When Ward's named Toyota Motor Corp.’s original hybrid-electric vehicle (HEV) powertrain as a 10 Best winner in 2001, nobody was sure if the thing even qualified as a competitor to standard internal-combustion engines. After all, HEV powertrains rely on an electric motor to in some way “aid” the IC engine; both engine and motor team to deliver torque to the drive wheels.

For 2004, we’re still not sure if Toyota’s super-intelligent Prius powertrain can – or should – be compared to standard IC engine-only drivelines. But we’re going out on a limb to say that the industry is going to see more HEV architectures – and we’d better decide to deal with them, because an HEV engineering priority always is going to be transparency to the driver and occupants.

Such is certainly the case with Toyota’s excellent second-generation HEV powertrain, Hybrid Synergy Drive. Apart from some of the antics the driver must perform to get the car moving, it’s not hard to imagine the uninitiated never knowing the Prius isn’t propelled by a standard IC-only driveline.

Toyota 1.5L DOHC I-4

Little about the Prius’ stout 1.5L DOHC I-4 has changed for Hybrid Synergy Drive. It’s a low-revving, Atkinson-cycle (extended expansion ratio) 4-cyl. cleverly designed for low-friction, maximum-efficiency operation. It is uprated to 76 hp from the previous 70-hp peak, but torque remains the same unremarkable 82 lb.-ft. (111 Nm) at 4,200 rpm. The engine’s primary attribute, apart from the previously mentioned efficiency, seems to be unobtrusiveness – Prius buyers presumably would prefer it wasn’t there at all.

Meanwhile, the piddling torque output doesn’t really matter, because that’s where the Prius’ electric motor comes in. The AC motor is rated at 67 hp and 295 lb.-ft. (400 Nm) of torque, and all that extra twist is fully available between 1,200-1,540 rpm to augment the gasoline engine. So the Prius steps off with more authority than many moderately powerful IC-only vehicles.

But more important, that extra-strong motor is supplied power by a new 500-volt electrical system, nearly twice the old car’s 273 volts. In fact, the much higher system voltage is the Hybrid Synergy Drive’s real secret for producing mileage figures of 60 mpg (3.9L/100 km) city and 51 mpg (4.6L/100 km) highway – electricians know higher voltage always means higher efficiency. Toyota says the Prius’ nickel-metal hydride battery pack also is uprated; it’s 15% smaller, 25% lighter and has 35% greater output.

Finally, an optimized continuously variable transmission and what’s said to be one of the world’s most sophisticated energy-management systems ensure the two Prius motivators – the IC engine and the electric motor – are optimally employed, either separately or in tandem.

Hybrid Synergy Drive is a remarkable achievement. We know much of the powertrain community says it wouldn’t get encouragement from management for a midsize-vehicle powertrain that gets the car to 60 mph (97 km/h) in 11.3 seconds. But the Prius obviously isn’t meant for power-craving buyers.

We simply say, for what the Prius is meant to achieve, it does it remarkably well. And at press time, Toyota announced a hike in Prius production to meet rising U.S. demand. The company just hiked projected full-year sales to 47,000 units – a number that makes Prius look less like a curiosity.

1.5L DOHC I-4 Hybrid
Engine type 1.5L DOHC I-4
Displacement (cc) 1,497
Block/head material aluminum/aluminum
Bore x stroke (mm) 75 x 84.7
Horsepower (SAE net) 76 @ 4,500 rpm
Torque 82 lb.-ft. (111 Nm) @ 4,200 rpm
Specific output 51 hp/L
Compression ratio 10.5:1
Application tested Prius
Fuel economy
for tested vehicle
(EPA city/hwy. mpg)