Acceleration Rate

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Summary: The Acceleration rates are controlled using CV 3 and CV 4. These CVs are mandatory. This feature is used to simulate momentum.

Acceleration and Deceleration

This feature allows the user to adjust how rapidly the vehicle responds to speed change instructions. A value of zero in these CVs results in instantaneous response to throttle changes. Increasing the value in the CV slows this response. The multifunction decoder software may impose limits on how large a value can be used with these CVs.

The responsiveness can be adjusted, and is dependent on the speed step mode in use. Setting CV3 to a high value can give the impression that the multifunction decoder is not working, as it will take a long time for the vehicle to begin moving. The same is true for CV4, which impacts deceleration.

If the speed table is enabled by CV29, CVs 3 – 6 are ignored.

Acceleration Adjustment and Deceleration Adjustment are optional features defined in the NMRA DCC Standard for Configuration Variables. They are defined with CVs 23 and 24.

CV 3 and 4

The contents of these CVs are multiplied by 0.896 then divided by the number of speed steps to determine the rate of acceleration or braking. A value of zero disables the feature.

For example, at 128 speed step mode, a value of 2 in CV3 equals 2 × 0.896 ÷ 128 = 1.792 ÷ 128 = 0.014.

This means the acceleration rate is 14mS per step. At this rate, the vehicle will take 1.8 seconds to reach its top speed. Increasing the value in CV3 to 10 will result in the vehicle reaching its top speed in 9 seconds (0.896 × 10).

The same is true for CV4.

Putting a large value, such as 100, in the CV results in a long acceleration time, in the order of 90 seconds to reach 100%. Avoid large numbers as it can give the illusion that the decoder is defective.

Optional Acceleration and Deceleration Adjustment CVs

The intended purpose of the adjustment CVs is to make changes to the momentum to simulate the train's load and length, usually when consisted with other locomotives.

Acceleration Adjustment (CV 23)

This CV contains additional acceleration rate data that is added to or subtracted from the value contained in CV 3. It is determined by (Value in CV 23) × 0.896 ÷ number of speed steps in use.

This vlaue is a 7 bit number (0-6) with the seventh bit reserved for the sign of the value.

Along with CV 24, CV 23 is used to fine tune the momentum features set in CVs 3 and 4. These come into play when multiple unit consists are assembled.

Deceleration / Braking (CV 24)

It is the same as CV 23, except CV 4 is acted on.

European Exception

Decoders manufactured by European companies use a different factor for calculating the acceleration rates.

Further Reading

Refer to the page on Configuration Variables and MU Consisting for more information on CVs and how these features are used.