Difference in Motor vs. Engine Specifications

Figure 5-6 Simplified EV drivetrain layout.

Difference in Motor vs. Engine Specifications

Comparing electric motors and internal combustion engines is not an “apples to apples” comparison. If someone offers you either an electric motor or an internal combustion engine with the same rated horsepower, take the electric motor—it’s far more powerful. Also, a series wound electric motor delivers peak torque upon startup (zero RPM), whereas an internal combustion engine delivers nothing until you wind up its RPMs. An electric motor is so different from an internal combustion engine that a brief discussion of terms is necessary before going further.

There is a substantial difference in the way an electric motor and an internal combustion engine are rated in horsepower. Figure 5-7’s purpose is to show at a glance that an electric motor is more powerful than an internal combustion engine of the same rated horsepower. All internal combustion engines are rated at specific RPM levels for maximum torque and maximum horsepower. Internal combustion engine maximum horsepower ratings are typically derived under idealized laboratory conditions (for the bare engine without accessories attached), which is why the rated HP point appears above the maximum peak of the internal combustion engine horsepower curve in Figure 5-7. Electric motors, on the other hand, are typically rated at the continuous

Drivetrain Manual Differential

Overall type transmission Driveshaft drive Drive axle efficiency Front wheel drive 0.96 not required .097 .098 0.91 Rear wheel drive 0.96 0.99 .097 .098 0.90

Table 5-8 Comparison of Front and Rear Wheel Drivetrain Efficiences

Figure 5-7 Comparison of electric motor versus internal combustion engine characteristics.

output level it can maintain without overheating. As you can see from Figure 5-7, the rated HP point for an electric motor is far down from its short-term output, which is typically two to three times higher than its continuous output.

There is another substantial difference. While an electric motor can produce a high torque at zero speed, an internal combustion engine produces negative torque until some speed is reached. An electric motor can therefore be attached directly to the drive wheels and accelerate the vehicle from a standstill without the need for the clutch, transmission, or torque converter required by the internal combustion engine. Everything can be accomplished by controlling the drive current to the electric motor. While an internal combustion engine can only deliver peak torque in a relatively narrow speed range, and requires a transmission and different gear ratios to deliver its power over a wide vehicle speed range, electric motors can be designed to deliver their power over a broad speed range with no need for transmission at all.

All these factors mean that current EV conversions put a lighter load on their borrowed-from-an-internal-combustion-engine-vehicle drivetrains, and future EV conversions eliminate the need for several drivetrain components altogether. Let’s briefly summarize:

• Clutch—Although basically unused, a clutch is handy to have in today’s EV conversions because its front end gives you an easy place to attach the electric motor, and its back end is already conveniently mated to the transmission. In short, it saves the work of building adapters, etc. In the future, when widespread adoption of AC motors and controllers eliminates the need for a complicated mechanical transmission, the electric motor can be directly coupled to a simplified, lightweight, one-direction, one- or two-gear ratio transmission, eliminating the need for a clutch.