Understanding Minimum Speed for Turbojet Functionality

Turbojet engines, whether found in cars, ocean-going ferries, or other applications, have specific requirements for optimal performance. The minimum speed necessary for a turbojet to function effectively varies depending on the engine design and operating conditions. This article will explore the factors that influence this speed and provide insights into how turbojet functionality is affected.

The Role of Engine Type and RPM

The minimum speed needed for a turbojet to function can be influenced by the type of engine and its operating parameters, such as RPM (Revolutions Per Minute). Diesel engines, for example, can have turbochargers that start spinning even at idle, albeit not providing significant boost until higher RPMs are reached.

Car Diesel Engines - A Closer Look

In a 1.6-liter diesel car, the turbocharger can be heard whistling as early as 1500 RPM. At these lower speeds, the turbocharger may be spinning but not offering any noticeable boost. Once the throttle is floor, the turbocharger will spool up due to increased heat in the air, which forces more air into the engine and allows for more controlled diesel injection. This process is regulated by the intake air sensor.

Ocean-Going Ferries and Turbochargers

For ocean-going ferries, the engine may run at a very low RPM, typically around 150 RPM for a four-stroke engine. Here, the concept of RPM is also closely tied to when the turbocharger becomes active. Once the ship reaches a higher RPM (about 200 RPM), the turbocharger may be activated. This is a critical point where additional air pressure becomes available to enhance engine performance.

Petrol Engine Considerations

Gasoline engines have different considerations. Boosting too early in the RPM range can lead to detonation. Therefore, the onset of boost is typically above 2000 RPM. This helps prevent premature activation of the turbocharger, reducing the risk of engine damage and suboptimal performance.

Diesel Engines and Turbochargers

In diesel engines, the importance of increased air intake is paramount. In a 4-stroke cycle, more air means more power, as the power is controlled by the amount of diesel injected. During my apprenticeship, I worked on IH crawler tractors, and the diesel engines were 4-stroke. Even at idle, the turbocharger could be heard whistling. The effectiveness of the turbocharger depends on the engine brand. For example, GM Detroit engines used superchargers as they were two-stroke, while Perkins or Cummins engines relied on slower boost onset unless the throttle was opened.

When Does a Turbocharger 'Kick In'?

The timing of turbocharger activation is often determined by exhaust pressure, though RPMs can provide a better indicator. For instance, at 60 mph and 1800 RPM, the turbo might not be active, but at 20 mph, a hard acceleration might trigger the turbo at 2200 RPM. These are illustrative examples; the exact RPMs for turbo activation vary by vehicle and turbocharger size. Some engines use dual turbos, where a smaller turbo activates at lower RPMs, followed by a larger one that takes over at higher RPMs.

Conclusion

The minimum speed for a turbojet to function effectively is a complex issue influenced by engine type, RPMs, and specific operating conditions. By understanding these factors, car owners and engineers can optimize their turbocharged engines for better performance and efficiency. Whether in a car, ferry, or any other application, the correct use of turbocharging can significantly enhance engine power and efficiency.