Mr. Kevin Ro
National Manager, Technical &
Regulatory Affairs, Safety
Toyota Motor North America, Inc.
601 13th St. NW, Suite 910 South
Washington, DC 20005
Dear Mr. Ro:
This letter responds to your request for interpretation of Federal Motor Vehicle Safety Standard (FMVSS) No. 135, Light Vehicle Brake Systems, on behalf of Toyota Motor Corporation (Toyota). You state that Toyota is currently developing an electric vehicle (EV) that falls within the applicability of FMVSS No. 135. You would like confirmation of your interpretation that the phrase accelerate as rapidly as possible found within the test procedure of the standard takes account of a thermal protection feature in an EV battery system. The issue raised by your letter is addressed below.
Among the performance requirements for light vehicle brake systems in FMVSS No. 135 is a hot performance test. Prior to conducting that test, there is a procedure, contained in S7.13 of FMVSS No. 135, for conducting heating snubs, which are designed to heat the brakes for the test. As set forth in the test procedure in S7.13.3, the vehicle is driven at an initial speed of 120 km/h (74.6 mph) or 80 percent of the vehicles maximum speed, whichever is slower. The brakes are then applied to slow the vehicle to one-half the initial speed at a constant deceleration rate of 3.0 m/s2 (9.8 fps2). The vehicle is then accelerated back to the initial speed, and the procedure is repeated for 15 snubs. Immediately after the completion of this procedure, the hot performance test is conducted.
Your question relates to acceleration of the vehicle back to the initial speed. The pertinent provisions of S7.13.3 are set forth below:
S7.13.3 Test conditions and procedures.
(f) Time interval: Maintain an interval of 45 seconds between the start of brake applications (snubs).
(g) Accelerate as rapidly as possible to the initial test speed immediately after each snub.
You note that, with EV battery systems, greater power discharge results in higher operating temperature. You also note that Toyotas EV battery system is thermally regulated; that is, when the internal temperature exceeds a pre-defined limit, the vehicles speed is limited to prevent thermal damage to the system.
You state that Toyotas EV is capable of achieving the initial test speed for all 15 high-speed snubs within the 45-second interval specified in S7.13.3(f). However, because of the thermal protection feature, the vehicle may not be able to attain the initial test speed for 15 consecutive snubs within 45 second intervals if the vehicle is accelerated at wide open throttle (WOT). You state that the only way to complete testing is to keep the throttle at less than WOT. You state that Toyota believes that it is within the language and intent of FMVSS No. 135 to interpret S7.13.3(g) to mean that the vehicle must be accelerated as rapidly as the motor will permit after each snub to the required test speed within the specified time interval and complete the testing.
In considering your question, we note S7.13.1 sets forth the purpose of the heating snubs, which is to heat up the brakes in preparation for the hot performance test which follows immediately. The test procedure does not specify the throttle position. However, it does specify the time interval between the start of snubs and states that the vehicle is accelerated as rapidly as possible to the initial test speed immediately after each snub.
In conducting the heating snubs, we would ordinarily accelerate a vehicle as rapidly as possible during each acceleration specified for this test. Although the manner in which the vehicle is accelerated as rapidly as possible is not specified in the regulatory text, in practice this is generally done with a full application of the accelerator pedal. For most vehicles, the rate of acceleration in the earlier of the 15 accelerations would not affect the ability of the vehicle to achieve the specified test speed in later accelerations. We understand your letter to indicate that when some electric vehicles are tested under this procedure, the rate of acceleration in the earlier accelerations may affect the ability of the vehicle to achieve the specified initial test speed in later accelerations, due to heat build-up and the thermal protection feature.
In this specific situation, we agree that the language as rapidly as possible in S7.13.3(g) should be interpreted in the context of the necessity of the vehicles being able to complete the test sequence at the specified initial speed and within the 45-second time intervals between snubs. This means that we would complete the S7.13 test sequence by accelerating the vehicle as rapidly as possible, consistent with the need to allow the test sequence to be completed, provided each acceleration could be completed within the 45-second time interval between snubs. In the case of the vehicle that you describe, the vehicle may not be accelerated at wide open throttle for each acceleration. In heating up the brakes in preparation for the hot performance test, the key factors in achieving proper heating are the test speed of the vehicle at the beginning of each snub and the time interval between snubs (along with the snubs themselves). This performance test is not intended to test a vehicles acceleration capability. If higher rates of acceleration for the earlier accelerations have the effect of preventing the vehicle from achieving the specified initial test speed in later accelerations within the 45-second time interval, the brakes would not be appropriately heated.
I hope this information is helpful. If you have any further questions, please feel free to contact David Jasinski of my office at (202) 366-2992.
O. Kevin Vincent
Ref: Standard No. 135