Confessions of an Automationeer, Part 172: Matters of the Heart

Confessions of an Automationeer, Part 172: Matters of the Heart

About a month or so ago, I was stricken by unusual and unexpected chest pains. I feared I was at rick of having a heart attack, but given that I had no prior history of heart disease (or any other cardiac disorders, for that matter), I realized that the problem originated in my digestive system; specifically, excess gas had accumulated in my stomach, which prompted me to get rid of it as best as I could. Not only that, I also had to increase stool bulk - the lack thereof was another symptom that annoyed me for weeks on end. Eventually, through careful management of my diet, I was back to full strength physically, and a visit to a cardiologist confirmed that my ailment had nothing to do with the heart whatsoever, much to my relief.

This brings us neatly to the topic for my latest post: idle speeds. Defined as the lowest rpm at which an engine can operate when it's running, it has a significant impact on most of a car's stats, especially the three most important ones (drivability, comfort, and safety). A lower engine idle speed (resulting from a lower cam profile and/or increased balancing mass, both of which will yield more smoothness) helps improve drivability and comfort. Comfort-focused and utility/offroad builds generally benefit from having engines with lower idle speeds, since they need more torque in the lower rev range for better drivability at lower speeds.

On the other hand, a higher idle speed will compromise drivability and comfort, but since it can be induced by reducing balancing mass or increasing cam profiles (both of which decrease smoothness), it generally yields more sportiness due to increased throttle response. As such, race cars and high-performance road cars tend to suffer less from an increased idle speed, since the aggressive cam profile required to raise the idle speed shifts the power and torque peaks towards the upper rev range, where such engines are expected to spend most of their time. Moreover, with sportiness being a key trait of these cars, lower balancing mass (and the associated reduction in drivability and comfort) is generally an acceptable tradeoff for superior throttle response, which is among the key sportiness criteria (as shown in the detailed stats tab).

Here's an example of an engine intended for use in premium and luxury cars, with an accordingly low idle speed, as shown in its dyno sheet:

Above and below: This engine has a low idle speed for improved drivability and comfort - perfect for low-speed urban driving and relaxed freeway cruising. To achieve this, the balancing mass was increased and the cam profile reduced, thereby sacrificing throttle response (and hence sportiness), which is not much of a priority in these applications. The upshot is a smoother engine that's better suited to everyday use.

And for comparison, here is the dyno sheet for a high-revving, ultra-high-performance engine with a high idle speed:

Above and below: In contrast to the previous engine, this one has a higher idle speed (1000 vs 500 rpm) and trades balancing mass (and hence drivability and comfort) for much sharper throttle response (and therefore sportiness) - a boon for high-performance and racing applications. Although it runs rougher than it otherwise would, this loss of smoothness is not as much of a hindrance in the high-performance driving conditions for which it was intended.

In short, different engine idle speeds work best for different situations: high idle speeds are best reserved for high-performance and racing applications, while most other cars (especially utility, offroad, and premium/luxury vehicles) deserve to have lower idle speeds. Finding the right idle speed range (i.e., the minimum and maximum recommended idle speeds) that works best for you is a crucial Automationeering skill - one that you should strive to learn the basics of at first, on your way to mastering it later on.