These days, multi-million mile commercial vehicles are no longer the exception to the rule. They are the rule, making engine durability even more important. One key component of durability—managing cylinder thermal loads—is a known engineering challenge for conventional engines. And, that challenge is even greater for an opposed-piston, two-stroke (OP2S) engine. Unlike four-stroke architectures, the OP2S needs efficient cooling at the center of the cylinder where the heat load is highly concentrated. However, the two-stroke engine’s double-firing frequency makes this difficult—since the cylinder liner does not benefit from the cooling effects of the four-stroke engine’s separate intake and exhaust cycles.
To overcome this challenge, we developed a thermally efficient OP2S cooling system that achieves superior engine durability by limiting the surface temperature of the cylinder liner to below 270°C—ultimately preserving the oil film at the top ring reversal zone. This patented system will be introduced next week at the SAE World Congress as part of a technical session on heat transfer and advances in thermal and fluid sciences.
The first step in designing the cooling system was to create an effective means of predicting the cylinder’s thermal response. We defined hot-side boundary conditions (convective heat transfer on the hot side dominates effects including radiation) using a heat flux model. This model was created to predict the heat flux distribution into the cylinder wall and ports and was based on a discretized Nusselt number correlation for heat transfer coefficients. Other modeling techniques were developed to determine the gas temperature during compression and combustion as well as to simulate blowdown and scavenging.
Once the models were complete, we studied several different “coolant jacket” designs. The goal was to find a “jacket” design that would provide approximate uniform temperature distribution along the length of the cylinder and circumferentially about the cylinder axis. Among the designs evaluated were the:
- Full “coolant jacket” – Allowing the coolant to flow freely across and around the outside diameter of the cylinder, except near the injector boss
- Center “impingement jet”/full jacket – Enabling the coolant to impinge directly on the cylinder wall, creating a stagnation flow field that continued longitudinally down the exhaust and intake sides
- Slotted center impingement – Replacing the circular inlet of the center “impingement jet” design with a slot
Results of the study revealed:
- Uneven cooling, and even overheating of the coolant in the boundary layer, is possible with a full “coolant jacket” that uses a large surface area.
- Preventing over-cooling of the cylinder ends and controlling axial temperature distribution is possible with coolant jackets that only partially extend down the length of the cylinder.
- Circumferential temperature control is best accomplished by distributed impingement at the cylinder center.
- Axial temperature is largely independent of circumferential temperature control.
Based on this data, we modified the cylinder design to convert the inlet slot into discreet impingement jets by inlaying an impingement band (a thin steel band with small holes) into the cylinder. The hole diameter size was varied to ensure balanced flow through all jets. On the cylinder’s exhaust side, the coolant jacket was converted to a series of independent grooved channels that lined up with every other impingement jet. On the intake side, the coolant jacket was also converted to independent grooved channels. The intake channels were lined up with the impingement jets not associated with the exhaust side of the cylinder.
In order to validate the new approach, we created the needed parts to run in a single-cylinder, OP2S engine that incorporated embedded thermocouples. Testing on the prototype engine revealed that:
- Circumferential temperatures are within 5°C around the cylinder.
- Axial temperatures are well below the target of 270°C.
For more details, attend the presentation, Cylinder Cooling for Improved Durability on an Opposed-Piston Engine (2012-01-1215), on April 26. Or, purchase the technical paper.
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