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Article by: Fabio Squadrani, Senior Manager, Braking Systems in Applus IDIADA
Review Part One
Modern braking systems development is moving towards the reduction of long tests on open roads. However one of the most critical items when reducing the mileage in open road is brake wear. This work is answering to a simple question about how to simulate brake patterns in dynamometers in order to reduce the mileage of brake tests to be performed on vehicle. In this first part of this article, a set of data coming from a brake durability test was analysed. Particularly this is needed in order to define the target wear value for the brake dynamometer procedure.
Brake Dynamometer Testing
Brake dynamometer characteristics
The fixture is mounted on Applus IDIADA PC2 (Passenger Car 2) dynamometer. This dynamometer is a full scale dyno, which is able to replicate the exact vehicle performance, at its corresponding inertia. The specifications of the dyno is as follows:
Route classification and selection
In order to reproduce the routes on the dyno the first important element is the selection of a representative route.
First of all, a standard Mojacar route is made of the following main sections:
- Bedding routes
- Country road routes (usually called Lubrin Routes)
- Highway routes.
In order to select representative routes for wear correlation purposes, both bedding routes and highway routes are not considered. The first ones because they are considered as not representative for the study. The motorway routes, instead, are assumed not to be contributing to the wear generation.
In addition to this, the following snubs are also eliminated:
- Snubs at low speed (<3 km/h)
- Snubs in noise search area
- Snubs where difference from initial and final speed is less then 3km/h
In this way, the average mileage for each route in the dyno is calculated to be 125 km.
Under this criterion, the mileage of the test are modified as follows:
No impact on the average wear is assumed.
A statistical analysis conducted on the relevant routes from the Mojácar durability test and it is aimed to identify the most representative route to be replicated in the brake dyno simulation. Various parameters were computed for each route, including net kinetic energy, thermal energy and power, brake load and power factors, number of brake applications, and average front temperatures. A Gaussian curve-based classification was performed to select the route that best represented the average figures across these criteria. This selected route was then used as the basis to develop a repeatable dyno test pattern.
In order to clarify the step for this statistical analysis are as follows:
Step 1: All the relevant parameters are calculated for each route
Step 2: All the routes are classified in a Gaussian distribution (per each parameter) in order to identify the most representative value per each parameter. For instance, for the Net kinetic energy [kJ], the gaussian distribution of the routes is as follows:
Step 3: Each route is classified according to its distance from the gaussian distribution for each parameter. The closest route to the gaussian distribution is selected and is called the “reference lap”.
Initially, the aim was to consider only one representative route to be repeated a certain number of time. This would be the so called Reference Lap: the selected lap, with its corresponding IBT (initial brake temperature) for each snub.
However, in order to guarantee the representativity of the variability coming from road testing, three level of initial brake temperature are defined and three different laps are defined:
- Reference Lap: the selected lap, with its corresponding IBT (initial brake temperature) for each snub
- Low Temperature Lap: the reference lap, where the IBT of each snub is reduced by 15%
- High Temperature Lap: the reference lap, where the IBT of each snub is increased by 15%
It is considered that this step is crucial to select the correct route for the right correlation.
In order to clarify how the limits for Low and High temperature are defined, it can be said that this is coming from analysis of experimental data. The reference lap temperature is the average coming from the Gaussian curve-based classification. The limits are coming again from the same curve and that’s why the distribution is purely symmetrical.
The importance of bedding section
For wear, but also for performance and noise purposes, the bedding section of a vehicle or dyno project is of paramount importance. For this particular application a very simple process of bedding the brake with 1 simple running in process according to SAE J2707 brake dynamometer procedure has been carried out.
The procedure for carrying on the bedding on the brakes is summarized in the following tables:
Scale factor calculation
The main objective of such a correlation study is obviously to reduce testing time and impact in development for right material selection at the very beginning of a DVP programme.
In this way a simple scaling factor is defined, considering the total mileage on vehicle divided by the total simulated mileage on the dyno. It has to be said that the total mileage on each route is calculated to be 125 km and that the bedding section is assumed to be slightly shorter than that.
The main aim is to define a quick procedure which could be run in a dyno, with an objective testing time of 3-4 days maximum (in a three shift dyno usage basis).
In order to do this a target scaling factor (total mileage on vehicle divided by the total calculated mileage on dyno) is between 4.5 and 5.
So, after some basic energy and power considerations, the following mix has been proposed as a structure for the dyno validation procedure:
- Bedding Section according to SAE J2707
- 2 Low Temperature Laps (the reference lap, where the IBT of each snub is reduced by 15%)
- 12 Reference Laps: the selected lap, with its corresponding IBT (initial brake temperature) for each snub
- 2 High Temperature Laps: the reference lap, where the IBT of each snub is increased by 15%
In this way the scaling factor is calculated as follows:
About Applus IDIADA
With over 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.
Applus IDIADA is located in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.
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