A strong trend in automotive design has long been to reduce the engine’s friction by downsizing it, then recovering the power needed for spirited acceleration by some form of supercharging—stuffing the fuel-air charge of a larger engine into a smaller one. It’s simple arithmetic—increase intake pressure by adding 5 psi of supercharger or turbo boost and you boost power by roughly one-third.
We wondered “Will this spread to motorcycling?” Honda’s 1978 CX500 V-twin was boosted by a tiny turbocharger, but there were problems. Turbo boost takes time to build up (“turbo lag”) and there’s very little initial exhaust flow to drive it. Also, turbo boost is roughly proportional to impeller rpm, squared. Efforts to ride in sporting fashion on twisty roads could give the rider a bad surprise as the boost came in with a rush—often at exactly the wrong time. Except for record-setting and drag racing, the turbo didn’t catch on in motorcycling.
The turbo experiments ceased, although more recently, Kawasaki’s new H2 with mechanically driven centrifugal blower has attracted attention because of its tremendous power (earlier, Kawasaki gained valuable experience with supercharging on its personal watercraft engines). Another point is that because motorcycle fuel consumption is not presently limited by clean-air legislation, there is no legislative motivation to boost small engines to give them big-engine power. Yamaha has been experimenting with a three-cylinder turbo in recent years.
Other motivations definitely exist—lighter, more compact motorcycles that maneuver quickly and easily are always desired. Electric supercharging could make this triple lighter than traditional literbikes, but with equal or superior performance and rideability.
This could explain the attention given to Honda’s electrically supercharged V-3 prototype, revealed at the Milan Show (EICMA). The compact engine is displayed in a trellis-like steel tube chassis, with the single-sided swingarm and male-slider fork of high-performance Hondas. The two outer cylinders incline forward, while the middle (third) cylinder inclines back at 75 degrees to them. We can see conventional throttle bodies—one per cylinder—located under a cast aluminum plenum, into which blows the electric-motor-driven centrifugal compressor, located just behind the steering head. Air entry to the compressor is on the right with the electric motor on the left. Suggestively, a pair of barbed hose connections point straight forward from this motor, suggesting it may be liquid-cooled. Many parts have been left off to reveal the essentials of this concept bike.
Now for some history and numbers. Electric blowers have found use on large marine two-stroke diesels, and in Formula 1. Ships need electric power, so it was a natural to spin an alternator with any unused exhaust flow. Conversely, electric power could assist in the task of delivering charge air to the engine. F1 too has used fast-accelerating electric blowers to boost torque more quickly than is possible with turbocharging.
Electric supercharging performs similarly in some production autos. Back in 2014 Mark Glasson of Valeo North America, said, “[The electric supercharger] answers the low-end torque question. The turbocharger does not” (Valeo is a French automotive supplier).
Honda applied a Valeo electric blower to an NSX auto entered for the Pikes Peak International Hill Climb back in 2014. Clearly, that led to further study.
Multiple makers are now supplying electric superchargers to the auto industry. The Mitsubishi Technical Review reveals that from stopped to 90 percent of maximum rotational speed requires 0.3 second or less, with the electric motor drawing up to 5kW from the car’s electrical system. One automotive system described could operate for 30 seconds at the 5kW level, but could operate continuously at about half that level. The insulation on motor windings is temperature limited, which is why this car application requires 48V power. Electrical heating (as in a kitchen toaster) is proportional to resistance, and to current, squared. By quadrupling the voltage, current and therefore heating are reduced 75 percent.
The type of electric motor often referenced for this application is the switched reluctance type, or SRM, which has no windings or permanent magnets in its multi-pole rotor—just iron laminations. This gives them great durability at high speed, and reduces cost. With all windings in the multiphase stator, liquid-cooling becomes easier.
Now let’s try to understand what Honda may be doing here. In the EICMA show coverage, we said, “Sources in Japan suggest the engine is around the 850cc mark.” An 850cc triple with DOHC and four valves per cylinder (see the centrally located spark plugs in photos) is a middleweight, but with a fast-responding electric supercharger providing 5 psi of boost, it would easily make the power of a literbike. Take the Yamaha MT-09 and the 107 rear-wheel horsepower it produced on the Cycle World dyno. Just 5 psi of boost could increase this to 140 rear-wheel horsepower. Does this interest us?
Why doesn’t this concept lead us right back to the CX500 and other such projects, whose throttle response could be uncivilized? Electrical machines can be instantly and accurately controlled—switched reluctance motors are driven by “switcher” power supplies, making quick, accurate boost control possible. Result? Not only strong bottom-end torque, but in fact, torque everywhere, without unwanted “turbo surprise.”
How much extra electric power will such a system require? This will scale with the smaller airflow required by a motorcycle engine. If an automotive unit sized for 3 liters requires a peak of 5kW, a system for an 850cc bike engine might need only a third of that, or 1,700W. That doesn’t look out of reach, considering the 1,550W output of late-model Honda Gold Wing alternators.
How much weight will be added? On one Audi model the system added 22 pounds. This too should scale with the smaller airflow needs of a motorcycle engine.
Because of the smaller-diameter compressor rotor required by a motorcycle electric supercharger, it should spin up at least as fast as the 0.25 second Audi claims for its systems.
Why limit boost at all? Why not let the sky be the limit? The problem is that the higher the boost, the more likely destructive detonating combustion becomes. To avoid this, high-boost engines are given lower compression ratios. But in everyday use, reduced compression increases fuel consumption and weakens bottom-end.
A constant concern in motorcycle engine design these days is to reduce emissions of unburned hydrocarbons (UHC) by minimizing the total sealed length of piston ring. Some unburned fuel-air mixture is forced into piston ring crevice volume during compression and combustion, emerging later—still unburned—to become part of the exhaust stream. This is pushing auto and motorcycle engines away from the previous large bore/short stroke, and toward smaller bores (less total piston ring sealed length) and longer strokes. We don’t know the bore and stroke of Honda’s project triple, but we can be pretty sure it has less piston ring sealed length than the literbike fours of the previous era.
To maximize efficiency, a turbocharger must be located as close as possible to the engine’s exhaust ports—something difficult to do on Vee engines. The electric supercharger bypasses that problem.
To sum up, sudden throttle opening on a turbocharged engine places it at a disadvantage, because the small initial exhaust volume being produced is unable to rapidly accelerate the rotor. But in an electrically driven system, full motor torque is available even from zero rotor speed.
Honda descriptive material assures us that “Development will continue toward mass production and toward Honda’s goal of enabling customers to further experience the unique joy of riding and owning a motorcycle.”