Side view of NHW11 Prius

A key technical advance of the Generation 1, 2001-03 Prius, the transaxle, is described in the paper,"Development of Electric Motors for the TOYOTA Hybrid Vehicle "PRIUS"" by Kazuaki Shingo, Kaoru Kubo, Toshiaki Katus, Yuji Hata, TOYOTA MOTOR CORPORATION, 1, Toyota-cho, Toyota, Aichi, 471-8572, Japan. This obscure, somewhat difficult to find paper, describes the differences between the NHW10, the original Prius, and the NHW11, the first Prius sold in North American and overseas. Significant changes were:

  • 33kW, 350Nm torque motor, MG2 - versus the 30kW, 305Nm torque of the first Prius
  • advanced power circuit - inverter operation and electronics
  • lower mechanical loss - elimination of internal seals
  • improved productivity and noise reduction
  • flux-weaking for high speed operation

Another excellent source that compares and contrasts the NHW11 to the earlier NHW10 is "Development of the Hybrid Vehicle and its Future Expectation" by Shinichi Abe, Toyota Motor Corp, SAE-2000-01-C042. The 1.5L 1NZ-FXE engine power increased by moving the rpm limit from 4,000 to 4,500 rpm. However, the delayed intake valve closing and expansion ratios were changed to support the higher power and reduced the Atkinson cycle effect. The same engine was used in the NHW10, NHW11 and NHW20 models. The overall impression is the whole car was tweaked and improved to provide more power, greater efficiency, and improved manufacturing.

The trunk has a functioning spoiler to reduce drag and the bottom of the front bumper has a small air dam that also reduces drag. Thanks to a Japanese language, Toyota sales brochure, the vehicle drag is:

newtons = 190 + 0.42*(V**2)
V = velocity is meters per second on a standard day

A sedan-style body, similar to the Ford Focus, the North American Prius came without a cruise control in the first year. Unlike the European and Japanese versions, the North American was never offered with a fold-down, rear seat. However, it was the first Prius to use a prismatic battery modules:

Upper module is the NHW11 model. The lower module is from the NHW20 with a silvery, mold release, non-conductive.

Unlike the earlier NHW10 that used "D" size cells arranged in six cell strings, the new battery module provides higher density and reduces parts count. It also led to the much improved NHW20 module.

The original EPA rating was 52 City and 45 highway (and 48 combined), however owners were reporting average MPG lower than advertised, so the EPA changed the rules and reclassified Prius G1 as 42 City and 41 highway. As of June 30, 2011, 25 vehicle owners reported an average of 45.4 MPG.

The new EPA tests added more high-speed operation with air conditioner and cold weather testing. Mitigating high speed by limiting highway speed to 65 mph, steep hill climbs to 55 mph, minimizing air conditioner use and cold-weather warm-up significantly improves performance closer to the original EPA rating. For example, blocking the lower bumper air inlet at temperatures under 70F slightly reduces cooling drag and improves warm-up. Also, Canadian Prius owners can get a block heater installed that in one hour can make a measurable improvement in warm-up.

Roughly 54,000 of these model Prius were sold in the USA. With an average fleet loss of ~3%, their total numbers are approaching 50,000. But the lost Prius often provide servicable parts to keep others on the road. Primary weaknesses:

  • traction batteries in high heat or power conditions - the module terminals are subject to heat stress from high local temperatures and charge-discharge cycle heating. The seals can leak the KOH electrolyte and this leads to resistance to ground, loss of water from hydrolysis gas leakage, and eventual, unrepairable, module failure. One fix is to rebuild or buy a rebuilt pack using 38, NHW20 modules. The NHW20 modules have half the internal resistance and significantly improved terminal seals.
  • transaxle MG2 stator short - we don't know the mechanism but a short can randomly develope that causes a current loop. This current loop couples with the spinning rotor to overheat, generate a speed related hum, and cook MG2. Normally MG2 runs cooler than MG1. Eventually a short to ground leads to a non-recoverable failure. Replacement with a salvage transaxle is the only known fix as rebuilding has not been very successful due to the additional labor involved.
  • power steering torque sensor noise - the early, mechanical torque sensors can get electrically noisy and this leads to irregular steering behavior as the power amplifier tries to follow the noisy signal. The whole steering assembly has to be replaced although some owners just pull the fuses and drive the car in manual steering mode. Toyota has extended its warranty on the power steering until December 31, 2013 with no limit on mileage (warranty campaign "ZTP"), so it's worth taking it in to your dealer and trying to have them fix it under that.
  • other problems include - noisy accelerator encorder, sticky throttle plate, and sensitivity to over filled engine oil that also reduces mileage.
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