Automotive Ethernet as an in-vehicle networking paradigm has gradually become the main automotive backbone network. However, with the ever-increasing time-critical in-vehicle traffic, it is being confronted with enormous challenges to realize deterministic-latency communications, due to the inherent limitations of its distributed network architecture and the adopted MAC protocols, including limited-computing power, low-speed and unreliable traffic transmission. Furthermore, it is often incompatible with emerging vehicular functions and protocols, which can provide tremendous potential for better vehicular Quality-of-Service (QoS). Therefore, in this paper, we first design a Future Automotive Ethernet (FAE) architecture and then, built on the representative Time-Sensitive Networking (TSN) and industrial summation frame, propose a Deterministic-Latency MAC (DLM) protocol running in FAE to tackle these issues. The FAE architecture includes three functional domains, three in-vehicle computing units, and no fewer than three intelligent network processing modules, which can integrate with the emerging LAN protocols to realize cross-domain and intra-domain Ethernet-based communications. Under the umbrella of this architecture, DLM classifies the driving situations into driving, reversing, left turn, right turn and parking states, following the real-world vehicle behaviors, and assigns all traffic associated with driving safety in five states different recommended priorities to be delivered. Furthermore, an optimized deterministic-latency mechanism integrated with TSN and the industrial summation frame is developed to realize the timely and accurate transmission of high-priority and medium/low-priority traffic. Simulation results obtained from diversified scenarios demonstrate that the proposed DLM running in FAE can significantly improve transmission latency determinacy and reliability compared with the existing technical strategies.