-earth elements required for the motor, lithium, and other pricey materials in the battery, plus the still-unresolved question of large-scale recycling of such components at the end of their service lives. As is familiar from the long-running debate over nuclear power, the “pro” group hires famous scientists to endorse its official doctrine, and the “anti” group hires scientists who are just as famous and went to the same universities to say the opposite.

How do we make sense of this? Is truth just another commodity? That leaves you and me to make up our own minds. You like that blue one? I rather fancy this silver one.