Acceleration Rate

Summary: The Acceleration/Deceleration 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 the 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 2, 5 & 6 are ignored.

CV 3 and 4

Multifunction Decoders intended for the European market use a different multiplier value.

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.

Example: Using the 128 speed step mode, a value of 2 in CV3 the acceleration rate equals:

2 × 0.896 = 1.792
1.792 ÷ 128 = 0.014 Seconds

The acceleration rate is 14mS per step.

At this rate, the vehicle will take 1.8 seconds to reach its top speed (128 steps, or 100%).

Using 28 speed steps:

2 × 0.896 = 1.792
1.792 ÷ 28 = 0.064 Seconds per step.

Increasing the value in CV3 to 100 will result in the vehicle reaching its top speed in 90 seconds

(0.896 × 100 ÷ 128) × 128.
Simplified, this is 0.896 × 100 or 89.6 seconds.

The same is true for CV4.

Putting a large value, such as 256, in the CV results in a long acceleration time, in the order of minutes to reach 100%. Avoid large numbers as gives the illusion the decoder is defective.

Optional Acceleration and Deceleration Adjustment CVs

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.

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 value 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 for sale in Europe use a different multiplier for calculating the acceleration rates.