The simplest way a hybrid engine beats a “regular” gasser is simply size. A small engine can hit several engineering targets an equivalent large engine can’t- ask a motorcycle racer. If electricity makes up the torque difference, then pairing an electric motor with a small gasser makes much sense.
Thermodynamics– on the most basic level, fewer cylinders make a heat engine more efficient. One big cylinder has much less surface area than its equivalent in two or more smaller cylinders. Surface is bad because heat is lost through the walls of the combustion chamber. Any combustion heat that escapes to the oil, the coolant, or the air around is heat that’s not pushing down the piston, and thus pushing the vehicle.
So why don’t more vehicles use one big cylinder? Above a certain size, one cylinder is too rough. (Or “NVH”- Noise, Vibration, Harshness to engineers.) That size is generally 400-500cc- not enough engine for a (non-hybrid) car. You typically see single cylinders in motorcycles only, and typically 450cc or less, which is still considered a small motorcycle in developed countries. Single-cylinder motorcycles are then referred to as “thumpers,” and typically not sold for long-distance riding or upscale models.
If you add a second cylinder, you can play with piston arrangements to interact with each other, and modify the sound and feel. Various arrangements of 2-cylinder engines are seen up to 1800cc, which is huge for a motorcycle, and now enough for a (non-hybrid) car. Meanwhile, two cylinders have a fraction of the surfaces and thermal losses of a 4- or 6-cylinder (depending on which surfaces you’re counting).
Friction– Some of that surface area just sits there, and some is oiled as a machine interface. The actual bore of the cylinder wastes fuel heat, through oil circulation, and by being a friction source even after using oil. Fewer bores generate less friction, into less oil, than many, smaller bores.
There’s no free lunch here, either. One could improve thermodynamics by decreasing the bore-to-stroke ratio… using smaller pistons, with a longer crank throw (an “undersquare” cylinder). The cylinder displacement stays the same, but the surface area of the cylinder head goes down. This helps because the surfaces of the combustion chamber are hot more of the time, while the area of the cylinder walls is only heated during a portion of the piston stroke. So, if you’re trying to increase efficiency, you sacrifice some bore friction for reduced head heating. It’s yet another tradeoff engineers have to make to reach a satisfactory final design.
There’s more than one tradeoff here, too. A small bore means small valves, and poorer cylinder breathing. Energy is lost in trying to force gas in and out through smaller valves- “pumping losses,” or friction of the air. To an extent, multivalve engines help breathing. But a small-bore, undersquare multivalve engine still breathes less than its equivalent large-bore (“oversquare”) multivalve engine. Also, choosing undersquare versus oversquare results in different driveability characteristics. One type is seen as torquey, but strained at high revs, while the other is speedy, but gutless at low revs.
Yet again, hybridization comes to the rescue. With a computer-controlled assist motor, the powerband of your gasser cylinders is less of an issue. The intake and exhaust can be tuned for fewer conditions, and better breathing in that limited band. Also, electric motors have perfect driveability; this eases the issue of low-rev performance versus high revs. With enough electrification, the gasser can run as a constant-speed generator. Breathing can then be tightly tuned; driveability can be forgotten completely.
In the case of the Chevrolet Volt, the gasser only operates at two speeds: medium and high rpm. This is certainly an improvement, and two discrete speeds can be finely tuned in a way that’s just not possible with a wide, continuous powerband. But the medium setting is likely not a peak of efficiency, while the high setting is avoided when possible. It’s considered buzzy and harsh. You can’t please everybody, it seems.
Transmission Gains– a gasser generally has a transmission, and a transmission generally has losses. But a smaller piston engine can use simpler transmissions, with lower losses. In the extreme case, a constant-speed generator has no transmission at all, using hybridization instead.
Stepping back from one speed, a tiny gasser might still have no transmission, at least as we’re used to the term. Smaller, lighter engines can rev through a wider band, without resorting to exotic technologies. That wide powerband means good driveability, and wideband engines need little (possibly no) gearshifting. Particularly if electric torque is used to fill in powerband gaps and limits- i. e., a hybrid. The issue of driveability and frequent shifting is then double-teamed, by electricity down low and wide gasser powerbands up high.
Entirely new transmissions become possible (or just easier) with enough hybridization. Continuously-variable transmissions (CVTs) are only possible with small piston engines, as today’s CV technology can’t take high torques. CVTs then keep you in the efficient part of the gasser’s powerband.
Note, however, that I consider the Chevrolet Volt drivetrain to be entirely old- it’s based on GM’s “Two-Mode” system. Not only does Two-Mode not eliminate the clutch, it arguably adds a second clutch. FAIL. I suspect GM wanted to go with a two-motor hybrid, like Toyota and Ford, but those two companies patented it too fast. Or GM worked too slow.
Packaging and Aerodynamics– As secondary effects, a smaller engine is simply easier to deal with, in terms of structures, aerodynamics, and accessories. A smaller engine fits better in a vehicle structure, making it simpler and lighter. A smaller engine bay yields a more aerodynamic vehicle, without a draggy hood/windshield jog. An engine with less surface area and friction, running in a limited powerband less often, needs a smaller radiator. Beyond a certain size, radiators are a major drag. Besides the radiator, other components such as the oil sump and the various pumps have more leeway for placement, making thermal management easier (and heat engines are all about the thermal power). This is particularly true if those pumps go electric, which is no problem with a high-voltage pack onboard. This is doubly true if there is no transmission or output shaft, i. e., a constant-speed generator, and power cables let you place the generator wherever it’s handy.
As a tertiary effect, a vehicle with a small engine block and distributed accessories can be safer. The placement leeway makes a structure much easier to design and build. With no huge block to deal with, engineers can pay more attention to protecting the passenger cage. A smaller engine block, with a battery pack mounted low, results in a low center of gravity, and can yield a vehicle that handles well and avoids accidents in the first place.
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Electric motors have excellent torque and responsiveness. Combine this with computing demands and other car “options” that are increasingly standard. Multiple auto execs have thus gone on record calling drivetrain electrification “inevitable.” We just have to start with hybrids (mild and strong, then plug-in) along the path.