There are still a good many people who refer to a Toyota Prius as an electric car, yet it most certainly is not: a Prius is a gasoline/electric hybrid, which runs on its internal combustion gasoline engine in those parts of its performance envelope where it is advantageous, and on a small (150-pound) battery and electric motor in conditions where they are more efficient.
Why not just run on the battery all the time? Because the market purpose of Prius was mileage improvement rather than perfection. It avoided the pure electric’s limited range by being able to cruise nearly 600 miles on its gasoline engine. When the Prius first came to market in 2000, coal was still generating 50 percent of US electricity, and was generating 48.2 percent of it in 2008. Today, though, coal has declined to generating 19 percent of our electrical power, while natural gas has risen from 15 percent to 40 percent of US electricity production in 2020.
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A hybrid’s gasoline engine gives it long range and the ability to refuel in minutes at existing gas stations. Its electric powertrain takes over in the operating conditions where the gasoline engine is least efficient—at low speeds and in stop-and-go traffic.
It’s All About Specific Fuel Consumption
If we graph a gasoline engine’s specific fuel consumption (SFC) versus its rpm and throttle opening, we get a concentric series of curves of constant SFC. SFC measures how much fuel (by weight) is required to produce one horsepower for one hour, so its unit is pounds per horsepower-hour (or a cgs/mks metric equivalent). SFC rises at higher rpm because of increasing friction losses, and rises at low throttle/idle conditions because of pumping losses. Somewhere in the middle is an island of lowest SFC—throttle open far enough to prevent most pumping loss, but at rpm moderate enough not to generate excessive friction loss. Worst of all is small throttle operation and idle (where the engine is running but produces no net power beyond that required to operate accessories such as lights).
Ducati’s CEO Claudio Domenicali has told me the ratio of best to worst SFC can be surprisingly large—as great as 2.5:1! While cruising on its “sweet spot” a gasoline engine may achieve an SFC of 0.42 pound per horsepower-hour, but creeping in slow traffic or even pootling along a residential street may push that to a pound or more per horsepower-hour. Using the electric motor for power in these conditions is what gives a hybrid vehicle its reduced fuel consumption. It has abundant, economical range when cruising on the gasoline engine’s sweet spot (modern autos have small engines and as many as 10 transmission speeds to keep them working in that sweet spot), but when in stop-and-go traffic or at steady low speeds, a hybrid vehicle shuts down the gas engine and runs more efficiently on the battery. The result is an attractively higher highway/city average fuel mileage. To further boost fuel economy hybrids may be given especially streamlined shape, reduced frontal area, and smaller-than-normal tires at increased inflation pressures to reduce rolling resistance.
Related: More About Engine Friction
Because a hybrid’s battery weight is about equal to that of one passenger, it can’t drive the car very far. But it provides efficient power adequate to creep through traffic, and uses no power at all when a gas engine would be idling. When the battery becomes discharged, the gasoline engine starts automatically and recharges it. This improves combined city/highway mileage. Unlike a full electric car, a hybrid need not carry with it a large battery (whose weight is typically equal to that of five to seven passengers). On the other hand, since it has two drive trains a hybrid naturally costs more than a conventional gasoline-fired auto of the same class.
Two-wheeled Hybrids
What does all this have to do with motorcycles?
There are 40 times more cars registered in the US than there are motorcycles, and bikes generally use less fuel than most cars. At present there are no laws requiring bikes to achieve specified fuel economy levels as cars must. A motorcycle’s compactness leaves little room for a hybrid’s second drive train. Because the present moment is not a golden age for motorcycle sales, the added cost of the second drive train will discourage economy-minded buyers. The result is less motivation for motorcycle manufacturers to develop hybrid motorcycles.
However, prototype development of hybrid motorcycles may be an example of what is called “contingency engineering.” If a manufacturer believes there is a possibility that motorcycles may one day be required to achieve legislated levels of fuel economy, it could make sense to begin developing the necessary technologies ahead of time. For some years (and perhaps to this day) Honda maintained what some called “Mr. Oguma’s refrigerator.” It was a collection of ready-for-production technologies, kept in “cold storage,” that might become useful in the future. Yoichi Oguma was for many years a prominent (some would add “colorful”) Honda engineer.
I leave to the readers the question of whether or not the extra cost of hybrid vehicles is offset by fuel savings over their operational lives.
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