One of the most frequent customer complaints about motorcycles is the heat plume that rises from the engine and hits the rider while idling in traffic or after getting off the highway. Because many engines are air-cooled, its heat will rise from the cooling fins in a thermo-buoyant manner and strike the rider when there is no airflow around the engine. This analysis was performed to simulate the heat plume and predict the increase in temperatures the rider will experience.
Mesh Geometry
The original mesh for the Harley was downloaded from the Internet and imported into Poser. In Poser a rider was created and posed on the motorcycle, then the rider was mounted onto the bike in Rhinoceros cad software. After the rider and bike were joined, the complete model was exported to ANSA and re-meshed, as shown (top).
After the model of a Harley Davidson and its rider were created, a drive cycle was chosen and then the corresponding boundary conditions were determined. MuSES Pro 8.0a1 was used to perform the thermal analysis, and verification testing was performed on an actual Harley Davidson Softail after a similar drive cycle. Verification included the use of an FLIR infrared camera as well as thermocouple readings taken from several locations on the engine.
Heat Production of Powertrain
To perform an analysis of the plume problem, the first task was to create a drive cycle for the biker that simulated starting the motorcycle, cruising on city streets, then getting onto the highway, and finally taking an off ramp and coming to a stop. The cruising and highway speeds were chosen to be 52.4 and 70 M.P.H. respectively, from which the rolling and wind resistances were calculated.
| Time (min) |
Speed (M.P.H.) |
Power Required (kW) |
Heat Rejected to Ambient (kW) |
Notes |
| 0 |
0.0 |
0.0 |
0.0 |
Off |
| 1 |
0.0 |
0.6 |
1.5 |
Idle |
| 3 |
52.4 |
4.3 |
10.0 |
City Streets |
| 6 |
52.4 |
4.3 |
10.0 |
City Streets |
| 7 |
70.0 |
12.1 |
28.2 |
Highway |
| 31 |
70.0 |
12.1 |
28.2 |
Highway |
| 32 |
0.0 |
0.6 |
1.5 |
Idle |
| 36 |
0.0 |
0.6 |
1.5 |
Idle |
Table 1. Drive Cycle, Power Production and Heat Produced in Engine.
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| Minute |
Flow Rate (L/min)
|
| 0 |
0
|
| 1 |
90
|
| 3 |
454
|
| 6 |
454
|
| 7 |
608
|
| 31 |
608
|
| 32 |
90
|
| 36 |
90
|
Table 2. Exhaust Flow Rate for Each Cylinder.
 Figure 1. The OLD approach. Exhaust Pipes and Their Different Subsections from version 7.1 setup. (Click to enlarge.) |
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Fluids and Convection
Exhaust gases were modeled using the assumed combustion temperature of 400 degrees centigrade and the flow rates shown in Table 2 (left). The front and rear exhaust pipe were each broken into five subsections, and had a fluid node attached to the inside of each subsection.
The muffler section of the exhaust only had one fluid node attached to the inside of the pipe, and it received 100% of the flow from the front and rear exhaust pipes. In Figure 1 (left), the different sections of the pipes are illustrated. Library circular duct convection was utilized on the insides of all the exhaust pipes to model the heat transfer between the exhaust gas and the pipes.
Similarly, the exhaust was modeled using MuSES’s automatic Fluid Stream Generator. The Fluid Stream Generator creates multiple fluid nodes which are distributed along the surface of one or more parts to simulate a flow system. As seen in the previous description of exhaust flow, without the Fluid Stream, the exhaust system needs to be broken into segments and each segment can only utilize one fluid node.
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 Figure 2. The NEW Approach in version 8.0 RadTherm and beyond: Exhaust Pipes Displaying Fluid Stream Location and Direction (Click to enlarge.) |
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With the automatic Fluid Stream, the exhaust can be modeled with as many fluid nodes as are needed to accurately model the distribution of heat throughout the pipes. As more fluid nodes are generated, the distribution becomes finer along the pipes. The exhaust system does not need to be separated into segments when using this method. The fluid nodes and their respective directions of flow are shown in Figure 2 (left). |
Thermal Prediction
During idle after highway speeds, the model predicted an average outer clothing temperature in the lower legs and feet of approximately five degrees centigrade higher than at highway speeds. Figure 3 (below) shows the locations of higher temperatures on the rider's right leg, with the spot on his upper leg caused by the cylinder head, and on his calf by the exhaust pipe. The model predicted a similar hot spot at the inside middle calf area of the right leg, due to the high exhaust pipe temperature.
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Figure 3. Comparison of Predicted Versus Actual High Temperature Locations on Rider's Leg
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| The model's temperature distribution is similar to the distribution captured by the infrared camera as shown in Figure 4 (below). |
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Figure 4. Side By Side Comparison of Predicted Temperatures and Infrared Signature
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