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Estimate of Truck Drag Forces from Roll-Down Tests Peter Hart Hartwood Consulting
What Is Roll-Down Testing ?
• The vehicle is rolled down (i.e. without engine power) from a high speed (100km/h) to a low speed (e.g. 20 km/h) on a flat road.
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What Is Roll-Down Testing ?
• The vehicle is rolled down (i.e. without engine power) from a high speed (100km/h) to a low speed (e.g. 20 km/h) on a flat road.
• The speed v ?me trace is captured.
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What Is Roll-Down Testing ?
• The vehicle is rolled down (i.e. without engine power) from a higher speed (100km/h) to a low speed (e.g. 20 km/h) on a flat road.
• The speed v ?me trace is captured.
• The drag force is es?mated from the computed dV/dt.
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So What?
• Want to determine the nature of the drag force speed dependence.
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So What?
• Want to determine the nature of the drag force speed dependence.
• Par?cular drag mechanisms are modeled by par?cular speed dependence.
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So What?
• Want to determine the nature of the drag force speed dependence.
• Par?cular drag mechanisms are modeled by par?cular speed dependence.
• Therefore it might be possible to quan?fy the the various drag force levels using simple tests.
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Fundamental Assumptions
• The speed dependence that exists for a transient test is the same as that for a con?nuous-‐speed test.
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Fundamental Assumption
• The speed dependence that exists for a transient test is the same as that for a con?nuous-‐speed test.
• The speed dependence is assumed to be:
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Loss Power Relationship
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Testing
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Testing
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Testing
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Testing
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Some Maths
• First order Ricca? non-‐linear DE. Count Ricca?, Italian noble circa 1650.
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Riccati Solution
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Riccati • According to Wikipedia:
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US EPA
• Requires car manufacturers to report the A , B and C coefficients
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The Riccati Map
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Practical Measurements
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Practical Measurements
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Drag Force Mechanisms
• Aerodynamic Drag Assumed to be with V2
A -‐ parameter
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Drag Force Mechanisms
• Aerodynamic Drag Assumed to be with V2
• Braking Drag Assumed to be constant force
C -‐ parameter
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Drag Force Mechanisms
• Aerodynamic Drag Assumed to be with V2
• Braking Drag Assumed to be constant force
• Engine, Bearings, etc may vary with V ?
B -‐ parameter ?
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Drag Force Mechanisms
• Tyres. Modeled as a constant drag force but this is an approxima?on. B , C -‐ parameters
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Drag Force Mechanisms
• Tyres. Modeled as a constant drag force but this is an approxima?on. B , C -‐ parameters
• Suspension Unknown varia?on with speed
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Drag Force Mechanisms
• Tyres. Modeled as a constant drag force but this is an approxima?on. B , C -‐ parameters.
• Suspension Unknown varia?on with speed. • Hill climbing / descending drag B and C -‐ parameters
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Comparative Tests
• Tests can be conducted with only one variable changed.
E.g. Laden and Unladen with no change in
aerodynamic profile.
With and without an extra trailer.
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Program Implementation
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Program Implementation
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Program Implementation
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Semi-trailer
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Semi-trailer (unladen)
• V3 3.8% -‐ • V2 54.4% 61.6%
• V 17.4% 18.9% • V1/2 4.8% -‐
• Constant 18.7% 19.5%
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Review
• Roll-‐down tes?ng needs perfect condi?ons
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Review
• Roll-‐down tes?ng needs perfect condi?ons • Can separate the various drag force components if the experiments are well-‐planned.
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Review
• Roll-‐down tes?ng needs perfect condi?ons • Can separate the various drag force components if the experiments are well-‐planned.
• Can be conduced with minimal test infra-‐structure.
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Review
• I’m looking for a rich patron to fund a project to quan?fy the drag force components on a Australia B-‐double trucks
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St Patrick’s Day
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Italian Contribution
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Finally
Thank you for listening!
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