Engine Idle Speed Explained: Why 700 RPM Is the Sweet Spot

Have you ever wondered why most cars sit idling at around 700 revolutions per minute (RPM)? Is there any specific reason for this common idle speed, or would it be better to choose a slower or faster RPM? This article will delve into the details of engine idle speed, exploring why manufacturers choose 700-800 RPM as the standard and how this impacts fuel efficiency, vibrations, comfort, and overall vehicle performance.

Why 700-800 RPM Is the Ideal Idle Speed

The ideal idle speed for a car engine, typically ranging between 700-800 RPM, is determined by a careful balance of various factors. The computer controlling the engine's operation will set the idle speed to maintain a smooth, quiet idle, prevent stalling, and minimize fuel consumption and vibrations.

Comfort and Vibrations

Engineers can design a small engine to idle at a lower RPM, such as 300, but this would require a larger flywheel to maintain the necessary inertia, particularly for the compression phase. A larger flywheel would make the engine less responsive to acceleration and slower to decelerate. Additionally, it would result in a rough idle, as the engine would be more susceptible to vibrations and pings from the cylinders. Therefore, 600-700 RPM is considered the ideal range for small flywheels and minimal vibrations while providing good acceleration performance.

Why Not Make Idle Speed 100 or 200 RPM?

Reducing idle speed to 100 or 200 RPM would have many drawbacks and necessitate significant design changes. For instance, the engine would need to perform well at higher RPMs, making it an "aerospace challenge" in terms of engineering complexity. While this would be beneficial for the environment and fuel efficiency, the added cost would be substantial, and the benefits would be marginal for the average consumer.

Furthermore, a lower idle speed would lead to a series of issues, such as insufficient power strokes at idle to power accessories and increased losses due to the engine's operation. For example, a car with the A/C on will periodically idle up and down, demonstrating the need for consistent idle speed to maintain driveability and responsiveness, especially in environmental conditions that vary slightly.

Drawbacks and Critical Aspects

Despite the potential benefits of lower idle speeds, there are several critical drawbacks that must be considered. Firstly, new car buyers and critics are very sensitive to any changes that might affect the performance and reliability of their vehicles. An engine that shudders or is unresponsive, even for brief moments, is unacceptable to most consumers. This sensitivity to the environment and fuel variations can make it difficult for car manufacturers to adopt a lower idle speed without losing market share.

At idle, a car's need to respond to the A/C and power accessories makes maintaining a consistent RPM crucial. If the A/C compressor starts and stops frequently, it can affect the engine's ability to maintain a smooth idle. Drivers appreciate consistency, as it makes them feel more comfortable and confident in their vehicles. In the context of vehicle performance, this consistency is not just a matter of comfort; it can also become a safety issue, as certain performance modifications can alter the car's behavior in ways that can be dangerous.

For example, suspension modifications that alter roll moment asymmetry can make a car less stable, especially during dynamic load transfers. If a car's suspension is modified in a way that affects one axle differently than the other, it can create dangerous, inconsistent behavior that could lead to loss of control. This is a far greater risk than simply changing the appearance of a car.

Conclusion

In conclusion, while there are some potential benefits to lowering the idle speed of an engine, the drawbacks and added complexity make 700-800 RPM the ideal range. Engineers must balance the need for fuel efficiency, smooth operation, and consistent performance, all while minimizing the impact on the driving experience. Choosing a lower idle speed is a trade-off that most car manufacturers are willing to make, as the added complexity and cost do not justify the marginal benefits in most cases. The auto start-stop feature is a much more effective and practical solution to save fuel than a lower idle speed.